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4. The gastro-splenic ligament
4. The gastro-splenic ligament
A. Introduction
1. Definitions
The coelomic cavities are the spaces which come to surround the various viscera of the body such as the pericardial cavity around the heart, the pleural
857
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THE DEVELOPMENT OF THE COELOMIC CAVITIES
cavities surrounding the lungs, and the peritoneal cavity in which lie the stomach, intestines, reproductive organs, etc. These coelomic spaces and recesses arise from a generalized basic condition known as the primitive splanchnocoelic coelom. The primitive splanchnocoelic coelom is the elongated cavity which extends throughout the trunk region beginning just anterior to the heart and continuing posteriorly to the base of the tail. It encloses the developing heart and the developing mesenteron (gut) from the esophageal region posteriorly to the anal region.
2. Origin of the Primitive Splanchnocoelic Coelom
As observed previously (Chapter 10) the elongated mesodermal masses lying along either side of the developing neural tube, notochord, and enteric tube have a tendency to hollow out to form a cavity within. That is, like the neural, gut, and epidermal areas of the late gastrula, the two mesodermal masses tend to assume the form of tubes.
In the case of Amphioxus, each individual somite forms a cavity, the myocoel. These myocoels merge on either side in their ventral halves to form an elongated splanchnocoel below the horizontal septum (see page 506). Later the two splanchnocoels fuse below the developing gut to form the single splanchnocoelic coelom which comes to surround the gut. In the vertebrate group, however, the two elongated splanchnocoels on either side of the developing gut tube and heart form directly in the hypomeric (lateral plate) area of the mesodermal masses without a process of secondary fusion as in Amphioxus. In the upper part of each mesodermal mass, that is in the epimere, and to some extent also in the mesomere (nephrotomic plate) in the vertebrate group as in A mphioxus, there is a tendency for the coelomic spaces to appear in segmental fashion within the primitive somites and within the anterior portion of the mesomere. These individual spaces within the somites are called myocoels, and the spaces which arise in the segmented portion of the nephrotome are called the nephrocoels.
In young shark embryos, such as the 3-4 mm. embryo of Squalus acanthias, and in amphibian embryos of the early post-gastrular period, the myocoelic and nephrocoelic portions of the coelom are continuous dorso-ventrally with the splanchnocoelic coelom (fig. 217G and H). (Actually, during the early stages of coelomic development within the mesodermal masses, in the shark and amphibian embryos, the coelom within the epimere and nephrotomic portions of the mesoderm is continuous antero-posteriorly and it is only after the appearance of the primitive somites and segmentation within the nephrotome that they become discontinuous.) On the other hand, in the embryos of higher vertebrates, the respective myocoels within the somites appear later in development, and in consequence they are always separated from the splanchnocoel. Similarly, the nephrocoelic coelom also arises later and only the separate nephrocoels which develop within the pronephric tubules
EARLY DIVISION OF SPLANCHNOCOELIC COELOM
859
and certain types of mesonephric tubules make contact with the splanchnocoelic portion of the coelom.
In all vertebrates (see figures 254, 332F-M) the formation of the primitive, generalized coeiomic cavity proper or generalized splanchnocoelic portion of the coelom is formed by the fusion around the developing heart and gut structures of the two elongated splanchnocoels present in the hypomeric portions of the mesodermal masses as described below.
B. Early Divisions of the Primitive Splanchnocoelic Coelom
1. Formation of Primitive Suspensory Structures
The splanchnic walls of the early coeiomic cavities (splanchnocoels) within the two hypomeres become apposed around the structures, lying in the median plane (fig. 254). In the region of the heart, this apposition gives rise to the dorsal and ventral mesocardia and to the epimyocardium of the heart itself (fig. 254A, B) and, in the region of the stomach and intestine, it produces the dorsal and ventral mesenteries of the gut tube and various ligaments, connecting one organ with another. The mesenchyme which arises from the two splanchnic layers also gives origin to the muscles and connective tissues of the gut and its evaginated structures (fig. 31 lA, B). The ventral mesocardium disappears in all vertebrates (Chap. 17). The dorsal mesocardium may persist for a while but eventually disappears entirely or almost entirely (Chap. 17). The dorsal mesentery is present constantly in reptiles and mammals but may be perforated and reduced in the intestinal area in other vertebrate classes, so that little of the dorsal mesentery remains to suspend the intestine in certain cases as, for example, in the shark. The dorsal mesentery above the stomach, the mesogastrium, and also the ventral mesentery in the immediate region between the stomach and liver and between the liver and the ventral body wall persist in all vertebrates. As a rule, however, the ventral mesentery disappears caudal to the liver with the exception of dipnoan and anguilliform fishes and the ganoid fish, Lepisosteus. In these forms the ventral mesentery tends to persist throughout the peritoneal cavity. It follows, therefore, that the two bilaterally developed, splanchnocoelic cavities tend to merge into one cavity or generalized splanchnocoel with a partial retention in certain areas of the splanchnic layers of the two hypomeres which act as suspensory ligamentous structures for the viscera.
2. Formation of the Primitive Transverse Division of the Body and the Primary Pericardial and Peritoneal Divisions of the Coelom
The primitive splanchnocoelic coelom soon becomes divided into the pericardial coelom, surrounding the heart, and the peritoneal or abdominal coelom, surrounding the digestive viscera, by the formation of the lateral mesocardia
NEURAL TUBE
NOTOCHORD
DORSAL PARIETAL RECESS OF HIS
PLEURO ERICAROIAL CANAL
NEURAL TUBE
DORSAL aorta DORSAL CLOSING FOLD CONTRIBUTION FROM SPLANCHNOPLEURIC
MESODERM DUCT OF CUVIER lORSAL CLOSING FOL CONTRIBUTION FROM SOMATOPLEURIC MESODERM LATERAL MESOCARDIUM SINUS VENOSUS heart (ATRIUM) LIVER
c DORSAL
MESOG^TRIUM
VENTRAL MESENTERY GASTROHEPATIC LIGAMENT 'LESSER OMENTUM)
dorsal pancreas
DORSAL MESENTERY
DORSAL aorta NEURAL TUBE
ventral
MESENTERY
Fig. 362. The lateral mesocardia form the initial division of the embryonic coelom (A-1 and A-2) represent idealized sections through the vertebrate embryonic body in £ plane bet\veen the caudal limits of the sinus venosus and the anterior extremity of th( potential liver region of the embryo. (A-1) Diagram of the initial stage of separatior of the pericardial and peritoneal coelomic cavities in many vertebrates. Two dorsal anc two ventral recesses or passageways above and below the lateral mesocardia and latera horns of the sinus venosus are evident. These passageways communicate with the peri cardial and peritoneal divisions of the primitive coelom. (A-2) Separation of primitive
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EARLY DIVISION OF SPLANCHNOCOELIC COELOM
861
and the primitive septum transversum which develop in relation to the converging veins of the sinus venosus and the ventro-cephalic growth of the liver rudiment. In other words, a ventral partition is established across the primitive splanchnocoelic coelom in a plane which separates the caudal end of the heart (i.e., sinus venosus) from the anterior limits of the liver. This primitive transverse partition partially separates the primitive splanchnocoelic coelom into two main divisions:
( 1 ) a cephalic compartment, the pericardial cavity, around the heart and
Fig. 362 — Continued
coelom into anterior pericardial and posterior peritoneal areas in early human embryo. The precocious development of the caudal wall of the parietal pericardium obliterates the ventral recesses shown in A-1 previous to septum transversum formation and the outgrowth of the liver rudiment. Communication between pericardial and peritoneal coelomic divisions is possible only through the dorsal parietal recesses (dorsal pericardioperitoneal canals). (B) Schematic diagram representing the initial division by the lateral mesocardia of the primitive coelomic cavity into anterior pericardial and posterior peritoneal divisions in an embryo of Squalus acanthias 10 mm. long. The liver outgrowth has been extended forward slightly for diagrammatic purposes. (C) Initial division, by the lateral mesocardia, of the primitive coelom in the 72 hr. chick embryo. Due to the depressed condition of the anterior end of the body much of the heart appears in the section below the sinus venosus and lateral mesocardia. However, if the embryo were straightened and the atrium, etc., of the heart pushed forward, the structural conditions would appear much the same as in B. The dorsal parietal recesses appear on either side of the esophagus. (D) Semidiagrammatic section through caudal end of sinus venosus of 22 mm. shark embryo. The dorsal closing folds are developing on either side of the esophagus, thus closing the dorsal recesses. The liver rudiment is expanding within the substance of the ventral mesentery caudal to the heart to form the liver’Septiirn transversum complex. The latter structure obliterates the ventral recesses below the lateral mesocardia. (E) Diagrammatic representation of the forward and ventral growth of the developing liver within the substance of the ventral mesentery to form the liverseptum transversum complex. (See fig. 363D.) Observe: ventral parietal recesses are obliterated by the forward growth of this complex of tissues. The arrow denotes the passageway from the pericardial coelom into the peritoneal coelom through the dorsal parietal recesses (dorsal pericardioperitoneal canals). (F) Early stage in development of human heart and septum transversum showing ingrowth of somatopleural mesoderm between the previously formed caudal wall of the parietal pericardial membrane (see A-2) and the entoderm of the anterior intestinal portal. (Redrawn from Davis, 1927, Carnegie Inst. Public. 380, Cont. to Embryology, 107.) (G) Later stage of human
heart development. Mesodermal partition (septum transversum) is present as a thickened mass of tissue below the developing sinus venosus and between the caudal wall of the parietal pericardium and the gut entoderm. (Redrawn from Davis, see fig. 362F, for reference.) (H) Lateral dissection of fifth week human embryo to show ingrowth of liver tissue into thickened septum transversum. (Redrawn from Patten, 1946, Human Embryology, Blakiston, Philadelphia.) Arrow denotes passageway (dorsal parietal recess; pericardioperitoneal canal; pleural canal) between pericardial and peritoneal coelomic cavities. (I-l) Sagittal section through 15 mm. pig embryo showing thickened anterior face of liver. This thickened anterior face of the liver later separates from the liver as the primary septum transversum (peritoneo-pericardial membrane). (1-2) Higher powered drawing to show condition of anterior face of liver shown in fig. 362, I-l. (J) Transverse section through thorax and pulmonary area of the body of a bird to show position of dorsal pulmonary diaphragm. (Redrawn from Goodrich, 1930, Studies on the Structure and Development of Vertebrates, Macmillan Co., Limited, London.) Observe position of liver lobes in relation to the heart. Compare with fig. 294, G-4 & G-5.
FOREBRAIN
STERNAL RIB
Fig. 362 — (Continued)
See legend on p. 860 .
EARLY DIVISION OF SPLANCHNOCOELIC COELOM
863
(2) a larger caudal compartment, the peritoneal cavity, around the digestive viscera and urogenital structures.
This primary division of the early coelomic cavity is accomplished by the formation of:
1 ) The lateral mesocardia, and
2) the primary (primitive) septum transversum.
The two lateral mesocardia are formed previous to the development of the primitive septum transversum. Eventually the lateral mesocardia fuse in part to the dorsal edge of the transverse septum and become a part of it. The lateral mesocardia thus, in reality, represent the initial stage in the division of the general coelomic cavity. In consequence we shall consider the lateral mesocardia as important structures which enter into the formation of the primary transverse division of the embryonic body, but they should not be confused with the primitive septum transversum in a strict sense.
a. Lateral Mesocardia
The lateral mesocardia (fig. 362A-1, A-2) are formed as follows:
A lateral bulging or growth from the splanchnopleure at the caudal limits of the developing sinus venosus extends dorso-laterad on each side to meet a somewhat similar though smaller growth mediad of the somatopleural mesoderm. These growths form a bridge on each side across the coelomic cavity, extending dorso-laterad from the posterior lateral edges of the ventrally situated sinus venosus to the somatic wall. The area of union of this bridge on either side with the lateral body wall is the lateral mesocardium. The lateral mesocardia, in other words, represent the areas of juncture between the lateral body walls and the lateral extensions of the sinus venosus. The common cardinal veins or ducts of Cuvier join these right and left lateral extensions or horns of the sinus venosus in the substance of the lateral mesocardia. Anterior to the lateral mesocardia is the pericardial coelom, while posterior to them is the peritoneal coelom. The two passageways dorsal to the lateral mesocardia, on either side, are called the dorsal parietal recesses of His, while those ventral to the lateral mesocardia and on either side of the ventral mesentery and developing liver constitute the ventral parietal recesses of His (fig. 362A).
b. Formation of the Liver-Septum Tramversum Complex
1) Formation of Liver-Septum Complex through Modification of the Ventral Mesentery by Liver Outgrowth. As the liver rudiment in the shark, chick, pig, etc., grows ventrally and forward between the two splanchnopleural layers of the ventral mesentery, it expands the ventral mesentery laterally as the liver substance forms within the mesenchyme between the two splanchnic layers. The expanding liver substance eventually reaches the ventral and lateral
PLEUROPERITONEAL
PLEUROPERITONEAL ' RUDIMENT OF DIAPHRAGM
PLEURAL CAVIT
hepato duodenal
LIGAMEm
A. -3.
Fig. 363 (A-1, 2, 3). Diagrams showing the invasion of the peritoneal coelom around the liver and relations of septum transversum and diaphragm to the liver. (A-1) The peritoneal invasion separates the liver substance away from the lateral body wall and also from the anterior face of the liver itself. The separated, thickened, anterior face of the liver (see fig. 362, I-l and 1-2) forms the primary septum transversum (peritoneopericardial membrane). (A-2) The relation of the liver and other viscera to the secondary septum transversum formed by the addition of the dorsal closing folds (see fig. 362D) to the primary septum transversum. (A-3) This is a diagrammatic representation of conditions shown in B. Observe position of various ligaments associated with the liver. (B) Sagittal section through opossum embryo presenting relation of the liver to diaphragm. The ventral part of the diaphragm is the remodeled primary septum transversum. Observe that the inferior vena cava perforates the diaphragm. The area of attachment of the liver to the diaphragm is the coronary ligament, (The preparation from which this drawing was made was loaned to the author by Dr. J. A. McClain.) (C) Pericardioperitoneal opening below the esophagus in the shark, Squalus acanthias. (See also fig. 362D.) (D) Schematic diagram, dorsal view, of initial stage of devel oping pleural cavities in the mammal showing the anterior and posterior lateral body folds. The anterior lateral body fold gives origin to the pulmonary ridge or rudiment of the pleuropericardial membrane and the posterior lateral body fold forms most of the pleuroperitoneal membrane. Cf. fig. 362E. (E-H) Schematic diagrams showing later
stages in separation of pleural cavities in the mammal, viewed from the dorsal aspect. Observe that the pleuroperitoneal membrane is formed from two rudiments, viz., the posterior lateral body fold and a very small splanchnopleuric contribution (fig. 363F).
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EARLY DIVISION OF SPLANCHNOCOELIC COELOM
865
CARDINAL VEIN
COMMON CARDINAL VEIN AND LATERAL ' MESOCARDIUM IN ANTERIOR LATERAL BODY FOLD
SINUS ^ VENOSUS SEPTUM TRANSVERSUM CUT EDGE OF LIVER
LEUROPERICARDIAL CANAL BELOW ESOPHAGUS
PULMONARY RIDGE GROWING MESAO BELOW LUNG BUD AS PLEURO PERICARDIAL MEMBRANE
ANTERIOR CARDINAL VEIN PERICARDIAL CAVITY ■ PULMONARY ridge. COMMON CARDINAL VEI COUCT OF CUVIER! PLEURAL CAVITY POSTERIOR LATERAL BODY FOLD • lateral RUDIMENT OF PLEUROPFRITONEAL MEMBRANE
SPI.ANCHNOPLEURIC CONTRIBUTION LIVER STOMACH
PLEUROPERICARDIAL MEMBRANE COMPLETE BELOW LUNG
- pleural CAVIT
POSTERIOR LATERAL BODY FOLD AND SPLANCHNOPLEURIC CONTRIBUTION FUSE TO FORM PLEUROPERITONEAL MEMBRANE
CUT DORSAL EDGE OF PLEUROPERITONEAL MEMBRANE STOMACH PERITONEAL cavity DORSAL mesentery
Fig. 363 — (Continued)
See legend on p. 864.
body wall, where it fuses with the somatopleure from the body wall. Since the lateral expansion of the developing liver is more rapid than its forward growth, the anterior face of the liver gradually becomes flattened in the area just below (ventral to) the lateral mesocardia and immediately posterior to the sinus venosus of the heart. The mesenteric tissue, covering the anterior face of the liver, then fuses with the more dorsally located, lateral mesocardia. A transverse division across the body is completed in this manner below the lateral mesocardia, and the ventral parietal recesses in consequence are closed. Passage from the pericardial cavity to the peritoneal (abdominal) cavity is now possible only by way of the pericardioperitoneal canals (dorsal parietal recesses) (fig. 362E).
Although liver-rudiment development in the embryo of the frog and in the embryos of other amphibians is precocious the essential procedure in the
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THE DEVELOPMENT OF THE COELOMIC CAVITIES
formation of the primitive liver-septum transversum complex is similar to that described above.
2) Formation of the Liver-Septum Complex in the Human Embryo. In the
developing human embryo, medial growths on either side from the somatopleural mesoderm occur in the region caudoventral to the forming sinus venosus, and below the developing gut tube. In this way ,a primitive transverse septum is formed below the lateral mesocardia and between the entoderm of the gut and the caudal wall of the parietal pericardium (fig. 362F, G). This septum fuses with the lateral mesocardia and caudal wall of the parietal pericardium. However, when the evaginating liver rudiment grows ventrad and forward into the splanchnopleural tissue below the gut, it ultimately appropriates the previously formed transverse septum as its anterior aspect. Consequently, the general result of the two methods is the same, namely, the transverse septum in its earlier stages of development appears as the thickened anterior face of the liver associated with the lateral mesocardia (figs. 261 A; 362H, I).
