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| [[File:Mark_Hill.jpg|90px|left]] This is a draft version of McMurrich's 1914 embryology textbook.
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[[Media:1907 The Development Of The Human Body.pdf|PDF 1907 edition]] | [https://archive.org/details/b21713157 Internet Archive - 1903 edition] | [https://archive.org/details/b21713145 1910 edition] | [https://archive.org/details/develohumanbody00mcmu c1920 edition]
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{| class="wikitable mw-collapsible mw-collapsed"
! McClendon - Book Review (1913) &nbsp;
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| [[Anatomical_Record_7_(1913)#BOOK_REVIEW_-_The_Development_of_the_Human_Body:_A_Manual_of_Human_Embryology|Book Review (1913)]]


{{Historic Disclaimer}}
The Development of the Human Body: A Manual of Human Embryology. By J. Playfair McMurrich. Philadelphia, P. Blakiston's Son and Company, fourth edition, 1913, pp. 8 + 495, 285 figures, $2.50 net.


=The Development Of The Human Body - A Manual Of Human Embryology=
A new edition of McMurrich's Embryology has just appeared. The general character of the previous editions is retained, particular attention being given to the development of organs and less space devoted to the early stages of the embryo. Parts of the book have been re-written and other parts revised. The numerous typographical errors so conspicuous in the third edition have been eliminated. The volume is of pocket size with flexible binding. The large, clear type has been retained. Perhaps the most important feature of the book is the author's clearness of expression. If embryology is to be an aid to anatomy it would appear desirable to find some royal road to this science, which is at present perhaps more difficult than adult anatomy. The student is usually bewildered by serial sections unless guided by a very clear presentation of the subject. McMurrich has achieved this end at the expense of many details, and no doubt rightly. Yet, the large mass of facts contained in this small book seems remarkable. A few passages are ambiguous if not erroneous, and may deserve mention. McMurrich describes the observations of Will on gastrulation in the Gecko, but transposes Will's terminology of primary and secondar}^ endoderm. If gastrulation in vertebrates means anything, certainly the formation of endoderm by invagination is the primitive process and is 'primary' from the standpoint of phylogeny, whereas delamination is a secondary method of formation although it does occur earliest in the development of mammalian embryo. On page 60 the author describes the origin of the mesoderm as follows : "the layer of enveloping cells splits into two concentric layers, the inner of which seems to be mesodermal in its nature and forms a layer lining the trophoblast," and again on page 110: "the extra-embryonic mesoderm, instead of growing out from the embryo to enclose the yolk sac, splits off directly from the enveloping layer." And on page 55: "a splitting of the enveloping layer has occurred, so that the wall of the ovum is now formed of three layers, an outer one which may be termed the trophoblast, a middle one which probably is transformed into the extra-embryonic mesoderm of later stages, though its significance is obscure, and an inner one which is the primary endoderm." I know of no primate, however, in which it has been demonstrated that any 102 BOOK REVIEW 103 part of the mesoderm is delaminated from the trophoblast. McMurrich supposes a separate origin of the extra-embryonic mesoderm. He may be correct, since the origin of the mesoderm is unknown in man. The relation of bloodvessels to endoderm in the liver anlage is described on page 308 as follows: "Shortly after the hepatic portion has been differentiated, its substance becomes permeated by numerous blood vessels (sinusoids) and so divided into anastomosing trabecular" Since these blood vessels arise by a breaking up of the vitelline veins, which are present before the liver anlage appears, it might seem more logical to say that the liver invades the vitelline veins than that the veins invade the liver. McMurrich describes the separation of the coelomic cavities as they occur in the rabbit. The separation of the pericardium from the other cavities is simpler in man in that there are no openings ventral to the viteline veins that have to be closed. This is described in Keibeland Mall's Handbook (vol. 1, p. 526) as follows: "In the rabbit the pericardial coelom ends in two dorsal and two ventral recesses, all four of which connect subsequently with the peritoneal coelom. However, only the dorsal recesses break into the peritoneal coelom in the human embryo." A description of the simpler condition in the human embryo might be preferable in a textbook. Finally, there is a point which is of no evident importance in embryology, but since McMurrich has brought it up, we will consider it very briefly. He describes protoplasm as having a visible reticular or possibly alveolar structure. This reticular ' theory' is based on fixed material, although is it well known that fixing fluids produce the same appearance in clear gelatine jelly or clear albuminous fluids. Formalin and osmium tetroxide do not produce a reticular structure in gelatine jelly and protoplasm fixed with them appears homogeneous. Butschli observed an alveolar structure in 'living' protoplasm under certain conditions, but this appearance seemsto be exceptional or possibly abnormal. With the ultra-microscope of Siedentopf and Zigsmondy, particles smaller than the largest molecules may be made very evident. With this instrument the protoplasm of erythrocytes, and both nucleus and cytoplasm of frog erythrocytes, appear absolutely homogeneous, until injured by abnormal conditions. When transferred from the normal medium to even so good an imitation as Ringer's solution coagulations begin to appear. The numerous deutoplasmic granules in many cells prevent a universal application of this method. To sum up: McMurrich's Embryology may be considered as a very convenient and desirable text book for medical students. It might also be used as a brief reference book provided the subject matter be verified by looking up the literature cited at the end of each chapter. J. F. McClendon.
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|}
{{Historic Disclaimer}}
=The Development of the Human Body - A Manual of Human Embryology=
[[File:J. Playfair McMurrich.jpg|thumb|alt=J. Playfair McMurrich|J. Playfair McMurrich (1859 – 1939)]]
By  
By  


