Embryology - User contributions [en-gb]

Template:Respiratory Links

2012-02-29T20:30:26Z

S8600021:

Talk:Respiratory System - Abnormalities

2012-02-27T21:16:44Z

S8600021: /* 2009 */

{{Talk Page}}

==2010==

===Interstitial lung diseases in children===
Orphanet J Rare Dis. 2010 Aug 20;5:22.

Clement A, Nathan N, Epaud R, Fauroux B, Corvol H.
Source
Pediatric Pulmonary Department, Reference Center for Rare Lung Diseases, AP-HP, Hôpital Trousseau, Inserm UMR S-938, Université Pierre et Marie Curie-Paris 6, Paris, F-75012 France. annick.clement@trs.aphp.fr

Abstract

Interstitial lung disease (ILD) in infants and children comprises a large spectrum of rare respiratory disorders that are mostly chronic and associated with high morbidity and mortality. These disorders are characterized by inflammatory and fibrotic changes that affect alveolar walls. Typical features of ILD include dyspnea, diffuse infiltrates on chest radiographs, and abnormal pulmonary function tests with restrictive ventilatory defect and/or impaired gas exchange. Many pathological situations can impair gas exchange and, therefore, may contribute to progressive lung damage and ILD. Consequently, diagnosis approach needs to be structured with a clinical evaluation requiring a careful history paying attention to exposures and systemic diseases. Several classifications for ILD have been proposed but none is entirely satisfactory especially in children. The present article reviews current concepts of pathophysiological mechanisms, etiology and diagnostic approaches, as well as therapeutic strategies. The following diagnostic grouping is used to discuss the various causes of pediatric ILD: 1) exposure-related ILD; 2) systemic disease-associated ILD; 3) alveolar structure disorder-associated ILD; and 4) ILD specific to infancy. Therapeutic options include mainly anti-inflammatory, immunosuppressive, and/or anti-fibrotic drugs. The outcome is highly variable with a mortality rate around 15%. An overall favorable response to corticosteroid therapy is observed in around 50% of cases, often associated with sequelae such as limited exercise tolerance or the need for long-term oxygen therapy.

PMID 20727133 PMCID: PMC2939531

==2009==

===Surfactant Metabolism Dysfunction and Childhood Interstitial Lung Disease (chILD)===
Ulster Med J. 2009 Jan;78(1):7-9.

McFetridge L, McMorrow A, Morrison PJ, Shields MD.
Source
Royal Belfast Hospital for Sick Children, Queens University Belfast, Grosvenor Road, Belfast BT12 6BA, UK.

Abstract
Surfactant deficiency and the resultant respiratory distress syndrome (RDS) seen in preterm infants is a major cause of respiratory morbidity in this population. Until recently, the contribution of surfactant to respiratory morbidity in infancy was limited to the neonatal period. It is now recognised that inborn errors of surfactant metabolism leading to surfactant dysfunction account for around 10% of childhood interstitial lung disease (chILD). These abnormalities can be detected by blood sampling for mutation analysis, thereby avoiding the need for lung biopsy in some children with chILD.

PMID 19252722

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2629012

===Genetic disorders of surfactant dysfunction===
Pediatr Dev Pathol. 2009 Jul-Aug;12(4):253-74.

Wert SE, Whitsett JA, Nogee LM.
Source
Perinatal Institute, Section of Neonatology, Perinatal and Pulmonary Biology, Cincinnati Children's Hospital Medical Center, and the Department of Pediatrics, University of Cincinnati College of Medicine, 3333 Burnet Avenue, Cincinnati, OH 45229-3039, USA. susan.wert@cchmc.org

Abstract

Mutations in the genes encoding the surfactant proteins B and C (SP-B and SP-C) and the phospholipid transporter, ABCA3, are associated with respiratory distress and interstitial lung disease in the pediatric population. Expression of these proteins is regulated developmentally, increasing with gestational age, and is critical for pulmonary surfactant function at birth. Pulmonary surfactant is a unique mixture of lipids and proteins that reduces surface tension at the air-liquid interface, preventing collapse of the lung at the end of expiration. SP-B and ABCA3 are required for the normal organization and packaging of surfactant phospholipids into specialized secretory organelles, known as lamellar bodies, while both SP-B and SP-C are important for adsorption of secreted surfactant phospholipids to the alveolar surface. In general, mutations in the SP-B gene SFTPB are associated with fatal respiratory distress in the neonatal period, and mutations in the SP-C gene SFTPC are more commonly associated with interstitial lung disease in older infants, children, and adults. Mutations in the ABCA3 gene are associated with both phenotypes. Despite this general classification, there is considerable overlap in the clinical and histologic characteristics of these genetic disorders. In this review, similarities and differences in the presentation of these disorders with an emphasis on their histochemical and ultrastructural features will be described, along with a brief discussion of surfactant metabolism. Mechanisms involved in the pathogenesis of lung disease caused by mutations in these genes will also be discussed.

PMID 19220077

===Secretory phospholipase A2 pathway in various types of lung injury in neonates and infants: a multicentre translational study===

BMC Pediatr. 2011 Nov 8;11:101.

De Luca D, Capoluongo E, Rigo V; Study group on Secretory Phospholipase in Paediatrics (SSPP).
Collaborators (19)
Source
Pediatric Intensive Care Unit, Dept of Emergency and Intensive Care, University Hospital A.Gemelli, Catholic University of the Sacred Heart-Rome, Italy. dm.deluca@fastwebnet.it

Abstract
BACKGROUND:
Secretory phospholipase A2 (sPLA2) is a group of enzymes involved in lung tissue inflammation and surfactant catabolism. sPLA2 plays a role in adults affected by acute lung injury and seems a promising therapeutic target. Preliminary data allow foreseeing the importance of such enzyme in some critical respiratory diseases in neonates and infants, as well. Our study aim is to clarify the role of sPLA2 and its modulators in the pathogenesis and clinical severity of hyaline membrane disease, infection related respiratory failure, meconium aspiration syndrome and acute respiratory distress syndrome. sPLA2 genes will also be sequenced and possible genetic involvement will be analysed.
METHODS/DESIGN:
Multicentre, international, translational study, including several paediatric and neonatal intensive care units and one coordinating laboratory. Babies affected by the above mentioned conditions will be enrolled: broncho-alveolar lavage fluid, serum and whole blood will be obtained at definite time-points during the disease course. Several clinical, respiratory and outcome data will be recorded. Laboratory researchers who perform the bench part of the study will be blinded to the clinical data.
DISCUSSION:
This study, thanks to its multicenter design, will clarify the role(s) of sPLA2 and its pathway in these diseases: sPLA2 might be the crossroad between inflammation and surfactant dysfunction. This may represent a crucial target for new anti-inflammatory therapies but also a novel approach to protect surfactant or spare it, improving alveolar stability, lung mechanics and gas exchange.

PMID 22067747

==2007==

===Bronchopulmonary dysplasia: where have all the vessels gone? Roles of angiogenic growth factors in chronic lung disease===
Am J Respir Crit Care Med. 2007 May 15;175(10):978-85. Epub 2007 Feb 1.

Thébaud B, Abman SH.
Source
Department of Pediatrics, Division of Neonatology, Vascular Biology Group, University of Alberta, HMRC 407, Edmonton, AB, T6G 2S2, Canada. bthebaud@ualberta.ca

Abstract

Bronchopulmonary dysplasia and emphysema are significant global health problems at the extreme stages of life. Both are characterized by arrested alveolar development or loss of alveoli, respectively. Both lack effective treatment strategies. Knowledge about the genetic control of branching morphogenesis in mammals derives from investigations of the respiratory system in Drosophila, but mechanisms that regulate alveolar development remain poorly understood. Even less is known about regulation of the growth and development of the pulmonary vasculature. Understanding how alveoli and the underlying capillary network develop, and how these mechanisms are disrupted in disease states, are critical for developing effective therapies for lung diseases characterized by impaired alveolar structure. Recent observations have challenged old notions that the development of the blood vessels in the lung passively follows that of the airways. Rather, increasing evidence suggests that lung blood vessels actively promote alveolar growth during development and contribute to the maintenance of alveolar structures throughout postnatal life. Our working hypothesis is that disruption of angiogenesis impairs alveolarization, and that preservation of vascular growth and endothelial survival promotes growth and sustains the architecture of the distal airspace. Furthermore, the explosion of interest in stem cell biology suggests potential roles for endothelial progenitor cells in the pathogenesis or treatment of lung vascular disease. In this Pulmonary Perspective, we review recent data on the importance of the lung circulation, specifically examining the relationship between dysmorphic vascular growth and impaired alveolarization, and speculate on how these new insights may lead to novel therapeutic strategies for bronchopulmonary dysplasia.
Comment in
Am J Respir Crit Care Med. 2007 Oct 1;176(7):724-5; author reply 725.

PMID 17272782

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2747658/

==2005==

===Interstitial lung disease in children -- genetic background and associated phenotypes===
Respir Res. 2005 Apr 8;6:32.

Hartl D, Griese M.
Source
Pediatric Pneumology, Childrens' hospital of the Ludwig-Maximilians-University, Munich, Germany. dominic.hartl@med.uni-muenchen.de

Abstract

Interstitial lung disease in children represents a group of rare chronic respiratory disorders. There is growing evidence that mutations in the surfactant protein C gene play a role in the pathogenesis of certain forms of pediatric interstitial lung disease. Recently, mutations in the ABCA3 transporter were found as an underlying cause of fatal respiratory failure in neonates without surfactant protein B deficiency. Especially in familiar cases or in children of consanguineous parents, genetic diagnosis provides an useful tool to identify the underlying etiology of interstitial lung disease. The aim of this review is to summarize and to describe in detail the clinical features of hereditary interstitial lung disease in children. The knowledge of gene variants and associated phenotypes is crucial to identify relevant patients in clinical practice.

PMID 15819986

===Outcomes of congenital diaphragmatic hernia: a population-based study in Western Australia===

Pediatrics. 2005 Sep;116(3):e356-63.
Outcomes of congenital diaphragmatic hernia: a population-based study in Western Australia.
Colvin J, Bower C, Dickinson JE, Sokol J.

Department of Neonatal Pediatrics, Women's and Children's Health Service, Perth, Australia.
Erratum in:

Pediatrics. 2006 May;117(5):1870.
Abstract
OBJECTIVES: There have been many recent reports of improved survival rates for congenital diaphragmatic hernia (CDH), largely derived from institution-based data. These are often flawed by case selection bias. The objectives of this study were to document the true incidence, management, and outcomes of CDH in a geographically defined population over a 12-year period and to determine the changing trends in these over time. We also sought to ascertain the prenatal and postnatal factors associated with morbidity and death among these infants.

METHODS: A retrospective study of all cases of CDH in Western Australia from 1991 to 2002 was conducted. Cases were identified from 5 independent databases within the Western Australian health network, including the Western Australian Birth Defects Registry. All fetuses and neonates diagnosed with CDH in Western Australia during this period were identified, including miscarriages, stillbirths, and terminations of pregnancies in which a diagnosis of fetal CDH had been made, as well as those diagnosed postnatally. Cases not known to involve CDH until diagnosis at autopsy were also included. Infants with diaphragmatic eventration were excluded from the study. Detailed information was obtained from review of maternal and infant medical records.

RESULTS: One hundred sixteen cases of CDH were identified. Of these, 71 (61%) infants were born alive and 37 survived beyond 1 year of age (52% of live-born infants, 32% of all cases of CDH). Pregnancies involving 38 (33%) fetuses were terminated electively, 4 (3%) fetuses were aborted spontaneously, and 3 (3%) fetuses were stillborn. Another major congenital anomaly was present in 54 (47%) cases. Twenty-one (18%) cases had other anomalies that were likely to be fatal. Of all cases with an additional major anomaly, 42 (78%) died. Twenty-seven (71%) of 38 fetuses for whom the pregnancy was terminated had another major anomaly. Twenty-three (32%) live-born infants had another major anomaly (4 of which were considered fatal conditions); however, this did not affect their survival rates. Fifty-three percent of cases were diagnosed prenatally, and 49% of these pregnancies were then terminated. Of live-born infants with prenatally diagnosed CDH, 10 (33%) survived beyond 1 year of age. The gestational age at diagnosis did not affect the survival rate for live-born infants. Postnatal diagnosis occurred in 55 (47%) cases. Of these, 41 (74%) case subjects were born alive and diagnosed on clinical grounds after birth. In the remaining 14 cases, the diagnosis was made in postmortem examinations of fetuses from pregnancies that were terminated for other reasons (8 cases) or after spontaneous abortion or stillbirth (5 cases). Significant differences were found between prenatally and postnatally diagnosed live-born infants. Among live-born infants, prenatal diagnosis was associated with a significantly reduced survival rate (33%, compared with 66% for postnatally diagnosed infants). Prenatally diagnosed live-born infants were of lower birth weight and were born at an earlier gestational age. There was no statistically significant difference between the 2 groups in the onset of labor (spontaneous or induced) or in the rate of elective cesarean sections. Prenatally diagnosed live-born infants were more likely to be delivered in a tertiary perinatal center and were intubated more commonly at delivery. No difference was found in the Apgar scores at either 1 or 5 minutes between the groups. Of 71 live-born infants, 37 (52%) survived to 1 year of age. The majority of deaths occurred within the first 7 days of life (44%). Preoperative air leaks occurred for 16 (22%) infants, of whom 14 (88%) died. Factors found to predict death of live-born infants included prenatal diagnosis, right-sided hernia, major air leak, earlier gestational age at birth, lower birth weight, and lower Apgar scores at 1 and 5 minutes. Over the course of the decade, there were significant increases in the proportion of cases in which the diagnosis of CDH was made with prenatal ultrasonography and in the number of live-born infants born at the tertiary perinatal center. The mortality rate for all cases, the mortality rate for live-born infants, and the proportion of pregnancies involving prenatally diagnosed cases that were terminated electively were all greater in the later epoch but not significantly so.

CONCLUSIONS: This was a comprehensive, population-based study of CDH, with full case ascertainment, large sample size, and complete outcome data for all cases. The majority of published studies of CDH examined specific patient populations, such as neonates referred to tertiary pediatric surgical centers. Invariably, those studies failed to detect the demise of cases with CDH before arrival at the referral center, whether through termination of pregnancy, in utero fetal demise, or postnatal death occurring before transfer. Exclusion of these cases from calculations of mortality rates results in significant case selection bias. In our study, 35% of live-born infants died before referral or transport. The population of infants reaching the tertiary surgical center represented only 40% of the total cases of CDH. Wide variations in reported survival rates occur throughout the literature. These differences reflect the influence of this case selection bias, as well as variable referral policies and management practices. For our study population, survival rates differed vastly depending on the subgroup analyzed. Ninety-two percent of postoperative infants survived beyond 1 year of age, as did 80% of infants who reached the surgical referral center. However, only 52% of live-born infants, 32% of all cases, and 16% of all prenatally diagnosed cases survived. Therefore, the overall mortality rate for this condition remains high, despite increased prenatal detection, transfer to tertiary institutions for delivery, and advances in neonatal care, and is influenced significantly by the rate of prenatal termination. In our study, 33% of all cases of CDH and 49% of prenatally diagnosed fetuses underwent elective termination of pregnancy. This large number of fetal terminations confounds the accurate assessment of the true outcomes of this condition.

PMID 16140678
http://www.ncbi.nlm.nih.gov/pubmed/16140678

Talk:Respiratory System - Abnormalities

2012-02-27T21:15:00Z

S8600021: /* 2009 */

{{Talk Page}}

==2010==

===Interstitial lung diseases in children===
Orphanet J Rare Dis. 2010 Aug 20;5:22.

Clement A, Nathan N, Epaud R, Fauroux B, Corvol H.
Source
Pediatric Pulmonary Department, Reference Center for Rare Lung Diseases, AP-HP, Hôpital Trousseau, Inserm UMR S-938, Université Pierre et Marie Curie-Paris 6, Paris, F-75012 France. annick.clement@trs.aphp.fr

Abstract

Interstitial lung disease (ILD) in infants and children comprises a large spectrum of rare respiratory disorders that are mostly chronic and associated with high morbidity and mortality. These disorders are characterized by inflammatory and fibrotic changes that affect alveolar walls. Typical features of ILD include dyspnea, diffuse infiltrates on chest radiographs, and abnormal pulmonary function tests with restrictive ventilatory defect and/or impaired gas exchange. Many pathological situations can impair gas exchange and, therefore, may contribute to progressive lung damage and ILD. Consequently, diagnosis approach needs to be structured with a clinical evaluation requiring a careful history paying attention to exposures and systemic diseases. Several classifications for ILD have been proposed but none is entirely satisfactory especially in children. The present article reviews current concepts of pathophysiological mechanisms, etiology and diagnostic approaches, as well as therapeutic strategies. The following diagnostic grouping is used to discuss the various causes of pediatric ILD: 1) exposure-related ILD; 2) systemic disease-associated ILD; 3) alveolar structure disorder-associated ILD; and 4) ILD specific to infancy. Therapeutic options include mainly anti-inflammatory, immunosuppressive, and/or anti-fibrotic drugs. The outcome is highly variable with a mortality rate around 15%. An overall favorable response to corticosteroid therapy is observed in around 50% of cases, often associated with sequelae such as limited exercise tolerance or the need for long-term oxygen therapy.

PMID 20727133 PMCID: PMC2939531

==2009==

===Genetic disorders of surfactant dysfunction===
Pediatr Dev Pathol. 2009 Jul-Aug;12(4):253-74.

Wert SE, Whitsett JA, Nogee LM.
Source
Perinatal Institute, Section of Neonatology, Perinatal and Pulmonary Biology, Cincinnati Children's Hospital Medical Center, and the Department of Pediatrics, University of Cincinnati College of Medicine, 3333 Burnet Avenue, Cincinnati, OH 45229-3039, USA. susan.wert@cchmc.org

Abstract

Mutations in the genes encoding the surfactant proteins B and C (SP-B and SP-C) and the phospholipid transporter, ABCA3, are associated with respiratory distress and interstitial lung disease in the pediatric population. Expression of these proteins is regulated developmentally, increasing with gestational age, and is critical for pulmonary surfactant function at birth. Pulmonary surfactant is a unique mixture of lipids and proteins that reduces surface tension at the air-liquid interface, preventing collapse of the lung at the end of expiration. SP-B and ABCA3 are required for the normal organization and packaging of surfactant phospholipids into specialized secretory organelles, known as lamellar bodies, while both SP-B and SP-C are important for adsorption of secreted surfactant phospholipids to the alveolar surface. In general, mutations in the SP-B gene SFTPB are associated with fatal respiratory distress in the neonatal period, and mutations in the SP-C gene SFTPC are more commonly associated with interstitial lung disease in older infants, children, and adults. Mutations in the ABCA3 gene are associated with both phenotypes. Despite this general classification, there is considerable overlap in the clinical and histologic characteristics of these genetic disorders. In this review, similarities and differences in the presentation of these disorders with an emphasis on their histochemical and ultrastructural features will be described, along with a brief discussion of surfactant metabolism. Mechanisms involved in the pathogenesis of lung disease caused by mutations in these genes will also be discussed.

PMID 19220077

===Secretory phospholipase A2 pathway in various types of lung injury in neonates and infants: a multicentre translational study===

BMC Pediatr. 2011 Nov 8;11:101.

De Luca D, Capoluongo E, Rigo V; Study group on Secretory Phospholipase in Paediatrics (SSPP).
Collaborators (19)
Source
Pediatric Intensive Care Unit, Dept of Emergency and Intensive Care, University Hospital A.Gemelli, Catholic University of the Sacred Heart-Rome, Italy. dm.deluca@fastwebnet.it

Abstract
BACKGROUND:
Secretory phospholipase A2 (sPLA2) is a group of enzymes involved in lung tissue inflammation and surfactant catabolism. sPLA2 plays a role in adults affected by acute lung injury and seems a promising therapeutic target. Preliminary data allow foreseeing the importance of such enzyme in some critical respiratory diseases in neonates and infants, as well. Our study aim is to clarify the role of sPLA2 and its modulators in the pathogenesis and clinical severity of hyaline membrane disease, infection related respiratory failure, meconium aspiration syndrome and acute respiratory distress syndrome. sPLA2 genes will also be sequenced and possible genetic involvement will be analysed.
METHODS/DESIGN:
Multicentre, international, translational study, including several paediatric and neonatal intensive care units and one coordinating laboratory. Babies affected by the above mentioned conditions will be enrolled: broncho-alveolar lavage fluid, serum and whole blood will be obtained at definite time-points during the disease course. Several clinical, respiratory and outcome data will be recorded. Laboratory researchers who perform the bench part of the study will be blinded to the clinical data.
DISCUSSION:
This study, thanks to its multicenter design, will clarify the role(s) of sPLA2 and its pathway in these diseases: sPLA2 might be the crossroad between inflammation and surfactant dysfunction. This may represent a crucial target for new anti-inflammatory therapies but also a novel approach to protect surfactant or spare it, improving alveolar stability, lung mechanics and gas exchange.

PMID 22067747

==2007==

===Bronchopulmonary dysplasia: where have all the vessels gone? Roles of angiogenic growth factors in chronic lung disease===
Am J Respir Crit Care Med. 2007 May 15;175(10):978-85. Epub 2007 Feb 1.

Thébaud B, Abman SH.
Source
Department of Pediatrics, Division of Neonatology, Vascular Biology Group, University of Alberta, HMRC 407, Edmonton, AB, T6G 2S2, Canada. bthebaud@ualberta.ca

Abstract

Bronchopulmonary dysplasia and emphysema are significant global health problems at the extreme stages of life. Both are characterized by arrested alveolar development or loss of alveoli, respectively. Both lack effective treatment strategies. Knowledge about the genetic control of branching morphogenesis in mammals derives from investigations of the respiratory system in Drosophila, but mechanisms that regulate alveolar development remain poorly understood. Even less is known about regulation of the growth and development of the pulmonary vasculature. Understanding how alveoli and the underlying capillary network develop, and how these mechanisms are disrupted in disease states, are critical for developing effective therapies for lung diseases characterized by impaired alveolar structure. Recent observations have challenged old notions that the development of the blood vessels in the lung passively follows that of the airways. Rather, increasing evidence suggests that lung blood vessels actively promote alveolar growth during development and contribute to the maintenance of alveolar structures throughout postnatal life. Our working hypothesis is that disruption of angiogenesis impairs alveolarization, and that preservation of vascular growth and endothelial survival promotes growth and sustains the architecture of the distal airspace. Furthermore, the explosion of interest in stem cell biology suggests potential roles for endothelial progenitor cells in the pathogenesis or treatment of lung vascular disease. In this Pulmonary Perspective, we review recent data on the importance of the lung circulation, specifically examining the relationship between dysmorphic vascular growth and impaired alveolarization, and speculate on how these new insights may lead to novel therapeutic strategies for bronchopulmonary dysplasia.
Comment in
Am J Respir Crit Care Med. 2007 Oct 1;176(7):724-5; author reply 725.

PMID 17272782

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2747658/

==2005==

===Interstitial lung disease in children -- genetic background and associated phenotypes===
Respir Res. 2005 Apr 8;6:32.

Hartl D, Griese M.
Source
Pediatric Pneumology, Childrens' hospital of the Ludwig-Maximilians-University, Munich, Germany. dominic.hartl@med.uni-muenchen.de

Abstract

Interstitial lung disease in children represents a group of rare chronic respiratory disorders. There is growing evidence that mutations in the surfactant protein C gene play a role in the pathogenesis of certain forms of pediatric interstitial lung disease. Recently, mutations in the ABCA3 transporter were found as an underlying cause of fatal respiratory failure in neonates without surfactant protein B deficiency. Especially in familiar cases or in children of consanguineous parents, genetic diagnosis provides an useful tool to identify the underlying etiology of interstitial lung disease. The aim of this review is to summarize and to describe in detail the clinical features of hereditary interstitial lung disease in children. The knowledge of gene variants and associated phenotypes is crucial to identify relevant patients in clinical practice.

PMID 15819986

===Outcomes of congenital diaphragmatic hernia: a population-based study in Western Australia===

Pediatrics. 2005 Sep;116(3):e356-63.
Outcomes of congenital diaphragmatic hernia: a population-based study in Western Australia.
Colvin J, Bower C, Dickinson JE, Sokol J.

Department of Neonatal Pediatrics, Women's and Children's Health Service, Perth, Australia.
Erratum in:

Pediatrics. 2006 May;117(5):1870.
Abstract
OBJECTIVES: There have been many recent reports of improved survival rates for congenital diaphragmatic hernia (CDH), largely derived from institution-based data. These are often flawed by case selection bias. The objectives of this study were to document the true incidence, management, and outcomes of CDH in a geographically defined population over a 12-year period and to determine the changing trends in these over time. We also sought to ascertain the prenatal and postnatal factors associated with morbidity and death among these infants.

METHODS: A retrospective study of all cases of CDH in Western Australia from 1991 to 2002 was conducted. Cases were identified from 5 independent databases within the Western Australian health network, including the Western Australian Birth Defects Registry. All fetuses and neonates diagnosed with CDH in Western Australia during this period were identified, including miscarriages, stillbirths, and terminations of pregnancies in which a diagnosis of fetal CDH had been made, as well as those diagnosed postnatally. Cases not known to involve CDH until diagnosis at autopsy were also included. Infants with diaphragmatic eventration were excluded from the study. Detailed information was obtained from review of maternal and infant medical records.

RESULTS: One hundred sixteen cases of CDH were identified. Of these, 71 (61%) infants were born alive and 37 survived beyond 1 year of age (52% of live-born infants, 32% of all cases of CDH). Pregnancies involving 38 (33%) fetuses were terminated electively, 4 (3%) fetuses were aborted spontaneously, and 3 (3%) fetuses were stillborn. Another major congenital anomaly was present in 54 (47%) cases. Twenty-one (18%) cases had other anomalies that were likely to be fatal. Of all cases with an additional major anomaly, 42 (78%) died. Twenty-seven (71%) of 38 fetuses for whom the pregnancy was terminated had another major anomaly. Twenty-three (32%) live-born infants had another major anomaly (4 of which were considered fatal conditions); however, this did not affect their survival rates. Fifty-three percent of cases were diagnosed prenatally, and 49% of these pregnancies were then terminated. Of live-born infants with prenatally diagnosed CDH, 10 (33%) survived beyond 1 year of age. The gestational age at diagnosis did not affect the survival rate for live-born infants. Postnatal diagnosis occurred in 55 (47%) cases. Of these, 41 (74%) case subjects were born alive and diagnosed on clinical grounds after birth. In the remaining 14 cases, the diagnosis was made in postmortem examinations of fetuses from pregnancies that were terminated for other reasons (8 cases) or after spontaneous abortion or stillbirth (5 cases). Significant differences were found between prenatally and postnatally diagnosed live-born infants. Among live-born infants, prenatal diagnosis was associated with a significantly reduced survival rate (33%, compared with 66% for postnatally diagnosed infants). Prenatally diagnosed live-born infants were of lower birth weight and were born at an earlier gestational age. There was no statistically significant difference between the 2 groups in the onset of labor (spontaneous or induced) or in the rate of elective cesarean sections. Prenatally diagnosed live-born infants were more likely to be delivered in a tertiary perinatal center and were intubated more commonly at delivery. No difference was found in the Apgar scores at either 1 or 5 minutes between the groups. Of 71 live-born infants, 37 (52%) survived to 1 year of age. The majority of deaths occurred within the first 7 days of life (44%). Preoperative air leaks occurred for 16 (22%) infants, of whom 14 (88%) died. Factors found to predict death of live-born infants included prenatal diagnosis, right-sided hernia, major air leak, earlier gestational age at birth, lower birth weight, and lower Apgar scores at 1 and 5 minutes. Over the course of the decade, there were significant increases in the proportion of cases in which the diagnosis of CDH was made with prenatal ultrasonography and in the number of live-born infants born at the tertiary perinatal center. The mortality rate for all cases, the mortality rate for live-born infants, and the proportion of pregnancies involving prenatally diagnosed cases that were terminated electively were all greater in the later epoch but not significantly so.

CONCLUSIONS: This was a comprehensive, population-based study of CDH, with full case ascertainment, large sample size, and complete outcome data for all cases. The majority of published studies of CDH examined specific patient populations, such as neonates referred to tertiary pediatric surgical centers. Invariably, those studies failed to detect the demise of cases with CDH before arrival at the referral center, whether through termination of pregnancy, in utero fetal demise, or postnatal death occurring before transfer. Exclusion of these cases from calculations of mortality rates results in significant case selection bias. In our study, 35% of live-born infants died before referral or transport. The population of infants reaching the tertiary surgical center represented only 40% of the total cases of CDH. Wide variations in reported survival rates occur throughout the literature. These differences reflect the influence of this case selection bias, as well as variable referral policies and management practices. For our study population, survival rates differed vastly depending on the subgroup analyzed. Ninety-two percent of postoperative infants survived beyond 1 year of age, as did 80% of infants who reached the surgical referral center. However, only 52% of live-born infants, 32% of all cases, and 16% of all prenatally diagnosed cases survived. Therefore, the overall mortality rate for this condition remains high, despite increased prenatal detection, transfer to tertiary institutions for delivery, and advances in neonatal care, and is influenced significantly by the rate of prenatal termination. In our study, 33% of all cases of CDH and 49% of prenatally diagnosed fetuses underwent elective termination of pregnancy. This large number of fetal terminations confounds the accurate assessment of the true outcomes of this condition.

PMID 16140678
http://www.ncbi.nlm.nih.gov/pubmed/16140678

Respiratory System - Abnormalities

2012-02-27T21:11:42Z

S8600021: /* Search Pubmed */

==Introduction==

Abnormalities of the respiratory system include not only lung development but also the upper respiratory tract, the supporting musculoskeletal system and the vascular system. In addition, some respiratory problems arise from prematurity of birth or difficulty with the birth process itself.