c. Formation of the Primary Septum Transversum
After the liver-septum transversum complex has been established and the potential ventral parietal recesses are closed by either of the two methods described above, the next stage in the development of the primitive septum transversum is correlated with the forward expansion of the peritoneal coelom around the sides and anterior face of the liver. In doing so, the peritoneal coelom on either side of the liver extends anteriad and mesiad and thus becomes involved in a secondary separation of the liver from the lateral and ventral body wall and also from the anterior face of the liver itself which becomes the primary septum transversum (fig. 363 A, B). A separation does not occur in the area traversed by the veins passing from the liver to the sinus venosus or slightly dorsal to this area. Here the liver remains attached directly to the septum transversum and is suspended literally from it. This attaching tissue forms the coronary ligament of the liver. The ingrowth of the two coelomic areas on either side of and ventral to the liver, by apposition of the coelomic epithelium in the median plane, forms a secondary ventral mesentery of the liver. This secondary ventral mesentery or falciform ligament ties the liver to the mid-ventral area of the body wall and to the septum transversum. {Note: The terms primary septum transversum and peritoneopericardial membrane are synonymous.)
C. Coelomic Changes in Fishes, Amphibians, Reptiles, and Birds 1. In Fishes
In the adult shark, and fishes in general, the fully developed adult form of the septum transversum forms a complete partition between the pericardial cavity and the peritoneal cavity. In fishes the pericardial cavity in the adult fish, as in the embryo, extends laterally and ventrally to the body wall in a
COELOMIC CHANGES IN FISHES, AMPHIBIANS, REPTILES, AND BIRDS
867
fashion similar to that of the peritoneal cavity. Also, the heart continues to lie posterioventrally to the pharyngeal region in a manner very similar to that of the basic, embryonic body plan (fig. 294G-I).
In the formation of the adult, piscine, septum transversum from the primary transverse septum two membranous partitions are developed which close the dorsal parietal recesses or the openings above the lateral mesocardia. These partitions are called the dorsal closing folds and they arise as follows:
The splanchnopleural tissue on either side of the foregut, just anterior to the stomach rudiment and above the primitive septum transversum, forms a thin fold of tissue. This fold grows laterad and ventrad and fuses ultimately with the lateral mesocardium and the somatopleuric tissue, which overlies the common cardinal vein, as this vein travels caudo-ventrally along the body wall to reach the lateral mesocardium and the sinus venosus. As a result of this splanchnopleuric and somatopleuric fusion of tissues with the dorsal edge of the primary septum transversum a dorsal closing fold is formed on either side of the esophagus, and the two dorsal parietal recesses are obliterated, separating completely the pericardial cavity from the peritoneal cavity (fig. 362D). However, a small pericardioperitoneal opening may be left below the esophagus in the shark.
The secondary septum transversum thus formed is a thickened transverse partition, composed of two walls, an anterior pericardial wall and a posterior peritoneal wall, with a loose tissue layer between these two coelomic membranes. The liver is suspended from the peritoneal or caudal aspect of the septum transversum in the region of the coronary ligament, while the posterior end of the sinus venosus is apposed against the anterior or pericardial face of the transverse septum. The common cardinal and other converging veins of the heart utilize the substance of the septum transversum as a support on their way to the sinus venosus. The hepatic veins (the right and left, embryonic vitelline veins) pass through the coronary ligament on their journey to the sinus venosus.
2. In Amphibians, Reptiles, and Birds
The conversion of the primary septum transversum in amphibians, reptiles, and birds into the secondary or adult septum transversum occurs essentially as described above. A dorsal closing fold, obliterating the dorsal parietal recess on either side of the gut, is developed, although, in reptiles and birds, the inward growth and contribution of somatopleuric tissue overlying the common cardinal ridge is more important than in fishes in effecting this closure.
However, one must keep in mind an important fact, namely, that, in amphibia, reptiles and birds, there is an extensive caudal migration of the heart, septum transversum, and liver complex from their original cephalic position just posterior to the pharyngeal area. This caudal migration produces a condition in which the primary septum transversum and the dorsal membranes,
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THE DEVELOPMENT OF THE COELOMIC CAVITIES
formed by the dorsal closing folds, are inclined to a great degree, with the ventral end of the primary septum transversum considerably more posterior in position than the dorsal edge of the dorsal membranes. Consequently, a secondary recess or pocket is formed on either side anterior and dorsal to the septum transversum. This secondary recess occurs on either side of the gut, and, into each of these recesses, a lung extends in many reptiles and in those amphibia which possess lungs. In this pocket also lie certain of the air sacs of birds. Thus, the general cavity back of the pericardioperitoneal membrane or secondary septum transversum (i.e., the primary septum transversum plus the two dorsal membranes, formed by the dorsal closing folds) is known as the pleuroperitoneal cavity in amphibia and many reptiles. In birds (see below), the respiratory part of the lung becomes enclosed dorsally near the vertebrae within a separate pleural cavity, separated from the peritoneal cavity by the dorsal diaphragm (fig. 362J). The thin air sacs of the bird’s lung (Chap. 14) project from the lung through the dorsal diaphragm into the peritoneal cavity and also into certain of the bones. In the turtle group, among the reptiles, a dorsal diaphragm is developed below each lung, segregating the lungs partly within dorsal cavities, thus simulating the bird condition.
D. Formation of the Coelomic Cavities in Mammals
In the mammalia, a pronounced caudal migration of the heart, liver, and developing diaphragm occurs. Also, as in birds, a further morphogenetic feature is present which results in the development of a pleural cavity for each lung in addition to the peritoneal and pericardial cavities present in fishes, amphibians, and reptiles. Thus it is that the development of two partitioning membranes on either side of the gut tube, the pleuropericardial membranes, which correspond to the dorsal closing membranes mentioned above, together with two additional membranes, the pleuroperitoneal membranes, are necessary to effect the division of the primitive splanchnocoelic coelom into the four main coelomic cavities in the Mammalia.
1. Formation of the Pleuropericardial Membrane
It so happens that the anterior cardinal vein develops slightly in advance of the posterior cardinal vein. As a result the common cardinal vein, which develops from the caudal end of the primitive anterior cardinal vein, travels along the lateral body wall in an inclined plane to reach the area of the lateral mesocardium and sinus venosus of the heart. This inclined pathway of the common cardinal vein is characteristic of the vertebrate embryo. As the common cardinal vein increases in size, a lateral ridge or elongated bulge is formed along the lateral body wall. This ridge projects inward into the coelomic cavity and inclines caudo-ventrally to reach the dorsal edge of the area of the primitive septum transversum (fig. 363D).
In the mammals, the mesonephric folds (ridges), in which the mesonephric
FORMATION OF COELOMIC CAVITIES IN MAMMALS
869
kidneys develop, are large and project downward into the coelomic cavity. The anterior ends of the mesonephric ridges continue along the lateral body wall on either side and follow an inclined plane antero-ventrally to the dorsal edge of the primitive septum transversum (fig. 363D). Two lateral body folds or ridges, which incline toward and fuse with the dorsal edge of the primitive septum transversum, are produced in this manner on either side. These folds are an anterior lateral body fold or ridge, overlying the common cardinal vein, and a posterior lateral body fold, which represents the antero-ventral continuation of the mesonephric ridge as it inclines ventrally to join the lateral edge of the primitive septum transversum (fig. 363D). A V-shaped pocket is formed between these two ridges. This pocket represents the primitive pleural cavity or pocket. The apex of this V-shaped pocket unites with the primitive septum transversum. As the lung buds grow out posteriorly below the foregut, each projects into a pleural pocket (fig. 363F).
The formation of the pleuropericardial membrane is effected by an ingrowth of tissue along the edge of the anterior, lateral body fold, the fold that overlies the common cardinal vein. This ingrowing tissue forms a secondary ridge, known as the pulmonary ridge, which continues to grow mesad below the developing lung until it reaches the splanchnopleure of the esophagus with which it fuses. A pleuropericardial membrane, in this way, is established which separates the pericardial cavity below from the pleural cavity above (fig. 363E-G). The pleuropericardial membranes probably arc homologous with the dorsal closing folds of the secondary septum transversum of the vertebrates below the mammals.
2. Development of the Pleuroperitoneal Membrane
As mentioned previously, the cephalic end of the mesonephric ridge projects forward and ventrad along the lateral body wall to unite with the primitive septum transversum to form the posterior, lateral body fold. The medial growth of this posterior, lateral body fold and ultimate fusion with a small splanchnopleural outgrowth, the splanchnopleural fold, forms a second partitioning membrane, the pleuroperitoneal membrane, which separates the pleural cavity from the general peritoneal cavity (fig. 363E-H). Contributions of the somatic mesoderm to the lateral body-fold tissue are significant in the formation of the pleuroperitoneal membrane. It is to be noted that the primitive pleural cavities of the mammalian embryo are small and dorsally placed, one on either side of the gut and dorsal to the pericardial cavity. Their later expansion is described below. To summarize the partitioning process of the primitive coelom in mammals, we find that the following membranes are formed:
( 1 ) the primary septum transversum,
(2) the two dorsal closing folds or pleuropericardial membranes, and
(3) two pleuroperitoneal membranes.
Fig. 364 (A). Transverse section of the thoracic area of opossum embryo showing the separation of the parietal pericardium from the lateral body walls by expanding pleural sacs. (The preparation from which this drawing was made was loaned to the author by Dr. J. A. McClain.) (B-1) Transverse section through lung buds and pleural
870
MAMMALIAN DIAPHRAGM
871
E. Development of Independent Pericardial Walls
1. The Arrangement of the Parietal Pericardial Wall in Fishes
The parietal pericardium of the fish embryo is fused with the lateral body wall. The caudal area of the sinus venosus is associated intimately with the anterior wall of the septum transversum. This condition is a primary one in all vertebrate embryos. It is retained in the adult fish.
2. Formation of an Independent Parietal Pericardial Wall
IN THE Chick
In the chick, two main processes occur in development which separate the septum transversum from the liver, and also the parietal pericardial membrane from the lateral body walls. These processes are:
(a) The peritoneal cavity on either side of the liver grows forward and separates the cardiac or anterior face of the liver from the posterior face of the septum transversum, with the exception of the area where the veins from the hepatic region perforate the septum. This process frees the septum transversum from the liver surface and permits it to function as a part of the pericardial sac as indicated in figure 294G-4; G-5.
(b) The extending peritoneal coelom not only separates the liver from the posterior face of the septum transversum, but it continues anteriad followed by the liver lobes along the ventral and lateral aspects of the body wall and splits the membranous pericardium away from the lateral body wall. Ventrally, a median septum unites the pericardium with the body wall (fig. 362J).
3. Formation of the Independent Parietal Pericardial Wall
IN Amphibians and Reptiles
A somewhat similar process to that described for the chick obtains in reptiles and, to a modified extent, in amphibia.
Fig. 364 — Continued
cavities of a 10 mm. pig embryo showing position of the primitive mediastinum. (B-2) Later mediastinal area development portraying adult position (black area) of the mediastinum. (Based on the cat.) Observe that fig. 364 (A) is an intermediate condition between figs. 364 (B-1) and 364 (B-2). (C) Probable origin of parts of the mammalian diaphragm. (D) The caudal migration of the septum transversum and developing diaphragm during development. 2-position = embryo of 2 mm.; 24-position = 24 mm. embryo. (Redrawn from F. P. Mall, 1910, Chap. 13, Vol. 1, Manual of Human Embryology, Lippincott, Philadelphia.) (E-H) Development of the mesenteries and omental bursa or lesser peritoneal cavity in the human. The cross-lined areas in H show areas of the mesentery which fuses with the body wall. The arrows in F-H denote development of the lesser peritoneal cavity.
872
THE DEVELOPMENT OF THE COELOMIC CAVITIES
4. Separation of the Parietal Pericardial Wall in Mammals
On the other hand, in the mammals, it is the pleural cavities, i.e., the pleural divisions of the splanchnocoelic coelom, which extend ventrally around the heart and thus separate the parietal pericardium from the thoracic body wall (fig. 364A and B) . Posteriorly, they separate the pericardium from the anterior face of the developing diaphragm (fig. 363B). The secondary condition of the mediastinum thus is established which extends dorsoventrally between the two pleural sacs (fig. 364B-2). It is to be observed that the medial walls of the pleural sacs fuse with the lateral walls of the pericardium by means of the connective tissue which forms between these two layers.
F. The Mammalian Diaphragm
The mammalian diaphragm is a musculotendinous structure, innervated by the phrenic nerve and developed from tissues around the gut, primary septum transversum, the two pleuroperitoneal membranes, and possibly also by contributions from the body wall. Study figure 364C. The exact origin of the voluntary musculature of the diaphragm is in doubt, but it is assumed to come from the cervical myotomes in the region of origin of the phrenic nerve, together with some invasion of muscle substance from the lateral body wall posterior to the cervical area. Successive caudal positions of the septum transversum and developing diaphragm, assumed during its recession in the body, are shown in figure 364D.
G. The Pulmonary Diaphragm or Aponeurosis of the Chick
The pulmonary diaphragm in the chick is a composite structure formed of two membranes which develop in a horizontal position in the dorsal region of the thoracic area below the lungs. Each of these two membranes fuses with the median mesentery and the lateral body wall and thus forms a partition separating the pleural cavities above from the peritoneal cavity below (fig. 362J). The development of this partitioning membrane is as follows:
In the four- to five-day chick as the lung buds grow out dorso-posteriad each lung bud pushes into a mass of mesenchyme which is continuous from the splanchnopleure around the esophagus to the dorsal region of the liver.
This connecting bridge of mesenchyme is the pleuro-peritoneal membrane and it extends from the region of the esophagus across the lower part of the lung bud tissue to the liver lobe on each side. The mesenchymal connection of this membrane with the liver then spreads laterally to unite with the lateral body wall. As a result, the pleural cavity above is shut off from the peritoneal cavity below. A continual growth dorsoposteriad of the pleuro-peritoneal membrane, and subsequent fusion with the dorsal body wall tissues, separates the pleural cavity completely from the peritoneal cavity. However, certain canals remain in this membrane for the passage of the air sacs (see Chapter 14) of the lungs. Striated musculature from the lateral body wall grows into
PULMONARY DIAPHRAGM OF CHICK
873
the pleuro-peritoneal membrane on either side and converts it into a muscular structure. These two muscular partitions thus form the pulmonary diaphragm.
H. The Omental Bursa
In all gnathostomous vertebrates, the mesogastrium is prone to form a primitive pocket, associated with the rotation of the stomach to the right. This pocket is quite prevalent in most gnathostomous embryos from the elasmobranch fishes to the mammals and is known as the primitive omental bursa. In mammals, the omental bursa is highly developed, and it gives rise to the lesser peritoneal cavity, retaining its connection with the greater peritoneal cavity by means of the foramen of Winslow. The lesser peritoneal cavity in the cat is extensive, filling the entire inside of the omental sac. In the human, however, the distal part of the lesser peritoneal cavity is reduced by the fusion of the omental layers. Though a rudimentary omental bursa is formed in the early embryonic condition of elasmobranch fishes (sharks), it soon disappears, so that, in the adult fish, the omental bursa is nonexistent. Figure 364E~H presents various stages in the development of the omental bursa in the human embryo.
I. The Formation of Various Ligaments in the Stomach-Liver Region
Ligaments are those specializations of the peritoneal tissue which unite various organs with each other or with the body wall.
1. The Gastro-hepatic and Hepato-duodenal Ligaments. These structures are derivatives of the ventral mesentery between the stomach-duodenal area and the liver. The gastro-hepatic ligament ties the stomach and liver together while the hepato-duodenal ligament unites the duodenum with the liver.
2. The Coronary Ligament of the Liver. This is the tissue which unites the liver with the caudal face of the septum transversum and in mammals with the later developed diaphragm. Its development is described on page 866.
3. The Falciform Ligament of the Liver. This unites the liver in the median plane to the ventral body wall and to the septum transversum or diaphragm.
4. The Gastro-splenic Ligament suspends the spleen from the stomach and it represents a modification of the mesogastrium (see Chapter 17).
(Note: Ligamentous structures associated with the reproductive organs are described in Chapter 18.)
Bibliography
Goodrich, E. S. 1930. Chap. XU in Studies on the Structure and Development of Vertebrates. Macmillan and Co., London.
Mall, F. P. 1910. Chap. 13, Vol. I, Manual of Human Embryology, Lippincott, Philadelphia.