J. Playfair McMurrich, A. M., Ph. D., Ll. D.  
J. Playfair McMurrich, A. M., Ph. D., Ll. D.
 


Professor Of Anatomy In The University Of Toronto  
Professor Of Anatomy In The University Of Toronto  
Line 19: Line 37:
With Two Hundred and Eighty-five Illustrations Several of which are Printed in Colors  
With Two Hundred and Eighty-five Illustrations Several of which are Printed in Colors  


Philadelphia


Philadelphia
P. Blakiston's Son & Co.


P. Blakiston's Son & Co.


1012 Walnut Street  
1012 Walnut Street  
Line 28: Line 46:
1914  
1914  


Copyright, 1913, By P. Blakiston's Son & Co.  
Copyright, 1913, By P. Blakiston's Son & Co.


{| class="wikitable mw-collapsible mw-collapsed"
! Book Review by Frederic T. Lewis - Anatomical Record 1 (1906)&nbsp;
|-
| '''The Development of the Human Body'''  by J. Playfair McMurrich. Third edition. P. Blakiston's Son & Co., Philadelphia, 1907. X + 528 pages, 277 figures. $3.00


==Preface to the Fourth Edition==
McMurrich's "Development of the Human Body" is the best brief didactic American text-book of embryology. In its third edition it has been thoroughly revised, but is no larger than before. The Basle nomenclature has been introduced, but here and there are found such rejected terms as lymph follicles, discus proligems, and uterus masculinm. The word anlage does not occur. In English we may say- that "' an organ arises (or begins) as an outgrowth " rather than "' the anlage of the organ is an outgrowth " ; and that " the liver at first is a diverticulum " rather than " the anlage of the liver is a diverticulum." Apparently no new terms have been proposed.


The increasing interest in human and mammalian embryology
The following changes and additions are of interest. In man the number of chromosomes is 24 (Duesberg) instead of 16. The studies of Heape on menstruation in lower mammals, of Marshall on the internal secretion of the ovaries, of Kirkham and of Gerlach showing that the mouse has a second polar body overlooked by Sobotta, and of several authors in favor of the epithelial origin of lutein cells have been added. Hubrecht's term trophohlast, corresponding with epiblast and hypoblast, is replaced by Minot's trophoderm, corresponding with ectoderm, etc.; it is described as a modified ectoderm adapted for implantation rather than as a distinct germ layer. The description of implantation has been modified with reference to Rejsek's study of Sperinophilus and Doria's account of a young human embryo. Assheton's and Peebles' experimental studies of growth in the chick's blastoderm have been substituted for the concrescence theory, which is not mentioned in the third edition. The existence of a primary chordal canal opening at Hensen's knot is now admitted (p. 60) ; in view of this change the secondary chordal canal (p. 100) should be more sharply defined.
which has characterized the last few years has resulted in many
additions to our knowledge of these branches of science, and has
necessitated not a few corrections of ideas formerly held. In this
fourth edition of this book the attempt has been made to incorporate the results of all important recent contributions upon the topics
discussed, and, at the same time, to avoid any considerable increase
in the bulk of the volume. Several chapters have, therefore, been  
almost entirely recast, and the subject matter has been thoroughly
revised throughout, so that it is hoped that the book forms an
accurate statement of our present knowledge of the development
of the human body.  