:{{Template:Respiratory Links}}

:{{Template:Abnormality Links}}

==Some Recent Findings==

* '''Surfactant Metabolism Dysfunction and Childhood Interstitial Lung Disease (chILD).'''<ref><pubmed>19252722</pubmed></ref> "Surfactant deficiency and the resultant respiratory distress syndrome (RDS) seen in preterm infants is a major cause of respiratory morbidity in this population. Until recently, the contribution of surfactant to respiratory morbidity in infancy was limited to the neonatal period. It is now recognised that inborn errors of surfactant metabolism leading to surfactant dysfunction account for around 10% of childhood interstitial lung disease (chILD)."

==Tracheoesophageal Fistula ==
(Tracheo-Oesophageal Fistula, Oesophageal Atresia) - Oesophageal Atresia with or without tracheo-oesophageal fistula

==Lobar Emphysema (Overinflated Lung)==
[[File:Congenital lobar emphysema.jpg|thumb|Congenital lobar emphysema]]
# There is an overinflated left upper lobe
# There is a collapsed lower lobe
# The left lung is herniating across the mediastinum

==Congenital Diaphragmatic Hernia==
[[File:Adult_diaphragm.jpg|thumb|Normal Adult Diaphragm]]
Failure of the pleuroperitoneal foramen (foramen of Bochdalek) to close allows viscera into thorax. Intestine, stomach or spleen can enter the pleural cavity, compressing the lung.

===Australian Statistics===

A recent Western Australian study<ref><pubmed>16140678</pubmed></ref> of congenital diaphragmatic hernia (CDH) outcomes showed:

* 35% of live-born infants died before referral or transport.
* population of infants reaching center represented only 40% of the total cases
* 92% percent of postoperative infants survived beyond 1 year of age
* 80% of infants who reached the surgical referral center
* only 52% of live-born infants, 32% of all cases, and 16% of all prenatally diagnosed cases survived.
* the overall mortality rate for this condition remains high
* 33% of all cases of CDH and 49% of prenatally diagnosed fetuses underwent elective termination of pregnancy
* the number of fetal terminations confounds the accurate assessment of the true outcomes of this condition

:'''Links:''' [[Musculoskeletal System - Abnormalities]] | [http://www.ncbi.nlm.nih.gov/books/NBK1359 GeneReviews]

==Azygos Lobe==
[[File:Lung_Azygos_Lobe.jpg|thumb|Lung azygos lobe in the adult.]]

[[File:Lung_Azygos_Lobe_02.jpg‎|300px]]

Common anatomical variation occurring in about 0.5% of the population. The right lung upper lobe expands either side of the posterior cardinal. There is also some course variability of the phrenic nerve in the presence of an azygos lobe.

==Congenital Laryngeal Webs==
Laryngeal abnormality due to embryonic (week 10) incomplete recanalization of the laryngotracheal tube during the fetal period. Rare abnormality occuring mainly at the level of the vocal folds (glottis).

==Meconium Aspiration Syndrome==
[[File:Meconium aspiration syndrome 01.jpg|thumb|Newborn X-ray Meconium aspiration syndrome]]
(MAS) Meconium is the gastrointestinal contents that accumulate in the intestines during the fetal period. Fetal stress in the third trimester, prior to/at/ or during parturition (birth) can lead to premature meconium discharge into the amniotic fluid and sunsequent ingestion by the fetus and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur.

* meconium is formed from gut and associated organ secretions as well as cells and debris from the swallowed amniotic fluid.
* Meconium accumulates during the fetal period in the large intestine (bowel). It can be described as being a generally dark colour (green black) , sticky and odourless.
* Normally this meconium is defaecated (passed) postnatally over the first 48 hours and then transitional stools from day 4.
* Abnormally this meconium is defaecated in utero, due to oxygen deprivation and other stresses. Premature discharge into the amniotic sac can lead to mixing with amniotic fluid and be reswallowed by the fetus. This is meconium aspiration syndrome and can damage both the developing lungs and placental vessels.

===Australian Statistics===

The following Australia and New Zealand (1995 - 2002) data is from a recent (2009) study, the epidemiology of meconium aspiration syndrome: incidence, risk factors, therapies, and outcome.<ref name="PMID16651329"><pubmed>16651329</pubmed></ref>

* Data were gathered on all of the infants in Australia and New Zealand who were intubated and mechanically ventilated with a primary diagnosis of MAS (MASINT) between 1995 and 2002, inclusive.
* MASINT occurred in 1061 of 2,490,862 live births (0.43 of 1000), with a decrease in incidence from 1995 to 2002.
* A higher risk of MASINT was noted at advanced gestation, with 34% of cases born beyond 40 weeks, compared with 16% of infants without MAS.
* Fetal distress requiring obstetric intervention was noted in 51% of cases, and 42% were delivered by cesarean section.
* There was a striking association between low 5-minute Apgar score and MASINT.
* Risk of MASINT was higher where maternal ethnicity was Pacific Islander or indigenous Australian and was also increased after planned home birth.
* Uptake of exogenous surfactant, high-frequency ventilation, and inhaled nitric oxide increased considerably during the study period, with >50% of infants receiving > or =1 of these therapies by 2002.
* Risk of air leak was 9.6% overall, with an apparent reduction to 5.3% in 2001-2002.
* The duration of intubation remained constant throughout the study period (median: 3 days), whereas duration of oxygen therapy and length of hospital stay increased.
* Death related to MAS occurred in 24 infants (2.5% of the MASINT cohort; 0.96 per 100,000 live births).

==Newborn Respiratory Distress Syndrome==
(Hyaline Membrane Disease) [http://www.nlm.nih.gov/MEDLINEPLUS/ency/article/001563.htm medline plus] | [http://www.medscape.com/article/976034-overview eMedicine]

==Surfactant Metabolism==
(pulmonary surfactant metabolism dysfunctions, surfactant dysfunction disorders) For review of genetic disorders of surfactant dysfunction<ref><pubmed>19220077</pubmed></ref>

Mutations in the genes encoding:
* surfactant protein B (SP-B)
* surfactant protein C ( SP-C)
* phospholipid transporter ABCA3

==Bronchopulmonary Dysplasia==
A chronic lung disease which can occur following premature birth and related lung injury. The definition of bronchopulmonary dysplasia (BPD) has in recent years changed from a severe lung injury and associated repair, to more of a disruption of lung growth in older infants.<ref><pubmed>19712501</pubmed></ref>

Most infants who develop BPD are born more than 10 weeks before their due dates, weigh less than 1,000 grams (about 2 pounds) at birth, and have breathing problems. Infections that occur before or shortly after birth also can contribute to BPD.

:'''Links:''' [http://www.nhlbi.nih.gov/health/health-topics/topics/bpd NIH - NHLBI]

==Cystic Fibrosis==
[[File:UK deaths from cystic fibrosis.jpg|thumb|UK deaths from cystic fibrosis <ref>PMID 21862532 | [http://www.bmj.com/content/343/bmj.d4662.full BMJ]</ref>]]

Cystic Fibrosis (CF) is a serious genetic disease due to abnormal chloride channel synthesis (cystic fibrosis transmembrane conductance regulator, CFTR), the impact occurs postnatally. Mucus accumulates mainly in the passages of the lungs and in the pancreas.

:'''Links:''' [http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001167/ PubMed Health] | [http://omim.org/entry/219700 OMIM] | [http://www.nhlbi.nih.gov/health/dci/Diseases/cf/cf_what.html USA National Heart Lung and Blood Institute] | [http://www.cysticfibrosis.org.au Cystic Fibrosis Australia]

==OMIM==

List of respiratory related abnormalities [[OMIM_References#Respiratory|Respiratory]] and [[OMIM_References#Diaphragmatic_Hernia|Diaphragmatic Hernia]].

==References==

<references/>

===Reviews===
<pubmed>20727133</pubmed>
<pubmed>17272782</pubmed>
<pubmed>15819986</pubmed>

===Articles===
<pubmed>19381312</pubmed>
<pubmed>20004027</pubmed>
<pubmed>20318245</pubmed>
<pubmed>2017954</pubmed>
<pubmed>6787889</pubmed>
<pubmed>19436785</pubmed>

===Search Pubmed===

'''Search Pubmed:''' [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=Respiratory%20System%20Developmental%20Abnormalities Respiratory System Developmental Abnormalities] | [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=Tracheoesophageal%20Fistula Tracheoesophageal Fistula] |
[http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=Bronchopulmonary%20Dysplasia Bronchopulmonary Dysplasia] | [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=Congenital%20Laryngeal%20Webs Congenital Laryngeal Webs] | [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=Hyaline+Membrane+Disease Hyaline Membrane Disease] | [http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=Meconium+Aspiration+Syndrome Meconium Aspiration Syndrome]

==External Links==

{{External Links}}

* [http://www.aafp.org/afp/2007/1001/p987.html AAFP - Respiratory Distress in the Newborn]
* [http://www.adhb.govt.nz/newborn/teachingresources/radiology/lungparenchyma.htm NZ - Parenchymal Lung Disease]
* [http://www.cysticfibrosis.org.au Cystic Fibrosis Australia]

{{Template:Glossary}}
{{Template:Footer}}

SH Lecture - Respiratory System Development

2012-02-27T21:10:01Z

S8600021: /* Fetal Respiratory Movements */

[[File:SHsmall.jpg]]
==Introduction==
[[Image:Mark Hill.jpg|100px|left]]
[[File:Respiratory tract.jpg|thumb|Respiratory tract]]
The lecture will introduce the development of the respiratory system and associated structures. The lecture will not cover adult anatomy, physiology of gas exchange and red blood cell function and cardiovascular development.

:{{Template:SH2011}}

Start Time/End Time: 10am to 11am Thursday 1 March 2012 Clancy Auditorium [http://emed.med.unsw.edu.au/Map.nsf/0/6FF17DFEF645DABACA2573390006292A?OpenDocument&login eMed Link to Learning Activity - Respiratory System Development]

The respiratory system does not carry out its physiological function (of gas exchange) until after birth, though the respiratory tract, diaphragm and lungs do begin to form early in embryonic development and continue through fetal development, only functionally maturing just before birth.

The respiratory tract is divided anatomically into 2 main parts:
# '''upper respiratory tract''' - consisting of the nose, nasal cavity and the pharynx.
# '''lower respiratory tract''' - consisting of the larynx, trachea, bronchi and the lungs.

The respiratory "system" usually includes descriptions of not only the functional development of the lungs, but also related musculoskeletal (diaphragm) and vascular (pulmonary) development.

===Aims===
[[File:Historic-lungs.jpg|thumb|adult lungs]]
To understand the prenatal and postnatal developmental anatomy of human respiratory organs.
===Key Concepts===
# Embryonic origin of respiratory components (tract, lungs, diaphragm, muscles)
# Key stages in respiratory development.
# Time course of respiratory development.
# Respiration at birth.
# Postnatal development of respiration.
# Developmental abnormalities.

[[File:Gray0974.jpg|thumb|lung structure]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]

==Textbooks==

{|
| [[File:The Developing Human, 8th edn.jpg|80px]]
| Moore, K.L. & Persuad, T.V.N. (2008). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-1-4160-3706-4&eid=4-u1.0-B978-1-4160-3706-4..50013-X Chapter 10 - The Respiratory System] (chapter links only work with a UNSW connection).
|-
| [[File:Larsen's human embryology 4th edn.jpg|80px]]
| Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-0-443-06811-9&eid=4-u1.0-B978-0-443-06811-9..10011-9 Chapter 11 - Development of the Respiratory System and Body Cavities] (chapter links only work with a UNSW connection).
|-
| [[File:Logo.png|80px]]
| Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

* {{Respiratory Links}}
|}

===Additional Textbooks===

* Before We Are Born (5th ed.) Moore and Persaud Chapter 13 p255-287
* Essentials of Human Embryology Larson Chapter 9 p123-146
* Human Embryology Fitzgerald and Fitzgerald Chapter 19,20 p119-123
* Developmental Biology 8e Online[http://8e.devbio.com/article.php?ch=15&id=157 Lung Branching Morphogenesis]
* Anatomy of the Human Body 1918 Henry Gray [http://www.bartleby.com/107/235.html 1. The Respiratory Apparatus]
* [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology - Respiratory]

==Lecture Objectives==
[[File:Gray0974.jpg|thumb|lung structure]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]
* Understanding of embryonic lung development
* Understanding of 4 stages of lung development
* Understanding of diaphragm development
* Brief understanding of respiratory vascular development
* Brief understanding of respiratory abnormalities
* Brief understanding of molecular mechanisms

{|
| [[File:Podcast_icon.jpg|link=ANAT2341_Embryology_2011_Lecture_Recordings]]
| '''Lectopia Lecture Audio'''

[http://lectopia.telt.unsw.edu.au/lectopia/lectopia.lasso?ut=153&id=110475 Lecture 10 Audio]
|}

==Textbooks==

{|
| [[File:The Developing Human, 8th edn.jpg|80px]]
| Moore, K.L. & Persuad, T.V.N. (2008). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-1-4160-3706-4&eid=4-u1.0-B978-1-4160-3706-4..50013-X Chapter 10 - The Respiratory System] (chapter links only work with a UNSW connection).
|-
| [[File:Larsen's human embryology 4th edn.jpg|80px]]
| Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-0-443-06811-9&eid=4-u1.0-B978-0-443-06811-9..10011-9 Chapter 11 - Development of the Respiratory System and Body Cavities] (chapter links only work with a UNSW connection).
|-
| [[File:Logo.png|80px]]
| Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

* {{Respiratory Links}}

* '''Anatomy of the Human Body''' 1918 Henry Gray [http://www.bartleby.com/107/235.html 1. The Respiratory Apparatus]
* '''Developmental Biology''' 8e Online [http://8e.devbio.com/article.php?ch=15&id=157 Lung Branching Morphogenesis]

|}

==Developmental Overview==
[[File:Stage14 respiratory tract.jpg|thumb|Week 5 Respiratory Development]]

===Lung Development===

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Germ Layers===
* Endoderm and splanchnic mesoderm form majority of conducting and alveoli.
* Ectoderm will contribute the neural innervation.
* Mesoderm also contributes the supporting musculoskeletal components.

===Events===
* '''Week 4''' - laryngotracheal groove forms on floor foregut.
* '''Week 5''' - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)
* '''Week 6''' - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.
* '''Week 7''' - enlargement of liver stops descent of heart and lungs.
* '''Month 3-6''' - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.
* '''Month 7''' - respiratory bronchioles proliferate and end in alveolar ducts and sacs.

==Lung Development Stages==
[[File:Lung_alveoli_development_cartoon.jpg|thumb|300px]]
The sequence is most important rather than the actual timing, which is variable in the existing literature.

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Embryonic===
* '''week 4 - 5'''
* Endoderm - tubular ventral growth from foregut pharynx.
* Mesoderm - mesenchyme of lung buds.
* Intraembryonic coelom - pleural cavities elongated spaces connecting pericardial and peritoneal spaces.
===Pseudoglandular stage===
* '''week 5 - 17'''
* tubular branching of the human lung airways continues
* by 2 months all segmental bronchi are present.
* lungs have appearance of a glandlike structure.
* stage is critical for the formation of all conducting airways.
** lined with '''tall columnar epithelium'''
** more distal structures are lined with '''cuboidal epithelium'''.

===Canalicular stage===

* '''week 16 - 24'''
* Lung morphology changes dramatically
* differentiation of the pulmonary epithelium results in the formation of the future air-blood tissue barrier.
* '''Surfactant''' synthesis and the canalization of the lung parenchyma by capillaries begin.
* future gas exchange regions can be distinguished from the future conducting airways of the lungs.

===Saccular stage===
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* '''week 24 to near term.'''
* most peripheral airways form widened "airspaces", termed saccules.
* saccules widen and lengthen the airspace (by the addition of new generations).
* future gas exchange region expands significantly.
* Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin.
** May have a role in epithelial differentiation and control of surfactant secretion.
* The vascular tree also grows in length and diameter during this time.

==Foregut development==
[[File:Head arches cartoon.jpg|thumb|Foregut cartoon]]
From the oral cavity the next portion of the foregut is initially a single gastrointestinal (oesophagus) and respiratory (trachea) common tube, the pharynx which lies behind the heart. Note that the respiratory tract will form from a ventral bud arising at this level.

* Oral cavity
* Pharynx (esophagus, trachea)
* Respiratory tract
* Stomach

==Upper respiratory tract==
[[File:Gray0961.jpg|thumb|Adult upper respiratory tract conducting system]]
<gallery>
File:Gitbpm.jpg|stage 11 foregut
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Stage_22_image_167.jpg|Stage 22 trachea
File:Head_arches_cartoon.jpg|Head arches cartoon
File:Pharynx_cartoon.jpg|Pharynx
File:Nasal cavities.jpg|Nasal cavities
File:Pharynx.jpg|Pharynx
File:Larynx.jpg|Larynx
</gallery>
* part of foregut development
* anatomically the nose, nasal cavity and the pharynx
* the pharynx forms a major arched cavity within the pharyngeal arches

'''MH''' - pharyngeal arches will be described in head development lecture

==Lower respiratory tract==
<gallery>
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Gray0982b.jpg|week 4 later ventral endoderm growth
File:Bronchi lungs.jpg|lower respiratory tract
File:Respiratory tract.jpg|conducting system bronchi to lungs
</gallery>
[[File:Stage14-22 lungs.jpg]] [[File:Stage_22_image_171.jpg|300px]]

[[File:Lung alveoli development cartoon.jpg|thumb|Lung alveoli development cartoon]]
[[File:Fetal lung histology.jpg|thumb|Fetal lung histology]]

* lung buds ( endoderm epithelial tubes) grow/push into mesenchyme covered with pleural cells (lung border)
* generates a tree-like network by repeated:
# elongation
# terminal bifurcation
# lateral budding
* The lungs go through an embryonic and 4 distinct histological phases of development

Growth initially of branched "conducting" system of bronchial tree, followed by later development of the "functional units" of the alveoli.

* '''embryonic''' - week 4 - 5 (stage 14 above)
* '''pseudoglandular''' - week 5 - 17 (stage 22 above)
* '''canalicular''' - week 16 - 25
* '''terminal sac''' - week 24 - 40
* '''alveolar''' - late fetal - 8 years (Latin, ''alveus'' = cavity or hollow)

===Fetal lung volume===
Each human lung volume as determined by ultrasound and matched to gestational age <ref><pubmed>16388511</pubmed></ref>
{|
| Weeks (gestational)
| Volume (ml)
|-
| 12 to 13
| 0.05
|-
| 19 to 22
| 0.5
|-
| 29 to 32
| 1.9
|}

== Pleural Cavity ==
[[File:Gray0965.jpg|thumb|pleura]]
[[File:Gray0968.jpg|thumb|pleura]]
* The anatomical body cavity in which the lungs develop and lie.
* The pleural cavity forms in the lateral plate mesoderm as part of the early single intraembryonic coelom.
* This cavity is initially continuous with pericardial and peritoneal cavities and form initially as two narrow canals
** later becomes separated by folding (pleuropericardial fold, pleuroperitoneal membrane) and the later formation of the diaphragm

pleuropericardial fold - (pleuropericardial membrane) An early embryonic fold which restricts the communication between pleural cavity and pericardiac cavity, contains both the cardinal vein and phrenic nerve.

pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity.

===Pleura===
* serous membrane covers the surface of the lung and the spaces between the lobes
* arranged as a closed invaginated sac
* two layers (pulmonary, parietal) continuous with each other, the potential space between them is the '''pleural cavity'''

==Diaphragm==
* Not respiratory tract but musculoskeletal development, there are '''5 embryonic elements''' that contribute to the diaphragm.
{|
| [[File:Diaphragm components.jpg|300px|Components of the diaphragm]]
|
# septum transversum- central tendon
# 3rd to 5th somite- musculature of diaphragm
# ventral pleural sac- connective tissue
# mesentry of oesophagus- connective tissue around oesophasus and IVC
# pleuroperitoneal membranes- connective tissue around central tendon
|}
[[File:Gray804.gif|thumb|Adult Cervical Plexus (phrenic nerve shown lower right)]]
[[File:Adult diaphragm.jpg|300px|adult diaphragm]]

* Innervation of the human diaphragm is by the '''phrenic nerves'''
** arising from the same segmental levels from which the diaphragm skeletal muscles arise, segmental levels C3 to C5.
* The paired phrenic nerves are '''mixed nerves'''
** motor neurons for the diaphragm
** sensory nerves for other abdominal structures (mediastinum, pleura, liver, gall bladder).

==Pulmonary Circulation==
[[File:Pulmonary circulation cartoon.jpg|thumb|300px|Pulmonary circulation]]
* the pulmonary system not "functional" until after birth
* pulmonary arteries - 6th aortic arch arteries
* pulmonary veins - are incorporated into the left atrium wall
* bronchial arteries - branches from dorsal aorta

==Fetal==
===Fetal Respiratory Movements===
* Fetal respiratory movements (FRM) or Fetal breathing movements (FBM) are regular muscular contrations occurring in the third trimester.
* preparing the respiratory muscular system for neonatal function.
* may also have a role in late lung development.

==The First Breath==
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* The respiratory system does not carry out its physiological function (gas exchange) prenatally and remain entirely fluid-filled until birth.
* At birth, fluid in the upper respiratory tract is expired and fluid in the lung aveoli is rapidly absorbed this event has also been called "dewatering of the lung".
** The lung epithelia has to now rapidly change from its prenatal secretory function to that of fluid absorbtion.

The exchange of lung fluid for air leads to:
* fall in pulmonary vascular resistance
* increase in pulmonary blood flow
* thinning of pulmonary arteries (stretching as lungs increase in size)
* blood fills the alveolar capillaries

In the heart, pressure in the right side of the heart decreases and pressure in the left side of the heart increases (more blood returning from pulmonary).
[[File:Neonatal rib orientation.jpg|thumb|Rib orientation]]
* Respiratory Rate is higher than adult (30 breaths/minute).

===Rib Orientation===
* Infant rib - is virtually horizontal, allowing diaphragmatic breathing only.
* Adult rib - is oblique (both anterior and lateral views), allows for pump-handle and bucket handle types of inspiration.

== Respiratory Tract Abnormalities ==
[[Respiratory System - Abnormalities]]
===Tracheoesophageal Fistula ===
(Tracheo-Oesophageal Fistula, Oesophageal Atresia) - Oesophageal Atresia with or without tracheo-oesophageal fistula

===Lobar Emphysema (Overinflated Lung)===
# There is an overinflated left upper lobe
# There is a collapsed lower lobe
# The left lung is herniating across the mediastinum

===Congenital Diaphragmatic Hernia===
Failure of the pleuroperitoneal foramen (foramen of Bochdalek) to close allows viscera into thorax. Intestine, stomach or spleen can enter the pleural cavity, compressing the lung.

===Azygos Lobe===
[[File:Lung_Azygos_Lobe_02.jpg|thumb|Lung Azygos Lobe]]
* Common condition (0.5% of population).
* The right lung upper lobe expands either side of the posterior cardinal.
* There is also some course variability of the phrenic nerve in the presence of an azygos lobe.

===Congenital Laryngeal Webs===
* Laryngeal abnormality due to embryonic (week 10) incomplete recanalization of the laryngotracheal tube during the fetal period.
* Rare abnormality occuring mainly at the level of the vocal folds (glottis).

===Meconium Aspiration Syndrome===

* (MAS) Meconium is the gastrointestinal contents that accumulate in the intestines during the fetal period.
* Fetal stress in the third trimester, prior to/at/ or during parturition can lead to premature meconium discharge into the amniotic fluid.
* Subsequent ingestion by the fetus and damage to respiratory function.
* Damage to placental vessels '''meconium myonecrosis''' may also occur.

===Newborn Respiratory Distress Syndrome===
* (Hyaline Membrane Disease) [http://www.nlm.nih.gov/MEDLINEPLUS/ency/article/001563.htm medline plus] | [http://www.medscape.com/article/976034-overview eMedicine]

===Bronchopulmonary Dysplasia===
* A chronic lung disease which can occur following premature birth.
* The definition of bronchopulmonary dysplasia (BPD) has in recent years changed.
* From a severe lung injury and associated repair, to more of a disruption of lung development.

{{Glossary}}

{{Footer}}

[[Category:Endoderm]] [[Category:Respiratory]]

SH Lecture - Respiratory System Development

2012-02-27T21:08:16Z

S8600021: /* Textbooks */

[[File:SHsmall.jpg]]
==Introduction==
[[Image:Mark Hill.jpg|100px|left]]
[[File:Respiratory tract.jpg|thumb|Respiratory tract]]
The lecture will introduce the development of the respiratory system and associated structures. The lecture will not cover adult anatomy, physiology of gas exchange and red blood cell function and cardiovascular development.

:{{Template:SH2011}}

Start Time/End Time: 10am to 11am Thursday 1 March 2012 Clancy Auditorium [http://emed.med.unsw.edu.au/Map.nsf/0/6FF17DFEF645DABACA2573390006292A?OpenDocument&login eMed Link to Learning Activity - Respiratory System Development]

The respiratory system does not carry out its physiological function (of gas exchange) until after birth, though the respiratory tract, diaphragm and lungs do begin to form early in embryonic development and continue through fetal development, only functionally maturing just before birth.

The respiratory tract is divided anatomically into 2 main parts:
# '''upper respiratory tract''' - consisting of the nose, nasal cavity and the pharynx.
# '''lower respiratory tract''' - consisting of the larynx, trachea, bronchi and the lungs.

The respiratory "system" usually includes descriptions of not only the functional development of the lungs, but also related musculoskeletal (diaphragm) and vascular (pulmonary) development.

===Aims===
[[File:Historic-lungs.jpg|thumb|adult lungs]]
To understand the prenatal and postnatal developmental anatomy of human respiratory organs.
===Key Concepts===
# Embryonic origin of respiratory components (tract, lungs, diaphragm, muscles)
# Key stages in respiratory development.
# Time course of respiratory development.
# Respiration at birth.
# Postnatal development of respiration.
# Developmental abnormalities.

[[File:Gray0974.jpg|thumb|lung structure]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]

==Textbooks==

{|
| [[File:The Developing Human, 8th edn.jpg|80px]]
| Moore, K.L. & Persuad, T.V.N. (2008). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-1-4160-3706-4&eid=4-u1.0-B978-1-4160-3706-4..50013-X Chapter 10 - The Respiratory System] (chapter links only work with a UNSW connection).
|-
| [[File:Larsen's human embryology 4th edn.jpg|80px]]
| Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-0-443-06811-9&eid=4-u1.0-B978-0-443-06811-9..10011-9 Chapter 11 - Development of the Respiratory System and Body Cavities] (chapter links only work with a UNSW connection).
|-
| [[File:Logo.png|80px]]
| Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

* {{Respiratory Links}}
|}

===Additional Textbooks===

* Before We Are Born (5th ed.) Moore and Persaud Chapter 13 p255-287
* Essentials of Human Embryology Larson Chapter 9 p123-146
* Human Embryology Fitzgerald and Fitzgerald Chapter 19,20 p119-123
* Developmental Biology 8e Online[http://8e.devbio.com/article.php?ch=15&id=157 Lung Branching Morphogenesis]
* Anatomy of the Human Body 1918 Henry Gray [http://www.bartleby.com/107/235.html 1. The Respiratory Apparatus]
* [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology - Respiratory]

==Lecture Objectives==
[[File:Gray0974.jpg|thumb|lung structure]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]
* Understanding of embryonic lung development
* Understanding of 4 stages of lung development
* Understanding of diaphragm development
* Brief understanding of respiratory vascular development
* Brief understanding of respiratory abnormalities
* Brief understanding of molecular mechanisms

{|
| [[File:Podcast_icon.jpg|link=ANAT2341_Embryology_2011_Lecture_Recordings]]
| '''Lectopia Lecture Audio'''

[http://lectopia.telt.unsw.edu.au/lectopia/lectopia.lasso?ut=153&id=110475 Lecture 10 Audio]
|}

==Textbooks==

{|
| [[File:The Developing Human, 8th edn.jpg|80px]]
| Moore, K.L. & Persuad, T.V.N. (2008). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-1-4160-3706-4&eid=4-u1.0-B978-1-4160-3706-4..50013-X Chapter 10 - The Respiratory System] (chapter links only work with a UNSW connection).
|-
| [[File:Larsen's human embryology 4th edn.jpg|80px]]
| Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-0-443-06811-9&eid=4-u1.0-B978-0-443-06811-9..10011-9 Chapter 11 - Development of the Respiratory System and Body Cavities] (chapter links only work with a UNSW connection).
|-
| [[File:Logo.png|80px]]
| Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

* {{Respiratory Links}}

* '''Anatomy of the Human Body''' 1918 Henry Gray [http://www.bartleby.com/107/235.html 1. The Respiratory Apparatus]
* '''Developmental Biology''' 8e Online [http://8e.devbio.com/article.php?ch=15&id=157 Lung Branching Morphogenesis]

|}

==Developmental Overview==
[[File:Stage14 respiratory tract.jpg|thumb|Week 5 Respiratory Development]]

===Lung Development===

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Germ Layers===
* Endoderm and splanchnic mesoderm form majority of conducting and alveoli.
* Ectoderm will contribute the neural innervation.
* Mesoderm also contributes the supporting musculoskeletal components.

===Events===
* '''Week 4''' - laryngotracheal groove forms on floor foregut.
* '''Week 5''' - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)
* '''Week 6''' - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.
* '''Week 7''' - enlargement of liver stops descent of heart and lungs.
* '''Month 3-6''' - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.
* '''Month 7''' - respiratory bronchioles proliferate and end in alveolar ducts and sacs.

==Lung Development Stages==
[[File:Lung_alveoli_development_cartoon.jpg|thumb|300px]]
The sequence is most important rather than the actual timing, which is variable in the existing literature.