==The Developing Endocrine Glands and Tlieir Possible Relation to Definitive Body Formation and the Differentiation of Sex==
==The Developing Endocrine Glands and Tlieir Possible Relation to Definitive Body Formation and the Differentiation of Sex==

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

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Part IV - Histogenesis and Morphogenesis of the Organ Systems

Part IV - Histogenesis and Morphogenesis of the Organ Systems: 12. Structure and Development of the Integumentary System | 13. Structure and Development of the Digestive System | 14. Development of the Respiratory-buoyancy System | 15. The Skeletal System | 16. The Muscular System | 17. The Circulatory System | 18. The Excretory and Reproductive System | 19. The Nervous System | 20. The Development of Coelomic Cavities | 21. The Developing Endocrine Glands and Their Possible Relation to Definitive Body Formation and the Differentiation of Sex


The Integumentary System

A. Introduction

1. Definition and general structure of the vertebrate integument or skin

2. General functions of the skin

3. Basic structure of the vertebrate skin in the embryo

a. Component parts of the developing integument

b. Origin of the component parts of the early integument

1 ) Origin of the epidermal component

2) Origin of the dermal or mesenchymal component

3) Origin of chromatophores

B. Development of the skin in various vertebrates

1. Fishes

a. Anatomical characteristics of the integument of fishes

b. Development of the skin in the embryo of the shark, Squalus acanthias

1 ) Epidermis

2) Dermis

3) Development of scales and glands

c. Development of the skin in the bony ganoid fish, Lepisosteus (Lepidosteus) osseus

d. Development of the skin in the teleost fish

2. Amphibia

a. Characteristics of the amphibian skin

b. Development of the skin in Necturus maculosus

c. Development of the skin in the frog, Rana pipiens

3. Reptiles

a. Characteristics of the reptilian skin

b. Development of the turtle skin

4. Birds

a. Characteristics of the avian skin

1 ) Kinds of feathers

2) General structure of feathers

a) Pluma or contour feather

b) Plumule or down feather

c) Filoplume or hair feather

d) Distribution of feathers on the body

b. Development of the avian skin

1) Development of the epidermis, dermis, and nestling down feather

2) Development of the contour feather

a) Formation of barbs during the primary or early phase of contour-feather formation

b) Secondary phase of contour-feather formation

c) Formation of the barbules and the feather vane

d) Later development of the feather shaft

3) Formation of the after feather

4) Development of the later down and filoplumous feathers

5. Mammals

a. Characteristics of the mammalian skin

b. Development of the skin

1 ) Development of the skin in general

2) Development of accessory structures associated with the skin

a) Development of the hair

b) Structure of the mature hair and the hair follicle

3) Development of nails, claws, and hoofs

4) Development of horns

5) Development of the skin glands

a) Sebaceous glands

b) Sudoriferous glands

c) Mammary glands

C. Coloration and pigmentation of the vertebrate skin and accessory structures

1. Factors concerned with skin color

2. Color patterns

3. Manner of color-pattern production

a. Role of chromatophores in producing skin-color effects

b. Activities of other substances and structures in producing color effects of the skin

c. Genic control of chromatophoric activity

d. Examples of hormonal control of chromatophoric activity

e. Environmental control of chromatophoric activity

The Digestive System

A. Introduction

1. General structure and regions of the early digestive tube or primitive metenteron

a. Definition

b. Two main types of the early metenteron

2. Basic structure of the early metenteron (gut tube)

a. Basic regions of the primitive metenteron

1 ) Stomodaeum

2) Head gut or Seessel’s pocket

3) Foregut

4) Midgut

5) Hindgut

6) Tail gut (post-anal gut)

7) Proctodaeum

b. Basic cellular units of the primitive metenteron

3. Areas of the primitive metenteron from which cvaginations (diverticula) normally arise

a. Stomodaeum

b. Pharynx

c. Anterior intestinal or pyloric area

d. Junction of midgut and hindgut

e. Cloacal and proctodaeal area

B. Development of the digestive tube 6r metenteron

1. General morphogenesis of the digestive tube

2. Histogenesis and morphogenesis of special areas a. Oral cavity

1 ) General characteristics of the stomodaeal invagination

2) Rudiments of the jaws

3) Development of the tongue

4) Teeth

a) General characteristics

b) Development of teeth in the shark embryo

c) Development of teeth in the frog tadpole

d) Development of the egg tooth in the chick

e) Development of teeth in mammals

5) Formation of the secondary palate

6) Formation of the lips

7) Oral glands


b. Development of the pharyngeal area

1 ) Pharyngeal pouches and grooves

2) Pharyngeal glands of internal secretion

3) Other respiratory diverticula

c. Morphogenesis and histogenesis of the esophagus and the stomach region of the metenteron

d. Morphogenesis and histogenesis of the hepato-pancreatic area

1 ) Development of the liver rudiment

a) Shark embryo

b) Frog embryo

c) Chick embryo

d) Pig embryo

e) Human embryo

2) Histogenesis of the liver

3) Development of the rudiments of the pancreas

a) Shark embryo

b) Frog embryo

c) Chick embryo

d) Pig embryo

e) Human embryo

4) Histogenesis of the pancreas

e. Morphogenesis and histogenesis of the intestine

1) Morphogenesis of the intestine in the fish group

2) Morphogenesis of the intestine in amphibia, reptiles, birds, and mammals

3) Torsion and rotation of the intestine during development

4) Histogenesis of the intestine

f. Differentiation of the cloaca

C. Physiological aspects of the developing gut tube

Respiratory and Buoyancy Systems

A. Introduction

1. External and internal respiration

2. Basic structural relationships involved in external respiration

a. Cellular relationships

b. Sites or areas where external respiration is accomplished

c. Main types of organs used for respiration

B. Development of bronchial or gill respiratory organs

1. Development of gills in fishes

a. Development of gills in Squalus acant/iias

b. Gills of teleost fishes

c. External gills

2. Development of gills in Amphibia

a. General features

b. Development of gills in Nectunis maculosus

c. Development of gills in the larva of the frog, Rana pipiens

1) Development of external gills

2) Formation of the operculum

3) Internal gills

4) Resorption and obliteration of gills

C. Development of lungs and buoyancy structures

1. General relationship between lungs and air bladders

2. Development of lungs

a. Development of lungs in the frog and other Amphibia

b. Lung development in the chick

1 ) General features of lung development

2) Formation of air sacs

3) Formation of the bronchi and respiratory areas of the chick’s lung

4) Trachea, voice box, and ultimate position of the bird’s lung in the body

5) Basic cellular composition of the trachea, lungs, and air sacs

c. Development of lungs in the mammal

1 ) Origin of the lung rudiment

2) Formation of the bronchi

3) Formation of the respiratory area of the lung

4) Development of the epiglottis and voice box

5) Cellular composition

6) Ultimate position of the mammalian lung in the body

3. Development of air bladders

4. Lunglessness

The Skeletal System

A. Introduction

1. Definition

2. Generalized or basic embryonic skeleton; its origin and significance

a. Basic condition of the skeletal system

b. Origin of the primitive ghost skeleton

1) Notochord and subnotochordal rod

2) Origin of the mesenchyme of the early embryonic skeleton

c. Importance of the mesenchymal packing tissue of the early embryo

B. Characteristics and kinds of connective tissues

1. Connective tissue proper

a. Fibrous types

1) Reticular tissue

2) White fibrous tissue

3) Elastic tissue

b. Adipose tissue

2. Cartilage

a. Hyaline cartilage

b. Fibrocartilage

c. Elastic cartilage

3. Bone

a. Characteristics of bone

b. Types of bone

c. Characteristics of spongy bone

d. Compact bone

C. Development of skeletal tissues

1. Formation of the connective tissue proper

a. Formation of fibrous connective tissues

b. Formation of adipose or fatty connective tissue

2. Development of cartilage

3. Development of bone

a. Membranous bone formation

b. Endochondral and perichondrial (periosteal) bone formation

1) Endochrondral bone formation

2) Perichondrial (periosteal) bone formation

c. Conversion of cancellous bone into compact bone

D. Development (morphogenesis) of the endoskeleton

1. Definitions

2. Morphogenesis of the axial skeleton

a. General features of the skeleton of the head

1 ) Neurocranium or cranium proper

2) Visceral skeleton or splanchnocranium

3) Development of the skull or neurocranium

4) Vicissitudes of the splanchnocranium

b. Ossification centers and the development of bony skulls

c. Development of the axial skeleton

1) Axial skeleton of the trunk

a) Notochord

b) Vertebrae

c) Divisions of the vertebral column

d) Ribs

e) Sternum

2) Axial skeleton of the tail

d. Development of the appendicular skeleton of the paired appendages

1) General features

2) Development of the skeleton of the free appendage

3) Formation of the girdles

e. Growth of bone

f. Formation of joints

1) Definitions

2) Ankylosis (synosteosis) and synarthrosis

3) Diarthroses

4) Amphiarthroses

g. Dermal bones

The Muscular System

A. Introduction

1. Definition

2. General structure of muscle tissue

a. Skeletal muscle

b. Cardiac muscle

c. Smooth muscle

B. Histogenesis of muscle tissues

1. Skeletal muscle

2. Cardiac muscle

3. Smooth muscle

C. Morphogenesis of the muscular system

1. Musculature associated with the viscera of the body

2. Musculature of the skeleton

a. Development of trunk and tail muscles

1 ) Characteristics of trunk and tail muscles in aquatic and terrestrial vertebrates

a) Natatorial adaptations

b) Terrestrial adaptations

c) Aerial adaptations

2) Development of trunk and tail musculature

a) General features of myotomic differentiation in the trunk

b) Differentiation of the myotomes in fishes and amphibia

c) Differentiation of the truncal myotomes in higher vertebrates and particularly in the human embryo

d) Muscles of the cloacal and perineal area

e) Development of the musculature of the tail region

b. Development of muscles of the head-pharyngeal area

1) Extrinsic muscles of the eye

2) Muscles of the visceral skeleton and post-branchial area

a) Tongue and other hypobranchial musculature

b) Musculature of the mandibular visceral arch

c) Musculature of the hyoid visceral arch

d) Musculature of the first branchial arch

e) Muscles of the succeeding visceral arches

f) Muscles associated with the spinal accessory or eleventh cranial nerve

g) Musculature of the mammalian diaphragm

c. Development of the musculature of the paired appendages

d. Panniculus carnosus

The Circulatory System

A. Introduction

1. Definition

2. Major subdivisions of the circulatory system

B. Development of the basic features of the arteriovenous system

1. The basic plan of the arteriovenous system

2. Development of the primitive heart and blood vessels associated with the primitive gut

3. Formation of the primitive blood vessels associated with the mesodermal and neural areas

4. Regions of the primitive vascular system

C. Histogenesis of the circulatory system

1. The heart

2. Formation of the primitive vascular channels and capillaries

3. Later development of blood vessels

a. Arteries

b. Veins

c. Capillaries

4. Hematopoiesis (Hemopoiesis)

a. Theories of blood-cell origin

b. Places of blood-cell origin

1 ) Early embryonic origin of blood cells

2) Later sites of blood-cell formation

3) Characteristics of development of the erythrocyte

4) Characteristics of various white blood cells

a) Granulocytes

b) Lymphoid forms

D. Morphogenesis of the circulatory system

1. Introduction

2. Transformation of the converging veins of the early embryonic heart into the major veins which enter the adult form of the heart

a. Alteration of the primitive converging veins of the heart in the shark, Squalus acanthids

b. Changes in the primitive converging veins of the heart in the anuran amphibia

1) The vitelline veins

2) Lateral (ventral abdominal) veins

3) Formation of the inferior vena cava

4) Formation of the renal portal system

5) Precaval veins

c. Changes in the primitive converging veins of the heart in the chick

1) Transformation of the vitelline and allantoic veins

a) Vitelline veins

b) Allantoic veins

2) Formation of the inferior vena cava

3) Development of the precaval veins

d. The developing converging veins of the mammalian heart

3. Development of the heart

a. General morphology of the primitive heart

b. The basic histological structure of the primitive embryonic heart

c. Importance of the septum transvcrsum to the early heart

d. Activities of early-heart development common to all vertebrates

e. Development of the heart in various vertebrates

1 ) Shark, Squaliis acanthias

2) Frog, Rana pipiens

3 ) Amniota

a) Heart of the chick

b) Mammalian heart

( 1 ) Early features

(2) Internal partitioning

(3) Fate of the sinus venosus

(4) The division of the bulbus cordis (truncus arteriosus and conus)

f. Fate of embryonic heart segments in various vertebrates

4. Modifications of the aortal arches

5. Dorsal aortae (aorta) and branches

E. Development of the Lymphatic System

F. Modifications of the circulatory system in the mammalian fetus at birth

G. The initiation of the heart beat

The Excretory and Reproductive Systems

A. Introduction

1. Developmental relationships

2. Functions of the excretory and reproductive systems

3. Basic embryonic tissues which contribute to the urogenital structures

B. Development of the excretory system

1. General description

a. Types of kidneys formed during embryonic development

b. Types of nephrons or renal units produced in developing vertebrate embryos

2. Functional kidneys during embryonic development

a. Pronephros

b. Mesonephros

c. Metanephros and opisthonephros

3. Development and importance of the pronephric kidney

a. General considerations

b. Shark, Squalus acanthias

c. Frog

d. Chick

e. Mammal (human)

4. Development of the mesonephric kidney

a. Squalus acanthias

b. Frog

c. Chick

d. Mammal

5. Development of the metanephric kidney

a. Chick

1) Metanephric duct and metanephrogenous tissue

2) Formation of the metanephric renal units

b. Mammal (human)

1) Formation of the pelvis, calyces, collecting ducts, and nephric units

2) Formation of the capsule

3) Changes in position of the developing kidney

6. Urinary ducts and urinary bladders

a. Types of urinary ducts

b. Urinary bladders

c. Cloaca


C. Development of the reproductive system

1. Early developmental features; the indifferent gonad

2. Development of the testis

a. Mammal

b. Chick

c. Frog

3. Development of the ovary

a. Mammal

b. Chick

c. Frog

4. Development of the reproductive ducts

a. Male reproductive duct

b. Female reproductive duct

5 . Development of intromittent organs

6. Accessory reproductive glands in mammals

a. Prostate glands

b. Seminal vesicles

c. Bulbourethral glands

7. Peritoneal supports for the reproductive structures

a. Testis and ovary

b. Reproductive ducts

The Nervous System

A. Introduction

1. Definition

2. Structural and functional features

a. The morphological and functional unit of the nervous system

b. The reflex arc

c. Structural divisions of the vertebrate nervous system

d. The supporting tissue

B. Basic developmental features

1. The embryonic origin of nervous tissues

2. The structural fundaments of the nervous system

a. The elongated hollow tube

b. The neural crest cells

c. Special sense placodes

3. The histogenesis of nervous tissue

a. The formation of neurons

1 ) General cytoplasmic changes - 2) Nuclear changes

3) Growth and development of nerve-cell processes

b. The development of the supporting tissue of the neural tube

c. Early histogenesis of the neural tube

d. Early histogenesis of the peripheral nervous system

C. Morphogenesis of the central nervous system

1. Development of the spinal cord

a. Internal changes in the cord

b. Enlargements of the spinal cord

c. Enveloping membranes of the cord

2. Development of the brain

a. The development of specialized areas and outgrowths of the brain

1 ) The formation of the five-part brain

2) The cavities of the primitive five-part brain and spinal cord

b. The formation of cervical and pontine flexures

c. Later development of the five-part brain

D. Development of the peripheral nervous system

1. Structural divisions of the peripheral nervous system

2. The cerebrospinal system

3. General structure and function of the spinal nerves

4. The origin, development and functions of the cranial nerves O. Terminal

I. Olfactory

II. Optic

III. Oculomotor

IV. Trochlear

V. Trigeminal

A. Ophthalmicus or deep profundus

B. M axillaris

C. Mandibularis

VI. Abducens

VII. Facial

VIII. Acoustic

IX. Glossopharyngeal

X. Vagus

XI. The spinal accessory

XII. Hypoglossal

5. The origin and development of the autonomic system

a. Definition of the autonomic nervous system

b. Divisions of the autonomic nervous system

c. Dual innervation of thoracicolumbar and craniosacral autonomic nerves

1) Autonomic efferent innervation of the eye

2) Autonomic efferent innervation of the heart

d. Ganglia of the autonomic system and their origin

E. The sense or receptor organs

1. Definition

2. Somatic sense organs

3. Visceral sense organs

4. The lateral-line system

5. The taste-bud system

6. The development of the olfactory organ

a. Development of the olfactory organs in Squalus acanthias

b. Development of the olfactory organs in the frog

c. Development of the olfactory organs in the chick

d. Development of the olfactory organs in the mammalian embryo

7. The eye

a. General structure of the eye

b. Development of the eye

c. Special aspects of eye development

1) The choroid fissure, hyaloid artery, pecten, etc.