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Embryonic===
* '''week 4 - 5'''
* Endoderm - tubular ventral growth from foregut pharynx.
* Mesoderm - mesenchyme of lung buds.
* Intraembryonic coelom - pleural cavities elongated spaces connecting pericardial and peritoneal spaces.
===Pseudoglandular stage===
* '''week 5 - 17'''
* tubular branching of the human lung airways continues
* by 2 months all segmental bronchi are present.
* lungs have appearance of a glandlike structure.
* stage is critical for the formation of all conducting airways.
** lined with '''tall columnar epithelium'''
** more distal structures are lined with '''cuboidal epithelium'''.

===Canalicular stage===

* '''week 16 - 24'''
* Lung morphology changes dramatically
* differentiation of the pulmonary epithelium results in the formation of the future air-blood tissue barrier.
* '''Surfactant''' synthesis and the canalization of the lung parenchyma by capillaries begin.
* future gas exchange regions can be distinguished from the future conducting airways of the lungs.

===Saccular stage===
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* '''week 24 to near term.'''
* most peripheral airways form widened "airspaces", termed saccules.
* saccules widen and lengthen the airspace (by the addition of new generations).
* future gas exchange region expands significantly.
* Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin.
** May have a role in epithelial differentiation and control of surfactant secretion.
* The vascular tree also grows in length and diameter during this time.

==Foregut development==
[[File:Head arches cartoon.jpg|thumb|Foregut cartoon]]
From the oral cavity the next portion of the foregut is initially a single gastrointestinal (oesophagus) and respiratory (trachea) common tube, the pharynx which lies behind the heart. Note that the respiratory tract will form from a ventral bud arising at this level.

* Oral cavity
* Pharynx (esophagus, trachea)
* Respiratory tract
* Stomach

==Upper respiratory tract==
[[File:Gray0961.jpg|thumb|Adult upper respiratory tract conducting system]]
<gallery>
File:Gitbpm.jpg|stage 11 foregut
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Stage_22_image_167.jpg|Stage 22 trachea
File:Head_arches_cartoon.jpg|Head arches cartoon
File:Pharynx_cartoon.jpg|Pharynx
File:Nasal cavities.jpg|Nasal cavities
File:Pharynx.jpg|Pharynx
File:Larynx.jpg|Larynx
</gallery>
* part of foregut development
* anatomically the nose, nasal cavity and the pharynx
* the pharynx forms a major arched cavity within the pharyngeal arches

'''MH''' - pharyngeal arches will be described in head development lecture

==Lower respiratory tract==
<gallery>
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Gray0982b.jpg|week 4 later ventral endoderm growth
File:Bronchi lungs.jpg|lower respiratory tract
File:Respiratory tract.jpg|conducting system bronchi to lungs
</gallery>
[[File:Stage14-22 lungs.jpg]] [[File:Stage_22_image_171.jpg|300px]]

[[File:Lung alveoli development cartoon.jpg|thumb|Lung alveoli development cartoon]]
[[File:Fetal lung histology.jpg|thumb|Fetal lung histology]]

* lung buds ( endoderm epithelial tubes) grow/push into mesenchyme covered with pleural cells (lung border)
* generates a tree-like network by repeated:
# elongation
# terminal bifurcation
# lateral budding
* The lungs go through an embryonic and 4 distinct histological phases of development

Growth initially of branched "conducting" system of bronchial tree, followed by later development of the "functional units" of the alveoli.

* '''embryonic''' - week 4 - 5 (stage 14 above)
* '''pseudoglandular''' - week 5 - 17 (stage 22 above)
* '''canalicular''' - week 16 - 25
* '''terminal sac''' - week 24 - 40
* '''alveolar''' - late fetal - 8 years (Latin, ''alveus'' = cavity or hollow)

===Fetal lung volume===
Each human lung volume as determined by ultrasound and matched to gestational age <ref><pubmed>16388511</pubmed></ref>
{|
| Weeks (gestational)
| Volume (ml)
|-
| 12 to 13
| 0.05
|-
| 19 to 22
| 0.5
|-
| 29 to 32
| 1.9
|}

== Pleural Cavity ==
[[File:Gray0965.jpg|thumb|pleura]]
[[File:Gray0968.jpg|thumb|pleura]]
* The anatomical body cavity in which the lungs develop and lie.
* The pleural cavity forms in the lateral plate mesoderm as part of the early single intraembryonic coelom.
* This cavity is initially continuous with pericardial and peritoneal cavities and form initially as two narrow canals
** later becomes separated by folding (pleuropericardial fold, pleuroperitoneal membrane) and the later formation of the diaphragm

pleuropericardial fold - (pleuropericardial membrane) An early embryonic fold which restricts the communication between pleural cavity and pericardiac cavity, contains both the cardinal vein and phrenic nerve.

pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity.

===Pleura===
* serous membrane covers the surface of the lung and the spaces between the lobes
* arranged as a closed invaginated sac
* two layers (pulmonary, parietal) continuous with each other, the potential space between them is the '''pleural cavity'''

==Diaphragm==
* Not respiratory tract but musculoskeletal development, there are '''5 embryonic elements''' that contribute to the diaphragm.
{|
| [[File:Diaphragm components.jpg|300px|Components of the diaphragm]]
|
# septum transversum- central tendon
# 3rd to 5th somite- musculature of diaphragm
# ventral pleural sac- connective tissue
# mesentry of oesophagus- connective tissue around oesophasus and IVC
# pleuroperitoneal membranes- connective tissue around central tendon
|}
[[File:Gray804.gif|thumb|Adult Cervical Plexus (phrenic nerve shown lower right)]]
[[File:Adult diaphragm.jpg|300px|adult diaphragm]]

* Innervation of the human diaphragm is by the '''phrenic nerves'''
** arising from the same segmental levels from which the diaphragm skeletal muscles arise, segmental levels C3 to C5.
* The paired phrenic nerves are '''mixed nerves'''
** motor neurons for the diaphragm
** sensory nerves for other abdominal structures (mediastinum, pleura, liver, gall bladder).

==Pulmonary Circulation==
[[File:Pulmonary circulation cartoon.jpg|thumb|300px|Pulmonary circulation]]
* the pulmonary system not "functional" until after birth
* pulmonary arteries - 6th aortic arch arteries
* pulmonary veins - are incorporated into the left atrium wall
* bronchial arteries - branches from dorsal aorta

==Fetal==
===Fetal Respiratory Movements===
* Fetal respiratory movements (FRM) or Fetal breathing movements (FBM) are regular muscular contrations occurring in the third trimester.
* thought to be preparing the respiratory muscular system for neonatal function
* thought to also have a role in late lung development.

==The First Breath==
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* The respiratory system does not carry out its physiological function (gas exchange) prenatally and remain entirely fluid-filled until birth.
* At birth, fluid in the upper respiratory tract is expired and fluid in the lung aveoli is rapidly absorbed this event has also been called "dewatering of the lung".
** The lung epithelia has to now rapidly change from its prenatal secretory function to that of fluid absorbtion.

The exchange of lung fluid for air leads to:
* fall in pulmonary vascular resistance
* increase in pulmonary blood flow
* thinning of pulmonary arteries (stretching as lungs increase in size)
* blood fills the alveolar capillaries

In the heart, pressure in the right side of the heart decreases and pressure in the left side of the heart increases (more blood returning from pulmonary).
[[File:Neonatal rib orientation.jpg|thumb|Rib orientation]]
* Respiratory Rate is higher than adult (30 breaths/minute).

===Rib Orientation===
* Infant rib - is virtually horizontal, allowing diaphragmatic breathing only.
* Adult rib - is oblique (both anterior and lateral views), allows for pump-handle and bucket handle types of inspiration.

== Respiratory Tract Abnormalities ==
[[Respiratory System - Abnormalities]]
===Tracheoesophageal Fistula ===
(Tracheo-Oesophageal Fistula, Oesophageal Atresia) - Oesophageal Atresia with or without tracheo-oesophageal fistula

===Lobar Emphysema (Overinflated Lung)===
# There is an overinflated left upper lobe
# There is a collapsed lower lobe
# The left lung is herniating across the mediastinum

===Congenital Diaphragmatic Hernia===
Failure of the pleuroperitoneal foramen (foramen of Bochdalek) to close allows viscera into thorax. Intestine, stomach or spleen can enter the pleural cavity, compressing the lung.

===Azygos Lobe===
[[File:Lung_Azygos_Lobe_02.jpg|thumb|Lung Azygos Lobe]]
* Common condition (0.5% of population).
* The right lung upper lobe expands either side of the posterior cardinal.
* There is also some course variability of the phrenic nerve in the presence of an azygos lobe.

===Congenital Laryngeal Webs===
* Laryngeal abnormality due to embryonic (week 10) incomplete recanalization of the laryngotracheal tube during the fetal period.
* Rare abnormality occuring mainly at the level of the vocal folds (glottis).

===Meconium Aspiration Syndrome===

* (MAS) Meconium is the gastrointestinal contents that accumulate in the intestines during the fetal period.
* Fetal stress in the third trimester, prior to/at/ or during parturition can lead to premature meconium discharge into the amniotic fluid.
* Subsequent ingestion by the fetus and damage to respiratory function.
* Damage to placental vessels '''meconium myonecrosis''' may also occur.

===Newborn Respiratory Distress Syndrome===
* (Hyaline Membrane Disease) [http://www.nlm.nih.gov/MEDLINEPLUS/ency/article/001563.htm medline plus] | [http://www.medscape.com/article/976034-overview eMedicine]

===Bronchopulmonary Dysplasia===
* A chronic lung disease which can occur following premature birth.
* The definition of bronchopulmonary dysplasia (BPD) has in recent years changed.
* From a severe lung injury and associated repair, to more of a disruption of lung development.

{{Glossary}}

{{Footer}}

[[Category:Endoderm]] [[Category:Respiratory]]

Talk:SH Lecture - Respiratory System Development

2012-02-27T21:07:49Z

S8600021:

==Virtual Slides==

===UNSW Virtual Slides===
http://vslides.unsw.edu.au/VirtualSlideV2.nsf/id/F51ED7

===Virtual Slidebox===

http://www.path.uiowa.edu/virtualslidebox/nlm_histology/content_index_db.html

http://www.path.uiowa.edu/cgi-bin-pub/vs/fpx_browse.cgi?cat=o_lung&div=nlm

===Blue Histology===

http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm

==Lung Development Stages==
Text from: <pubmed>10852845</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1637815 PMC1637815] | [http://ehpnetl.niehs.nih.gov/docs/2000/suppl-3/457462pinkerton/abstract.html Environ Health Perspect.]

Embryogenesis

The lungs in humans first appear at the end of the first month of gestation as an evagination of epithelium from the foregut. The bud rapidly divides as a series of branching tubes in a dichotomous pattern. These tubular branches invade and interdigitate with mesenchymal tissues. Branching morphogenesis during this period forms the most proximal portions of the future tracheobronchial tree. As these tissues grow, they push into the future pleuroperitoneal cavity of the embryo. During embryogenesis, transcription factors play an important role in gene expression and regulation. Transcription factors are essential in both the stimulation and inhibition of gene expression to regulate the proper temporal and spatial patterning of lung development. Hepatocyte nuclear factor- 3 (12) and the homeobox gene TTF-1 (13) are examples of transcription factors serving as important regulators of early differentiation of the pulmonary epithelium during this period. Lung development is also highly dependent on interactions between the epithelium and mesenchyme. This dual origin of lung tissues is critical in development. Removal of mesenchyme from the tip of a lung bud during early phases of development with transplantation to the side of a higher ordered segment abolishes further branching at the site of removal while stimulating growth of a

Pseudoglandular stage.

Tubular branching of the human lung airways continues from the fifth to the seventeenth week of gestation. As early as 2 months of gestational age, all segmental bronchi are present. During this period, the lungs take on the appearance of a glandlike structure. This stage is the most critical for the formation of all conducting airways. During this period, the airway tubular structures are lined with tall columnar epithelium, whereas the more distal structures are lined with cuboidal epithelium. A number of signals arising from epithelial mesenchymal interactions during this time continue to modulate cellular proliferation temporally as well as spatially (4). These regulatory signals lead to further branching morphogenesis by affecting the rate of cellular proliferation (15). The presence of extracellular matrix molecules, including collagen, fibronectin, laminin, glycosaminoglycans, and proteoglycans, as well as cell membrane-bound integrins, also plays an important role in directing lung development by influencing the rates of cellular proliferation and differentiation (3,16,1/). Mechanical distention exerted on the lung as well as on specific cell types can also significantly affect gene expression and, ultimately, lung growth and development (4). A variety of growth factors and growth factor receptors are also important in controlling cellular functions (3). Epidermal growth factor, transforming growth factor-a, and retinoic acid all act to affect branching morphogenesis and cellular differentiation (18,19). Epithelial differentiation of ciliated, goblet, and basal cells first appears in the most central airways during this stage of development. Cartilage and smooth muscle cells are also first noted in the trachea and extend more peripherally with progressive growth of the lungs. During this stage of

Canalicular stage.

This stage lasts from week 16 to week 24 in the human fetus. Lung morphology changes dramatically during this time because of differentiation of the pulmonary epithelium, resulting in the formation of the future air-blood tissue barrier. Surfactant synthesis and the canalization of the lung parenchyma by capillaries begin. During this stage, the future gas exchange regions can be easily distinguished from the future conducting airways of the lungs.

Saccular stage.

The saccular stage of lung development in humans lasts from week 24 to near term. The most peripheral airways form widened airspaces, termed saccules. These saccules widen and lengthen the airspace, in large measure by the addition of new generations. During this stage, the future gas exchange region expands significantly. Populations of fibroblastic cells also undergo differentiation during this stage. These fibroblast-like cells are responsible for the production of the extracellular matrix, collagen, and elastin. It is also presumed that they play an important role in epithelial differentiation and control of surfactant secretion in connection with the growth of the gas exchange region during this stage. The vascular tree also grows in length and diameter during this time.

Columnar cells that are undifferentiated characterize the first epithelial cells lining fetal lung tubules. The first epithelial cells to differentiate in the trachea are neuroendocrine cells, followed closely by ciliated cells, and finally basal and secretory cells in rapid sequence. This process of differentiation covers a developmental period ranging from days to months. In rodents including the mouse, rat, and hamster, complete epithelial differentiation of the trachea occurs in as little as 2 days. In primate trachea, cellular differentiation takes up to 6 months to be complete. In most species, epithelial cell differentiation of the trachea usually is not complete until just before birth. For more peripheral airway generations, cellular differentiation is likely to continue into the early postnatal period. Fetal epithelial cells are typically filled with glycogen that is gradually replaced with a granular cytoplasm filled with numerous organelles during cellular differentiation. These glycogen-filled cells are found throughout the tracheobronchial tree as well as into the most peripheral saccules. Differentiation of the epithelium is highly site specific, giving rise to more than 10 different cell types. For example, within the saccules of the lungs, cells lining these surfaces differentiate to form both squamous type 1 cells as well as cuboidal type 2 cells. The presence of glycogen within these cells may persist into early postnatal life.

==Original Lecture Page==

==Developmental Overview==
'''Week 4''' - laryngotracheal groove forms on floor foregut.

'''Week 5''' - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

'''Week 6''' - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.

'''Week 7''' - enlargement of liver stops descent of heart and lungs.

'''Month 3-6''' - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.

'''Month 7''' - respiratory bronchioles proliferate and end in alveolar ducts and sacs.

===Lung Development===

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Germ Layers===
* Endoderm form epithelium of conducting and alveoli.
* Ectoderm will contribute the neural innervation.
* Mesoderm contributes connective tissues, blood vessels, smooth muscle, also contributes the supporting musculoskeletal components.

==Foregut development==
From the oral cavity the next portion of the foregut is initially a single gastrointestinal (oesophagus) and respiratory (trachea) common tube, the pharynx which lies behind the heart. Note that the respiratory tract will form from a ventral bud arising at this level.

* Oral cavity
* Pharynx (esophagus, trachea)
* Respiratory tract
* Stomach

==Upper respiratory tract==
<gallery>
File:Gray0961.jpg|Adult upper respiratory tract conducting system
File:Gitbpm.jpg|stage 11 foregut
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Head_arches_cartoon.jpg|Head arches cartoon
File:Pharynx_cartoon.jpg|Pharynx
File:Nasal cavities.jpg|Nasal cavities
File:Pharynx.jpg|Pharynx
File:Larynx.jpg|Larynx
</gallery>
* part of foregut development
* anatomically the nose, nasal cavity and the pharynx
* the pharynx forms a major arched cavity within the pharyngeal arches (pharyngeal arches will be described in head development lecture)

{|
| <wikiflv width="280" height="322" autostart="true" position="left">Endoderm 002.flv|File:Endoderm 002 icon.jpg</wikiflv>
| [[File:Endoderm_cartoon.jpg]]

[[Quicktime Development_Animation_-_Endoderm|Quicktime]] | [[Development_Animation_-_Endoderm|Flash]]
|}

==Lower respiratory tract==
<gallery>
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Gray0982b.jpg|week 4 later ventral endoderm growth
File:Stage14 respiratory tract.jpg|Developing lung buds
File:Bronchi lungs.jpg|lower respiratory tract
</gallery>
[[File:Stage14-22 lungs.jpg]] [[File:Stage_22_image_171.jpg|300px]]

* The lungs go through an embryonic and 4 distinct histological phases of development

Growth initially of branched "conducting" system of bronchial tree, followed by later development of the "functional units" of the alveoli.

* '''embryonic''' - week 4 - 5 (stage 14 above)
* '''pseudoglandular''' - week 5 - 17 (stage 22 above)
* '''canalicular''' - week 16 - 25
* '''terminal sac''' - week 24 - 40
* '''alveolar''' - late fetal - 8 years (Latin, ''alveus'' = cavity or hollow)

===Lung morphogenesis===
====Embryonic stage====
[[File:Stage_13_image_070.jpg|600px]]

[[Movie_-_Gastrointestinal_Tract_3D_stage_13|Stage 13 movie]]

* Lung buds ( endoderm epithelial tubes) grow/push into splanchnic mesenchyme covered with pleural cells (lung border)
** '''embryonic tissue''' (adult tissue) organised inside to out
** '''endoderm''' (future respiratory epithelia) - '''splanchnic mesoderm''' (connective tissue, blood vessels, smooth muscle) - '''splanchnic mesothelium''' (visceral pleura) - pericardioperiotoneal canals (pleural cavity) - '''somatic mesothelium''' (parietal pleura) - '''somatic mesoderm''' (body wall CT, skeleton)
* generates a tree-like network by repeated:

# elongation
# terminal bifurcation
# lateral budding

===Pseudoglandular stage===
[[File:Fetal lung histology.jpg|thumb|Fetal lung histology ([[:File:Fetal_lung_histology_01.jpg|large image]])]]
* week 5 - 17 (late embryonic, fetal)
* tubular branching of the human lung airways continues (16-25 generations of branching)
* by 2 months all segmental bronchi are present.
* lungs have appearance of a glandlike structure.
* stage is critical for the formation of all conducting airways.
* lined with tall columnar epithelium, the more distal structures are lined with cuboidal epithelium.

===Canalicular stage===

* week 16 - 24 (fetal to end of second trimester)
* Lung morphology changes dramatically
* differentiation of the pulmonary epithelium results in the formation of the future air-blood tissue barrier.
* Surfactant synthesis and the canalization of the lung parenchyma by capillaries begin (week 17).
* future gas exchange regions can be distinguished from the future conducting airways of the lungs.
* 1 respiratory bronchiole gives rise to 3-6 alveolar ducts.

===Saccular stage===
[[File:Lung alveoli development cartoon.jpg|thumb|Lung alveoli development cartoon]]
* week 24 to near term (fetal third trimester)
* most peripheral airways form widened airspaces, termed saccules.
* saccules widen and lengthen the airspace (by the addition of new generations).
* future gas exchange region expands significantly.
* Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin.
** May have a role in epithelial differentiation and control of surfactant secretion
* The vascular tree also grows in length and diameter during this time.

===Alveolar stage===
[[File:Pig lung alveolarization.jpg|thumb|Fetal lung alveolarization]]
[[File:Alveolar-sac-01.jpg|Alveolar sac structure|600px]]

* Thyroid hormone required for differentiation and stimulate surfactant production.
====Alveolar type I cells====
* small alveolar cells, type I pneumocytes
* very flat cells (thin as 0.05 µm)
* form most of the surface of the alveolar walls
* may contribute epithelium on both faces of the alveolar wall

====Alveolar type II cells====
* large alveolar cells, type II pneumocytes
* irregular to cuboidal shaped cells
* contain large number of granules called lamellar bodies, these are the precursors to pulmonary surfactant (phospholipid mixture).

* end month 6 alveolar cells type 2 appear and begin to secrete surfactant - premature babies have difficulties associated with insufficient surfactant.

====Alveolar macrophages====
* remove particulate matter that enters the alveoli with inspired air
* migrate over alveolar epithelium and phagocytose particulate matter

===Fetal lung volume===
Each lung volume as determined by ultrasound and matched to gestational age (PMID: 16388511)
* 12-13 weeks 0.05 mL
* 19-22 weeks 0.5 mL
* 29-32 weeks 1.9 mL

== Pleural Cavity ==
[[File:Gray0965.jpg|thumb|pleura]]
[[File:Gray0968.jpg|thumb|pleura]]
* The anatomical body cavity in which the lungs develop and lie.
* The pleural cavity forms in the lateral plate mesoderm as part of the early single intraembryonic coelom.
* This cavity is initially continuous with pericardial and peritoneal cavities and form initially as two narrow canals
** later becomes separated by folding (pleuropericardial fold, pleuroperitoneal membrane) and the later formation of the diaphragm

pleuropericardial fold - (pleuropericardial membrane) An early embryonic fold which restricts the communication between pleural cavity and pericardiac cavity, contains both the cardinal vein and phrenic nerve.

pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity.

===Pleura===
* serous membrane covers the surface of the lung and the spaces between the lobes
* arranged as a closed invaginated sac
* two layers (pulmonary, parietal) continuous with each other, the potential space between them is the '''pleural cavity'''

==Diaphragm==
Not respiratory tract but musculoskeletal development, there are 5 elements that contribute to the diaphragm.
{|
| [[File:Diaphragm components.jpg|300px|Components of the diaphragm]]
|
# septum transversum- central tendon
# 3rd to 5th somite- musculature of diaphragm
# ventral pleural sac- connective tissue
# mesentry of oesophagus- connective tissue around oesophasus and IVC
# pleuroperitoneal membranes- connective tissue around central tendon
|}
[[File:Gray804.gif|thumb|Adult Cervical Plexus (phrenic nerve shown lower right)]]
[[File:Adult diaphragm.jpg|300px|adult diaphragm]]

Innervation of the human diaphragm is by the phrenic nerves, arising from the same segmental levels from which the diaphragm skeletal muscles arise, segmental levels C3 to C5.

The paired phrenic nerves are mixed containing motor neurons for the diaphragm and sensory nerves for other abdominal structures (mediastinum, pleura, liver, gall bladder).

==Pulmonary Circulation==
[[File:Pulmonary circulation cartoon.jpg|thumb|300px|Pulmonary circulation]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]
* the pulmonary system not "functional" until after birth
* pulmonary arteries - (deoxygenated blood to lung) 6th aortic arch arteries
* pulmonary veins - (oxygenated blood from lung) are incorporated into the left atrium wall
* bronchial arteries - branches from dorsal aorta

==Fetal Respiratory Movements==
* Fetal respiratory movements (FRM) or Fetal breathing movements (FBM) are regular muscular contrations occurring in the third trimester.
* thought to be preparing the respiratory muscular system for neonatal function (and amniotic fluid are thought to have a role in lung maturation)
* thought to also have a role in late lung development.

==The First Breath==
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* The respiratory system does not carry out its physiological function (gas exchange) prenatally and remain entirely fluid-filled until birth.
* At birth, fluid in the upper respiratory tract is expired and fluid in the lung aveoli is rapidly absorbed this event has also been called "dewatering of the lung".
** The lung epithelia has to now rapidly change from its prenatal secretory function to that of fluid absorbtion.

===Exchange of Fluid for Air===
* fall in pulmonary vascular resistance
* increase in pulmonary blood flow
* thinning of pulmonary arteries (stretching as lungs increase in size)
* blood fills the alveolar capillaries

In the heart, pressure in the right side of the heart decreases and pressure in the left side of the heart increases (more blood returning from pulmonary).
[[File:Neonatal rib orientation.jpg|thumb|Neonatal rib orientation]]
* Respiratory Rate is higher than adult (30 breaths/minute).
===Rib Orientation===
* Infant rib is virtually horizontal, allowing only diaphragmatic breathing
* Adult rib orientation is oblique (both anterior and lateral views), allows for pump-handle and bucket handle types of inspiration.

== Respiratory Tract Histology==
<gallery>
File:Respiratory_histology_05.jpg|Trachea
File:Respiratory_histology_06.jpg|Trachea
File:Respiratory_histology_01.jpg|Bronchiole
File:Respiratory histology 09.jpg|Bronchiole ciliated simple columnar epithelium
File:Respiratory histology 07.jpg|Lung structure
File:Respiratory_histology_02.jpg|Alveoli and Duct
File:Respiratory_histology_03.jpg|Alveoli
File:Respiratory_histology_04.jpg|Alveoli elastin
</gallery>

:'''Histology Links:''' [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology] | [http://vslides.unsw.edu.au/VirtualSlideV2.nsf/id/F51ED7 UNSW Virtual Slides] | [http://www.path.uiowa.edu/cgi-bin-pub/vs/fpx_browse.cgi?cat=o_lung&div=nlm UIOWA Virtual Slidebox]

== Respiratory Tract Abnormalities ==
[[File:Respiratory tract.jpg|thumb|conducting system bronchi to lungs]]
[[Respiratory System - Abnormalities]]
===Tracheoesophageal Fistula ===
* Tracheo-Oesophageal Fistula, Oesophageal Atresia - Oesophageal Atresia with or without tracheo-oesophageal [[F#fistula|fistula]]
** '''Fistula''' - an abnormal communication between 2 structures (organs, vessels, cavities) that do not normally connect.

===Lobar Emphysema (Overinflated Lung)===
# There is an overinflated left upper lobe
# There is a collapsed lower lobe
# The left lung is herniating across the mediastinum

===Congenital Diaphragmatic Hernia===
* Failure of the pleuroperitoneal foramen (foramen of Bochdalek) to close (left side)
* allows viscera into thorax -iIntestine, stomach or spleen can enter the pleural cavity, compressing the lung.

* rare (Morgagni hernia) -an opening in the front of the diaphragm.

:'''Links:''' [http://www.ncbi.nlm.nih.gov/books/NBK1359 GeneReviews]

===Azygos Lobe===
[[File:Lung_Azygos_Lobe_02.jpg|thumb|Lung Azygos Lobe]]
* Common condition (0.5% of population).
* The right lung upper lobe expands either side of the posterior cardinal.
* There is also some course variability of the phrenic nerve in the presence of an azygos lobe.

===Congenital Laryngeal Webs===
* Laryngeal abnormality due to embryonic (week 10) incomplete recanalization of the laryngotracheal tube during the fetal period.
* Rare abnormality occuring mainly at the level of the vocal folds (glottis).

===Meconium Aspiration Syndrome===

(MAS) Meconium is the gastrointestinal contents that accumulate in the intestines during the fetal period. Fetal stress in the third trimester, prior to/at/ or during parturition can lead to premature meconium discharge into the amniotic fluid and sunsequent ingestion by the fetus and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur.

===Newborn Respiratory Distress Syndrome===
* Hyaline Membrane Disease (membrane-like substance from damaged pulmonary cells)
* absence of surfactant, if prolonged can be irreversible
* intrauterine asphyxia, prematurity and maternal diabetes

:'''Links:''' [http://www.nlm.nih.gov/MEDLINEPLUS/ency/article/001563.htm medline plus] | [http://www.medscape.com/article/976034-overview eMedicine]

===Bronchopulmonary Dysplasia===
* A chronic lung disease which can occur following premature birth.
* The definition of bronchopulmonary dysplasia (BPD) has in recent years changed from a severe lung injury and associated repair, to more of a disruption of lung development.