2) The formation of the lens

3) The choroid and sclerotic coat of the eyeball; the cornea

4) Contributions of the pars caeca

5) The origin of the ciliary muscles

6) Accessory structures of the eye

8. Structure and development of the ear

a. Structure

1 ) Three semicircular canals

2) An endolymphatic duct

3) A cochlear duct or lagena

b. Development of the internal ear

c. Development of the middle ear

d. Development of the external auditory meatus and pinna

F. Nerve-fiber-effector organ relationships

The Development of the Coelomic Cavities

A. Introduction

1. Definitions

2. Origin of the primitive splanchnocoelic coelom

B. Early divisions of the primitive splanchnocoelic coelom

1. Formation of primitive suspensory structures

2. Formation of the primitive transverse division of the body and the primary pericardial and peritoneal divisions of the coelom

a. Lateral mesocardia

b. Formation of the liver-septum transversum complex

1) Foritiation of the liver-septum complex through modification of the ventral mesentery by liver outgrowth

2) Formation of the liver-septum complex in the human embryo

c. Formation of the primary septum transversum

C. Coelomic changes in fishes, amphibians, reptiles, and birds

1. In fishes

2. In amphibians, reptiles, and birds

D. Formation of the coelomic cavities in mammals

1. Formation of the pleuropericardial membrane

2. Development of the pleuroperitoneal membrane

E. Development of independent pericardial walls

1. The arrangement of the parietal pericardial wall in fishes

2. Formation of an independent parietal pericardial wall in the chick

3. Formation of the independent parietal pericardial wall in amphibians and reptiles

4. Separation of the parietal pericardial wall in mammals

F. The mammalian diaphragm

G. The pulmonary diaphragm or aponeurosis of the chick

H. The omental bursa

I. The formation of various ligaments in the stomach-liver region

1. The gastro-hepatic and hepato-duodenal ligaments

2. The coronary ligament of the liver

3. The falciform ligament of the liver

4. The gastro-splenic ligament

The Developing Endocrine Glands and Tlieir Possible Relation to Definitive Body Formation and the Differentiation of Sex

A. Introduction

B. Morphological features and embryological origin of the endocrine glands

1. Pancreas

2. Pituitary gland (hypophysis cerebri)

a. Anterior lobe

b. Posterior lobe

c. Pars intermedia

3. Thyroid gland

4. Parathyroid glands

5. Thymus gland

6. Pineal body

7. Adrenal (suprarenal) glands

8. Gonads

C. Possible influence of endocrine secretions on the development of definitive body form

1. Thyroid and pituitary glands and anuran metamorphosis

2. Tliyroid and pituitary glands in relation to the development of other vertebrate embryos

a. Chick

1) Thyroid gland

2) Pituitary gland

b. Mammal

1) Thyroid gland

2) Pituitary gland

c. Fishes

3. General conclusions relative to the influence of the thyroid and pituitary glands in vertebrate embryology

D. Possible correlation of the endocrine glands with sex differentiation

1. Differentiation of sex •

a. General sex features in the animal kingdom

b. Chromosomal, sex-determining mechanisms

c. Possible influence of the sex field in sex determination

2. Influence of hormones on the differentiation of sex

3. General summary of the factors involved in sex differentiation in the vertebrate group


874


INTRODUCTION


875


A. Introduction

The endocrine glands are those glands which produce hormonal secretions. The term hormone is derived from a Greek word meaning to stimulate or to stir up. Selye in 1948 (p. 11) defined hormones as physiologic, organic compounds produced by certain cells for the sole purpose of directing the activities of distant parts of the same organism.”

The endocrine organs may be separated into two main groups:

(1) purely endocrine glands, and

(2) mixed endo-exocrine glands.

Purely endocrine glands have as their sole function the production of hormones. Under this heading are included the pituitary (hypophysis), thyroid, parathyroid, pineal, adrenal (suprarenal), and thymus glands.

Mixed endo-exocrine glands are exemplified by the pancreas, liver, duodenum, and reproductive organs. Parts of these organs are purely exocrine, e.g., the pancreas where pancreatic juice is produced by the acinous cells but which elaborates, at the same time, insulin from the islets of Langerhans. The liver elaborates the exocrine secretion, bile, which is discharged through the bile ducts and, concurrently, manufactures the antipernicious-anemia factor which is dispensed into the blood stream directly. The duodenum produces digestive substances and also secretin. Secretin is elaborated by the epithelial lining cells of this area, and it stimulates the pancreas to secrete its pancreatic juice.

Relative to their secretory activities all endocrine glands have this physiomorphological feature in common: They discharge the hormonal or endocrine substance directly into the blood stream without the mediation of a duct system. Endocrine glands, therefore, are distinguished by this process from exocrine glands, which exude the secretory product into a duct system from whence the secretion passes to the site of activity.

B. Morphological Features and Embry ological Origin of the Endocrine Glands

1. Pancreas

The islets of Langerhans are small masses of cells or islands scattered among the acini (alveoli) of the general pancreatic tissue. The pancreatic islets appear to arise as specialized buds from the same entodermal cords which give origin to the alveoli. The islets separate early from the entodermal cords and produce isolated cellular cords. Blood capillaries form a meshwork within these cords of cells (figs. 295G; 365A). Their secretion, insulin, is concerned with sugar metabolism and prevents the malfunction known as diabetes.

Pancreatic islets are found extensively in the vertebrates and generally are


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Fig. 365. The pancreatic islets and pituitary gland. (A) Origin of islet tissue from developing pancreatic ducts and acini. 1 == young bud; 5 = older bud. (Modified from Arey, ’46, Developmental Anatomy, Philadelphia, Saunders.) (B-E) Diagrams of pituitary gland conditions in Petromyzon (B), Rana (C), Reptile (D), and Man(E). (Modified from Neal and Rand, 1939, Chordate Anatomy, Philadelphia, Blakiston.) (F) Origin of Rathke’s pouch material from inner layer of epidermal ectoderm in early tadpole of Rana. (G-I) Developmental stages of hypophysis in human embryo.

associated with the pancreas. In some teleost fishes, the two glands are separated although both are derived from the entoderm. The pancreatic islets are classified as belonging to the solid, non-storage type of endocrine gland.

2. Pituitary Gland (Hypophysis Cerebri)

Previous to the latter part of the last century, the function of the pituitary gland was presumed to be one of mucous secretion, hence the name pituitary from the Latin, pituita, a nasal secretion. It was so regarded by Vesalius in 1543. The English anatomist, Willis, believed that the pituitaly gland secreted the cerebrospinal fluid.

The pituitary gland (fig. 365E and I) is composed of three main parts as follows:

a. Anterior Lobe

The anterior lobe (pars anterior) is composed of two subdivisions:

( 1 ) a large anterior lobe (pars distalis), and

(2) a smaller glandular mass (pars tuberalis).


ORIGIN OF THE ENDOCRINE GLANDS


877


b. Posterior Lobe

The posterior lobe (lobus nervosus, pars neuralis) is derived from the distal part of the infundibulum.

c. Pars Intermedia

The pars intermedia or intermediate lobe is associated closely with the posterior lobe but has the same embryonic origin as the pars distalis and pars tuberalis of the anterior lobe.

In Petromyzon fiuviatilis, the hypophysis is a flat, tube-like organ attached to the infundibular evagination of the floor of the diencephalon. The anterior lobe is represented by the hypophyseal duct which ends blindly below the infundibulum. From this duct are proliferated the cells of the intermediate lobe (tig. 365B). The pituitary gland shows great similarity, in all higher vertebrates, being composed of three main parts, viz., pars anterior, pars intermedia, and pars posterior (fig. 365C-E). However, in the chicken, whale, manatee, and armadillo, the intermediate lobe is missing (Selye, ’48).

The pars anterior and the pars intermedia of the pituitary gland develop from Rathke’s pouch as evaginations of the middorsal area of the stomodaeal pocket, although in the frog Rathke’s pouch develops precociously from the so-called neural ectoderm above the stomodaeal invagination (fig. 365F-I). Rathke’s pouch gradually comes into contact with the ventrally directed infundibular evagination from the diencephalon. The distal part of the infundibular evagination forms the pars neuralis, while Rathke’s pouch differentiates into the pars distalis, pars intermedia, and pars tuberalis.

3. Thyroid Gland

The thyroid gland (fig. 366B) was described first in 1656 by Thomas Wharton, the English anatomist, who called it the thyroid gland because of its association with the thyroid or shield-shaped cartilage of the larynx.

After about 50 years of work by many observers on the thyroid gland and its activities, the crystalline form of the secretory principle of the thyroid gland was isolated by Kendall in 1919, and he called it thyroxine. This compound contained 65 per cent of iodine by weight and its empirical formula was subsequently determined as C, 5 H„ 04 Nl 4 .

One of the thyroid’s functions is to govern carbohydrate metabolism, and, in general, the gland controls the basal metabolism of the animal together with growth processes. In man and the cat, the thyroid gland is in the form of two lateral lobes, located on the ventro-lateral aspect of the thyroid cartilage of the larynx, the two lobes being joined by an isthmus. In birds, there are two glands, both being located within the thoracic cavity; in fishes, including the Cyclostomes, the thyroid is an unpaired structure and is to be found generally between and near the posterior ends of the lower jaws. The gland, therefore, is a constant feature of all vertebrates.


878


THE DEVELOPING ENDOCRINE GLANDS


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Fig. 366. Thyroid, parathyroid, and thymus glands in human embryo. (A) The loci of origin of thyroid, parathyroid, thymus, and ultimobranchial bodies. (B) Late Stage (somewhat abnormal) of thyroid, parathyroid, and thymus gland development in human. (C) Early stage of thyroid follicle differentiation. (D) Later stage of thyroid follicle differentiation.

In the embryos of all vertebrates the thyroid gland appears as a pharyngeal derivative. In the human as in fishes and amphibia (Lynn and Wachowski, ’51), it arises as a midventral outpocketing of the anterior pharyngeal floor. In the human embryo, this outpocketing occurs between the first and second branchial pouches at about the end of the fourth week of development (fig. 3 66 A). Its point of origin is observable during later development as a small indentation, the foramen caecum, in the region between the root and body of the tongue (fig. 285). It is a bilobed evagination which soon loses its connection with the pharyngeal floor and migrates caudally to the laryngeal area where it differentiates into a double-Iobed structure, connected by a narrow bridge of thyroid tissue, the isthmus. Occasionally, a persistent thyroglossal duct, connecting the foramen caecum with the thyroid gland, remains (fig. 366B). While the thyroid rudiment migrates posteriad, the post-branchial (ultimobranchial) bodies, which take their origin from the caudal margin of the fourth branchial pouch, become incorporated within the thyroid tissue.


ORIGIN OF THE ENDOCRINE GLANDS


879


The significance of this incorporation is unknown, and evidence of functional thyroid tissue, being derived from the post-branchial body cells, is lacking.

When the cellular masses of the developing thyroid gland reach the site of the future thyroid gland, the cells multiply and break up into cellular strands, surrounded by mesenchyme and blood vessels (fig. 366C). These strands in turn break up into small, rounded, bud-like masses of epithelial cells, the young thyroid follicles (fig. 366D). During the third month of development in the human, colloidal substance begins to appear within the young thyroid follicles. The colloid increases during the fourth month, and the surrounding cells of the follicle appear as a single layer of low columnar cells. Each thyroid follicle as a whole assumes the typical appearance of a functioning structure. Blood capillaries ramify profusely between the respective follicles.

The colloidal substance within each thyroid follicle presumably represents stored thyroid secretion, and the thyroid gland is regarded, therefore, as a “storage type” of endocrine gland. The theory relative to thyroid gland function is set forth that the follicle cells may secrete directly into the capillaries and, hence, into the blood stream, or the secretion may be stored as colloid within the follicles. Later this reserve secretion in the form of colloid may be resorbed by the cells in times of extreme activity and passed on into the region of the capillaries. In certain instances, e.g., dog and rat, individual thyroid follicles may be lined with stratified squamous epithelium (Selye, ’48, p. 695).