==Terms==
* '''antenatal''' before birth.
* '''alveoli number at birth''' - from 20 - 50 million and eventually in the adult 300 million.
* '''Bronchopulmonary dysplasia''' - (BPD) the most common serious sequela of premature birth.
* '''Bronchiolitis''' - is a viral infection of the lower respiratory tract and most common lower respiratory tract infection in infants. Respiratory syncytial virus (RSV) is responsible for 70 percent of all cases overall and Parainfluenza, adenovirus and influenza account for most of the remaining cases. (HSTAT Management of Bronchiolitis in Infants and Children)
* '''Chronic obstructive pulmonary disease''' (COPD) causes include smoking (85–90 percent of all cases), genetic factors (alpha-1 antitrypsin deficiency), passive smoking (children), occupational exposures, air pollution, and hyperresponsive airways. (HSTAT Management of Acute Exacerbations of Chronic Obstructive Pulmonary Disease)
* '''Clara cells''' non-ciliated cell found in the small airways (bronchioles) consisting of ciliated simple epithelium, these cells secrete glycosaminoglycans (Clara cell secretory protein, CCSP) to protect the bronchiole lining.
* '''Congenital Diaphragmatic Hernia''' (CDH) disorder with an incidence of 1 in 2500 live births.
* '''fetal breathing-like movements''' (FBMs) or Fetal respiratory movements are thought to be regular muscular contrations occurring in the third trimester, preparing the respiratory muscular system for neonatal function and to have a role in late lung development.
* '''glucocorticoid treatment''' - antenatal therapy to promote the maturation of the human fetal lung. Given as a synthetic glucocorticoid between 24 and 32 weeks of pregnancy to promote lung maturation in fetuses at risk of preterm delivery.
* '''lamellar bodies''' the storage form of surfactant in type II alveolar cells, seen as centrically layered "packages" of phospholipid. A count of lamellar bodies can be used as an assay for measuring fetal lung maturity.
* '''maternal diabetes''' if not controlled in pregnancy may delay fetal pulmonary maturation.
* '''Persistent Pulmonary Hypertension of the Newborn''' (PPHN) serious newborn condition due to due to the failure of closure one of the prenatal circulatory shunts, the ductus arteriosus. Occurs in about 1-2 newborns per 1000 live births and results in hypoxemia. (More? Respiratory Development - Birth)
* '''Pharyngitis''' inflammation of the pharynx involving lymphoid tissues of the posterior pharynx and lateral pharyngeal bands.
* '''pneumocyte''' or alveolar type I and type II cells.
* '''pulmonary hypoplasia''' can be due to anencephaly, renal hypoplasia or abnormalities of the thoracic cage
* '''pulmonary neuroendocrine cells''' (PNEC) single or innervated clusters of cells (neuroepithelial bodies) that line the airway epithelium, thought to have a role in regulating fetal lung growth and differentiation. At birth may also act as airway oxygen sensors involved in newborn adaptation. These cells synthesis and release amine (serotonin, 5-HT) and a several neuropeptides (bombesin).
* '''Respiratory distress syndrome''' (RDS) due to a surfactant deficiency at birth, particulary in preterm birth.
* '''secondary alveolar septa''' formed during the alveolar stage and are formed by projections of connective tissue and a double capillary loop.
*''' surfactant''' produced by alveolar type II cells is a mixture of lipids and proteins that both maintains alveolar integrity and plays a role in the control of host defense and inflammation in the lung.
* '''Surfactant therapy''' ([http://aappolicy.aappublications.org/cgi/content/full/pediatrics;121/2/419 American Academy of Pediatrics Policy] | [http://www.cps.ca/english/statements/fn/fn05-01.htm Canadian Paediatric Society Recommendations])
*''' thyroid hormone''' involved in the regulation of fetal lung development.
* '''vascular endothelial growth factor''' (VEGF) a secreted growth factor acting through receptors on endothelial cells to regulate vasculogenesis through their development, growth and function.

SH Lecture - Respiratory System Development

2012-02-27T21:07:16Z

S8600021:

SH Lecture - Respiratory System Development

2012-02-27T21:05:04Z

S8600021: /* Textbooks */

[[File:SHsmall.jpg]]
==Introduction==
[[Image:Mark Hill.jpg|100px|left]]
[[File:Respiratory tract.jpg|thumb|Respiratory tract]]
The lecture will introduce the development of the respiratory system and associated structures. The lecture will not cover adult anatomy, physiology of gas exchange and red blood cell function and cardiovascular development.

:{{Template:SH2011}}

Start Time/End Time: 10am to 11am Thursday 1 March 2012 Clancy Auditorium [http://emed.med.unsw.edu.au/Map.nsf/0/6FF17DFEF645DABACA2573390006292A?OpenDocument&login eMed Link to Learning Activity - Respiratory System Development]

The respiratory system does not carry out its physiological function (of gas exchange) until after birth, though the respiratory tract, diaphragm and lungs do begin to form early in embryonic development and continue through fetal development, only functionally maturing just before birth.

The respiratory tract is divided anatomically into 2 main parts:
# '''upper respiratory tract''' - consisting of the nose, nasal cavity and the pharynx.
# '''lower respiratory tract''' - consisting of the larynx, trachea, bronchi and the lungs.

The respiratory "system" usually includes descriptions of not only the functional development of the lungs, but also related musculoskeletal (diaphragm) and vascular (pulmonary) development.

===Aims===
[[File:Historic-lungs.jpg|thumb|adult lungs]]
To understand the prenatal and postnatal developmental anatomy of human respiratory organs.
===Key Concepts===
# Embryonic origin of respiratory components (tract, lungs, diaphragm, muscles)
# Key stages in respiratory development.
# Time course of respiratory development.
# Respiration at birth.
# Postnatal development of respiration.
# Developmental abnormalities.

[[File:Gray0974.jpg|thumb|lung structure]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]

==Textbooks==

{|
| [[File:The Developing Human, 8th edn.jpg|80px]]
| Moore, K.L. & Persuad, T.V.N. (2008). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-1-4160-3706-4&eid=4-u1.0-B978-1-4160-3706-4..50013-X Chapter 10 - The Respiratory System] (chapter links only work with a UNSW connection).
|-
| [[File:Larsen's human embryology 4th edn.jpg|80px]]
| Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-0-443-06811-9&eid=4-u1.0-B978-0-443-06811-9..10011-9 Chapter 11 - Development of the Respiratory System and Body Cavities] (chapter links only work with a UNSW connection).
|-
| [[File:Logo.png|80px]]
| Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

* {{Respiratory Links}}
|}

===Additional Textbooks===

* Before We Are Born (5th ed.) Moore and Persaud Chapter 13 p255-287
* Essentials of Human Embryology Larson Chapter 9 p123-146
* Human Embryology Fitzgerald and Fitzgerald Chapter 19,20 p119-123
* Developmental Biology 8e Online[http://8e.devbio.com/article.php?ch=15&id=157 Lung Branching Morphogenesis]
* Anatomy of the Human Body 1918 Henry Gray [http://www.bartleby.com/107/235.html 1. The Respiratory Apparatus]
* [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology - Respiratory]

==Developmental Overview==
'''Week 4''' - laryngotracheal groove forms on floor foregut.

'''Week 5''' - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

'''Week 6''' - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.

'''Week 7''' - enlargement of liver stops descent of heart and lungs.

'''Month 3-6''' - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.

'''Month 7''' - respiratory bronchioles proliferate and end in alveolar ducts and sacs.

===Lung Development===

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Germ Layers===
* Endoderm form epithelium of conducting and alveoli.
* Ectoderm will contribute the neural innervation.
* Mesoderm contributes connective tissues, blood vessels, smooth muscle, also contributes the supporting musculoskeletal components.

==Foregut development==
From the oral cavity the next portion of the foregut is initially a single gastrointestinal (oesophagus) and respiratory (trachea) common tube, the pharynx which lies behind the heart. Note that the respiratory tract will form from a ventral bud arising at this level.

* Oral cavity
* Pharynx (esophagus, trachea)
* Respiratory tract
* Stomach

==Upper respiratory tract==
<gallery>
File:Gray0961.jpg|Adult upper respiratory tract conducting system
File:Gitbpm.jpg|stage 11 foregut
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Head_arches_cartoon.jpg|Head arches cartoon
File:Pharynx_cartoon.jpg|Pharynx
File:Nasal cavities.jpg|Nasal cavities
File:Pharynx.jpg|Pharynx
File:Larynx.jpg|Larynx
</gallery>
* part of foregut development
* anatomically the nose, nasal cavity and the pharynx
* the pharynx forms a major arched cavity within the pharyngeal arches (pharyngeal arches will be described in head development lecture)

{|
| <wikiflv width="280" height="322" autostart="true" position="left">Endoderm 002.flv|File:Endoderm 002 icon.jpg</wikiflv>
| [[File:Endoderm_cartoon.jpg]]

[[Quicktime Development_Animation_-_Endoderm|Quicktime]] | [[Development_Animation_-_Endoderm|Flash]]
|}

==Lower respiratory tract==
<gallery>
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Gray0982b.jpg|week 4 later ventral endoderm growth
File:Stage14 respiratory tract.jpg|Developing lung buds
File:Bronchi lungs.jpg|lower respiratory tract
</gallery>
[[File:Stage14-22 lungs.jpg]] [[File:Stage_22_image_171.jpg|300px]]

* The lungs go through an embryonic and 4 distinct histological phases of development

Growth initially of branched "conducting" system of bronchial tree, followed by later development of the "functional units" of the alveoli.

* '''embryonic''' - week 4 - 5 (stage 14 above)
* '''pseudoglandular''' - week 5 - 17 (stage 22 above)
* '''canalicular''' - week 16 - 25
* '''terminal sac''' - week 24 - 40
* '''alveolar''' - late fetal - 8 years (Latin, ''alveus'' = cavity or hollow)

===Lung morphogenesis===
====Embryonic stage====
[[File:Stage_13_image_070.jpg|600px]]

[[Movie_-_Gastrointestinal_Tract_3D_stage_13|Stage 13 movie]]

* Lung buds ( endoderm epithelial tubes) grow/push into splanchnic mesenchyme covered with pleural cells (lung border)
** '''embryonic tissue''' (adult tissue) organised inside to out
** '''endoderm''' (future respiratory epithelia) - '''splanchnic mesoderm''' (connective tissue, blood vessels, smooth muscle) - '''splanchnic mesothelium''' (visceral pleura) - pericardioperiotoneal canals (pleural cavity) - '''somatic mesothelium''' (parietal pleura) - '''somatic mesoderm''' (body wall CT, skeleton)
* generates a tree-like network by repeated:

# elongation
# terminal bifurcation
# lateral budding

===Pseudoglandular stage===
[[File:Fetal lung histology.jpg|thumb|Fetal lung histology ([[:File:Fetal_lung_histology_01.jpg|large image]])]]
* week 5 - 17 (late embryonic, fetal)
* tubular branching of the human lung airways continues (16-25 generations of branching)
* by 2 months all segmental bronchi are present.
* lungs have appearance of a glandlike structure.
* stage is critical for the formation of all conducting airways.
* lined with tall columnar epithelium, the more distal structures are lined with cuboidal epithelium.

===Canalicular stage===

* week 16 - 24 (fetal to end of second trimester)
* Lung morphology changes dramatically
* differentiation of the pulmonary epithelium results in the formation of the future air-blood tissue barrier.
* Surfactant synthesis and the canalization of the lung parenchyma by capillaries begin (week 17).
* future gas exchange regions can be distinguished from the future conducting airways of the lungs.
* 1 respiratory bronchiole gives rise to 3-6 alveolar ducts.

===Saccular stage===
[[File:Lung alveoli development cartoon.jpg|thumb|Lung alveoli development cartoon]]
* week 24 to near term (fetal third trimester)
* most peripheral airways form widened airspaces, termed saccules.
* saccules widen and lengthen the airspace (by the addition of new generations).
* future gas exchange region expands significantly.
* Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin.
** May have a role in epithelial differentiation and control of surfactant secretion
* The vascular tree also grows in length and diameter during this time.

===Alveolar stage===
[[File:Pig lung alveolarization.jpg|thumb|Fetal lung alveolarization]]
[[File:Alveolar-sac-01.jpg|Alveolar sac structure|600px]]

* Thyroid hormone required for differentiation and stimulate surfactant production.
====Alveolar type I cells====
* small alveolar cells, type I pneumocytes
* very flat cells (thin as 0.05 µm)
* form most of the surface of the alveolar walls
* may contribute epithelium on both faces of the alveolar wall

====Alveolar type II cells====
* large alveolar cells, type II pneumocytes
* irregular to cuboidal shaped cells
* contain large number of granules called lamellar bodies, these are the precursors to pulmonary surfactant (phospholipid mixture).

* end month 6 alveolar cells type 2 appear and begin to secrete surfactant - premature babies have difficulties associated with insufficient surfactant.

====Alveolar macrophages====
* remove particulate matter that enters the alveoli with inspired air
* migrate over alveolar epithelium and phagocytose particulate matter

===Fetal lung volume===
Each lung volume as determined by ultrasound and matched to gestational age (PMID: 16388511)
* 12-13 weeks 0.05 mL
* 19-22 weeks 0.5 mL
* 29-32 weeks 1.9 mL

== Pleural Cavity ==
[[File:Gray0965.jpg|thumb|pleura]]
[[File:Gray0968.jpg|thumb|pleura]]
* The anatomical body cavity in which the lungs develop and lie.
* The pleural cavity forms in the lateral plate mesoderm as part of the early single intraembryonic coelom.
* This cavity is initially continuous with pericardial and peritoneal cavities and form initially as two narrow canals
** later becomes separated by folding (pleuropericardial fold, pleuroperitoneal membrane) and the later formation of the diaphragm

pleuropericardial fold - (pleuropericardial membrane) An early embryonic fold which restricts the communication between pleural cavity and pericardiac cavity, contains both the cardinal vein and phrenic nerve.

pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity.

===Pleura===
* serous membrane covers the surface of the lung and the spaces between the lobes
* arranged as a closed invaginated sac
* two layers (pulmonary, parietal) continuous with each other, the potential space between them is the '''pleural cavity'''

==Diaphragm==
Not respiratory tract but musculoskeletal development, there are 5 elements that contribute to the diaphragm.
{|
| [[File:Diaphragm components.jpg|300px|Components of the diaphragm]]
|
# septum transversum- central tendon
# 3rd to 5th somite- musculature of diaphragm
# ventral pleural sac- connective tissue
# mesentry of oesophagus- connective tissue around oesophasus and IVC
# pleuroperitoneal membranes- connective tissue around central tendon
|}
[[File:Gray804.gif|thumb|Adult Cervical Plexus (phrenic nerve shown lower right)]]
[[File:Adult diaphragm.jpg|300px|adult diaphragm]]

Innervation of the human diaphragm is by the phrenic nerves, arising from the same segmental levels from which the diaphragm skeletal muscles arise, segmental levels C3 to C5.

The paired phrenic nerves are mixed containing motor neurons for the diaphragm and sensory nerves for other abdominal structures (mediastinum, pleura, liver, gall bladder).

==Pulmonary Circulation==
[[File:Pulmonary circulation cartoon.jpg|thumb|300px|Pulmonary circulation]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]
* the pulmonary system not "functional" until after birth
* pulmonary arteries - (deoxygenated blood to lung) 6th aortic arch arteries
* pulmonary veins - (oxygenated blood from lung) are incorporated into the left atrium wall
* bronchial arteries - branches from dorsal aorta

==Fetal Respiratory Movements==
* Fetal respiratory movements (FRM) or Fetal breathing movements (FBM) are regular muscular contrations occurring in the third trimester.
* thought to be preparing the respiratory muscular system for neonatal function (and amniotic fluid are thought to have a role in lung maturation)
* thought to also have a role in late lung development.

==The First Breath==
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* The respiratory system does not carry out its physiological function (gas exchange) prenatally and remain entirely fluid-filled until birth.
* At birth, fluid in the upper respiratory tract is expired and fluid in the lung aveoli is rapidly absorbed this event has also been called "dewatering of the lung".
** The lung epithelia has to now rapidly change from its prenatal secretory function to that of fluid absorbtion.

===Exchange of Fluid for Air===
* fall in pulmonary vascular resistance
* increase in pulmonary blood flow
* thinning of pulmonary arteries (stretching as lungs increase in size)
* blood fills the alveolar capillaries

In the heart, pressure in the right side of the heart decreases and pressure in the left side of the heart increases (more blood returning from pulmonary).
[[File:Neonatal rib orientation.jpg|thumb|Neonatal rib orientation]]
* Respiratory Rate is higher than adult (30 breaths/minute).
===Rib Orientation===
* Infant rib is virtually horizontal, allowing only diaphragmatic breathing
* Adult rib orientation is oblique (both anterior and lateral views), allows for pump-handle and bucket handle types of inspiration.

== Respiratory Tract Histology==
<gallery>
File:Respiratory_histology_05.jpg|Trachea
File:Respiratory_histology_06.jpg|Trachea
File:Respiratory_histology_01.jpg|Bronchiole
File:Respiratory histology 09.jpg|Bronchiole ciliated simple columnar epithelium
File:Respiratory histology 07.jpg|Lung structure
File:Respiratory_histology_02.jpg|Alveoli and Duct
File:Respiratory_histology_03.jpg|Alveoli
File:Respiratory_histology_04.jpg|Alveoli elastin
</gallery>

:'''Histology Links:''' [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology] | [http://vslides.unsw.edu.au/VirtualSlideV2.nsf/id/F51ED7 UNSW Virtual Slides] | [http://www.path.uiowa.edu/cgi-bin-pub/vs/fpx_browse.cgi?cat=o_lung&div=nlm UIOWA Virtual Slidebox]

== Respiratory Tract Abnormalities ==
[[File:Respiratory tract.jpg|thumb|conducting system bronchi to lungs]]
[[Respiratory System - Abnormalities]]
===Tracheoesophageal Fistula ===
* Tracheo-Oesophageal Fistula, Oesophageal Atresia - Oesophageal Atresia with or without tracheo-oesophageal [[F#fistula|fistula]]
** '''Fistula''' - an abnormal communication between 2 structures (organs, vessels, cavities) that do not normally connect.

===Lobar Emphysema (Overinflated Lung)===
# There is an overinflated left upper lobe
# There is a collapsed lower lobe
# The left lung is herniating across the mediastinum

===Congenital Diaphragmatic Hernia===
* Failure of the pleuroperitoneal foramen (foramen of Bochdalek) to close (left side)
* allows viscera into thorax -iIntestine, stomach or spleen can enter the pleural cavity, compressing the lung.

* rare (Morgagni hernia) -an opening in the front of the diaphragm.

:'''Links:''' [http://www.ncbi.nlm.nih.gov/books/NBK1359 GeneReviews]

===Azygos Lobe===
[[File:Lung_Azygos_Lobe_02.jpg|thumb|Lung Azygos Lobe]]
* Common condition (0.5% of population).
* The right lung upper lobe expands either side of the posterior cardinal.
* There is also some course variability of the phrenic nerve in the presence of an azygos lobe.

===Congenital Laryngeal Webs===
* Laryngeal abnormality due to embryonic (week 10) incomplete recanalization of the laryngotracheal tube during the fetal period.
* Rare abnormality occuring mainly at the level of the vocal folds (glottis).

===Meconium Aspiration Syndrome===

(MAS) Meconium is the gastrointestinal contents that accumulate in the intestines during the fetal period. Fetal stress in the third trimester, prior to/at/ or during parturition can lead to premature meconium discharge into the amniotic fluid and sunsequent ingestion by the fetus and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur.

===Newborn Respiratory Distress Syndrome===
* Hyaline Membrane Disease (membrane-like substance from damaged pulmonary cells)
* absence of surfactant, if prolonged can be irreversible
* intrauterine asphyxia, prematurity and maternal diabetes

:'''Links:''' [http://www.nlm.nih.gov/MEDLINEPLUS/ency/article/001563.htm medline plus] | [http://www.medscape.com/article/976034-overview eMedicine]

===Bronchopulmonary Dysplasia===
* A chronic lung disease which can occur following premature birth.
* The definition of bronchopulmonary dysplasia (BPD) has in recent years changed from a severe lung injury and associated repair, to more of a disruption of lung development.

==Terms==
* '''antenatal''' before birth.
* '''alveoli number at birth''' - from 20 - 50 million and eventually in the adult 300 million.
* '''Bronchopulmonary dysplasia''' - (BPD) the most common serious sequela of premature birth.
* '''Bronchiolitis''' - is a viral infection of the lower respiratory tract and most common lower respiratory tract infection in infants. Respiratory syncytial virus (RSV) is responsible for 70 percent of all cases overall and Parainfluenza, adenovirus and influenza account for most of the remaining cases. (HSTAT Management of Bronchiolitis in Infants and Children)
* '''Chronic obstructive pulmonary disease''' (COPD) causes include smoking (85–90 percent of all cases), genetic factors (alpha-1 antitrypsin deficiency), passive smoking (children), occupational exposures, air pollution, and hyperresponsive airways. (HSTAT Management of Acute Exacerbations of Chronic Obstructive Pulmonary Disease)
* '''Clara cells''' non-ciliated cell found in the small airways (bronchioles) consisting of ciliated simple epithelium, these cells secrete glycosaminoglycans (Clara cell secretory protein, CCSP) to protect the bronchiole lining.
* '''Congenital Diaphragmatic Hernia''' (CDH) disorder with an incidence of 1 in 2500 live births.
* '''fetal breathing-like movements''' (FBMs) or Fetal respiratory movements are thought to be regular muscular contrations occurring in the third trimester, preparing the respiratory muscular system for neonatal function and to have a role in late lung development.
* '''glucocorticoid treatment''' - antenatal therapy to promote the maturation of the human fetal lung. Given as a synthetic glucocorticoid between 24 and 32 weeks of pregnancy to promote lung maturation in fetuses at risk of preterm delivery.
* '''lamellar bodies''' the storage form of surfactant in type II alveolar cells, seen as centrically layered "packages" of phospholipid. A count of lamellar bodies can be used as an assay for measuring fetal lung maturity.
* '''maternal diabetes''' if not controlled in pregnancy may delay fetal pulmonary maturation.
* '''Persistent Pulmonary Hypertension of the Newborn''' (PPHN) serious newborn condition due to due to the failure of closure one of the prenatal circulatory shunts, the ductus arteriosus. Occurs in about 1-2 newborns per 1000 live births and results in hypoxemia. (More? Respiratory Development - Birth)
* '''Pharyngitis''' inflammation of the pharynx involving lymphoid tissues of the posterior pharynx and lateral pharyngeal bands.
* '''pneumocyte''' or alveolar type I and type II cells.
* '''pulmonary hypoplasia''' can be due to anencephaly, renal hypoplasia or abnormalities of the thoracic cage
* '''pulmonary neuroendocrine cells''' (PNEC) single or innervated clusters of cells (neuroepithelial bodies) that line the airway epithelium, thought to have a role in regulating fetal lung growth and differentiation. At birth may also act as airway oxygen sensors involved in newborn adaptation. These cells synthesis and release amine (serotonin, 5-HT) and a several neuropeptides (bombesin).
* '''Respiratory distress syndrome''' (RDS) due to a surfactant deficiency at birth, particulary in preterm birth.
* '''secondary alveolar septa''' formed during the alveolar stage and are formed by projections of connective tissue and a double capillary loop.
*''' surfactant''' produced by alveolar type II cells is a mixture of lipids and proteins that both maintains alveolar integrity and plays a role in the control of host defense and inflammation in the lung.
* '''Surfactant therapy''' ([http://aappolicy.aappublications.org/cgi/content/full/pediatrics;121/2/419 American Academy of Pediatrics Policy] | [http://www.cps.ca/english/statements/fn/fn05-01.htm Canadian Paediatric Society Recommendations])
*''' thyroid hormone''' involved in the regulation of fetal lung development.
* '''vascular endothelial growth factor''' (VEGF) a secreted growth factor acting through receptors on endothelial cells to regulate vasculogenesis through their development, growth and function.
{{Glossary}}

{{Footer}}

[[Category:Endoderm]] [[Category:Respiratory]]

SH Lecture - Respiratory System Development

2012-02-27T21:04:45Z

S8600021: /* Textbook References */

[[File:SHsmall.jpg]]
==Introduction==
[[Image:Mark Hill.jpg|100px|left]]
[[File:Respiratory tract.jpg|thumb|Respiratory tract]]
The lecture will introduce the development of the respiratory system and associated structures. The lecture will not cover adult anatomy, physiology of gas exchange and red blood cell function and cardiovascular development.

:{{Template:SH2011}}

Start Time/End Time: 10am to 11am Thursday 1 March 2012 Clancy Auditorium [http://emed.med.unsw.edu.au/Map.nsf/0/6FF17DFEF645DABACA2573390006292A?OpenDocument&login eMed Link to Learning Activity - Respiratory System Development]

The respiratory system does not carry out its physiological function (of gas exchange) until after birth, though the respiratory tract, diaphragm and lungs do begin to form early in embryonic development and continue through fetal development, only functionally maturing just before birth.

The respiratory tract is divided anatomically into 2 main parts:
# '''upper respiratory tract''' - consisting of the nose, nasal cavity and the pharynx.
# '''lower respiratory tract''' - consisting of the larynx, trachea, bronchi and the lungs.

The respiratory "system" usually includes descriptions of not only the functional development of the lungs, but also related musculoskeletal (diaphragm) and vascular (pulmonary) development.

===Aims===
[[File:Historic-lungs.jpg|thumb|adult lungs]]
To understand the prenatal and postnatal developmental anatomy of human respiratory organs.
===Key Concepts===
# Embryonic origin of respiratory components (tract, lungs, diaphragm, muscles)
# Key stages in respiratory development.
# Time course of respiratory development.
# Respiration at birth.
# Postnatal development of respiration.
# Developmental abnormalities.

[[File:Gray0974.jpg|thumb|lung structure]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]

==Textbooks==

{|
| [[File:The Developing Human, 8th edn.jpg|80px]]
| Moore, K.L. & Persuad, T.V.N. (2008). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-1-4160-3706-4&eid=4-u1.0-B978-1-4160-3706-4..50013-X Chapter 10 - The Respiratory System] (chapter links only work with a UNSW connection).
|-
| [[File:Larsen's human embryology 4th edn.jpg|80px]]
| Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

* [http://www.mdconsult.com/books/linkTo?type=bookPage&isbn=978-0-443-06811-9&eid=4-u1.0-B978-0-443-06811-9..10011-9 Chapter 11 - Development of the Respiratory System and Body Cavities] (chapter links only work with a UNSW connection).
|-
| [[File:Logo.png|80px]]
| Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

* {{Respiratory Links}}

===Additional Textbooks===

* Before We Are Born (5th ed.) Moore and Persaud Chapter 13 p255-287
* Essentials of Human Embryology Larson Chapter 9 p123-146
* Human Embryology Fitzgerald and Fitzgerald Chapter 19,20 p119-123
* Developmental Biology 8e Online[http://8e.devbio.com/article.php?ch=15&id=157 Lung Branching Morphogenesis]
* Anatomy of the Human Body 1918 Henry Gray [http://www.bartleby.com/107/235.html 1. The Respiratory Apparatus]
* [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology - Respiratory]

==Developmental Overview==
'''Week 4''' - laryngotracheal groove forms on floor foregut.

'''Week 5''' - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

'''Week 6''' - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.

'''Week 7''' - enlargement of liver stops descent of heart and lungs.

'''Month 3-6''' - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.

'''Month 7''' - respiratory bronchioles proliferate and end in alveolar ducts and sacs.

===Lung Development===

* week 4 - 5 embryonic
* week 5 - 17 pseudoglandular
* week 16 - 25 canalicular
* week 24 - 40 terminal sac
* late fetal - 8 years alveolar

===Germ Layers===
* Endoderm form epithelium of conducting and alveoli.
* Ectoderm will contribute the neural innervation.
* Mesoderm contributes connective tissues, blood vessels, smooth muscle, also contributes the supporting musculoskeletal components.

==Foregut development==
From the oral cavity the next portion of the foregut is initially a single gastrointestinal (oesophagus) and respiratory (trachea) common tube, the pharynx which lies behind the heart. Note that the respiratory tract will form from a ventral bud arising at this level.

* Oral cavity
* Pharynx (esophagus, trachea)
* Respiratory tract
* Stomach

==Upper respiratory tract==
<gallery>
File:Gray0961.jpg|Adult upper respiratory tract conducting system
File:Gitbpm.jpg|stage 11 foregut
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Head_arches_cartoon.jpg|Head arches cartoon
File:Pharynx_cartoon.jpg|Pharynx
File:Nasal cavities.jpg|Nasal cavities
File:Pharynx.jpg|Pharynx
File:Larynx.jpg|Larynx
</gallery>
* part of foregut development
* anatomically the nose, nasal cavity and the pharynx
* the pharynx forms a major arched cavity within the pharyngeal arches (pharyngeal arches will be described in head development lecture)

{|
| <wikiflv width="280" height="322" autostart="true" position="left">Endoderm 002.flv|File:Endoderm 002 icon.jpg</wikiflv>
| [[File:Endoderm_cartoon.jpg]]

[[Quicktime Development_Animation_-_Endoderm|Quicktime]] | [[Development_Animation_-_Endoderm|Flash]]
|}

==Lower respiratory tract==
<gallery>
File:Gray0982a.jpg|week 4 early respiratory endodermal bud
File:Gray0982b.jpg|week 4 later ventral endoderm growth
File:Stage14 respiratory tract.jpg|Developing lung buds
File:Bronchi lungs.jpg|lower respiratory tract
</gallery>
[[File:Stage14-22 lungs.jpg]] [[File:Stage_22_image_171.jpg|300px]]

* The lungs go through an embryonic and 4 distinct histological phases of development

Growth initially of branched "conducting" system of bronchial tree, followed by later development of the "functional units" of the alveoli.

* '''embryonic''' - week 4 - 5 (stage 14 above)
* '''pseudoglandular''' - week 5 - 17 (stage 22 above)
* '''canalicular''' - week 16 - 25
* '''terminal sac''' - week 24 - 40
* '''alveolar''' - late fetal - 8 years (Latin, ''alveus'' = cavity or hollow)

===Lung morphogenesis===
====Embryonic stage====
[[File:Stage_13_image_070.jpg|600px]]

[[Movie_-_Gastrointestinal_Tract_3D_stage_13|Stage 13 movie]]

* Lung buds ( endoderm epithelial tubes) grow/push into splanchnic mesenchyme covered with pleural cells (lung border)
** '''embryonic tissue''' (adult tissue) organised inside to out
** '''endoderm''' (future respiratory epithelia) - '''splanchnic mesoderm''' (connective tissue, blood vessels, smooth muscle) - '''splanchnic mesothelium''' (visceral pleura) - pericardioperiotoneal canals (pleural cavity) - '''somatic mesothelium''' (parietal pleura) - '''somatic mesoderm''' (body wall CT, skeleton)
* generates a tree-like network by repeated:

# elongation
# terminal bifurcation
# lateral budding

===Pseudoglandular stage===
[[File:Fetal lung histology.jpg|thumb|Fetal lung histology ([[:File:Fetal_lung_histology_01.jpg|large image]])]]
* week 5 - 17 (late embryonic, fetal)
* tubular branching of the human lung airways continues (16-25 generations of branching)
* by 2 months all segmental bronchi are present.
* lungs have appearance of a glandlike structure.
* stage is critical for the formation of all conducting airways.
* lined with tall columnar epithelium, the more distal structures are lined with cuboidal epithelium.

===Canalicular stage===

* week 16 - 24 (fetal to end of second trimester)
* Lung morphology changes dramatically
* differentiation of the pulmonary epithelium results in the formation of the future air-blood tissue barrier.
* Surfactant synthesis and the canalization of the lung parenchyma by capillaries begin (week 17).
* future gas exchange regions can be distinguished from the future conducting airways of the lungs.
* 1 respiratory bronchiole gives rise to 3-6 alveolar ducts.

===Saccular stage===
[[File:Lung alveoli development cartoon.jpg|thumb|Lung alveoli development cartoon]]
* week 24 to near term (fetal third trimester)
* most peripheral airways form widened airspaces, termed saccules.
* saccules widen and lengthen the airspace (by the addition of new generations).
* future gas exchange region expands significantly.
* Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin.
** May have a role in epithelial differentiation and control of surfactant secretion
* The vascular tree also grows in length and diameter during this time.

===Alveolar stage===
[[File:Pig lung alveolarization.jpg|thumb|Fetal lung alveolarization]]
[[File:Alveolar-sac-01.jpg|Alveolar sac structure|600px]]

* Thyroid hormone required for differentiation and stimulate surfactant production.
====Alveolar type I cells====
* small alveolar cells, type I pneumocytes
* very flat cells (thin as 0.05 µm)
* form most of the surface of the alveolar walls
* may contribute epithelium on both faces of the alveolar wall

====Alveolar type II cells====
* large alveolar cells, type II pneumocytes
* irregular to cuboidal shaped cells
* contain large number of granules called lamellar bodies, these are the precursors to pulmonary surfactant (phospholipid mixture).

* end month 6 alveolar cells type 2 appear and begin to secrete surfactant - premature babies have difficulties associated with insufficient surfactant.

====Alveolar macrophages====
* remove particulate matter that enters the alveoli with inspired air
* migrate over alveolar epithelium and phagocytose particulate matter

===Fetal lung volume===
Each lung volume as determined by ultrasound and matched to gestational age (PMID: 16388511)
* 12-13 weeks 0.05 mL
* 19-22 weeks 0.5 mL
* 29-32 weeks 1.9 mL

== Pleural Cavity ==
[[File:Gray0965.jpg|thumb|pleura]]
[[File:Gray0968.jpg|thumb|pleura]]
* The anatomical body cavity in which the lungs develop and lie.
* The pleural cavity forms in the lateral plate mesoderm as part of the early single intraembryonic coelom.
* This cavity is initially continuous with pericardial and peritoneal cavities and form initially as two narrow canals
** later becomes separated by folding (pleuropericardial fold, pleuroperitoneal membrane) and the later formation of the diaphragm

pleuropericardial fold - (pleuropericardial membrane) An early embryonic fold which restricts the communication between pleural cavity and pericardiac cavity, contains both the cardinal vein and phrenic nerve.

pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity.

===Pleura===
* serous membrane covers the surface of the lung and the spaces between the lobes
* arranged as a closed invaginated sac
* two layers (pulmonary, parietal) continuous with each other, the potential space between them is the '''pleural cavity'''

==Diaphragm==
Not respiratory tract but musculoskeletal development, there are 5 elements that contribute to the diaphragm.
{|
| [[File:Diaphragm components.jpg|300px|Components of the diaphragm]]
|
# septum transversum- central tendon
# 3rd to 5th somite- musculature of diaphragm
# ventral pleural sac- connective tissue
# mesentry of oesophagus- connective tissue around oesophasus and IVC
# pleuroperitoneal membranes- connective tissue around central tendon
|}
[[File:Gray804.gif|thumb|Adult Cervical Plexus (phrenic nerve shown lower right)]]
[[File:Adult diaphragm.jpg|300px|adult diaphragm]]

Innervation of the human diaphragm is by the phrenic nerves, arising from the same segmental levels from which the diaphragm skeletal muscles arise, segmental levels C3 to C5.

The paired phrenic nerves are mixed containing motor neurons for the diaphragm and sensory nerves for other abdominal structures (mediastinum, pleura, liver, gall bladder).

==Pulmonary Circulation==
[[File:Pulmonary circulation cartoon.jpg|thumb|300px|Pulmonary circulation]]
[[File:Gray0975.jpg|thumb|alveoli and blood vessels]]
* the pulmonary system not "functional" until after birth
* pulmonary arteries - (deoxygenated blood to lung) 6th aortic arch arteries
* pulmonary veins - (oxygenated blood from lung) are incorporated into the left atrium wall
* bronchial arteries - branches from dorsal aorta

==Fetal Respiratory Movements==
* Fetal respiratory movements (FRM) or Fetal breathing movements (FBM) are regular muscular contrations occurring in the third trimester.
* thought to be preparing the respiratory muscular system for neonatal function (and amniotic fluid are thought to have a role in lung maturation)
* thought to also have a role in late lung development.

==The First Breath==
[[File:Alveolar-sac-01.jpg|thumb|Alveolar sac structure]]
* The respiratory system does not carry out its physiological function (gas exchange) prenatally and remain entirely fluid-filled until birth.
* At birth, fluid in the upper respiratory tract is expired and fluid in the lung aveoli is rapidly absorbed this event has also been called "dewatering of the lung".
** The lung epithelia has to now rapidly change from its prenatal secretory function to that of fluid absorbtion.

===Exchange of Fluid for Air===
* fall in pulmonary vascular resistance
* increase in pulmonary blood flow
* thinning of pulmonary arteries (stretching as lungs increase in size)
* blood fills the alveolar capillaries

In the heart, pressure in the right side of the heart decreases and pressure in the left side of the heart increases (more blood returning from pulmonary).
[[File:Neonatal rib orientation.jpg|thumb|Neonatal rib orientation]]
* Respiratory Rate is higher than adult (30 breaths/minute).
===Rib Orientation===
* Infant rib is virtually horizontal, allowing only diaphragmatic breathing
* Adult rib orientation is oblique (both anterior and lateral views), allows for pump-handle and bucket handle types of inspiration.

== Respiratory Tract Histology==
<gallery>
File:Respiratory_histology_05.jpg|Trachea
File:Respiratory_histology_06.jpg|Trachea
File:Respiratory_histology_01.jpg|Bronchiole
File:Respiratory histology 09.jpg|Bronchiole ciliated simple columnar epithelium
File:Respiratory histology 07.jpg|Lung structure
File:Respiratory_histology_02.jpg|Alveoli and Duct
File:Respiratory_histology_03.jpg|Alveoli
File:Respiratory_histology_04.jpg|Alveoli elastin
</gallery>

:'''Histology Links:''' [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Respiratory/respir.htm Blue Histology] | [http://vslides.unsw.edu.au/VirtualSlideV2.nsf/id/F51ED7 UNSW Virtual Slides] | [http://www.path.uiowa.edu/cgi-bin-pub/vs/fpx_browse.cgi?cat=o_lung&div=nlm UIOWA Virtual Slidebox]

== Respiratory Tract Abnormalities ==
[[File:Respiratory tract.jpg|thumb|conducting system bronchi to lungs]]
[[Respiratory System - Abnormalities]]
===Tracheoesophageal Fistula ===
* Tracheo-Oesophageal Fistula, Oesophageal Atresia - Oesophageal Atresia with or without tracheo-oesophageal [[F#fistula|fistula]]
** '''Fistula''' - an abnormal communication between 2 structures (organs, vessels, cavities) that do not normally connect.

===Lobar Emphysema (Overinflated Lung)===
# There is an overinflated left upper lobe
# There is a collapsed lower lobe
# The left lung is herniating across the mediastinum

===Congenital Diaphragmatic Hernia===
* Failure of the pleuroperitoneal foramen (foramen of Bochdalek) to close (left side)
* allows viscera into thorax -iIntestine, stomach or spleen can enter the pleural cavity, compressing the lung.

* rare (Morgagni hernia) -an opening in the front of the diaphragm.

:'''Links:''' [http://www.ncbi.nlm.nih.gov/books/NBK1359 GeneReviews]

===Azygos Lobe===
[[File:Lung_Azygos_Lobe_02.jpg|thumb|Lung Azygos Lobe]]
* Common condition (0.5% of population).
* The right lung upper lobe expands either side of the posterior cardinal.
* There is also some course variability of the phrenic nerve in the presence of an azygos lobe.

===Congenital Laryngeal Webs===
* Laryngeal abnormality due to embryonic (week 10) incomplete recanalization of the laryngotracheal tube during the fetal period.
* Rare abnormality occuring mainly at the level of the vocal folds (glottis).

===Meconium Aspiration Syndrome===

(MAS) Meconium is the gastrointestinal contents that accumulate in the intestines during the fetal period. Fetal stress in the third trimester, prior to/at/ or during parturition can lead to premature meconium discharge into the amniotic fluid and sunsequent ingestion by the fetus and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur.

===Newborn Respiratory Distress Syndrome===
* Hyaline Membrane Disease (membrane-like substance from damaged pulmonary cells)
* absence of surfactant, if prolonged can be irreversible
* intrauterine asphyxia, prematurity and maternal diabetes

:'''Links:''' [http://www.nlm.nih.gov/MEDLINEPLUS/ency/article/001563.htm medline plus] | [http://www.medscape.com/article/976034-overview eMedicine]

===Bronchopulmonary Dysplasia===
* A chronic lung disease which can occur following premature birth.
* The definition of bronchopulmonary dysplasia (BPD) has in recent years changed from a severe lung injury and associated repair, to more of a disruption of lung development.

==Terms==
* '''antenatal''' before birth.
* '''alveoli number at birth''' - from 20 - 50 million and eventually in the adult 300 million.
* '''Bronchopulmonary dysplasia''' - (BPD) the most common serious sequela of premature birth.
* '''Bronchiolitis''' - is a viral infection of the lower respiratory tract and most common lower respiratory tract infection in infants. Respiratory syncytial virus (RSV) is responsible for 70 percent of all cases overall and Parainfluenza, adenovirus and influenza account for most of the remaining cases. (HSTAT Management of Bronchiolitis in Infants and Children)
* '''Chronic obstructive pulmonary disease''' (COPD) causes include smoking (85–90 percent of all cases), genetic factors (alpha-1 antitrypsin deficiency), passive smoking (children), occupational exposures, air pollution, and hyperresponsive airways. (HSTAT Management of Acute Exacerbations of Chronic Obstructive Pulmonary Disease)
* '''Clara cells''' non-ciliated cell found in the small airways (bronchioles) consisting of ciliated simple epithelium, these cells secrete glycosaminoglycans (Clara cell secretory protein, CCSP) to protect the bronchiole lining.
* '''Congenital Diaphragmatic Hernia''' (CDH) disorder with an incidence of 1 in 2500 live births.
* '''fetal breathing-like movements''' (FBMs) or Fetal respiratory movements are thought to be regular muscular contrations occurring in the third trimester, preparing the respiratory muscular system for neonatal function and to have a role in late lung development.
* '''glucocorticoid treatment''' - antenatal therapy to promote the maturation of the human fetal lung. Given as a synthetic glucocorticoid between 24 and 32 weeks of pregnancy to promote lung maturation in fetuses at risk of preterm delivery.
* '''lamellar bodies''' the storage form of surfactant in type II alveolar cells, seen as centrically layered "packages" of phospholipid. A count of lamellar bodies can be used as an assay for measuring fetal lung maturity.
* '''maternal diabetes''' if not controlled in pregnancy may delay fetal pulmonary maturation.
* '''Persistent Pulmonary Hypertension of the Newborn''' (PPHN) serious newborn condition due to due to the failure of closure one of the prenatal circulatory shunts, the ductus arteriosus. Occurs in about 1-2 newborns per 1000 live births and results in hypoxemia. (More? Respiratory Development - Birth)
* '''Pharyngitis''' inflammation of the pharynx involving lymphoid tissues of the posterior pharynx and lateral pharyngeal bands.
* '''pneumocyte''' or alveolar type I and type II cells.
* '''pulmonary hypoplasia''' can be due to anencephaly, renal hypoplasia or abnormalities of the thoracic cage
* '''pulmonary neuroendocrine cells''' (PNEC) single or innervated clusters of cells (neuroepithelial bodies) that line the airway epithelium, thought to have a role in regulating fetal lung growth and differentiation. At birth may also act as airway oxygen sensors involved in newborn adaptation. These cells synthesis and release amine (serotonin, 5-HT) and a several neuropeptides (bombesin).
* '''Respiratory distress syndrome''' (RDS) due to a surfactant deficiency at birth, particulary in preterm birth.
* '''secondary alveolar septa''' formed during the alveolar stage and are formed by projections of connective tissue and a double capillary loop.
*''' surfactant''' produced by alveolar type II cells is a mixture of lipids and proteins that both maintains alveolar integrity and plays a role in the control of host defense and inflammation in the lung.
* '''Surfactant therapy''' ([http://aappolicy.aappublications.org/cgi/content/full/pediatrics;121/2/419 American Academy of Pediatrics Policy] | [http://www.cps.ca/english/statements/fn/fn05-01.htm Canadian Paediatric Society Recommendations])
*''' thyroid hormone''' involved in the regulation of fetal lung development.
* '''vascular endothelial growth factor''' (VEGF) a secreted growth factor acting through receptors on endothelial cells to regulate vasculogenesis through their development, growth and function.
{{Glossary}}

{{Footer}}

[[Category:Endoderm]] [[Category:Respiratory]]

File:Gray0974.jpg

2012-02-27T21:02:27Z

S8600021:

==Lung Anatomy==

Part of a secondary lobule from the depth of a human lung, showing parts of several primary lobules. Camera drawing of one 50 μ section. X 20 diameters. (Miller.)

# bronchiole
# respiratory bronchiole
# alveolar duct
# atria
# alveolar sac
# alveolus or air cell

* m - smooth muscle
* a - branch pulmonary artery
* v - branch pulmonary vein
* s - septum between secondary lobules

[[Category:Respiratory]] [[Category:Historic Embryology]] [[Category:Gray's 1918 Anatomy]]

BGDA Practical 3 - Quiz

2012-02-22T00:49:54Z

S8600021: /* Take the Quiz */

Here are a few simple questions that relate to your BGD practical, this page is not a part of today's Practical class. You should try in your own time after completing the Practical today.Take the quiz and see what you know, if you get some wrong, try working through through the [[BGDA Practical - Fertilization to Implantation]].

==Take the Quiz==

<quiz display=shuffle>

{Which of the following statements is most correct concerning early development in female gametogenesis:
|type="()"}
- the total number of ooctyes is regulated by follicle stimulating hormone
- oocyte numbers increase prenatally and begin to decrease at puberty
+ less than 0.1% of all oocytes formed are released during reproductive life
- oocytes within all antral follicles are released in sequence at ovulation
- oocyte selection occurs at the primordial follicle stage
||Of the 7,000,000 million oocytes initially formed only 400-500 will be released during reproductive life from puberty to menopause. [[F#follicle stimulating hormone|Follicle stimulating hormone]] (from the pituitary) is involved in the development of a number of oocytes in primordial follicles from the total cohort. Oocyte number decreases from birth onward. A number of antral follicles will develop, with only one released each cycle, a few of the remainder could develop for future release the majority will degenerate by [[A#atresia|atresia]]. Oocyte selection occurs at the antral stage to form the preovulatory follicle. (More? [[Ovary Development]])

{The spermatogenic epithelium is stimulated by follicle stimulating hormone (FSH)
|type="()"}
+ true
- false
|| Follicle stimulating hormone (FSH) does stimulate the spermatogenic epithelium and luteinizing-hormone (LH) stimulates testosterone production by Leydig cells.

{Gamete differentiation occurs while diploid in
|type="()"}
+ oocytes
- spermatozoa
||Oocyte differentiation occurs while diploid (in first meiotic prophase), while spermatozoa differentiation occurs while haploid (after meiosis ends) in spermiogenesis.

{Which of the following is incorrect about the block to polyspermy
|type="()"}
- occurs after fertilization
+ occurs when meiosis II is completed
- occurs initially when sperm and oocyte membranes fuse
- occurs when cortical granules are released

||Meiosis II does completes at fertilization, but it has no role in blocking polyspermy.

{Which of the following statements about the blastocyst is most correct
|type="()"}
- the blastocyst forms from the 2 blastomere stage
- the blastocyst has a cavity lined with endoderm
- the blastocyst stage occurs after hatching from the zona pellucida
+ the blastocyst has an embryoblast and trophoectoderm layer

||The blastocyst has an [[E#embryoblast|embryoblast]], also called an inner cell mass, and a [[T#trophoectoderm|trophoectoderm]] or trophoblast layer. The 2 blastomere stage develops into the morula and at the formation of the blastocoel is then the blastocyst. The cavity lined by endoderm is the yolk sac and occurs after the blastocyst stage. The blastocyst stage occurs both before and after after hatching from the zona pellucida.

{Select the correct options below for the process of implantation
|type="[]"}
+ is driven by the trophoblast layer
+ occurs following adplantation
+ can occur inside and outside the uterine body
+ allows endocrine support of the corpus luteum

||Yes, all the above relate to the process of implantation.

{The extraembryonic coelom refers too
|type="()"}
+ amniotic cavity, yolk sac and chorionic cavity
- pericardial cavity, pleural cavity and peritoneal cavity
- blastocoel, somitocoel and lateral plate coelom
- maternal lacunae, uterine gland lumen, uterine body cavity

|| The extraembryonic coelom is the cavity lying outside the embryo forming the amniotic cavity, yolk sac and chorionic cavity. The intraembryonic coelom is the space inside the embryo lateral plate mesoderm forming the pericardial cavity, pleural cavity and peritoneal cavity. The blastocoel is the cavity inside the blastocyst before embryo formation. The somitocoel is the cavity forming inside the early somite before cell proliferation fills the space. The lateral plate coelom is the horseshoe space forming in the embryonic disc that gives rise to the the pericardial cavity, pleural cavity and peritoneal cavity. The maternal lacunae is the blood-filled space formed by the implantation process. The uterine gland lumen and uterine body cavity have nothing to do with embryonic spaces.

{Which of the following statements is incorrect about the process of gastrulation
|type="()"}
- gives rise to the trilaminar embryo
- involves cell migration from the epiblast layer
+ extends from the primitive node to the buccopharyngeal membrane
- occurs at the region known as the primitive streak
- generates endoderm and then mesoderm layers

{What are the two main early embryonic developmental roles of the notochord
|type="()"}
+ mechanical role in embryonic disc folding and signaling for tissue patterning
- formation of the nucleus pulposis and separating cloacal and buccopharyngeal membranes
- gastrulation and neuralation
- formation of the nucleus pulposis and intervertebral disc
|| The notochord has a mechanical role in how the embryonic disc folds and releases a signal (sonic hedgehog) that patterns surrounding tissues. The nucleus pulposis does form from the notochord (much later in embryonic development) but the notochord does not separate the cloacal and buccopharyngeal membranes or form the intervertebral disc (from sclerotome of somite). Gastrulation and neuralation, you were asleep in the practical class or checking your facebook status.

{Ectoderm refers only to the neural plate region of the trilaminar embryo
|type="()"}
- true
+ false
|| The entire layer of the trilaminar embryo is the '''ectoderm''' (meaning outer layer), the neural plate is only the central portion of this layer. Ectoderm forms the central [[N#neural plate|neural plate]] and the lateral parts form the [[E#epidermis|epidermis]].

</quiz>

{{Template:Quizzes}}

[[Category:Quiz]] [[Category:BGD]]

SH Lecture - Lymphatic Structure and Organs

2012-02-22T00:27:26Z

S8600021: /* Lymphatic System */

[[File:SHsmall.jpg]]
==Introduction==
[[Image:Mark Hill.jpg|100px|left]]
[[File:Adult lymphatic system.jpg|thumb|Adult lymphatic system]]
--[[User:S8600021|Mark Hill]] 10:23, 4 January 2012 (EST) This lecture is currently being reviewed and revised for 2012.
The lecture will introduce the anatomical and cellular components of the immune system. The lecture will not cover development of the immune system or details on specific immune cell function.

:{{Template:SH2011}}

===Aim===
This lecture will provide an overview of the histology of key lymphoid organs, including the lymph nodes, spleen and thymus, as well as extranodal lymphoid tissues including mucosal associated lymphoid tissues (MALT)

===Key Concepts===
# Lymphatic System
# Organs - Thymus, Spleen
# Lymph Nodes and Nodules
# Bone Marrow
# Extranodal Lymphoid Tissues
# Mucosal Associated Lymphoid Tissues (MALT)

==Textbook References==
* [[SH_Practical_-_Lymphatic_Structure_and_Organs|SH Laboratory this week]]
* Janeway’s Immunobiology [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=imm.TOC&depth=2 NCBI Bookshelf] | [http://www.garlandscience.co.uk/textbooks/0815341237.asp Publisher page]
* '''Histology and Cell Biology''' - A.L. Kiersenbaum (2001)
** [http://books.google.com.au/books?id=_ElRdXfE0cYC&lpg=PA68&ots=5lZxyJl86x&dq=Histology%20and%20Cell%20Biology%20-%20A.%20L.%20Kierszenbaum&lr&pg=PA147#v=onepage&q=blood&f=false Chapter 6: Blood]
** [http://books.google.com.au/books?id=_ElRdXfE0cYC&lpg=PA68&ots=5lZxyJl86x&dq=Histology%20and%20Cell%20Biology%20-%20A.%20L.%20Kierszenbaum&lr&pg=PA147#v=snippet&q=immune&f=false Chapter 10: Immune-Lymphatic]
* [http://php.med.unsw.edu.au/cellbiology/index.php?title=2010_Society_and_Health_-_Lymphatic_organs_histology Online - Lecture 2010] | [http://cellbiology.med.unsw.edu.au/units/medicine/SHlymph.htm Online - Lecture 2008]
* {{Template:Immune Links}}

==Two Systems==
* '''Lymphoid System''' - three major types of lymphocytes (T, B, and NK), tissues, organs and vessels

* '''Mononuclear Phagocytic System''' (MPS, also called Lymphoreticular System or Reticuloendothelial System, RES) - circulating monocytes of peripheral blood and non-circulating (fixed) tissue macrophages found throughout the body

[http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4423 MBoC Figure 24-3 Human lymphoid organs]

==Lymphatic System==
* Connective Tissue Embryonic origin- Mesoderm
* Consists of Cells, tissues and organs
* Immune “monitor” of body surfaces, internal fluids

Immune System

Note: Immunity is covered in detail elsewhere in the course, current lecture is about Lymphoid Organ structure/location

* Tissues and Organs
** Thymus, spleen, lymph nodes, lymphatic nodules, diffuse lymphatic tissues, bone marrow
** Organs consist also of structural cells and extracellular matrix
* Lymphatic vessels connect system parts
* Cells are Lymphocytes
* B Lymphocytes and T Lymphocytes [http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4430 MBoC Figure 24-7. Electron micrographs of nonactivated and activated lymphocytes]
* White blood cells, leukocytes

These are blood cells

==Blood Cells==

[[File:Hematopoietic_and_stromal_cell_differentiation.jpg|500px]]

[[SH Lecture - Lymphatic Structure and Organs#Blood_Cells|Blood Cell proportions]]

[[File:lymphocyte 01.jpg|200px]] [[File:Lymphocyte_02.jpg|400px]]

==Central/Peripheral Lymphoid Organs==

Central lymphoid organs
* Lymphocytes develop from precursor cells

Peripheral lymphoid organs
* Lymphocytes respond to antigen
* lymph nodes or spleen

==Mononuclear Phagocytic System==

(Mononuclear Phagocytic System MPS, also called Lymphoreticular System or Reticuloendothelial System, RES)

[[File:Monocyte 01.jpg|300px]] [[File:Liver- Kupffer cell and reticular fibre.jpg]]

Mononuclear Phagocytes 2 types:
# Circulating '''monocytes''' of peripheral blood (monocytes entering the connective tissue differentiate into macrophages)
# Non-circulating (fixed) tissue '''macrophages''' (MΦ) found throughout the body (Liver (Kuffler cells), spleen and other tissues)

==Lymph==
[[File:GIT- Jejunum 01.jpg|thumb|Jejunum]]
* Fluid portion of lymphatic circulation
* blood plasma will leave blood vessels into surrounding tissues
* adds to normal tissue interstitial fluid
* surplus of liquid needs to be returned to circulation
* Lymph vessels are provide unidirectional flow of this liquid

==Lymph Vessels==
[[File:Lymphatic capillary.jpg|thumb|Lymph capillary]]
[[File:Gray0599.jpg|thumb|Thoracic and right lymphatic ducts]]
Three types based on size and morphology

* '''Lymph capillaries''' begin as blind-ending tubes in connective tissue, larger than blood capillaries, very irregularly shaped
* '''Lymph collecting vessels''' larger and form valves, morphology similar to lymph capillaries
* '''Lymph ducts''' 1 or 2 layers of smooth muscle cells in wall

(Remember anatomy '''NAVL''' = Nerve, Artery, Vein and Lymph)

==Lymphocyte Circulation==

[http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4442 MBoC Figure 24-14. The path followed by lymphocytes as they continuously circulate between the lymph and blood]
* The circulation through a lymph node is shown.
* Microbial antigens are carried into the lymph node by dendritic cells, which enter via afferent lymphatic vessels draining an infected tissue.
* T and B cells, by contrast, enter the lymph node via an artery and migrate out of the bloodstream through postcapillary venules.
* Unless they encounter their antigen, the T and B cells leave the lymph node via efferent lymphatic vessels, which eventually join the thoracic duct.
* The thoracic duct empties into a large vein carrying blood to the heart.
* A typical circulation cycle takes about 12–24 hours.

'''Links:''' [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A4419 MBoC Chapter 24 - The Adaptive Immune System] | [http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4442 MBoC Figure 24-14. The path followed by lymphocytes as they continuously circulate between the lymph and blood] | [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=imm Immunobiology]

==Diffuse Lymphatic Tissue==

Alimentary canal, respiratory passage, urogenital tract
* Not enclosed by a capsule
* Located in subepithelial tissue - '''Lamina propria'''

'''Lymphocytes'''
* travel to nodes and back again
* proliferation and differentiation

'''Effector cells'''
* B Cell secreting antibody = '''Plasma Cell'''
* T Cell = '''Memory Cell'''

* Diffuse lymphatic tissue + nodules
* Reactive - enlarge when activated (by antigen)

==MALT, BALT and GALT==
{|
| [[File:oesophagus MALT.jpg]]
| Internal epithelia Associated Lymphoid Tissue - naming based upon the anatomical locations

* MALT - Mucosa Associated Lymphoid Tissue
** BALT - Bronchus Associated Lymphoid Tissue
** GALT - Gut Associated Lymphatic Tissue
|}

==Immune Responses==
Adaptive immunity has 2 main classes

# '''Antibody-mediated''' - B Lymphocyte
# '''Cell-mediated''' - T Lymphocyte

==Lymph Nodules==
(or follicles)
* Organized concentrations of Lymphocytes
** No capsule, covered by epithelia
* Nodules are also unit structure seen in a node
* Oval concentrations in meshwork of reticular cells

'''Reticular cell'''
* produces reticular fibers (collagen type III) and surrounds the fibers with its cytoplasm
* reticular fibers are also produced by fibroblasts

===Gastrointestinal Tract===
* Oropharynx - Tonsils
* Distal small intestine (ilieum) - Peyer’s Patches
* Appendix, cecum

===Nodule States===
* '''Primary Nodule''' - Mainly small lymphocytes
* '''Secondary Nodule'''
** Central pale region (germinal centre) - Effector cells and macrophages
** Dark outer ring (small lymphocytes)

'''Links:''' [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A49 Immnuobiology - Figure 1.10. Organization of typical gut-associated lymphoid tissue]

==Tonsils==
Anatomical location - Palatine ('''tonsils'''), Lingual and Pharyngeal ( '''adenoids''' )

===Palatine Tonsils===
[[File:Tonsil_01.jpg|300px]] [[File:Tonsil_02.jpg|300px]]

* the "tonsils", lateral wall of oropharynx
* covered by stratified squamous epithelium
* numerous crypts (10-20) infolds of surface epithelium
* Afferent lymph vessels absent
* Efferent lymph vessels are present

===Lingual Tonsils===
* lamina propria root of tongue
* covered by stratified squamous epithelium
* salivary glands and skeletal muscle are directly adjacent

===Pharyngeal Tonsils===
* '''adenoids''' or nasopharyngeal tonsils, upper posterior part of throat
* covered by a pseudostratified ciliated epithelium with goblet cells

==Peyer's Patch==

[[File:Peyer's patch 01.jpg|200px]] [[File:Peyer's patch 02.jpg|200px]]

Peyer's Patch, Ileum

microfold cells or M-cells

==Lymph Nodes==

[[Quicktime Movie - Mouse Lymph Node 7]] | [[Quicktime_Movies#Immune|more lymph node movies]]
[[File:Lymph_node_structure.jpg|thumb]]
[[File:Lymph node structure 01.png|thumb|Schematic of lymph node showing lymph sinuses]]
[http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A47 Immunobiology - Figure 1.8. Organization of a lymph node]

[http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4444 MBoC Figure 24-16. A simplified drawing of a human lymph node]

{|
| * Encapsulated organ (1 mm - 2 cm)
* In lymph vessel pathways “filter”
* Afferent- towards node
* Efferent- away from node
* Location throughout the entire body - Concentrated in axilla, groin, mesenteries
* Antigen transformed lymphocytes from the blood
| [[File:Lymph_node_histology_06.jpg|200px]]
|}
===Lymph Node Structure===
[[File:Lymph_node_histology01.jpg|thumb|Lymph node cortex histology]]
* Capsule - dense connective tissue
* Trabeculae - dense connective tissue
* Reticular Tissue - Reticular cells and fibers, supporting meshwork

[[File:Lymph node histology 02.jpg]] [[File:Lymph node histology 03.jpg]]

Lymph

* enters the node through '''afferent vessels'''
* filters through the '''sinuses'''
* leaves through '''efferent vessels'''

Subcapsular sinus = marginal sinus

[[File:Lymph_node_histology_01.jpg]] [[File:Lymph_node_histology_04.jpg]]

Continuation of trabecular sinus

Lymphocyte (T and B) Traffic

# Enter from high endothelial venules (HEVs also called post-capillary venules)
# Spend 8 to 24 h in the lymph node interstitium.
# Enter a network of medullary sinuses.
# Drain from sinuses into efferent lymphatic vessels.