In the larvae of the cyclostome, Petromyzon, the so-called endostyle is lined with rows of mucus-secreting cells, alternating with ciliated cells. This endostylar organ becomes transformed into the thyroid gland upon metamorphosis. A localization of iodine in certain of the endostylar cells in the larva has been demonstrated (Lynn and Wachowski, ’51, p. 146).

4. Parathyroid Glands

The parathyroid glands in man are four, small, rounded bodies, located along the dorsal (posterior) median edges of the two thyroid lobes of the thyroid gland (fig. 366B). Unlike the storage type of endocrine gland, such as the thyroid gland with its follicles, the parathyroids contain no follicles and, therefore, represent the solid type of endocrine gland. Blood capillaries ramify through its substance which is composed of closely packed masses of polyhedral epithelial cells, arranged in small cords or in irregular clumps. Two main cell types are present in mammals, the chief or principal cells with a clear cytoplasm and the oxyphil cells whose granules stain readily with acid stains. The chief cells are common to all vertebrate parathyroids and thus may represent the essential cellular type of the parathyroid gland (Selye, ’48, p. 540).

The removal of the parathyroid glands results in a reduction of the calcium content of the blood, muscular tetany, convulsions, and ultimate death. The


880


THE DEVELOPING ENDOCRINE GLANDS


parathyroid glands in some way regulate calcium metabolism to keep the calcium content in the blood stream at its proper level.

Parathyroid structures may be present in fish (Selye, ’48), but it is generally believed that true parathyroid tissue is confined to the Tetrapoda. Two parathyroid glands on each side are found in most urodeles and other amphibia, and in reptiles. The birds have relatively large parathyroid glands, attached to the two thyroid glands located in the thoracic cavity. All mammals possess parathyroid glands which, in some instances, are located internally within the thyroid gland as well as externally. Accessory parathyroid glands, apart from the two parathyroids attached to the thyroid gland, are found in rats and mice and, consequently, may not be disturbed if the thyroid gland is removed in these rodents.

The parathyroid glands arise in the human embryo from proliferations of the dorso-lateral walls of the third and fourth branchial pouches (fig. 366A). The parathyroids which arise from the third pair of pouches are known as parathyroids III, while those from the fourth pair of branchial pouches are called parathyroids IV. Parathyroids III arise in close proximity to the thymusgland rudiments (fig. 366A). However, it is to be observed that the thymus rudiments arise from the ventral aspect of the third pair of pouches. The parathyroid-III rudiments move caudally with the thymus gland rudiments and come to lie in relation to the lateral lobes of the thyroid, posterior to parathyroids IV which take their origin in close relation to the post-branchial (ultimobranchial) bodies (fig. 3 66 A and B).

Parathyroids IV appear to be a constant feature of all Tetrapoda. In those species having but two parathyroids, it is probable that their origin is from the fourth branchial pouches.

5. Thymus Gland

The thymus gland or “throat sweetbread” (the pancreas is referred to commonly as the “stomach sweetbread”) lies in the anterior portion of the thoracic cavity and posterior neck region (fig. 366B). In some cases, it may extend well along in the neck region toward the thyroid gland. In the thoracic area, it lies between the two pleural sacs, that is, within the mediastinum, and reaches as far caudally as the heart. Histologically, it is composed of two parts:

( 1 ) a cortex and

(2) a medulla.

The cortex contains masses of thymocytes or lymphocyte-like cells, while the medulla contains thymocytes, reticular cells, and the so-called Hassall’s corpuscles, composed of stratified, squamous, epithelial cells.

In man, the thymus gland arises from the ventral portion of the third


ORIGIN OF THE ENDOCRINE GLANDS


881


branchial pouches during the sixth week. These epithelial derivatives of the third branchial pouch become solid masses of cells which migrate posteriad into the anterior thoracic area.

The thymus gland is found in all vertebrates, but its morphology is most variable. In birds, it is situated in the neck region in the form of isolated, irregular nodules. The bursa of Fabricius, previously mentioned (Chap. 13) as an evagination in the cloacal-proctodaeal region of the chick, is a “thymuslike organ” (Selye, ’48, p. 681 ). Thymus glands in reptiles are located in the neck region, and, in amphibians the two thymus glands lie near the angle of the jaws. In fishes several small, thymus-gland nodules arise from the dorsal portions of the gill pouches and come to lie dorsal to the gill slits in the adult.

The function of the thymus gland is not clear. It appears to have some relationship to sexual maturity. (For thorough discussion, see Selye, ’48, Chap. IX.)


6. Pineal Body

The pineal gland appears to have been first described by Galen, the Greek scientist and physician (130-ca.200 A.D.), who believed it to function in relation to the art of thinking. Descartes (1596-1650) considered it to be the “seat of the soul.”

During development, two fingerlike outgrowths of the thin roof of the diencephalon of the brain occur in many vertebrates, namely, an anterior paraphysis or parietal organ, and a more posteriorly situated epiphysis. In certain Cycles tomes (Petromyzon), the posterior pineal body or epiphysis is associated with the formation of a dorsal or pineal eye, while the anterior pineal organ or paraphysis forms a rudimentary eyelike structure. In Sphe nodon and in certain other lizards, the paraphysis or anterior pineal evagination develops an eyelike organ. Also, in various Amphibia (frogs; Ambystoma) rudimentary optic structures arise from the fused epiphyseal and paraphyseal diverticula. In consequence, we may assume that a primary function in some vertebrates of the dorsal, median pineal organs is to produce a dorsal, lightperceiving organ. In certain extinct vertebrates, a fully developed median dorsal eye appears to have been formed in this area.

On the other hand, the epiphysis (fig. 366A) in some reptiles, in birds and in mammals has been interpreted as a glandular organ. Various investigators have suggested different metabolic functions. However, an endocrine or essential secretory function remains to be demonstrated. (Consult Selye, ’48, p. 595.)

Many types of cells enter into the structure of the pineal gland. Among these are the chief cells, which are large and possess a clear cytoplasm. Nerve cells and neuroglial elements also are present. Various other cell types possessing granules of various kinds in the cytoplasm are recognized.


882


THE DEVELOPING ENDOCRINE GLANDS


7. Adrenal (Suprarenal) Glands

The adrenal bodies are associated, as the name implies, with the renal organs or kidneys. In fishes, definite adrenal bodies are not present, but cellular aggregates, corresponding to the adrenal cells of higher vertebrates, are present and associated with the major blood vessels.

In man and other mammals, the adrenal body is composed of:

(1) an outer, yellow-colored cortex and

(2) an inner medullary area.

The medulla contains the chromaffin cells — cells which have a pronounced affinity for chromium salts, such as potassium dichromate, which stain them reddish brown and produce the so-called ‘‘chromaffin reaction.”

The hormone, secreted by the medulla, is adrenaline (epinephrine). It has marked metabolic and vasoconstrictor effects. The smooth muscle tissue of the arrector pili muscles associated with the hairs in mammals contract and raise the hair as a result of adrenaline stimulation.

The morbid state, known as Addison’s disease and named after the English physician, Thomas Addison, who first described this fatal illness, arises from decreased function of the adrenal cortex. Various types of hormones have been discovered which arise from the cortical layer of the adrenal body, and a large number of steroid substances have been isolated from this area of the adrenal gland (Selye, ’48, p. 89). In fishes, the cortical cell groups are isolated from those of the medulla, and, in the elasmobranch fishes, the cortex forms a separate organ. Its removal may be effected without injury to the medulla but with resulting debility, ending in death.

Embryologically, the adrenal cortex and medulla take their origin from two distinct sources. The cortex arises as a proliferation of the dorsal root of the dorsal mesentery in the area near the anterior portion of the mesonephric kidney and liver on either side (fig. 367A, B). These two proliferations give origin to two cortical masses, each lying along the anterior mesial edge of the mesonephric kidney. Further growth of these masses produces two rounded bodies, the adrenals (suprarenals), lying between the anterior portions of the mesonephric kidneys (figs. 3 A and B; 367B) and later in relation to the antero-mesial portion of the metanephric kidneys (fig. 3B-E). After the cortical masses are established, the chromaffin cells invade them from the medial side (fig. 367C). The potential chromaffin cells migrate from the sympathetic ganglia in this area. Upon reaching the site of the developing adrenal gland they move inward between the cortical cells to the center of the gland where they give origin to the medulla. With the diverse embryological origins of the cortex and the medulla, it is seen readily why two separate glandular structures are present in lower vertebrates.

In man and other mammals, a later developed secondary cortex is laid down around the primary cortex. The primary cortex, characteristic of fetal


DEVELOPMENT OF DEFINITIVE BODY FORM


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medial side of primitive cortical tissue of adrenal body to form chromaffin cells of adrenal medulla.

life, then comes to form the “inner cortical zone” or androgenic zone (Howard, ’39).


8. Gonads

The developing gonads were described in Chapter 18, and their hormonal functions were outlined in Chapters 1 and 2.

C. Possible Influence of Endocrine Secretions on the Development of Definitive Body Form

1. Thyroid and Pituitary Glands and Anuran Metamorphosis

One of the earlier studies in this field of development was that by Gudernatsch (’12 and ’14) which showed that mammalian thyroid gland fed to anuran, and urodele larvae stimulated growth, differentiation, and metamorphosis. In a later series of studies by Allen (see Allen, ’25, for references and review) and by Hoskins (’18 and ’19), it was demonstrated that the removal of the thyroid gland in young tadpoles of Rana and Bufo prevents metamorphosis from the larval form into that of definitive body form (i.e.,


884


THE DEVELOPING ENDOCRINE GLANDS


the adult body form). Similar results were obtained as a result of hypophysectomy (i.e., removal of the hypophysis). (See Allen, ’29, and Smith, ’16 and ’20. ) The work of these observers clearly demonstrates that the thyroid and pituitary glands are instrumental in effecting the radical transformations necessary in the assumption of definitive body form in the Anura.

2. Thyroid and Pituitary Glands in Relation to the Development of Other Vertebrate Embryos

a. Chick

1) Thyroid Gland. Studies relative to the possible effect of the thyroid gland upon the developing chick embryo are complicated by the fact that the yolk of the chick egg is composed of many other factors besides fats, proteins, and carbohydrates. The yolk is a veritable storehouse for vitamins and for thyroid, sex, and possibly other hormones. Just what effect these substances have upon development is problematical. Some experiments, however, have been suggestive. Wheeler and Hoffman (’48, a and b), for example, produced goitrous chicks and retarded the hatching time of chicks from eggs laid by hens which were fed thyroprotein. Thyroprotein feeding seemingly reduced the amount of thyroid hormone deposited in the egg with subsequent deleterious effects upon the developing chicks. In normal development, the thyroid gland of the chick starts to develop during the third day and produces follicles which contain colloid by the tenth and eleventh days of incubation. Furthermore, Hopkins (’35) showed that thyroids from chick embryos of 10 days of incubation hastened metamorphosis in frog larvae. From days 8 to 14 the chick embryo undergoes the general changes which transform it from the larval form which is present during incubation days 6 to 8 into the definitive body form present at the beginning of the third week of incubation.

The foregoing evidence, therefore, while it does not demonstrate that thyroid secretion actually is being released by the developing thyroid gland into the chick’s blood stream, does suggest that the thyroid gland may be a factor in chick development and differentiation. If the chick’s thyroid gland is secreting the thyroid hormone into the chick’s blood stream during the second week of the incubation period, it is evident that the developing chick during the period when it is assuming the definitive body form has two sources of thyroid hormone to draw upon:

( 1 ) that contained within the yolk of the egg and

(2) that produced by its own thyroid gland.

2) Pituitary Gland. Relative to the development of the pituitary gland in the chick, Rahn (’39) showed that the anterior lobe develops both acidophilic and basophilic cells by the tenth day of incubation. Also, Chen, Oldham, and Ceiling (’40) demonstrated that the pituitary of chicks from eggs incubated


DEVELOPMENT OF DEFINITIVE BODY FORM


885


for five days possessed a melanophore-expanding principle when administered to hypophysectomized frogs.