'''Links:''' [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A47 Immunobiology - Figure 1.8. Organization of a lymph node] | [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Lymph Blue Histology - Lymph Nodes]

==Thymus==
{|
| [[File:Thymus_cartoon.jpg|300px]]
| [http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4429 MBoC Figure 24-6. The development and activation of T and B cells]

[http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4430/ Figure 24-7. Electron micrographs of nonactivated and activated lymphocytes]
|}
===Development Changes===
[[File:Gray1178.jpg|thumb|Fetal thymus anatomy]]
[[File:Fetal thymus.jpg|thumb|Fetal thymus]]
Changes with age
Overall Size
* birth 10-15 g
* puberty 30-40 g
* after puberty - involution
** Replaced by adipose tissue
** middle-aged 10 g

===Thymus Anatomy===

* Superior mediastinum, anterior to heart
* Bilobed lymphoepithelial organ
** Contains reticular cells but no fibers
* Stem lymphocytes
** proliferate and differentiate
** forms long-lived T- lymphocytes

===Thymus Cells===

* '''Reticular cells'''
** Abundant, eosinophilic, large, ovoid and light nucleus 1-2 nucleoli
** sheathe cortical capillaries
** form an epitheloid layer
** maintain microenvironment for development of T-lymphocytes in cortex (thymic epitheliocytes)
* '''Macrophages'''
** cortex and medulla
** difficult to distinguish from reticular cells in H&E
* '''Lymphocytes'''
** cortex and medulla - more numerous (denser) in cortex
** majority of them developing T-lymphocytes (= thymic lymphocytes or thymocytes)

===Fetal/Young Thymus===
{|
| [[File:Thymus - young 01.jpg‎|300px]]
| [[File:Thymus - young 02.jpg‎|300px]]
|-
|Young medulla
|Young cortex
|}
Thymic corpuscle

Hassall’s corpuscle - Mass of concentric epithelioreticular cells

===Adult Thymus===
[[File:Thymus adult.jpg]]

* Cortical lymphoid tissue is replaced by adipose tissue
* Increase in size of thymic corpuscles

'''Links:''' [http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus Blue Histology - Thymus]

==Spleen==
[[File:Spleen anatomy.jpg]] [[File:Gray1039.jpg|300px]]

===Functions===

'''1. Immune'''
* filters blood in much the way that the lymph nodes filter lymph.
* '''Lymphocytes''' in the spleen react to pathogens in the blood and attempt to destroy them.
* '''Macrophages''' then engulf the resulting debris, the damaged cells, and the other large particles.

'''2. Red Blood Cell Removal'''

* The spleen (and liver) removes old and damaged erythrocytes from the circulating blood.
* Like other lymphatic tissue, it produces lymphocytes, especially in response to invading pathogens.

'''3. Blood Reservoir'''
* The sinuses in the spleen also act as a reservoir for blood.
** In emergencies, such as hemorrhage, smooth muscle in the vessel walls and in the capsule of the spleen contracts.
** This squeezes the blood out of the spleen into the general circulation.

===Structure===
[[File:Spleen_histology_01.jpg]] [[File:Spleen_histology_02.jpg]]
* Capsule, trabeculae (dense connective tissue)
* Splenic pulp White pulp, red pulp - based on appearance and cell content

[[File:Spleen_histology_05.jpg|thumb|White pulp -periarterial lymphoid sheath (PALS)]]
'''White Pulp'''
* lymphocytes surround central arteries
* as periarterial lymphoid sheath (PALS)

'''Red Pulp'''
* Red blood cells
* Splenic sinuses
* Splenic cords

[[File:Spleen_histology_03.jpg]]

'''Reticular Fibers'''

[[File:Spleen_histology_05.jpg]]
[[File:Spleen_histology_04.jpg]]

===Spleen Reticular Fibres===

'''Links:''' [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A48 Immunobiology - Figure 1.9. Organization of the lymphoid tissues of the spleen]

==B Cell Development==
* Bone marrow
* blood
* Lymph node, nodule
* Lymphatic vessel
* Bone marrow

Germinal Centres
* Bone Marrow
* Medullary cords contain plasma cells

Plasma cells
* secrete antibody directly into blood for distribution to all body
* in local extrafollicular sites are short lived 2–4 days
* longer-lived plasma cells in bone marrow 3 weeks to 3 months+

==Additional Information==
''The following is not part of the lecture and is for reference purposes only.''
* Blood Cells
* Florence R. Sabin (1871-1953)

==Associated Practical==

[[SH Practical - Lymphatic Structure and Organs]]

==Blood Cell Numbers==
===Red Blood Cells===
* Male: 4.32 - 5.66 x 1012/l
* Female: 3.88 - 4.99 x 1012/l
===Leukocytes===
* Male: 3.7 - 9.5 x 109/l
* Female: 3.9 - 11.1 x 109/l

'''Granulocytes''' (1.8 - 8.9 x 109/l)
* Neutrophils: 1.5 - 7.4 x 109/l
* Eosinophils: 0.02 - 0.67 x 109/l
* Basophils: 0 - 0.13 x 109/l

'''Lymphocytes''' (1.1 - 3.5 x 109/l)
* B-cells: 0.06 - 0.66 x 109/l
* T-cells: 0.77 - 2.68 x 109/l
* CD4+: 0.53 - 1.76 x 109/l
* CD8+: 0.30 - 1.03 x 109/l
* NK cells: 0.20 - 0.40 x 109/l

'''Monocytes''' (0.21 - 0.92 x 109/l)

===Platelets===
* 140 - 440 x 109/l

===Textbook===
'''Immunobiology''' by Janeway, Charles A.; Travers, Paul; Walport, Mark; Shlomchik, Mark New York and London: Garland Science; c2001

* [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=imm.TOC&depth=2 Immunobiology]
* [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A45 Figure 1.7. The distribution of lymphoid tissues in the body]
* [http://www.ncbi.nlm.nih.gov/books/NBK27169/figure/A1359 Figure 10.18. Anatomy of mucosal immune responses]

* [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A40/ Figure 1.3. All the cellular elements of blood, including the lymphocytes of the adaptive immune system, arise from hematopoietic stem cells in the bone marrow]

* [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A41/ Figure 1.4. Myeloid cells in innate and adaptive immunity]

* [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A42/ Figure 1.5. Lymphocytes are mostly small and inactive cells]

'''Anatomy of the Human Body''' (Gray)1918
* Historic anatomy is good, there are there are some functional inaccuracies.
* [http://www.bartleby.com/107/175.html The Lymphatic System] | [http://www.bartleby.com/107/176.html The Thoracic Duct] | [http://www.bartleby.com/107/278.html The Spleen] | [http://www.bartleby.com/107/274.html The Thymus] |

Lymph Node Analysis
* 3D Reconstruction Pelvic Node Anatomy
* Reconstruction of Axillary Node Anatomy

==Additional Images==

<gallery>
File:Lymph_node_cartoon.jpg|Lymph node cartoon
File:Lymph_nodes_head_neck_superficial.jpg|Lymph nodes - head neck superficial
File:Gray1192.jpg|Section of the spleen, showing the termination of the small blood vessels
File:Spleen_histology_06.jpg|human spleen histology
File:Spleen_histology_07.jpg|human spleen histology
File:Spleen_histology_08.jpg|human spleen histology
File:Lymph node 05.jpg|rabbit lymph node subcapsular sinus
</gallery>

== Terms ==
A few key terms associated with the Lymphoid system.

* '''adenoid''' - (Greek " +''-oeides ''<nowiki>= in form of) in the form of a gland, glandular; the pharyngeal tonsil. </nowiki>
* '''Afferent lymph''' - vessel carrying lymph towards a node.
* '''Antibody mediated immunity''' - the immune function of plasma cells (active B lymphocytes) secreting antibody which binds antigen.
* '''antibodies''' - mammals have five classes (IgA, IgD, IgE, IgG, and IgM)
* '''antigen''' - any substance that is recognised by the immune system and stimulates antibody production.
* '''appendix''' - is a gut-associated lymphoid tissue located at the beginning of the colon. The anatomy is as a finger-like structure that arises from the cecum. The length (2.5-13 cm) is longer in both infants and children and also has more abundant lymphatic tissue in early life. The wall structure is similar to the small intestine (though with no villi), nor plicae circularis. Lymph nodules surround the lumen of the gastrointestinal tract and extend from the mucosa into the submucosa.
* '''B lymphocyte (cell)''' - historically named after a structure called the '''b'''ursa of Fabricius in birds, a source of antibody-producing lymphocytes. These cells develop in the bone marrow. (More? [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.4430 Electron micrographs of nonactivate and activated lymphocytes])
* '''BALT''' - Bronchus Associated Lymphoid Tissue
* '''band cell''' - (band neutrophil or stab cell) seen in bone marrow smear, a cell undergoing granulopoiesis, derived from a metamyelocyte, and leading to a mature granulocyte. Also occasionally seen in circulating blood.
* '''cecum''' - (caecum, Latin, ''caecus'' = "blind") within the gastrointestinal tract a pouch that connects the ileum with the ascending colon of the large intestine.
* '''cell''' - has a specific cell biology definition, but is often used instead of "lymphocyte" when describing B and T cells.
* '''Cell-mediated immunity''' - the immune function of T lymphocytes.
* '''"clockface"''' - a term used to describe the appearance of plasma cell nuclei due to the clumping of the chromatin at the nucleus periphery. More clearly seen in tissue plasma cells that the bone marrow smear, where they are sometimes confused with the basophilic erythroblasts.
* '''cords of Billroth''' - spleen cellular columns located in red pulp. surrounded by splenic sinusoids. Cords contain reticular cells, macrophages, lymphocytes, plasma cells and erythrocytes.
* '''cortex''' - outer layer, used in association with medulla (innner layer or core) a general description that can be applied to describing an organ with a layered structure.
* '''Effector cells''' - the immune functioning (active) B and T lymphocytes.
* '''Efferent lymph''' - vessel carrying lymph away from a node.
* '''GALT''' - Gut Associated Lymphatic Tissue
* '''haemopoiesis''' (hemopoiesis) formation of blood cells.
* '''Hassall's corpuscle''' - thymic corpuscle.
* '''IgA''' - the main class of antibody in secretions (saliva, tears, milk, and respiratory and intestinal secretions).
* '''IgD''' - the immunoglobulin B cell starts to produce as a cell-surface molecule after leaving the bone marrow.
* '''IgE''' - bind Fc receptors (surface of mast cells in tissues and basophils in the blood).
* '''IgG''' - the major class of immunoglobulin in the blood.
* '''IgM''' - the first class of antibody made by a developing B cell, which may switch to making other classes of antibody.
* '''immunodeficiency''' - when one or more components of the immune system is defective. (More? [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&rid=imm.section.1494 Immunobiology - immunodeficiency])
* '''involution''' - in the Thymus refers to the replacement, mainly in the cortex, of cells by adipose tissue. (More? [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&term=thymus+involution&doptcmdl=Books PubMed- thymus involution]) | [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&rid=cmed6.section.23856#23857 Cancer Medicine - Thymomas and Thymic Tumors])
* '''lamina propria''' - a layer of loose connective tissue found underneath the epithelium of mucosa.
* '''Leukocyte-''' (Greek, lukos= clear, white) white blood cell.
* '''lingual'''- related to the tongue.
* '''lymph node''' - connective tissue encapsulated lymphoid organ (1mm - 2cm in size), positioned in the pathway of lymph vessels.
* '''macrophage''' - a large highly motile white blood cell which engulfs foreign material (bacteria etc) and both degenerating cells and cell fragments. Found in many different tissues and locations. (More? [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=imm.figgrp.1508 Immunobiology - Defects in phagocytic cells are associated with persistence of bacterial infection])
* '''MALT''' - Mucosa Associated Lymphoid Tissue
* '''medulla''' - inner layer or core, used in association with cortex (outer layer) a general description that can be applied to describing an organ with a layered structure.
* '''Memory Cell''' - effector T cell (lymphocyte)
* '''normoblast''' - seen in bone marrow smear, a developing erythroblast (red blood cell) that still retains a nucleus.
* '''parenchyma''' - (Greek = ''enkeim'' "to pour in") cells forming the functional cells of an organ or tissue. These cells carry out the function of the organ at a cellular level, and are not the structural cells, connective tissue, extracellular matrix (stromal).
* '''periarterial lymphoid sheath''' - (PALS) in the spleen the white pulp that surrounds the central arteries. (T-lymphocytes,macrophages and plasma cells)
* '''Plasma Cell''' - active B cell (lymphocyte) which is secreting antibody. Located in either bone marrow or peripheral lymphoid tissues, these cells have and increased cytoplasmic volume (due to increase rough endoplasmic reticulum) in comparison to the inactive (non-secreting) lymphocyte.
* '''sentinel lymph node''' - the hypothetical first lymph node or group of nodes reached by metastasizing cancer cells from a primary tumour.
* '''splenic sinusoids''' - enlarged spleen capillary spaces located in red pulp and surrounding cords of Billroth.
* '''stroma''' - (Greek = "a cover, table-cloth, bedding") tissue forming the framework/support of an organ or tissue. That is the structural cells which form connective tissue and secrete extracellular matrix, rather than the functional cells (parenchymal). All organs can therefore be functionally divided into these 2 components, stromal/parenchymal.
* '''Subcapsular sinus''' (=marginal sinus) space lying under the connective tissue capsule which receives lymph from afferent lymphatic vessels.
* '''Thymic corpuscle''' (=Hassall's corpuscle) a mass of concentric epithelioreticular cells found in the thymus. The number present and size tend to increase with thymus age. (see classical description of Hammar, J. A. 1903 Zur Histogenese und Involution der Thymusdriise. Anat. Anz., 27: 1909 Fiinfzig Jahre Thymusforschung. Ergebn. Anat. Entwickl-gesch. 19: 1-274.)
* '''thymic epitheliocytes''' - reticular cells located in the thymus cortex that ensheathe the cortical capillaries, creating and maintain the microenvironment necessary for the development of T-lymphocytes in the cortex.
* '''T lymphocyte (cell)''' - named after '''t'''hymus, where they develop, the active cell is responsible for cell-mediated immunity. (More? [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.4430 Electron micrographs of nonactivate and activated lymphocytes])

thymus

tonsils

*''' vermiform appendix''' - see appendix, anatomical region containing gut-associated lymphoid tissue located within the gastrointestinal tract at the beginning of the colon. The anatomy is as a finger-like structure that arises from the cecum. The length (2.5-13 cm) is longer in both infants and children and also has more abundant lymphatic tissue in early life. The wall structure is similar to the small intestine (though with no villi), nor plicae circularis. Lymph nodules surround the lumen of the gastrointestinal tract and extend from the mucosa into the submucosa.

{{Glossary}}

{{Footer}}

[[Category:Immune]] [[Category:Histology]]

Paper - Observations upon Young Human Embryos

2012-02-22T00:25:20Z

S8600021: /* Characters and Dimensions of Chorionic Vesicle */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

{{Historic Disclaimer}}

{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]] [[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]] [[File:Wilson1914-09.jpg|400px|Fig. 9.]]

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connecting with the entoderm at the base of the allantoic duct (text-fig.9). The position of this cloacal membrane maybe compared with that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far in front of this point. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figure shows the degree of vertical curvature of the several regions of the embryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

[[File:Wilson1914-10.jpg|400px|Fig. 10.]] [[File:Wilson1914-11.jpg|400px|Fig. 11.]]

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's monograph L'ceuf human (5) (Geneve, 1909), illustrating respectively Selenka's outline figure of HylobatesRaflesi,and Eternod's outline of his 1-3 mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region in these embryos ascompared with my specimen "H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). In embryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." In the "Klb" embryo, the fore-gut was closed in throughout the extent of 32 sections. Grosser states (6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"represents the anterior portion of the future pharynx. It has the usual laterally expanded form (text-fig. 6). It gradually diminishes in width when traced forwards until the cranial limit of the first primary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. No such marked ventral flexion of the narrow terminal segment of the gut as that seen in embryo "Klb" is recognizable. At a distance of 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullary plate suddenly disappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

[[File:Wilson1914-12.jpg|400px|Fig. 12.]] [[File:Wilson1914-13.jpg|400px|Fig. 13.]]

I have made a rough wax-plate reconstruction of the entire pharynx, but unfortunately certain dislocations of structure detract from its value for purposes of publication. In most respects, however, it is reliable enough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. On the exterior of the model this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Though present throughout practically its entire length, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudally from the level of the oral sinus it becomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,as are the paired pericardial cavities themselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventral pharyngeal groove," in Grosser's sense (cf. (6), pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." It makes its appearance in the same section (142) which shows the first trace of the definite thyroid rudiment. This is the eighth section behind the cranial limit of the first pharyngeal pouch. It may be noted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the first pharyngeal pouch, as described by Grosser in embryo "Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary plate medially, and the two dorsal aorta lateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. The entoderm covering this median prominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexed condition of the entire primitive-streak region. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly (text-figs. 8 and 9), this groove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. This turns dorsally (text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differently in different regions of their extent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianin position. In no one section does this appear as wholly complete,but there is little doubt that it was actually completein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

[[File:Wilson1914-14.jpg|400px|Fig. 14.]] [[File:Wilson1914-15.jpg|400px|Fig. 15.]]

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

[[File:Wilson1914-16.jpg|400px|Fig. 16.]][[File:Wilson1914-17.jpg|400px|Fig. 17.]]

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17), but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16) the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig. 14' shows only the right heart. The plane of the latter section lies quite in front of the looped arterial end of the left heart, which is cut through in the section shown in text-fig. 6. To establish the connexions of this looped arterial left hear tone must pass caudally from the plane of the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is due mainly to the obliquity of the plane of sectional ready referred to. It is somewhat difficult to state accurately the extent of this obliquity, but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individual flexure. The relations of each heart-tube to the interpericardial septum may be easily recognised in the text-figs.6 and 14. It is evident that each heart-tube,in so far as it is separate, occupies its own pericardial cavity.

[[File:Wilson1914-18.jpg|400px|Fig. 18.]]

I have not yet been able to undertake a plastic reconstruction of this highly interesting state in cardiac development. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will only be approximate. The section series is complete and quite intelligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aortic arch system.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhaps this relationship would also best explain the condition met with by Eternod in his 1.3 mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured by Eternod in that interesting specimen. Inanycase,it is clear to me that there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappearing, while the me8ocardium posteriuos persists for sometime longer. The closely approximated but not yet fused endothelial tubes are now surrounded by a continuous myo-epicardial mnarntle (Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

The fusion of the right and left pericardial cavities is, as we have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium" atal in the ordinary sense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appear to be of essentially similar character to those of embryo "H 3," and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation to commencement of abortion, 40 days. Periods ince end of last menstruation, 36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21 days. No conjugal history. Menstrual history as above.

Body Form.-No dorsal bend of embryonic body.

Primitive-streak region (= tail end) deeply furrowed, and forming only a very slight angle with the body proper. No.tail-prominence other than caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.

Entire primitive-streak region still appears as the nearlydirectcaudal continuation of the embryonic body proper. Cloacal membrane posteriorly.

Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not less than two.

Chorda.-Chorda entirely absent as such. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in front of the neurenteric aperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend.

Medullary groove well formed throughout but no where closed. No clear evidence of intrinsic brain segmentation. No traces of commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, which are imperfectly separated by an incomplete septum propriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division into venous and arterial segments. Paired dorsal aortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. It tapers at its hindered, first gradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. The cloacal membranous connexion with the ectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsal enteric groove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. The total length of the canal,including the vesicle, is about 0.6mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
----

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Paper - Observations upon Young Human Embryos

2012-02-22T00:24:40Z

S8600021: /* Summary of Characters of Embryo "H3" */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]] [[File:Wilson1914-09.jpg|400px|Fig. 9.]]

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connecting with the entoderm at the base of the allantoic duct (text-fig.9). The position of this cloacal membrane maybe compared with that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far in front of this point. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figure shows the degree of vertical curvature of the several regions of the embryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

[[File:Wilson1914-10.jpg|400px|Fig. 10.]] [[File:Wilson1914-11.jpg|400px|Fig. 11.]]

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's monograph L'ceuf human (5) (Geneve, 1909), illustrating respectively Selenka's outline figure of HylobatesRaflesi,and Eternod's outline of his 1-3 mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region in these embryos ascompared with my specimen "H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). In embryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." In the "Klb" embryo, the fore-gut was closed in throughout the extent of 32 sections. Grosser states (6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"represents the anterior portion of the future pharynx. It has the usual laterally expanded form (text-fig. 6). It gradually diminishes in width when traced forwards until the cranial limit of the first primary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. No such marked ventral flexion of the narrow terminal segment of the gut as that seen in embryo "Klb" is recognizable. At a distance of 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullary plate suddenly disappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

[[File:Wilson1914-12.jpg|400px|Fig. 12.]] [[File:Wilson1914-13.jpg|400px|Fig. 13.]]

I have made a rough wax-plate reconstruction of the entire pharynx, but unfortunately certain dislocations of structure detract from its value for purposes of publication. In most respects, however, it is reliable enough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. On the exterior of the model this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Though present throughout practically its entire length, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudally from the level of the oral sinus it becomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,as are the paired pericardial cavities themselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventral pharyngeal groove," in Grosser's sense (cf. (6), pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." It makes its appearance in the same section (142) which shows the first trace of the definite thyroid rudiment. This is the eighth section behind the cranial limit of the first pharyngeal pouch. It may be noted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the first pharyngeal pouch, as described by Grosser in embryo "Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary plate medially, and the two dorsal aorta lateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. The entoderm covering this median prominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexed condition of the entire primitive-streak region. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly (text-figs. 8 and 9), this groove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. This turns dorsally (text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differently in different regions of their extent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianin position. In no one section does this appear as wholly complete,but there is little doubt that it was actually completein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

[[File:Wilson1914-14.jpg|400px|Fig. 14.]] [[File:Wilson1914-15.jpg|400px|Fig. 15.]]

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

[[File:Wilson1914-16.jpg|400px|Fig. 16.]][[File:Wilson1914-17.jpg|400px|Fig. 17.]]

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17), but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16) the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig. 14' shows only the right heart. The plane of the latter section lies quite in front of the looped arterial end of the left heart, which is cut through in the section shown in text-fig. 6. To establish the connexions of this looped arterial left hear tone must pass caudally from the plane of the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is due mainly to the obliquity of the plane of sectional ready referred to. It is somewhat difficult to state accurately the extent of this obliquity, but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individual flexure. The relations of each heart-tube to the interpericardial septum may be easily recognised in the text-figs.6 and 14. It is evident that each heart-tube,in so far as it is separate, occupies its own pericardial cavity.

[[File:Wilson1914-18.jpg|400px|Fig. 18.]]

I have not yet been able to undertake a plastic reconstruction of this highly interesting state in cardiac development. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will only be approximate. The section series is complete and quite intelligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aortic arch system.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhaps this relationship would also best explain the condition met with by Eternod in his 1.3 mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured by Eternod in that interesting specimen. Inanycase,it is clear to me that there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappearing, while the me8ocardium posteriuos persists for sometime longer. The closely approximated but not yet fused endothelial tubes are now surrounded by a continuous myo-epicardial mnarntle (Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

The fusion of the right and left pericardial cavities is, as we have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium" atal in the ordinary sense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appear to be of essentially similar character to those of embryo "H 3," and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation to commencement of abortion, 40 days. Periods ince end of last menstruation, 36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21 days. No conjugal history. Menstrual history as above.

Body Form.-No dorsal bend of embryonic body.

Primitive-streak region (= tail end) deeply furrowed, and forming only a very slight angle with the body proper. No.tail-prominence other than caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.

Entire primitive-streak region still appears as the nearlydirectcaudal continuation of the embryonic body proper. Cloacal membrane posteriorly.

Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not less than two.

Chorda.-Chorda entirely absent as such. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in front of the neurenteric aperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend.

Medullary groove well formed throughout but no where closed. No clear evidence of intrinsic brain segmentation. No traces of commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, which are imperfectly separated by an incomplete septum propriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division into venous and arterial segments. Paired dorsal aortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. It tapers at its hindered, first gradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. The cloacal membranous connexion with the ectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsal enteric groove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. The total length of the canal,including the vesicle, is about 0.6mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-22T00:20:08Z

S8600021: /* Heart */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]] [[File:Wilson1914-09.jpg|400px|Fig. 9.]]

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connecting with the entoderm at the base of the allantoic duct (text-fig.9). The position of this cloacal membrane maybe compared with that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far in front of this point. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figure shows the degree of vertical curvature of the several regions of the embryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

[[File:Wilson1914-10.jpg|400px|Fig. 10.]] [[File:Wilson1914-11.jpg|400px|Fig. 11.]]

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's monograph L'ceuf human (5) (Geneve, 1909), illustrating respectively Selenka's outline figure of HylobatesRaflesi,and Eternod's outline of his 1-3 mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region in these embryos ascompared with my specimen "H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). In embryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." In the "Klb" embryo, the fore-gut was closed in throughout the extent of 32 sections. Grosser states (6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"represents the anterior portion of the future pharynx. It has the usual laterally expanded form (text-fig. 6). It gradually diminishes in width when traced forwards until the cranial limit of the first primary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. No such marked ventral flexion of the narrow terminal segment of the gut as that seen in embryo "Klb" is recognizable. At a distance of 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullary plate suddenly disappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

[[File:Wilson1914-12.jpg|400px|Fig. 12.]] [[File:Wilson1914-13.jpg|400px|Fig. 13.]]

I have made a rough wax-plate reconstruction of the entire pharynx, but unfortunately certain dislocations of structure detract from its value for purposes of publication. In most respects, however, it is reliable enough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. On the exterior of the model this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Though present throughout practically its entire length, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudally from the level of the oral sinus it becomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,as are the paired pericardial cavities themselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventral pharyngeal groove," in Grosser's sense (cf. (6), pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." It makes its appearance in the same section (142) which shows the first trace of the definite thyroid rudiment. This is the eighth section behind the cranial limit of the first pharyngeal pouch. It may be noted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the first pharyngeal pouch, as described by Grosser in embryo "Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary plate medially, and the two dorsal aorta lateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. The entoderm covering this median prominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexed condition of the entire primitive-streak region. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly (text-figs. 8 and 9), this groove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. This turns dorsally (text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differently in different regions of their extent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianin position. In no one section does this appear as wholly complete,but there is little doubt that it was actually completein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

[[File:Wilson1914-14.jpg|400px|Fig. 14.]] [[File:Wilson1914-15.jpg|400px|Fig. 15.]]

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

[[File:Wilson1914-16.jpg|400px|Fig. 16.]][[File:Wilson1914-17.jpg|400px|Fig. 17.]]

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17), but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16) the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig. 14' shows only the right heart. The plane of the latter section lies quite in front of the looped arterial end of the left heart, which is cut through in the section shown in text-fig. 6. To establish the connexions of this looped arterial left hear tone must pass caudally from the plane of the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is due mainly to the obliquity of the plane of sectional ready referred to. It is somewhat difficult to state accurately the extent of this obliquity, but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individual flexure. The relations of each heart-tube to the interpericardial septum may be easily recognised in the text-figs.6 and 14. It is evident that each heart-tube,in so far as it is separate, occupies its own pericardial cavity.

[[File:Wilson1914-18.jpg|400px|Fig. 18.]]

I have not yet been able to undertake a plastic reconstruction of this highly interesting state in cardiac development. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will only be approximate. The section series is complete and quite intelligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aortic arch system.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhaps this relationship would also best explain the condition met with by Eternod in his 1.3 mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured by Eternod in that interesting specimen. Inanycase,it is clear to me that there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappearing, while the me8ocardium posteriuos persists for sometime longer. The closely approximated but not yet fused endothelial tubes are now surrounded by a continuous myo-epicardial mnarntle (Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

The fusion of the right and left pericardial cavities is, as we have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium" atal in the ordinary sense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appear to be of essentially similar character to those of embryo "H 3," and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-22T00:13:18Z

S8600021: /* Atlimentary Canal */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

{{Historic Disclaimer}}

{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]] [[File:Wilson1914-09.jpg|400px|Fig. 9.]]

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connecting with the entoderm at the base of the allantoic duct (text-fig.9). The position of this cloacal membrane maybe compared with that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far in front of this point. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figure shows the degree of vertical curvature of the several regions of the embryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

[[File:Wilson1914-10.jpg|400px|Fig. 10.]] [[File:Wilson1914-11.jpg|400px|Fig. 11.]]

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's monograph L'ceuf human (5) (Geneve, 1909), illustrating respectively Selenka's outline figure of HylobatesRaflesi,and Eternod's outline of his 1-3 mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region in these embryos ascompared with my specimen "H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). In embryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." In the "Klb" embryo, the fore-gut was closed in throughout the extent of 32 sections. Grosser states (6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"represents the anterior portion of the future pharynx. It has the usual laterally expanded form (text-fig. 6). It gradually diminishes in width when traced forwards until the cranial limit of the first primary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. No such marked ventral flexion of the narrow terminal segment of the gut as that seen in embryo "Klb" is recognizable. At a distance of 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullary plate suddenly disappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

[[File:Wilson1914-12.jpg|400px|Fig. 12.]] [[File:Wilson1914-13.jpg|400px|Fig. 13.]]

I have made a rough wax-plate reconstruction of the entire pharynx, but unfortunately certain dislocations of structure detract from its value for purposes of publication. In most respects, however, it is reliable enough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. On the exterior of the model this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Though present throughout practically its entire length, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudally from the level of the oral sinus it becomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,as are the paired pericardial cavities themselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventral pharyngeal groove," in Grosser's sense (cf. (6), pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." It makes its appearance in the same section (142) which shows the first trace of the definite thyroid rudiment. This is the eighth section behind the cranial limit of the first pharyngeal pouch. It may be noted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the first pharyngeal pouch, as described by Grosser in embryo "Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary plate medially, and the two dorsal aorta lateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. The entoderm covering this median prominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexed condition of the entire primitive-streak region. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly (text-figs. 8 and 9), this groove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. This turns dorsally (text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differently in different regions of their extent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianin position. In no one section does this appear as wholly complete,but there is little doubt that it was actually completein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

[[File:Wilson1914-14.jpg|400px|Fig. 14.]] [[File:Wilson1914-15.jpg|400px|Fig. 15.]]

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

[[File:Wilson1914-16.jpg|400px|Fig. 16.]][[File:Wilson1914-17.jpg|400px|Fig. 17.]]

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17), but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16) the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-22T00:06:19Z

S8600021: /* Form and Characters of Embryo and its Appendages */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]] [[File:Wilson1914-09.jpg|400px|Fig. 9.]]

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connecting with the entoderm at the base of the allantoic duct (text-fig.9). The position of this cloacal membrane maybe compared with that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far in front of this point. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figure shows the degree of vertical curvature of the several regions of the embryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

[[File:Wilson1914-10.jpg|400px|Fig. 10.]] [[File:Wilson1914-11.jpg|400px|Fig. 11.]]

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's monograph L'ceuf human (5) (Geneve, 1909), illustrating respectively Selenka's outline figure of HylobatesRaflesi,and Eternod's outline of his 1-3 mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region in these embryos ascompared with my specimen "H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differently in different regions of their extent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianin position. In no one section does this appear as wholly complete,but there is little doubt that it was actually completein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

[[File:Wilson1914-14.jpg|400px|Fig. 14.]] [[File:Wilson1914-15.jpg|400px|Fig. 15.]]

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

[[File:Wilson1914-16.jpg|400px|Fig. 16.]][[File:Wilson1914-17.jpg|400px|Fig. 17.]]

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17), but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16) the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:58:53Z

S8600021: /* Pericardum */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

{{Historic Disclaimer}}

{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]]
Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differently in different regions of their extent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianin position. In no one section does this appear as wholly complete,but there is little doubt that it was actually completein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

[[File:Wilson1914-14.jpg|400px|Fig. 14.]] [[File:Wilson1914-15.jpg|400px|Fig. 15.]]

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

[[File:Wilson1914-16.jpg|400px|Fig. 16.]][[File:Wilson1914-17.jpg|400px|Fig. 17.]]

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17), but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16) the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:56:35Z

S8600021: /* Mesodermal Somites */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

{{Historic Disclaimer}}

{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]]
Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Not withstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. The other features of the embryo seemed to indicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,the histological condition of the paraxial mesoderm is not so satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. I have only been able to recognize in the section seriesonelineofsegmentalcleavagewithtolerablecertainty. I dare not assert that it is the only one present, and a critical examination of good silver prints of the photograph here reproduced on P1.I.fig.2 suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the number of somites could only have beensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:54:54Z

S8600021: /* Form and Characters of Embryo and its Appendages */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance of a more or less vascular membrane (text-figs.4 and 5).

[[File:Wilson1914-04.jpg|400px|Fig. 4.]] [[File:Wilson1914-05.jpg|400px|Fig. 5.]]

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one (cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

[[File:Wilson1914-06.jpg|400px|Fig. 6.]] [[File:Wilson1914-07.jpg|400px|Fig. 7.]]

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region, there is no posterior tail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinder region of the amniotic cavity. This latter tapers into a narrow-pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. It has no continuation into the body-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

[[File:Wilson1914-08.jpg|400px|Fig. 8.]]
Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then, from the posterior limit of the primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. It may be looked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateral view of the specimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. The approximate position of the neurenteric aperture is indicated by the arrow. The region a to b is the primitive-streak region: the angle of its inclination to the main embryonic axis is to be noted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative position of the body-stalk. The absence of a ventral flexure of the primitive-streak region in embryo "H 3" permits of a rather acute reflexion,in a cranial direction, of the body-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmental phase to that of the embryo under consideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especially as the flexed attitude of the primitive-streak region was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true length of the embryo. Ifareliable comparison is to be instituted with other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deduction here suggested as necessary includes nearly the whole of the primitive-streak region. In the total of 143 sections of the embryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over the hinder portion of the amniotic sac. Some indications of the chorio-placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. The wide bulging area below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:48:45Z

S8600021: /* Characters and Dimensions of Chorionic Vesicle */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|400px|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-02.jpg|400px|Fig. 2.]]

[[File:Wilson1914-03.jpg|400px|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:47:54Z

S8600021: /* Characters and Dimensions of Chorionic Vesicle */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

[[File:Wilson1914-01.jpg|Fig. 1.]]

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

[[File:Wilson1914-03.jpg|Fig. 3.]]

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:45:03Z

S8600021: /* PLATE III. FIG. 6. */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

{{Historic Disclaimer}}

{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn from photographs and stereo-photographs by Mr Herbert Beecroft, Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:44:23Z

S8600021: /* EXPLANATION OF PLATES I.-III. FIGS. 1-6. */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrograph of chorionic vesicle of human embryo "H3." x 7 (reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with hematoxylin and subsequent cleaving of the entire vesicle in cedar oil.

Note the very slightly branched vili on the exterior of the vesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. The yolk-sac is the larger crumpled sac mainly to the left of the embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn fromphotographsandstereo-photographsbyMrHerbertBeecroft,Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
----

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Paper - Observations upon Young Human Embryos

2012-02-21T23:34:05Z

S8600021: /* Dimensions of Embryo "H 98." */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment - 1.8 mm.

Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measured in a straight line - 1.27

Greatestcranio-caudal length of amniotic sac - 1.38

Greatest cranio-caudal length of yolk-sac - 1.46

Greatest dorsoventral extent of amnion in head region - 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion - 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed 8 pairs of somites completely differentiated. My original notes of the examination of the specimen contain the statement that there are "probably 9 or 10 pairs of somites, of which the first pair appear to ne very small."

The figures, cf. especially fig. 6, PI. I., show indications of the existence of 10 pairs. But a plane reconstruction of the segmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. I can therefore only indicate the stage of somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were 7 pairs of somites differentiated. Embryo"H98" thus appears to be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation of a well-marked, dorsally concave body-flexure (the "dorsal kink," "dorsal flexure," or "Knickung"). This was present in almost ashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrographofchorionicvesicleofhumanembryo"H3." x7(reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with bkematoxylin and subsequentcleavingoftheentirevesicleincedaroil.

Notetheveryslightlybranchedviliontheexteriorofthevesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. Theyolk-sacisthelargercrumpledsacmainlytotheleftofthe embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn fromphotographsandstereo-photographsbyMrHerbertBeecroft,Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T23:30:48Z

S8600021: /* Technique */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandes brought the perfectly fresh and intact specimen direct to my laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. After fixation in this solution the specimen was transferred to alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocular microscope, and the embryo, with body-stalk, amnion, and yolk-sac, was isolated from the greater part of the chorion. Micrometer measurements were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under the stereo-binocular microscope. It was also repeatedly photographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidin and paraffin, cut in faultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. The quality of the sectional series thus obtained is exceptionally good for early human embryonic material. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one area of the interior of the yolk-sac wall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the right embryonic surface is to his right,and the left to his left. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment . . . . 1.8 mm.
Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measuredinastraightline . . . . 1.27

Greatestcranio-caudallengthofamnioticsac 1.38

Greatest cranio-caudallengthofyolk-sac . 1.46

Greatest dorsoventral extent of amnion in head region....... 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion . . . 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed8pairsofsomitescompletelydifferentiated. Myoriginalnotes of the examination of the specimen contain the statement that there are "probably9or10pairsofsomites,ofwhichthefirstpairappeartobevery small."

The figures, cf. especially fig. 6, PI. I., show indications of the existenceof10pairs. Butaplanereconstructionofthesegmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. Icanthereforeonlyindicatethestageof somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were7pairsofsomitesdifferentiated. Embryo"H98"thusappearsto be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation ofawell-marked,dorsallyconcavebody-flexure(the"dorsalkink,""dorsal flexure,"or"Knickung"). Thiswaspresentinalmostashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrographofchorionicvesicleofhumanembryo"H3." x7(reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with bkematoxylin and subsequentcleavingoftheentirevesicleincedaroil.

Notetheveryslightlybranchedviliontheexteriorofthevesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. Theyolk-sacisthelargercrumpledsacmainlytotheleftofthe embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn fromphotographsandstereo-photographsbyMrHerbertBeecroft,Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T19:38:26Z

S8600021: /* PLATE III. FIG. 6. */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

{{Historic Disclaimer}}

{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandesbroughttheperfectlyfreshandintactspecimendirecttomy laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. Afterfixationinthissolutionthespecimenwastransferredto alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocularmicroscope,andtheembryo,withbody-stalk,amnion,andyolk-sac, wasisolatedfromthegreaterpartofthechorion. Micrometermeasure- ments were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under thestereo-binocularmicroscope. Itwasalsorepeatedlyphotographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidinandparafin,cutinfaultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. Thequalityofthesectionalseriesthusobtainedisexceptionally goodforearlyhumanembryonicmaterial. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one areaoftheinterioroftheyolk-sacwall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the rightembryonicsurfaceistohisright,andthelefttohisleft. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment . . . . 1.8 mm.
Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measuredinastraightline . . . . 1.27

Greatestcranio-caudallengthofamnioticsac 1.38

Greatest cranio-caudallengthofyolk-sac . 1.46

Greatest dorsoventral extent of amnion in head region....... 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion . . . 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed8pairsofsomitescompletelydifferentiated. Myoriginalnotes of the examination of the specimen contain the statement that there are "probably9or10pairsofsomites,ofwhichthefirstpairappeartobevery small."

The figures, cf. especially fig. 6, PI. I., show indications of the existenceof10pairs. Butaplanereconstructionofthesegmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. Icanthereforeonlyindicatethestageof somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were7pairsofsomitesdifferentiated. Embryo"H98"thusappearsto be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation ofawell-marked,dorsallyconcavebody-flexure(the"dorsalkink,""dorsal flexure,"or"Knickung"). Thiswaspresentinalmostashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrographofchorionicvesicleofhumanembryo"H3." x7(reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with bkematoxylin and subsequentcleavingoftheentirevesicleincedaroil.

Notetheveryslightlybranchedviliontheexteriorofthevesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. Theyolk-sacisthelargercrumpledsacmainlytotheleftofthe embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn fromphotographsandstereo-photographsbyMrHerbertBeecroft,Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

<gallery>
File:Wilson1914-01.jpg|Fig. 1.
File:Wilson1914-02.jpg|Fig. 2.
File:Wilson1914-03.jpg|Fig. 3.
File:Wilson1914-04.jpg|Fig. 4.
File:Wilson1914-05.jpg|Fig. 5.
File:Wilson1914-06.jpg|Fig. 6.
File:Wilson1914-07.jpg|Fig. 7.
File:Wilson1914-08.jpg|Fig. 8.
File:Wilson1914-09.jpg|Fig. 9.
File:Wilson1914-10.jpg|Fig. 10.
File:Wilson1914-11.jpg|Fig. 11.
File:Wilson1914-12.jpg|Fig. 12.
File:Wilson1914-13.jpg|Fig. 13.
File:Wilson1914-14.jpg|Fig. 14.
File:Wilson1914-15.jpg|Fig. 15.
File:Wilson1914-16.jpg|Fig. 16.
File:Wilson1914-17.jpg|Fig. 17.
File:Wilson1914-18.jpg|Fig. 18.
</gallery>
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Paper - Observations upon Young Human Embryos

2012-02-21T19:36:56Z

S8600021: /* PLATE III. FIG. 6. */

===James Thomas Wilson===

Challis Professor of Anatomy in the University of Sydney, Australia.

[http://sydney.edu.au/medicine/museum/mwmuseum/index.php/Wilson,_James_Thomas Medicine Museum]

<pubmed>17233002</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1288949 PMC1288949]

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{{Historic Papers}}

==Part 1==

The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.

The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.

Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.

In the present paper I propose to give an account of the three youngest human embryos in my collection.

Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.

A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.

It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."

Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").

I am inclined to believe that the embryo "E," No.1 of His' ''Normentafel'' (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).

Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.

The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."

==History of the Human Embryo "H3"==
The specimen<ref>A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.</ref> was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.

From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.

According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.<ref>Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."</ref>

As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.

After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.

===Characters and Dimensions of Chorionic Vesicle===
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.

The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."

As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.

The vili showed a very moderate degree of branching (cf. P1. I. fig.1).

After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)

===Micrometer Measurements===

The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.

:Maximum cranio-caudal length of amnion - 1.78 mm.
:Maximum cranio-caudal length of yolk-sac - 2.26 mm.
:Apparent cranio-caudal length of embryo - 1.64 mm.
:Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
:Dorso-ventral extent o fyolk-sac 1.66 mm.

N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).

The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.

===Form and Characters of Embryo and its Appendages===

In PI. I. fig. 2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, ''in toto'', in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

===Mesodermal Somites===

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.<ref>It is perhaps necessary here to point out that although in the later embryo the neurenteric aperture was closed, yet it is quite easy to determine the site corresponding to it, for here the chorda merges in mesodermn which cuts into the overlying medullary plate like an inverted keel (text-fig. 10).</ref>

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

===Chorda===

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

===Excretory Apparatus===

There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.

===Medullary Plate===

The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

===Absence of Sense Organs===

There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.

===Oral Region===

On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.

This gently recessed area alone represents the oral sinus (stomodaeum), which is thus very imperfectly formed.

The ectoderm of this oral sinus is separated from the entoderm of the pharynx by abundant mesoderm. Nowhere do ectoderm and entoderm even closely approach one another, so that the "primary pharyngeal mem- brane" ishere quite thick (0.06 mm.), and composed of al three germ layers. In fact, the mesoderm is here disposed as a specially thick compact mass of cells between the ectoderm and entoderm (text-fig. 11), and is continued backwards as such to the very caudal limit of the region of the rudimentary oral sinus. The area of this imperfectly formed oral sinus extends through sections 159-153 inclusive.

There are no evidences of any hypophysial formation.

===Atlimentary Canal===

The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel. The pharynx of this embryo has been described, modelled, and figured by Grosser (6) (also in the Keibel-Mall Manual, vol i. pp.446-7). Inembryo"H3"thefore-gut is closed in for a distance represented by 30 sections, in front of the plane of its continuity with the yolk-sacatthe"Darmpforte." Inthe"Klb"embryo,thefore-gut was closedinthroughouttheextentof32sections. Grosserstates(6) that its actuallength,allowingforcurvature,was400micra. Thefore-gutin
H3"representstheanteriorportionofthefuturepharynx. Ithasthe usuallaterallyexpandedform(text-fig.6). Itgraduallydiminishesin widthwhen tracedforwardsuntilthecraniallimitofthefirstprimary pharyngealpouchisreached. Herethereisamore abruptdiminutionin width (text-fig. 12) and then a tolerably rapid tapering into the attenuated blind extremity of the gut,stil retaining its dorso-ventrally compressed character. Thistaperingterminalsegmentextendsinacranialdirection for several sections beyond the ascertainable anterior limit of the oral sinus. Nosuchmarkedventralflexionofthenarrowterminalsegmentof thegutasthatseeninembryo"Klb"isrecognizable. Atadistanceof 5 sections from its cranial limit, however, the contact which has up to that point been maintained between the dorsal wall of the pharynx and the medullaryplatesuddenlydisappears.' This separation corresponds with the now rapidly shallowing character of the medullary groove, as it is traced intherostraldirection. There is no recognisable trace of any Seessel's pouch.

I have made a rough wax-plate reconstruction of the entire pharynx, butunfortunatelycertaindislocationsofstructuredetractfromitsvaluefor purposesofpublication. Inmostrespects,however,itisreliableenough, and it brings into prominence the tolerably close similarity in various features between embryo "H 3" and embryo "Klb" in respect of the pharynx.

Throughout nearly its entire length the ventral pharyngeal wall is depressed into a median V-shaped furrow. Ontheexteriorofthemodel this expresses itself in the ventrally keeled appearance which the pharynx thuspresents. Thoughpresentthroughoutpracticallyitsentirelength, the keel-like prominence varies in its degree of salience at different levels. It is already visible even in the region of the "primary pharyngeal membrane." Caudallyfromtheleveloftheoralsinusitbecomes accentuated where it overlies the paired pericardial cavities (text-fig. 12). But it is recognizable even where the dorsal pericardial wall is perfectly level. Nevertheless, text-fig. 18 inevitably suggests that this keel-like form of the early ventral pharyngeal wall is somehow a result of the bilateral constitution oftheheadregion,asarethepairedpericardialcavitiesthemselves.

In the reconstruction model two maxima are apparent for the keeled ventralpharyngealwall. One of these (text-fig. 6) appears to coincide with that in embryo " Klb" in the region between the first and second pharyngeal pouches, which Grosser has determined as the thyreoid rudiment(6).

The second maximal development of the median ventral pharyngeal keel lies further forward (cf. text-figs. 12 and 18), in the region opposite the anterior end of the first pharyngeal pouch, and it is this accentuated portion of the keel which seems related to the imperfect interpericardial septumseeninthesetext-figures. Insucceedingsections(proceedingina caudal direction) there is a diminution of prominence of the ventral keel until it somewhat abruptly develops the second or thyreoid prominence seen in text-fig. 6. This then continues in a caudal direction until it merges in the median cranial boundary of the yolk-sac opening ("Darm- pforte"). Itthusappearsthatthethyreoidrudiment,thoughperhaps tolerably definitely localized in the cranial direction, has at the present stage no definite caudal limit.

There is seen on each side of the thyreoid rudiment in text-fig. 6, a "ventralpharyngealgroove,"inGrosser'ssense(cf.(6),pp.274-75). This, however, begins more abruptly and perhaps more caudally than in embryo "Klb." Itmakesitsappearanceinthesamesection(142)whichshowsthe firsttraceofthedefinitethyreoidrudiment. Thisistheeighthsection behindthecraniallimitofthefirstpharyngealpouch. Itmaybenoted that this bilateral " ventral pharyngeal groove," appearing as it does very abruptly, shows no sign of being continued from the ventral aspect of the firstpharyngealpouch,asdescribedbyGrosserinembryo"Klb." Further, it is more sharply defined, is of less cranio-caudal extent, and is somewhat more medial in position than would appear to have been the case in the latter embryo.

The first primary pharyngeal pouch is first met with in section 150 (text-fig. 12), in the form of an abrupt lateral extension from the side of the pharynx, exactly as in embryo " Klb " (Grosser's figs. 1 and 2 (6), and in Keibel-Mall(1),figs.314-5). The latter dilatation of the pharynx thus arising, is continued caudally, showing no definite limit in this direction. It does not exhibit the pronounced and progressive diminution in its dorso- ventral dimension visible in Grosser's model, but there is a slight indication of expansion inthe region corresponding to the "second pharyngeal pouch" of embryo " Klb, " as shown in that model.

The dorsal wall of the pharynx is bulged towards its lumen along three longitudinal zones, corresponding respectively to the grooved medullary platemedially,andthetwodorsalaortaelateraly. Grosser'sdescription seems to imply that dorsal aortic impressions on the dorsal pharyngeal wall were absent in embryo "Klb"; but Keibel and Elze's fig. 5a, p. 19 (2), shows them quite definitely.

It has already been stated that, from a point 5 sections behind the anterior limit of the pharynx, the dorsal wall of the pharynx is in close appositionwiththegroovedmedullaryplate. Thiscontactdeterminesthe median of the three longitudinal prominences which are evident along the dorsalwallofthepharynx. Theentodermcoveringthismedianprominence isthatofthechorda-plate(cf.text-figs.6,14,etc.),butitishardly,ifatal, distinguishablefromtherestoftheentodermatthisstage. Intheserial sections there are some ruptures of the continuity of the pharyngeal entoderm, but this chorda-entoderm has very generally preserved its intimate relation to the medullary plate, which has afforded it a firm support. Thereisasyetnoclosed-inhind-gut. Thisconditionistobe correlated with the absence of any true caudal prominence and the non- flexedconditionoftheentireprimitive-streakregion. Underlyingthe extremehinderend ofthelater,thecavityoftheyolk-sacisprolonged, unilaterally, in a dorsal direction on the right side, into an asymmetrically placeddorsalentericgroove. Posteriorly(text-figs.8and9),thisgroove leads into the somewhat dilated proximal portion or vestibule of the allantoicduct. Thisturnsdorsally(text-fig.9),andrapidlynarrowsinto a canal of practically uniform fine calibre, being carried at first forwards into the cranially reflexed body-stalk.

The more medial wall of the asymmetrically placed dorsal enteric groove of the yolk-sac aforesaid exhibits, close to the proximal end of the allantoic vestibule, a well-marked depression which is closed by the thick cloacal membrane already referred to.

===Allantoic Duct===

The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk. Afterpassinginacranio-dorsal directionforsome considerabledistance(025 mm.),itbendsfurtherdorsally and then finally in the caudal direction and is now very slightly more attenuated. At a further distance of about 0.15 from its latest flexure, it now ratherrapidlyincreasesinsize,andends byexpandingintoa distinct vesicle. Throughout its entire tubular portion,it is lined by relatively thick cubical entoderm. Atitsexpansion,this quite abruptly passes into very thin entodermal epithelium with which the terminal vesicle is lined. The maximum diameter of the lumen of the vesicular expansion is about eight times that of the tubular allantoic canal. Eternodhasnotedthatthe allantoic cord of his 1'3 mm. embryo had "no appreciable vesicular enlargement." A distance of 14 sections, or 0.14mm., intervenes between the commencement of the dilatation and the terminal fundus of the vesicle. The total length of the allantois, exclusive of what I have termed the allantoicvestibule,but including the terminal vesicle and allowingforthe flexure of the canal is over 0.6 mm. It maybe said to follow a U-shaped course,the concavity of the U looking int he caudal direction.

===Pericardum===

The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later. It consists of paired cavities which are in actual communication with one another for a distance of only 9 sections,whilst it is extremely probable that in the case of the three more cranial of these sections the communica- tion is artificial and due to rupture of a delicate septum (cf. text-fig. 12).

The separation, partial or total, of the two pericardial cavities is effected differentlyindifferentregionsoftheirextent.

Cranially and for a distance backwards of 15 sections (sections 152-137) there exists a genuine septUm proprium interpericardiacum, which is approximatelymedianinposition. Innoonesectiondoesthisappearas whollycomplete,butthereislitledoubtthatitwas actuallycompletein its most cranial portion, as already indicated (text-fig. 12). As itistraced caudallythisseptumproprium becomeslesandlescomplete(text-fig.14), and gradually undergoes reduction (text-fig. 6) to the condition of a mere vestigial ridge of the ventral pericardial wall.

ut already, considerably in front of the plane at which there is any marked reduction of the septum, the appearance of the heart rudiments, with their dorsal and ventral mesocardial connexions (cf. text-figs. 14 and 6), effects the complete temporary separation of the more caudal regions of the two pericardial cavities.

These now, as they are traced further in the caudal direction, diverge gradually (text-fig. 15, right side) more and more from one another and are continued as the parietal (pleuropericardial) recesses.

The parietal recesses end blindly without establishing any communica- tion with any other coelomic cavity such as occurs later, e.g. in Mall's embryo,N~o.391 (Dandy (7), pl.ii. fig.11).

Their lumina are only traceable for a distance of about 8 sections behind the termination of the crescentic curve of the cephalic medullary plate, or for a distance of about 12 sections behind the present posterior limit of the foregut (cranial margin of the "Darmpforte").

The-paired pericardial cavities are linedthroughoutbya definitelayer of ccelomic mesothelium, which in many of the sections has shrunk away to -some- extent from the mesoderm surrounding it (text-figs. 16 and 17).

The most cranialportionsofthe-rightand leftpericardialcavities.which are unoccupied by and wholly in front of the heart, occupy a position in thethick,plate-likemass of mesoderm uniting amnion and yolk-sac (cf. text-figs.1, 12, and 16). The dorsoventral thickness of this mesodermal mass is maintained through out at about 0.3 mm. In the cranial direction, it merges in an accumulation of loose vascular mesoderm which lies in the angle between yolk-sac and amnion, supporting, dorsally, the cephalic fold ofthelater,andcontinuous,ventrally,withthevascularmesoderm ofthe yolk-sac.

The presence of this mesodermal mass occasions a slight bulging ventral to the head of the embryo between amnion and yolk-sac (cf.text-figs. 2 and 3). Thismesodermalmassmayappropriatelybestyledthe"pericardial plate," inasmuch as it soon gives place, and probably with great rapidity at this juncture,to a bulging pericardium as found in such a closely succeeding stage of development as that represented by embryo"H 98,"v.infra,oras in Mall's embryo No. 391, F. T. Lewis' reconstruction, fig. 232, Keibel and Mall, vol.2.

In the present case of embryo " H3 " the pairedpericardialcavities have already appeared in this pericardial plate (text-figs. 16 and 17),but they have not yet so completely excavated it as to form a bulbous pericardial protrusion anteriorly, as is the case in the succeeding stage.

The pericardial cavity is first met within the serial sections at a plane lying 5 sections caudally to that of the anterior end of the medullary plate. Here (text-fig.16)the colomic mesothelium lining the fore end of the right pericardialcavityisfirstencountered. Thattherightcavityappearsas much as 12 sections before the left (text-fig. 17) is largely, but perhaps not wholly, to be accounted for by the obliquity of the sectional plane, which is considerable.

As the serial sections are traced in the caudal direction, the pericardial cavities are found to expand so as to occupy the place of the thick mesoderm of the " pericardial plate," and they gradually supplant that tissue, the interval between the amniotic and yolk-sacs remaining practically unaltered (text-fig. 14).

===Heart===

The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components. The duplicity of the latter may be recognised from a comparison of-text-figs. 6 and 14. Text- fig. 6 shows portions of both right and left heart-tubes (myo-epicardial). Text-fig.14'showsonlytherightheart. Theplaneofthelattersection lies quite in front of the looped arterial end of the left heart, which is cut throughinthesectionshownintext-fig.6. Toestablishtheconnexionsof thisloopedarterialleftheartone mustpasscaudallyfromtheplaneof the last-named text-figure.

The marked dissimilarity of the right and left hearts in that figure is duemainlytotheobliquityoftheplaneofsectionalreadyreferredto. It is somewhat difficult to state accurately heextentofthisobliquity,but it may be indicated by the statement that the cranial limit of the first primary pharyngeal pouch on the right side lies some 8 or 9 sections in front of the corresponding point on the left. Were it not for this obliquity the right and left heart-tubes would appear tolerably symmetri- cally arranged, though both of them already show a marked degree of individualflexure. Therelationsofeachheart-tubetotheinterpericardial septummaybeeasilyrecognisedinthetext-figs.6 and 14. Itisevident thateachheart-tube,insofarasitisseparate,occupiesitsown pericardial cavity.

I have not yet been able to undertake a plastic reconstruction of this highlyinterestingstateincardiacdevelopment. This I hope to see carried out later on, although I fear that the degree of accuracy attainable will onlybeapproximate. Thesectionseriesiscompleteandquiteinteligible, but there are histological imperfections which may be troublesome. This will be more especialythe case with the endothelial heart-tube and the aorticarchsystem.

The arrangement of the latter I have not been able thus far to clear up satisfactorilyfrommere examinationofthesectionseries. Sofarasmy observationsgoatpresent,Iam disposedtoregardtheconnexionsbetween the arterial heart-tubes and the dorsal aortme as stil of a more or les plexiformcharacter. Perhapsthisrelationshipwouldalsobestexplainthe conditionmetwithbyEternodinhis1-3mm. embryo. And I can well imagine the cardiac condition I find in embryo "H 3 " to be immediately preceded by some such duplex tubular cardiac condition as that figured byEternodinthatinterestingspecimen. Inanycase,itiscleartomethat there are several highly interesting pages of the history of early human cardiac development which are yet unwritten.

It is clear from the conditions met with in embryo "H 3" that the ordinarily accepted account of the origin of the single pericardium and heart in humandevelopmentwillnot hold good without considerable modification. Thus Tandler,in Keibel and Mall's Manual (English edition, vol.2), says:-" On the closure of the fore-gut ventrally the hitherto symmetrical pleuropericardial cavities come together anteriorly and fuse in this region, the median partition between them, the mesocardiuqm anterius, disappear- ing,whiletheme8ocardiumposteriuospersistsforsometimelonger. The closely approximated but not yet fused endothelial tubes are now surroundedbyacontinuousmyo-epicardialmnarntle(Molier). Finally, the two endothelial tubes come into contact, their partition wall disappears, and the unpaired heart cylinder is formed from the paired heart-tubes. This stage of the development of the heart occurs in the Kromer-Pfannenstiel embryo 'Klb,' etc. etc."

The final result here referred to as manifest in embryo "Klb " may be quite correctly described, but the hypothetical preliminary phases leading up to itare in alprobability neither so simple nor so direct as Tandler represents them.

Thefusionoftherightandleftpericardialcavitiesis,aswe have seen, for a time incomplete anteriorly, and the imperfect septum separating them isnota "mesocardium"atalintheordinarysense,butaninterpericardial septum of mesoderm, quite independent of the heart-tubes themselves. Further, there occurs, prior to the state of single and continuous myo- epicardial mantle, a stage of an at least nearly complete duplex heart in which both right and left hearts possess myo-epicardial mantles which are, at least throughout the greater part of their extent, independent of one another, and which cannot be regarded as mere semitubular elements completing a simple median heart-tube by their coalescence.