This general evidence, relative to the developing pituitary gland in the chick, suggests that the cells of the pituitary gland may be active functionally during the latter part of the first week and during the second week of incubation. If so, the pituitary gland may be a factor in inducing the rapid growth and changes which occur during the second week of incubation. It suggests further, that a possible release of a thyrotrophic principle may be responsible for the presence of colloid within the developing thyroid follicles during the second week of incubation.


b. Mammal

As in the chick, the developing embryo of the placental mammal is in contact with hormones from extraneous sources. Hormones are present in the amniotic fluid, while the placenta is the seat of origin of certain sex and gonadotrophic hormones. Also, the maternal blood stream, which comes in contact with embryonic placental tissues, is supplied with pituitary, thyroid, adrenal, and other hormonal substances. This general hormonal environment of the developing mammalian embryo complicates the problem of drawing actual conclusions relative to the effect of the embryo’s developing endocrine system upon the differentiation of its own organ systems and growth. Nevertheless, there is circumstantial evidence, relating to possible activities of the developing, embryonic, endocrine glands upon development.

1) Thyroid Gland. Colloid storage within the follicles of the developing, human, thyroid gland is evident at 3 to 4 months. In the pig embryo, Rankin (’41 ) detected thyroxine and other iodine-containing substances in the thyroid at the 90-mm. stage, and Hall and Kaan (’42) were able to induce metamorphic effects in amphibian larvae from thyroids obtained from the fetal rat at 18 days. The foregoing studies suggest that the thyroid gland is able to function in the fetal mammal at an early stage of development. (For further references, consult Moore, ’50.)

2) Pituitary Gland. Similarly, in the pituitary gland, granulations within the cells of the anterior lobe are present in the human embryo during the third and fourth months (Cooper, ’25). Comparable conditions are found in the pituitary of the pig from 50 to 170 mm. in length (Rumph and Smith, ’26).

c. Fishes

The relationship between the thyroid and pituitary glands in the development of fishes is problematical. There is evidence in favor of a positive influence of endostylar cells and of the cells of the developing thyroid gland in the transformation of the ammocoetes larva of the cyclostome, Petromyzon, into the definitive or adult body form. Similar evidence suggests a thyroid activity relationship in the transformation of the larvae of the trout and


886


THE DEVELOPING ENDOCRINE GLANDS


the bony eel. However, this evidence is not indisputable, and more study is necessary before definite conclusions are possible. (Consult Lynn and Wachowski, ’51, for discussion and references.)

3. General Conclusions Relative to the Influence of the Thyroid and Pituitary Glands in Vertebrate Embryology

These conclusions are:

(a) Positive activities of the thyroid and pituitary glands are demonstrated in the transformation of the larval form into the definitive or adult form in the Anura.

(b) Suggestive evidence in favor of such an interpretation has been accumulated in fishes.

(c) Circumstantial evidence, relative to the possible activities of the thyroid and pituitary glands during the period when the embryos of the chick and mammal are transforming into the adult form, is present. With the evidence at hand, however, it is impossible to conclude definitely that these glands are a contributing factor to a change in body form (metamorphosis) in chick and mammalian embryos (fig. 256).

D. Possible Correlation of the Endocrine Glands with Sex Differentiation

1. Differentiation of Sex a. General Sex Features in the A nimal Kingdom

Many animal groups are hermaphroditic, that is, both sexes occur in the same individual. Flatworms, roundworms, oligochaetous annelids, leeches, many mollusks, and certain fishes are representatives of this condition, whereas most vertebrates, insects, and echinoderms are bisexual. If one examines the developing gonads in insects or vertebrates, it is evident that, fundamentally, the potentialities for both sexes exist in the same individual. As observed previously (Chap. 18), the early gonad is bipotential in most vertebrates, and two sets of reproductive ducts are formed. As sex is differentiated, the gonadal cortex and the Mullerian duct assume dominance in the female, while the gonadal medulla and Wolffian duct become functional if the animal is a male. Generality, therefore, gives way to specificity. Conditions thus are established in the developing reproductive system, similar to the generalized conditions to be found in other systems. If we take into consideration the fact that in a large number of animals both sexes are present in a functional state in one individual and in many bisexual species both sexes are present in a rudimentary condition in the early embryo, we arrive at the conclusion that both sexes are fundamentally present in a large majority of animal species. Sex, therefore, tends to be an hermaphroditic matter among many species of animals. The problem of sex differentiation, consequently, resolves itself into this: Why do both sexes emerge in the adult condition in a large number of


CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION


887


animals, whereas in the development of many other animal species, only one of the two sex possibilities becomes functional?

b. Chromosomal, Sex-determining Mechanisms

A considerable body of information has been obtained which demonstrates a fundamental relationship between certain chromosomes and sex determination. The general topography of chromosomal sex-determining mechanisms has been established for a large number of species. A pair of homologous chromosomes, the so-called sex chromosomes, apparently have become specialized in carrying the genic substances directly concerned with sex determination. In many species, the members of this pair of sex-determining chromosomes appear to be identical throughout the extent of the chromosomes in one of the sexes. In the other sex, on the other hand, the two sex-determining chromosomes are not identical. When two identical chromosomes are present in a particular sex, that sex is referred to as the homogametic sex, for the reason that all of the gametes derived from this condition will possess identical sex chromosomes. However, that sex which possesses the two dissimilar chromosomes is called the heterogametic sex for it produces unlike gametes, Often the heterogametic condition is represented by one chromosome only, the other chromosome being absent. If under the above circumstances the normally appearing chromosome is called X, and the deleted, diminutive or strangely appearing chromosome is called Y, while the chromosome which is absent be designated as O, we arrive at the following formula:

XX = the homogametic sex and either XY or XO = the heterogametic sex. In many (probably in most) animal species the male is the heterogametic sex (fig. 36^A~C).

In some animal groups, however, such as the butterflies, the moths, possibly the reptiles, the birds, some fishes, and probably urodele amphibia, the female is the heterogametic sex, and the male is homogametic. In these particular groups, many authors prefer to use the designation ZZ for the homogametic sex (i.e., the male) and ZO or ZW for the female or heterogametic sex. The sex-determining mechanism in these groups, according to this arrangement, will be ZZrZW or ZZ:ZO (fig. 368D).

In endeavoring to explain the action of these chromosomal mechanisms, one of the underlying assumptions is that the genic composition of the chromosomes actively determines the sex. For example, in cases where the female sex is homogametic it is assumed that the X-chromosome contains genes which are female determining; when two (or more) X’s are present, the female sex is determined automatically. When, however, one X-chromosome is present, the determining mechanism works toward male determination. In those species where the female sex is the heterogametic sex it may be assumed that the Z-chromosome (or X-chromosome, depending upon one’s preference) contains genes which are male determining. When only one of these Z-chromo


888


THE DEVELOPING ENDOCRINE GLANDS


Fig. 368. The sex chromosomes in man, opossum, chick, and Drosophila; parabiotic experiments in Amphibia. (A) Late primary spermatocyte in human. (A') First maturation spindle in human spermatocyte. (Redrawn from Painter, ’23, J. Exper. Zool., 37.) (B) Dividing spermatogonium in opossum testis. (B') First maturation spindle

in spermatocyte of opossum. (Redrawn from Painter, ’22, J. Exper. Zool., 35.) (C)

Sex chromosomes in female Drosophila. (C') Sex chromosomes in male Drosophila. (Redrawn from Morgan, Embryology and Genetics, 1934, Columbia University Press, N. Y., after Dobzhansky.) (D) Sex chromosomes in common fowl, male. (D') Sex chromosomes in common fowl, female. (Redrawn from Bridges, 1939, Chap. 3, Sex and Internal Secretions, edited by Allen et al., Baltimore, Williams and Wilkins, after Sokolow, Tiniakow, and Trofimov.) (E-G) Diagrams illustrating the spreading of gonadal substances in frogs, toads, and salamanders. In toads, E, the gonadal influences (antagonisms) are evident only when the gonads actually are in contact. In the frogs, F, the range of influence is wider but its effect falls off peripherally. Figure G represents the condition in newts and salamanders. It is evident that in this group, some substance is carried in the blood stream which suppresses the gonads in the two females as indicated in the diagram. (Redrawn and modified slightly from Witschi, 1939, Chap. 4, Sex and Internal Secretions, edited by Allen et al., Baltimore, Williams and Wilkins.)

somes is present the developmental forces swing in the direction of the female sex. Sex, from this point of view, is determined by a genic balance, a balance which in turn is governed by the quality of certain genes as well as the quantitative presence of genes. (For detailed discussion consult Bridges, ’39, and White, ’48.)


CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION


889


c. Possible Influence of the Sex Field in Sex Determination

Two gonadal sex fields, the cortical field and the medullary field, are present in the early vertebrate gonad in amphibians, reptiles, birds, and mammals. This condition is true also of many fishes. Sex differentiation primarily is a question as to which one of these fields will assume dominance. During development in various instances, sex differentiation is clearly the result of only partial dominance on the part of one sex field, the other field emerging partly or almost completely. As a result, various types of intersexes may appear. For example, in the male toad, Bidder’s organ at the anterior part of the testis represents a suppressed cortical or ovarian field, held in abeyance by the developing testis. Surgical removal of the two testes permits the cortical field or Bidder’s organ to become free from its suppressed state. As a result, functional ovaries are developed, and the animal reverses its sex, becoming a functional female (Witschi, ’39).

One of the classical examples which demonstrates the dependence of the developing sex field upon surrounding environmental factors is the freemartin. The freemartin appears in cattle when twins of the opposite sex develop in such a manner that an anastomosis or union of some of the fetal blood vessels occurs (Lillie, ’17). Under these circumstances the female twin always experiences a transformation in the direction of maleness in the gonad and sex ducts. In those instances of freemartin development where the cortical field of the developing ovary is suppressed and the medullary area is hypertrophic, a partial or fairly well-developed testis may be formed. Under these conditions it is presumed that some substance is elaborated within the medullary field of the developing gonad of the male twin which enhances the development of the similar field in the freemartin ovary and suppresses, at the same time, the cortical field. The development of fully differentiated gametes (i.e., sperm) in the freemartin “testis” has not been demonstrated, but, on the whole, the more normally developed freemartin testis shows conditions at the time of birth which are comparable to a similar gonad of the normal male at about the same age, with the questionable presence or absence of very young germ cells. Gametogenesis in the developing testis of the bull occurs after birth. Consequently, the development of gametes in the freemartin of cattle cannot be ascertained because the freemartin gonad remains in the position of the normal ovary and does not descend into the scrotum as it does in the male (Willier, ’21). A scrotal residence (Chap. 1) is necessary for spermatogenesis in all males, possessing the scrotal condition.

A particularly interesting case of intersexuality, resulting from the lack of complete supremacy on the part of one sex field, is shown in the fowl described by Hartman and Hamilton (’22). A brief resume of its behavior and anatomy, as described by the authors, is presented herewith.

The bird was hatched as a robust chick and developed into an apparently normal Rhode Island Red pullet. The following spring the comb and wattles began to


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THE DEVELOPING ENDOCRINE GLANDS


enlarge, and the bird after a few abortive attempts, learned to give the genuine crow of a rooster. ... It was often seen scratching on the ground and calling the flock to an alleged morsel of food, and though it was never seen to tread hens it would strut and make advances after the manner of cocks. . . . The female behavior of the bird was as follows. For years it would sing like a laying hen. On two occasions it adopted incubator chicks, caring for them day and night and clucking like a normal hen. ... On one occasion it dropped an egg, which though small and elongated, showed the bird to be in possession of functional ovary and oviduct.

Its internal anatomy demonstrated the presence of a left ovotestis and a right testis. An oviduct was present on the left side and a vas deferens on both sides. The right testis contained tubules, and within the tubules were ripe sperm. The ovotestis on the left side contained a cortex studded “with oocytes of every size up to a diameter of 20 mm.” and “not unlike the ovary of a normal hen approaching the laying season” (Hartman and Hamilton, ’22) . Seminiferous tubules also were present in the ovotestis which was filled with sperm.

An interesting example of complete sex reversal was produced experimentally in the axolotl, Siredon (Ambystoma) mexicanurn, by Humphrey (’41). In doing so, Humphrey orthotopically implanted an embryonic testis of Ambystoma tigrinum into an axolotl embryo of similar age. After the ovary on the opposite side of the host (i.e., the young axolotl) had changed to a testis, the implanted testis was removed. Somewhat later, the sexually reversed female axolotl was bred with other females with success. The and F 2 generations suggest that the female axolotl is heterogametic whereas the male is homogametic, with a possible XY or ZW condition in the female and an XX (or ZZ) arrangement in the male. It is interesting to observe that Humphrey obtained YY (or WW) females which were fertile.