I may say here that through the courtesy of Professor J. P. Hill,F.R.S., I have had the opportunity of looking through a superb section-series of an embryo of Peraineles in which the stage of cardiac development- evident with diagrammatic clearness-is obviously essentially identical with that which obtains in my embryo"H 3."

The condition in Perameles clearly shows that fore-gut closure in a mammalian embryo does not of itself at once determine median fusion of the primitively paired heart-tubes, for in this mammalian embryo one inds a completely closed fore-gut very greatly expanded in the transverse direction, and, attached to its floor but widely separated from one another, there are seen independent right and left myo-epicardial tubes. These appeartobeofessentiallysimilarcharactertothoseofembryo"H 3,"and, like them, each shows a definite and more or less symmetrical (in Perameles apparently quite symmetrical) primary flexure, definitely marking the venous and arterial subdivisions of the tube.

No further discussion of either cardiac or vascular systems can be undertaken on the present occasion. Many details of the vascular arrangements have not yet been ascertained with suficient precision.

It may be stated that the vascular network in the wall of the yolk-sac is not only well established, but many of its channels show clear lumina with a definite endothelial lining.

===Relative Position of Embryo "H 3" in the Human Embryonic Series===

The relative position, in a "Normentafel" of human embryonic development, to be assigned to embryo " H 3," has already been discussed inanearliersectionofthispaper. Iplaceitintheintervalbetweenstages 2 and 3 of Keibel and Elze's Normentafel (2).

Although His' embryo "E" (No.1 of his Normentafel (4) is only known tousbyitsexternalcharacters,Ibelievethat,notwithstandingits greaterlength,itoffersthenearestparallelto"H3." Bothofthese embryos I believe to be slightly more advanced than Eternod's "No. 7 Vuill." of 1.3mm. length.

I have attempted to summarise the characteristics of embryo " H 3 " on the general lines of the paradigm of Keibel and Elze's Notrmerntafel, with a few modifications, so that the corresponding characters of various known embryosmaybereadilycompared. That"H3"isdistinctlyinadvance of stage 2 of the Normentctfel (Spee's embryo " Gle ") is at once evident on comparisonofthefiguresanddescriptions. In particular the medullary plate and the pericardium and heart in " H 3" show a marked advance on the earlier stage.

The more outstanding characters whichseem todifferentiate"H 3,"as earlier, from embryo " Klb," representing stage 3 of the Normentafel, are the following:-

# In " H 3 " the embryonic length, as interpreted in the body of this paper,is slightly less than that of "Klb."
# In the former embryo no brain subdivisions were apparent in the otherwisewell-markedcephalicmedullaryplate. Ontheotherhand,the brain region seems rather more prominent in " H 3 " (fig. 2, PI. I.) than inthepublishedfiguresof"Klb."
# The neurenteric aperture was widely patent, and was both relatively andabsolutelyclosertotheheadregionin"H 3"thanin"Klb."
# In embryo "H3" there was no hind-gut. Ontheotherhand,the fore-gut, both in its dimensions and its degree of differentiation, differed onlyslightlyfromthatof"Klb."
# The chorda rudiment is in a much more backward stage in "H 3." Hardly any indication of it is visible, even in the form of a chorda-plate.
# The somites, although not quite conclusively determined in "H 3," were certainly fewer (not more than three) in number.
# The pericardium in "H 3" is stil in the condition of bilaterally paired and nearly symmetrical cavities, separated by a septum proprium interpericardiacum. The extent of the intercommunication between the two pericardial cavities is very limited.
# The heart of "H 3" is still largely duplex, consisting of right and left myo-epicardial mantles.

===Summary of Characters of Embryo "H3"===
Desqgnation. - Human embryo "H 3"(Hdr). CollectionJ.T.Wilson.

Dimensions.-Chorionic vesicle6.4 x 5.7 x 5mm., ex.vili.
, , 85 x5,imld.,

Length, amniotic sac 1.78 mm., yielding 151 sections.
embryonic region 1.64 mm., yielding 143 sections.
yolk-sac (distorted) 2.26 mm.

Depth, yolk-sac (dorsoventrally) 166 mm.

Length (cranio-caudally) of cephalic medullary plate, estimated as to the caudal limit of its prominent fold, 0.6mm. i (plus or minus).

Age.-Period elapsed since beginning of last menstruation tocommence- mentofabortion,40days. Periodsinceendoflastmenstruation,36 days. Period elapsed since due date of lapsed menstruation, 12 days. Estimated age of embryo, 18-21days. No conjugal history. Menstrual historyasabove.

Body Form.-No dorsal bend of embryonic body.
Primitive-streak region (= tail end) deeply furrowed, and forming only averyslightanglewiththebodyproper. No.tail-prominenceotherthan caudal cushions bordering the furrow of the primitive-streak region.

Headendofbrain-plateflexedventrallyoverapericardial-plate-prominence, which is already visible.

Primnitive-Streak Region.- Neurenteric canal stil widely patent Primitive-streakregion tolerablylengthy(047mm.inthesectionseries) and deeply furrowed, and bordered by elongated, cushion-like caudal swellings.
Entireprimitive-streakregionstilappears as the nearlydirectcaudal continuationoftheembryonicbodyproper. Cloacalmembraneposteriorly. Mesodermal Segments.-Somites not visible on external examination; certainly fewer than five pairs, probably not more than two pairs, and not
lessthantwo.

Chorda.-Chordaentirelyabsentassuch. Median strip of entoderm forming chorda-plate only recognizable with any degree of certainty just in frontoftheneurentericaperture. But probably also in head region.

Nervous System.-Medullary plate expanded in front to form a brain- plate elevated laterally into prominent crescentic folds and flexed ventrally atitsanteriorend. Medullarygroovewellformedthroughoutbutnowhere closed. Noclearevidenceofintrinsicbrainsegmentation. Notracesof commencing formation of eye, ear, or nose.

Hypophysis.-Noindicationofhypophysis.

Mouth Cavity.-Veryfaintindicationoforalsinus. Primarypharyn- geal membrane shows abundant mesoderm between ectoderm and entoderm. DigestireTract.-Fore-gutendsblindlyatcranialend. Consistssolely
ofwidepharynx. Extendsfor30sections(0-3mm.)infrontof"Darm- pforte.'" Hind-gutnotyetclosedin. Aproximaldilatationor"allantoic vestibule" at root of allantoic duct opens widely into a unilateral dorsal groove of the yolk-sac, or "dorsal enteric groove," whose medial wall exhibits a well-marked depression closed by a cloacal membrane.

Pharyngeal Derivatives.-First primary pharyngeal pouch is differenti- ated, but is separated from the ectoderm of the head by thick layer of mesoderm. Thereisageneralventralmediangrooveofthepharyngeal wall, whose maximal point of depression answers to the median thyreoid rudiment. "Ventralpharyngealgrooves"(paramedian)arealsopresent. There is a possible faint indication of the appearance of a second primary pharyngeal pouch.

Urogenital System.-No indication of any urogenital apparatus. Heart, Pericardium, and Vascwtar Systemr-Heart largely duplex; right and left myo-epicardial mantles occupy paired pericardial cavities, whichareimperfectlyseparatedbyan incompleteseptumpropriuminter- pericardiacumn. Right and left epimyocardial hearts each exhibit sub-division intovenousandarterialsegments. Paireddorsalaortae.

Blood-vessels of yolk-sac contain corpuscles, but many show clear lumina lined by endothelium.

Amnion.-Amniotic sac extends for a distance of 8 sections in front of anterior.endofhead. Ittapersatitshinderend,firstgradually,then abruptly, into a narrow-pointed recess floored by the continuation of the primitivestreak. Theeloacalmembranousconnexionwiththeectoderm lies at the right dorsolateral wall of this recess, very nearly at its apex. Mesoderm of the body-stalk extends like a hood over the hinder third of the amnion, and even in-front of the body-stalk, the mesoderm of the amnion is thickened.

Allantois.-The allantoic duct opens by a dilated " allantoic vestibule" intoa"dorsalentericgroove."
Sinuous in its course, it ends in a definite thin-walled, terminal vesicle. Thetotallengthofthecanal,includingthevesicle,isabout06mm.

==Second Embryo "H 98"==

For the second of mys specimens, to be referred to as "H 98," I am indebted to the kindness of Dr F.P. Sandes, who obtained itasthe resultof abortion in a patient who had exceeded her menstrual period by 10 days.

===Technique===

Dr Sandesbroughttheperfectlyfreshandintactspecimendirecttomy laboratory, where itwas at once placed in isotonic 10 per cent. solution of formalin. Afterfixationinthissolutionthespecimenwastransferredto alcohol. Aftercompletedehydration,itwas cleareden blocincedaroil, when the embryo in its interior became clearly discernible.

The chorionic vesicle was then carefully opened up under the stereo- binocularmicroscope,andtheembryo,withbody-stalk,amnion,andyolk-sac, wasisolatedfromthegreaterpartofthechorion. Micrometermeasure- ments were made of the various dimensions of the embryo, etc.,and the specimen was subjected to very careful and prolonged examination under thestereo-binocularmicroscope. Itwasalsorepeatedlyphotographed. On completion of the examination of the specimen in toto, it was double- embedded incedar-oil-celloidinandparafin,cutinfaultless series of 10 AL sections transversely to its long axis, in caudo-cranial succession, mounted on albuminised slides, and double-stained in Benda's hoematoxylin, followed byeosin. Thequalityofthesectionalseriesthusobtainedisexceptionally goodforearlyhumanembryonicmaterial. A rent in the right wall of the yolk-sac is visible in the sections, but this must have occurred prior to fixation, since there is a small collection of effused blood adherent to one areaoftheinterioroftheyolk-sacwall.

The sections being cut in caudo-cranial succession and mounted with their latest cut surfaces downwards, it will be evident that when the sections are looked at with the dorsal aspect away from the observer, the rightembryonicsurfaceistohisright,andthelefttohisleft. This relationship is preserved in the photomicrographs and drawings of the sections.

It should be noticed that the sections, though cut in caudo-cranial succession, are not numbered in this order as they are in "H 3." The numbers of the sections in "H 98" run cranio-caudally.

===Chorionic Vesicle of "H98."===

The chorionic vesicle of "H 98" (stereographic fig. 3, PI. I.) was somewhat oval and flattened, and whilst stil in the formalin fixative, it measured 6.6 x 5.3 x 4mm., exclusive of the vili. Ifthevilibeincluded inthemeasurements, the dimensions were 9 x 8 x 5 mm.

The chorionic wall was more or less villous throughout, except over partofoneofthepolarareas. Asissofrequentlythecasewithvesicles ofapproximatelythesamesize,theviliwere more luxuriantlydeveloped aroundtheequatorialzoneoftheflattenedvesicle. Asisalsousual,the comparatively bald polar area of the vesicle was ant embryonic.

The stereographic fig.3,PI.I.,shows the chorionicvesicle,trans- lucent in cedar oil,viewed by transmitted light. The embryo with its yolk-sac is visible in the interior.
When the chorionic vesicle was opened the interior was found to be filedwiththenormal"magma reticularis,"clearandtransparentandof semigelatinousconsistency.

===Appearance and Configuration of the Embryo and its Immediate Appendages===

As was to be expected from the circumstances of its reception and subsequent treatment, the condition of the embryo, etc., when isolated from the general chorion, appeared to be quite perfect. This judgment was borne out by the subsequent microscopical examination of the sectional series.

Apart from the one fact of its having been procured from a case of abortion (causation undetermined), there is no ground for regarding the embryo as in any way abnormal.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,made with the aid of various photographs both stereoscopic and other.

Figs. 4, 5, and 6,PI. I. and I.,illustrate the appearance presented bytheembryo,etc.,afterisolationfromthegeneralchorion. Infig.4, PI. I., is reproduced a stereo-photograph obtained by enlarging the negatives taken with Zeiss' "a 3" paired objectives in the Braus-Druner camera.

Fig.5,PI.I.,representsaphotomicrograph ofthe same at a magnifica- tion of 28 diameters; whilst fig. 6, PI. I.,represents a very careful drawingbyMr HerbertBeecroft,madewiththeaidofvariousphotographs both stereoscopic and other.

===Dimensions of Embryo "H 98."===

Length from anterior limit of amniotic collaring front of head to posterior limit of body-stalk at chorionic attachment . . . . 1.8 mm.
Length of embryo, without body-stalk, from tip of anterior end of brain-plate to tip of tail, measuredinastraightline . . . . 1.27

Greatestcranio-caudallengthofamnioticsac 1.38

Greatest cranio-caudallengthofyolk-sac . 1.46

Greatest dorsoventral extent of amnion in head region....... 0.86

Greatest dorsoventral extent of yolk-sac measured from line of reflection of amnion . . . 1.32

From the illustrations given, it will be seen that embryo "H 98" possessed8pairsofsomitescompletelydifferentiated. Myoriginalnotes of the examination of the specimen contain the statement that there are "probably9or10pairsofsomites,ofwhichthefirstpairappeartobevery small."

The figures, cf. especially fig. 6, PI. I., show indications of the existenceof10pairs. Butaplanereconstructionofthesegmented mesoderm, since made, indicates that neither the first nor the last of the 10 segments is definitely segmented off from the axial mesoderm in front of,andbehind,thesomites. Icanthereforeonlyindicatethestageof somite formation by the statement that there are 8-10 pairs.

In Mall's embryo No. 391, reconstructed and described by Dandy (7), and also by F. T. Lewis (Keibel and Mall's Manual, vol. 2, fig. 232), there were7pairsofsomitesdifferentiated. Embryo"H98"thusappearsto be more advanced than the latter embryo, which is otherwise remarkably similar to it-to the extent of the appearance of one additional somite pair.

A feature of considerable interest in embryo., 'H 98" is its manifestation ofawell-marked,dorsallyconcavebody-flexure(the"dorsalkink,""dorsal flexure,"or"Knickung"). Thiswaspresentinalmostashighdegreein theMallembryoNo.391. InDandy'sopinionitwas"partlynaturaland partlyanexaggeratedpostmortemcondition." He remarks(loc.cit.)that "we should naturally expect a dorsal concavity due to the greater develop- ment of the structures in both the anterior and posterior regions of the embryo," and then proceeds to account for the supposed post-mortem accentuationofthischaracter. IcannothelpfeelingthatDandyhasbeen influenced in his attitude towards the question of this dorsal flexure or kink by the weight of Keibel's authority against considering it as a normal feature. Ithereforedesirespeciallytoemphasisethefollowingconsidera- tionsinregardtothesamefeatureasoccurringinembryo"H 98":--

(a) The chorionic vesicle was obtained fresh and unopened; (b) it was fixed in 10 per cent. isotonic formalin, in accordance with the recommenda- tions of Mann, in order to obviate or minimise to the utmost the tendency to tissue distortion; (c) the photographs taken through the walls of the cleared, but as yet unopened, chorionic vesicle already showed the well- marked dorsal flexure; (d) a consideration of the later photograph (fig. 4-6, P1.I.andI.)wil,I think,convincetheunprejudicedobserver that the existence of the developmental phase of the heart and pericardium shown in the photograph must almost necessarily have determined the temporarily erect attitude of the cranial end of the body, or, more strictly speaking; of thecephalicportionofthemedullaryplate. Anditisthiserectattitude of the latter which is the main (though not the only) factor in the produc- tionofthedorsalkink. Theotherfactor,asindeedhasbeenalready impliedbyDandy,istheelevationofthetailend. Andthisisinturnno doubtduetotheventralattachmentoftheembryonicstalk.

It is, of course, open to anyone to throw doubt upon the normal characteroftheseandsimilarembryos. Butitwillnotdotobasesuch doubts solely upon the ground of the existence of the flexure, whose validityasanormalfeatureissubjuice. Anditistobenotedthatthe Mall-Dandy embryo, although aborted, was from a case of traumatic abortion, in which there was therefore no reason to suspect any natural or inherentabnormalityoftheovum.

With regard to the normal occurrence of the flexure in question in the humanembryo,Keibelhasrepeatedlyexpressedhimselfasskeptical. In hislatestutteranceon thissubject(Manual,vol.1,1910,pp.66-7)heagain recordshisjudgmentthatthenormaloccurrence ofa dorsalflexurein embryosofthestageofKollmann's"von Bulle"embryo,orlater,hasbeen disproved. Buthestilleavesopenthequestionofitsoccurrenceasa normalfeatureinembryosoffrom6to12pairsofsomites. At the same time, he regards the evidence for its normal occurrence even in these stagesasinsufficientlybased upon Eternod's 211 mm. embryo and Spee's embryo with 7 somites.

But, in the light of the condition met with in the Mall-Dandy embryo, andnowagaininmy specimen"H 98,"Isubmitthatthenormalexistence of the dorsal flexure during stages of from 7 to 10 pairs of somites should beadmitted. Itistruethatthesetwolatestspecimensarenotbythem- selveswhollyunimpeachable,beingabortedspecimens. Butthetraumatic character of the abortion in the one case, and the extreme care taken to avoid the possibility of distortion in the other, tend to establish the naturalnessoftheappearance. Eachcorroboratestheother. And,inview of the facts as now before us, it is rather too much to ask us to set aside the positive evidence of flexure in a whole series of cases, viz. Spee's embryo with 7 somites, Eternod's embryo of 8 somites, the Unger embryo of Keibel himself with 9 somites, the Mall-Dandy embryo of 7somites, and "H 98" with 8 to 10 somites,aseachandalabnormal,whereas the fact of the matter now is that, so far as I am aware, no embryo of the stage of 7 to 10 somites has ever been recorded without the flexure in dispute.

Further,embryo"SR" No.2,ofHis'Normentafel,is(Ithinkun- warrantably) set aside by Keibel as probably abnormal, apparently solely onaccountofitspossessingadorsalflexure. Now,thedimensionsofthis embryo would tend to indicate that, although His does not show nor describe somites in this stage, the embryo must have possessed them. Embryo " SR," in fact, shows a tolerably close resemblance, so far as one cantel,to the stage of 7 to 12 somites,andmay beconfidentlyaddedto the list of embryos of this stage exhibiting the dorsal flexure.

Other features which may be noted in the figs.4 to 6, PI. I. and I., are (1) the differentiation of the cephalic portion of the medullary plateintofore-,mid-,andhind-brainregions. Itistobenotedthatwhat appears to be a primary cerebral flexure is at this stage intrinsic to the fore-brainregion. Themid-brainshowsnotraceofflexureatal. One can recognise the same characteristic in the Mall-Dandy specimen and in Kollmann's"vonBulle"embryo; perhapsalso,butmoredoubtfully,inthe Kroemer-Pfannenstielembryo"Klb." Ihavenotseenattentiondrawnto this distinction, which is very striking in view of the fact that, later on, what we cal the "primary cerebral flexure" lies in the region of the mid-brain.

(2) The contour of the fore-gut may be followed in the figs. 5 and 6 from its blind anterior extremity to its communication with the yolk-sac throughthewidevitellinestalk. The posterior continuation of the hind-gut from the caudal limit of the vitelline stalk into the tail region of the embryo may also be perceived. Incidentally, in connexion with the problem of the flexure, attention has been directed to the extremely prominentpericardialdevelopment. Thisconditionrepresentsadistinct advance on that seen in Mall's embryo No. 391, cf. especially F. T. Lewis' fig.232intheKeibel-MallManual,vol.2. Butitisanadvanceinthe samedirectionasthatoftheprogressfromthestageofmy "H 3"through thatofembryo"Klb"tothatoftheMallembryoNo.391. Itisaprogress which consists of a continuing process of excavation of what I have called the "pericardial plate" in embryo "H 3," and when that process of excavation has reached its limit, of a further process of bulbous expansion ofthepericardialcavity.

I imagine that the condition seen in "H 98" must represent the maximumstageofthisrapidexpansion. WhenthestageofKollmann's embryo"von Bulle"isreached,thedevelopmentoftheheadhasalready overtaken the pericardium and the latter is forced ventrally by the now moreorlessoverhanginghead. Theentireprocessseemstomeaperfectly obvious and intelligible swaying in the balance of competing developmental processes. Idonotevenseethatoneisforcedtobelievethatthe equilibrium isalways preciselysimilarlymaintained at alcorresponding movements in the otherwise perfectly normal course of development.

It is quite likely that another factor in the maintenance of develop- mentalequilibriuminthisregionistherateofamnioticexpansion. We know perfectly well that, at least in later stages, the rate of growth and expansion of the amnion varies within tolerably wide limits without being intheleastabnormal.

Iseelitlereasontodenythatthesemaybequite normal variations in a similar sense, though of lesser degree, even at the earlyperiodnow underconsideration.
The carrying out of an adequate structural analysis of this embryo from the serial sections has been largely in suspense during my personal absencefromSydney. EreIleftIhaddonesomeworkinthedirection of reconstruction of the heart, but I do not propose now to enter upon the discussionofthesectionalanatomyofthisembryo. Thiswil,Ihope,form a portion of the subject-matter of Part I. of this contribution to our knowledge of early human embryos.

The sequel will also, I hope, contain a descriptive account of the third embryo of the series, viz."H 86" in my list.

==LIST OF REFERENCES.==

(1) KEIBEL and MALL, Manual of Human Embryology, Philadelphia and London, 1910-12.

(2) KEIBEL and ELZE, Normentafeln zur Entwicklungsgeschichte des Menschen, Jena, 1908.

(3) SPEE, v., "Neue Beobachtungen fiber sehr friuhe Entwickluiigsstufen, etc.," Arch. f. Anat. u. Physiol., Analt. Abt., 1896.

(4) His, W., Anat. menschl. Embryonen, 1880-5, Taf. x.

(5) ETERNOD, L'wrufhuman, Genbve, 1909.

(6) GROSSER, O., "Zur Entwicklung des Vorderdarmes menschlicher Embryonen,etc.," Sitzungsber. d. kaiserl. Akad. d. Wiss. in Wien. Math.-Naturw. Klasse, Bd. cxx.,Abt.i.,1911.

(7) DANDY, W. E., " A Human Embryo with Seven Pairs of Somites, measuring about 2 mm. in Length," American Journal of Anatomy, vol. x. p. 85, 1910.

(8) GROSSER, O., "Ein menschlicher Embryo mit Chordakanal," Anat. Hefte, Heft 143, p. 653, 1913.

==EXPLANATION OF PLATES I.-III. FIGS. 1-6==

===EXPLANATION OF TEXT-FIG. 18.===
Photomicrograph(x 100)ofsectionNo.149ofhumanembryo"H3,"atthe level of the cranial limit of the 1st right primary pharyngeal pouch.

Thesectionisratherobliquelycut. Itpasses throughthecephalicportionof themedullaryplate,whoseedgesare welldefined. Incontactwiththemedian ectodermofthefloorofthemedullarygroove may be seen the adherent strip of pharyngeal entoderm which represents the chorda-plate. The pharynx shows its medio-ventral keel immediately dorsal and corresponding to the septum proprium inteirericardiacum (see text). The latter nearly completely separates right and left pericardial chambers, which show a very definite coelomic epithelial lining. The right chamber contains portions of tissue from tangential grazing of the cranial bulgingoftherightmyo-epicardialtube. Thedorsalaortrearevisibledorsalof thepharynx.

Note the entoderm lining the roof of the yolk-sac in the lower part of the figure, and the amnion on either side in the upper part.

===EXPLANATION OF PLATES I.-III. FIGS. 1-6.===

Photomicrographofchorionicvesicleofhumanembryo"H3." x7(reduced from x14).

Zeiss' 35 mm. micro-projection objective.

The photograph was taken after slight surface staining with bkematoxylin and subsequentcleavingoftheentirevesicleincedaroil.

Notetheveryslightlybranchedviliontheexteriorofthevesicle. Theembryo, with its more intimate appendages, amnion and yolk-sac, are visible in the interior. Theyolk-sacisthelargercrumpledsacmainlytotheleftofthe embryo. The sharply defined clear outline of the more anterior portion of the amnion may be seen to the right and (in the figure) above the nearly vertically directed embryonic rudiment.

===PLATE I. FIG. 2.===
Photomicrograph(x155)ofhuman embryo"H3,"withitsamnion,yolk-sac, body-stalk, and a portion of its chorion, viewed from the right dorsolateral aspect.

Zeiss' 35 mm. micro-projection objective.

The somewhat collapsed and crumpled yolk-sac is seen in the lower part of the figure. Thepatchofchoriongivingattachmenttotheembryo,etc.,intheupper part of the figure presents its edge, for the most part, to the observer and shows no detail. Descendingfromthistowardstheembryoisthebody-stalkandtail-halfof theamnion. Thislatterappearsasifseparatedfromtheanteriorportionofthe amnionbyadeepcreaseofthemembrane. Thebody-stalkisevidentlydirected
dorsallyandcraniallyfrom.thecaudalendoftheembryonicrudiment. Thie medullary folds of the embryo are visible within the amnion anteriorly, and the leaf- likeareatraversedbytheprimitivestreakposteriorly.

===PLATE II. FIG. 3.===

Stereo-photomicrographofchorionicvesicleofhuman embryo"H98." The photographwas takenfromthespecimenincedaroilbytransmittedlight,under the Braus-Druner camera, Zeiss' stereo-objectives a', magnification x 2-8.

The vesicle is translucent, and shows the embryo and yolk-sac in its interior.

===PLATE I. FIG. 4.===

Stereo-photomicrograph(x16)ofhumanembryo"H 98,"withamnioii,body-stalk, andyolk-sac. EnlargedfromnegativetakenwithZeiss'stereo-objectivea2at fourdiameters.
The specimen was photographed in cedar oil by transmitted light.

Compare with figs. 5 and 6, P1. III.

The body-stalk is practically complete, although the portion of clhorion which
hadbeenleftinconnexionwithitbecamedetachedwhilstinthecedaroil. The darkpatchesinthebody-stalkrepresentthebloodcontentofvessels.
The pericardium and heart form a prominent bulging mass ventrally to the head, which is, in consequence, erect in attitude.
The amnion and yolk-sac are both almost wholly free from folds, and there is no evidenceofshrinkage.

===PLATE III. FIG. 5.===
Photomicrograph (x 28) of human embryo " H 98," with amnion, body-stalk, and yolk-sac.

See description of fig. 4 and text, and compare with figs. 4 and 6, PI. II. and III.

===PLATE III. FIG. 6.===

Human embryo " H 98," with amnnion, body-stalk, and yolk-sac.

Drawn fromphotographsandstereo-photographsbyMrHerbertBeecroft,Sydney. See description of fig. 4 and text, and compare with figs. 4 and 5, P1. I.
and III.

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{{Historic Disclaimer}}

{{Glossary}}

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File:Wilson1914-18.jpg

2012-02-21T19:34:09Z

S8600021: ==Fig. 18.== {{Wilson1914}}

==Fig. 18.==

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S8600021: ==Fig. 17.== {{Wilson1914}}

==Fig. 17.==

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S8600021: ==Fig. 16.== {{Wilson1914}}

==Fig. 16.==

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S8600021: ==Fig. 15.== {{Wilson1914}}

==Fig. 15.==

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S8600021: ==Fig. 14.== {{Wilson1914}}

==Fig. 14.==

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S8600021: ==Fig. 13.== {{Wilson1914}}

==Fig. 13.==

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S8600021: ==Fig. 12.== {{Wilson1914}}

==Fig. 12.==

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S8600021: ==Fig. 11.== {{Wilson1914}}

==Fig. 11.==

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S8600021: ==Fig. 10.== {{Wilson1914}}

==Fig. 10.==

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S8600021: ==Fig. 9.== {{Wilson1914}}

==Fig. 9.==

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S8600021: ==Fig. 8.== {{Wilson1914}}

==Fig. 8.==

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S8600021: ==Fig. 7.== {{Wilson1914}}

==Fig. 7.==

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S8600021: ==Fig. 6.== {{Wilson1914}}

==Fig. 6.==

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S8600021: ==Fig. 5.== {{Wilson1914}}

==Fig. 5.==

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S8600021: ==Fig. 4.== {{Wilson1914}}

==Fig. 4.==

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S8600021: ==Fig. 3.== {{Wilson1914}}

==Fig. 3.==

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S8600021: ==Fig. 2.== {{Wilson1914}}

==Fig. 2.==

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S8600021: ==Fig. 1.== {{Wilson1914}}

==Fig. 1.==

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File:Low 14.jpg

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S8600021: /* Fig. 14 Section through Head of Embryo */

==Fig. 14 Section through Head of Embryo==

The hind-brain is of interest in that it shows certain neuromeres very clearly; these neuromeres form folds involving the whole thickness of the lateral wall of the hind-brain, and are convex externally, with corresponding concavities internally (fig.14).

Fb., fore-brain, Mb. mid-brain; II., III. and IV., 2nd, 3rd and 4th neuromeresofhinzd-brain;Op., opticvesicle;Au.,auditorypit; G~af.,ganglion acustico-facialis.

{{Low 1908}}

[[Category:Head]]

File:Low 15.jpg

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S8600021:

==Fig. 15 Section through Region of First Visceral Cleft==

Pb., fore-brain- Hb. hind brain- Op., optic vesicle; Ph., pharynx; I.V.Cl., first visceral cleft; Au., auditory pit; tuo., dorsal aorta; Ch., notochord;
Ao. I., first aortic arch vessel.

{{Low 1908}}

File:Low 15.jpg

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S8600021:

==Fig. 15 Section through Region of First Visceral Cleft==

Pb., fore-brain- Hb. hind brain- Op., optic vesicle; Ph., pharynx- I.V.Cl., first visceral cleft; Au., auditory pit; tuo., dorsal aorta; Ch., notochord;
Ao~l., firstaorticarchvessel.

{{Low 1908}}