Many other studies have been made along the lines of experimental transformation of sex. Of these, the careful studies of Witschi (’39) are illuminating. The method, employed by Witschi, was to join two embryos of opposite sex before the period of sex differentiation. In his studies, he used toad, frog, and urodele embryos. Three different results were obtained, in which the medulla or developing testicular rudiment tended to dominate and suppress the cortex or developing female sex field. For example, in toads, it was evident that the medulla suppressed the cortex only if the two fields came into actual contact; in frogs, the effect of suppression was inversely proportional to the distance of the two sex fields from each other; on the other hand, in urodeles, the substance produced by the medulla evidently circulated in the blood stream and produced its effects at a distance (fig. 368E-G). Witschi postulated the presence of two, not readily diffusible, “activator” substances, cortexin, formed by the cortex, and medullarin, elaborated by the medulla, to account for the results in the toad and frog embryos, and, in urodeles, he assumed a hormonal substance to be present.


CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION


891


The foregoing examples and many others (Witschi, ’39) suggest the following interpretations relative to sex determination and differentiation:

(1) The germ cell, regardless of its genetic constitution, develops into an egg or a sperm, depending upon whether it lies in a developing cortex or in a developing medulla. That is, the influence of the sex field governs the direction of germ-cell differentiation (fig. 22).

(2) The sex field is a powerful factor in determining sex. A factor (or factors) which enables an elevation to partial or complete dominance on the part of one sex field, which under normal conditions is suppressed, may result in the partial or complete reversal of sex.

(3) Differentiation of sex is dependent upon an interplay between the genes of the sex chromosomes and the bio-chemical forces present in the gonadal sex field. This interplay may be considered to work as follows; (a) If the male-sex field or medulla in a particular species is stronger than the female field or cortex, that is, if it is able to compete for substrate substances more vigorously and successfully and to produce diffusible hormonal substance more plentifully, it will suppress the female sex field. Under these conditions, the chromosomal sex-determining mechanism is established in such a way that the male is the heterogametic sex, composed of XY or XO chromosomal combinations, and the female is XX, the genes of the extra X chromosome being necessary to override the male tendency present normally in the male sex field, (b) On the other hand, if the female sex field or cortex is stronger physiologically, then the female is the heterogametic sex (XO or ZW), the homozygous condition of the sex chromosomes in the male being necessary to suppress the natural tendencies toward supremacy of the stronger female sex field, (c) It may be that the general characteristics and strength of the sex field are controlled by genes present in certain autosomal chromosomes, whereas the specific role which the particular sex field takes normally in sex differentiation is controlled by the genes in the sex chromosomes.

2. Influence of Hormones on the Differentiation of Sex

The possible effects of hormones upon sex differentiation, particularly upon the development of the accessory ducts, have been studied with great interest since F. R. Lillie’s (T7) description of freemartin development in cattle. He tentatively made the assumption that the male fetal associate of the freemartin produces a hormonal substance which, through the medium of vascular anastomoses within the placentae of the two fetuses, brings about a partial suppression of the developing ovary and effects, in part, a sex reversal in the developing reproductive organs of the female. The female member of this heterosexual relationship, therefore, is more or less changed in the direction of the male; hence, the common name freemartin.


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THE DEVELOPING ENDOCRINE GLANDS


It should be mentioned in this connection that in the marmoset, Oedipomidas geoffroyi, similar anastomoses between the placental blood vessels of heterosexual twins fail to produce the freemartin condition, both twins being normal. Species differences in the response to hormones or other sex-modifying substances therefore occur (Wislocki, ’32).

The studies made in an endeavor to ascertain the influences which sex hormones play in the development of the reproductive system and in sexual differentiation have produced the following general results.

Developing ovaries and testes and the reproductive ducts of birds, frogs, and urodeles may show various degrees of sex reversal when the developing young are exposed to hormones or other humeral substances of the opposite sex. There is some evidence to the effect that sex reversal by sex hormones is accomplished more readily and completely from the homogametic sex to the heterogametic sex, suggesting, possibly, that the sex field of the heterogametic sex is the stronger and more resistant. The reproductive ducts are more responsive to change than are the gonads (Burns, ’38, ’39a; Domm, ’39; Mintz, Foote, and Witschi, ’45; Puckett, ’40; Willier, ’39; and Witschi, ’39).

In mammals, the gonads (ovary and testis) appear quite immune to the presence of sex hormones, whereas the reproductive ducts respond partially to the sex hormone of the opposite sex. The caudal parts of the genital passages are more sensitive to change than are the more anterior portions (Burns, ’39b, ’42; Greene, Burrill, and Ivy, ’42; and Moore, ’41, ’50).

Castration experiments before and shortly after birth in mammals produce the following effects:

(1) Removal of the testis results in retardation and suppression of the male duct system, while it allows the female duct system to develop.

(2) Removal of the ovary does not affect the female duct system until the time of puberty.

(See LaVelle, ’51, and Moore, ’50, for extensive references and discussion.)

The general conclusions to be drawn from the above experiments, relative to the differentiation of the reproductive ducts, are as follows:

(1) The reproductive ducts are responsive to sex hormones after they are formed in the embryo.

(2) The male duct system normally responds to humeral substances, elaborated by the developing testis soon after it is formed.

(3) The female duct system probably is not dependent upon hormonal secretion for its development until about the time of sexual maturity.

(4) The developing ovary, unlike the developing testis, probably under normal conditions does not elaborate sex hormones in large amounts until about the time of sexual maturity.


CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION


893


3. General Summary of the Factors Involved in Sex Differentiation in the Vertebrate Group

The sex glands (gonads) and the reproductive ducts appear to arise independently of each other.

The primitive gonad is composed of two main parts:

( 1 ) the primordial germ cells and

(2) cellular structures which act as supporting and enveloping structures for the germ cells.

The presence of the primitive germ cells probably is a primary requisite for the development of a functional reproductive gland (see p. 121).

In the differentiation of the gonad, two basic sex fields or territories appear to be involved in Tetrapoda and probably also in most fishes. These territories are:

( 1 ) the medulla or testis-forming territory and

(2) the cortex or ovary -forming area.

The sex fields may be controlled by the genes in the autosomal chromosomes, and there probably is a tendency for one or the other of these fields to be functionally stronger than the other. The heterogametic (XY, XO, ZW or ZO) conditions of the sex chromosomes appear to be associated with the stronger sex field, and the homogametic (i.e., XX or ZZ) combination is associated with the weaker sex field.

During development, presumably, there is a struggle for supremacy through competition for substrate substances (see Dalcq, ’49) by these two sex fields and, under normal conditions, the sex chromosomal mechanism determines which of the two sex fields shall be suppressed and which shall rise to domination. The sex chromosomes thus control the direction of sex differentiation, whereas the field or territory elaborates the power of differentiation.

Disturbing influences may upset the sex-determining mechanism set forth above, and various degrees of hermaphroditism may arise in the same individual in proportion to the degree of escape permitted the normally suppressed sex field.

The sex ducts arise in association with the pronephric kidney and its duct, the pronephric (mesonephric) duct. The Mullerian or female duct arises by a longitudinal splitting of the original pronephric (mesonephric) ducts (e.g., in elasmobranchs) or by an independent caudal growth of a small invagination of the coelomic epithelium at the anterior end of the mesonephric kidney (e.g., reptiles, birds, and mammals). This independent caudal growth is dependent, however, upon the pre-existence of the mesonephric duct (Chap. 18). In the urodeles, the Mullerian duct appears to arise partly from an independent origin and in part from contributions of the mesonephric duct.


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THE DEVELOPING ENDOCRINE GLANDS


Two sets of primitive ducts thus are established in the majority of vertebrates in each sex, the Mullerian or female duct and the mesonephric (pronephric) or male duct

During later normal development, the Mullerian duct is developed in the female, while, in the male, the mesonephric duct is retained and elaborated as the functional, male reproductive duct.

The male duct system is dependent upon secretions from the developing testis for its realization during the later embryonic period and during postnatal development, whereas the female duct develops independently of the ovary up to the time of sexual maturity when its behavior is altered greatly by the presence of the ovarian hormones.

Bibliography


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Cooper, E. R. A. 1925. The histology of the more important human endocrine organs at various ages. Oxford University Press, Inc., New York.

Dalcq, A. M. 1949. The concept of physiological competition (Spiegelman) and the interpretation of vertebrate morphogenesis. Exp. Cell Research, Supplement 1, Bonnier, Stockholm and Academic Press, New York.

Domm, L. V. 1939. Chap. V. Modifications in sex and secondary sexual characters in birds in Sex and Internal Secretions by Allen, et al., 2d ed.. The Williams & Wilkins Co., Baltimore.

Greene, R. R., Burrill, M. W., and Ivy, A. C. 1942. Experimental intersexuality. The relative sensitivity of male and female rat embryos to administered estrogens and androgens. Physiol. Zodl. 15:1.

Gudernatsch, J. F. 1912. Feeding experiments on tadpoles. I. The influence of specific organs given as food on growth and differentiation. A contribution to the knowledge of organs with internal secretion. Arch. f. Entwicklngsmech. d. Organ. 35:457.

, 1914. Feeding experiments on tadpoles. II. A further contribution to the knowledge of organs with internal secretion. Am. Jour. Anat. 15:431.

Hall, A. R., and Kaan, H. W. 1942. Anatomical and physiological studies on the thyroid gland of the albino rat. Anat. Rec. 84:221.


BIBLIOGRAPHY


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Hartman, C. G., and Hamilton, W. F. 1922. A case of true hermaphroditism in the fowl, with remarks upon secondary sex characters. J. Exper. ZooL 36:185.

Hopkins, M. L. 1935. Development of the thyroid gland in the chick embryo. J. Morphol. 58:585.

Hoskins, E. R. and M. M. 1918. Further experiments with thyroidectomy in Amphibia. Proc. Soc. Exper. Biol. & Med. 15:102.

. 1919. Growth and development of

Amphibia as affected by thyroidectomy. J. Exper. Zool. 29:1.

Howard, E. 1939. Effects of castration on the seminal vesicles as influenced by age, considered in relation to the degree of development of the adrenal X zone. Am. J. Anat. 65:105.

LaVelle, F. W. 1951. A study of hormonal factors in the early sex development of the golden hamster. Contrib. to Embryol. Carnegie Inst., Washington, Publ. 34:223.

Lillie, F. R. 1917. The free-martin; a study of the action of sex hormones in the fetal life of cattle. J. Exper. Zool. 23:371.

Lynn, W. G., and Wachowski, H. E. 1951. The thyroid gland and its functions in cold-blooded vertebrates. Quart. Rev. Biol. 26:123.

Mintz, B., Foote, C. L,, and Witschi, E. 1945. Quantitative studies on response of sex characters of differentiated Rana clamitans larvae to injected androgens and estrogens. Endocrinology. 37:286.

Moore, C. R. 1941. On the role of sex hormones in sex differentiation in the opossum {Didelphys virginiana). Physiol. Zool. 14:1.

. 1950. The role of the fetal endocrine glands in development. J. Clin. Endocrinol. 10:942.

Puckett, W. O. 1940. Some effects of crystalline sex hormones on the differentiation of the gonads of an undifferentiated race of Rana catesbiana tadpoles. J. Exper. Zool. 84:39.

Rahn, H. 1939. The development of the chick pituitary with special reference to the cellular differentiation of the pars buccalis. J. Morph. 64:483.


Rankin, R. M. 1941. Changes in the content of iodine compounds and in the histological structure of the thyroid gland of the pig during fetal life .Anat. Rec. 80:123.

Rumph, P., and Smith, P. E. 1926. The first occurrence of secretory products and of a specific structural differentiation in the thyroid and anterior pituitary during the development of the pig foetus. Anat. Rec. 33:289.

Selye, H. 1948. Textbook of Endocrinology. Universite de Montreal, Montreal, Canada.

Smith, P. E. 1916. The effect of hypophysectomy in the early embryo upon growth and development of the frog. Anat. Rec. 11:57.

. 1920. The pigmentary growth and

endocrine disturbances induced in the anuran tadpole by the early ablation of the pars buccalis of the hypophysis. Am. Anat. Memoirs. 11, The Wistar Institute of Anatomy and Biology, Philadelphia.

Wheeler, R. S., and Hoffman, E. 1948a. Goitrous chicks from thyroprotein-fed hens. Endocrinology. 42:326.

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quantitative thyroprotein treatment of hens on length of incubation period and thyroid size of chicks. Endocrinology. 43:430.

White, M. J. D. 1948. Animal Cytology and Evolution, Chap. XI. Cambridge University Press, London.

Willier, B. E. 1921. Structures and homologies of free-martin gonads. J. Exper. Zool. 33:63.

. 1939. Chap. III. The embryonic

development of sex in Sex and Internal Secretions by Allen, et al., 2d ed.. The Williams & Wilkins Co., Baltimore.

Wislocki, G. B. 1932. Placentation in the marmoset {Oedipomidas geoffroyi) with remarks on twinning in monkeys. Anat. Rec. 52:381.

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