To several colleagues the author is indebted for valuable suggestions, and in this connection he desires especially to thank Dr.
In connection with the development of the skeleton, a new figure is introduced illustrating the relation of vertebrae to segments; ]\Iairs work on the ossification centers of the interparietal and maxilla, and Fawcett's on the pterygoid process have been cited.
J. C. Watt for much generous assistance in the revision of the manuscript and for undertaking the correction of the proof-sheets.  


In addition to the works mentioned in the preface to the first
Red blood corpuscles are described as discs. Since photographs of the cup-shaped form have not yet been published, two are insei-ted in this review. They represent a few corpuscles of the foi'm in question among a large number of shadows from which the luemoglobin has disappeared. It is improbable that IJetterer is correct in regarding the cup as being only apparent and due to the distribution of haemoglobin. Such cupshaped appearances as are shown in the photograph are characteristic of circulating and of well-preserved blood. They appear in embryos wlien the nuclei of the erythroblasts are eliminated.
edition as of special value to the student of Embryology, mention
should be made of the Handbuch der vergleichenden mid experimentellen Entwickhmgslehre der Wirbeltiere edited by Professor Oscar
Hertwig and especially of the Manual of Human Embryology edited
by Professors F. Keibel and F. P. Mall.
University of Toronto.  


Of the origin of blood plates it is said that " the most plausible suggestion is that they are the fragmented nuclei of broken-down leucocytes." The fine investigation of James 11. Wright is not mentioned.


There are many clianges in the chapter on blood-vessels. A new diagram shows the questionable fifth aortic arch as equivalent to the other five. Six aortic arches require the presence of five pharyngeal pouches, yet according to McMurrich, there are but four. The fifth arch is however, said to be " rudimentary." Mall's important work on the cerebral veins in man replaces Selzer's description of these veins in the guinea-pig, but the complex story of development is rendered confusing by a misplacement of figures, Fig. 151 B (p. 271) being younger than Fig. 151 A. The supracardinal veins of Huntington and McClure are described and figured. These include the dorsal limbs of the loop formed by the cardinal veins around the ureter, and other more anterior vessels of a different origin; the anterior portion is shown in the figures of the cat, but not of the rabbit. Tlie loop around the ureter is well known from the studies of Tloehstetter. His diagrams (Hertwig's Handhuch, Vol. 3, p. 142) show the extent of the loop more accurately than McMurrich's figure, and correctly indicate the relation of the genital veins to its ventral limb (compare also Anat. Anz., Vol. 25, p. 271). If the term supracardinal vein could be restricted in its application, so as to be synon}anous with dorsal limh of the ureteric loop, it would be more readily adopted. The veins of the limbs are described at greater length than in the previous edition, but without the necessary figures.


==Preface to the First Edition==
Photographs of human red blood corpuscles within a blood vessel. Fixing reagent unknown. Weigert's stain. Those retaining their haemoglobin are cup-shaped. A, X 315 diams. B, X 630 diams.


The assimilation of the enormous mass of facts which constitute
what is usually known as descriptive anatomy has always been a
difficult task for the student. Part of the difficulty has been due to
a lack of information regarding the causes which have determined
the structure and relations of the parts of the body, for without some
knowledge of the why things are so, the facts of anatomy stand as so
many isolated items, while with such knowledge they become bound
together to a continuous whole and their study assumes the dignity
of a science.


The great key to the significance of the structure and relations
In the account of the lymphatic vessels interpretations rather than observations are considered, and thus the " discordant " element is emphasized. A figure of a section through the jugular lympli sac, such as Professor Sabin has published, is much needed, and reconstructions are better than diagrams. Sabin's studies of lymph glands and ]\rairs work on the spleen have been incorporated. The spleen has been transferred from the chapter on mesenteries to that on the circnlatorv system, and as a result of Stoerk's investigations, the coccygeal gland is placed with the lymphoid organs.
of organs is their development, recognizing by that term the historical
as well as the individual development, and the following pages constitute an attempt to present a concise statement of the development
of the human body and a foundation for the proper understanding of
the facts of anatomy. Naturally, the individual development claims
the major share of attention, since its processes are the more immediate forces at work in determining the conditions in the adult, but
where the embryological record fails to afford the required data,  
whether from its actual imperfection or from the incompleteness
of our knowledge concerning it, recourse has been had to the facts of
comparative anatomy as affording indications of the historical development or evolution of the parts under consideration.  


It has not seemed feasible to include in the book a complete list
The description of the entodermal tract now includes Flint's conclusion that the hmg in the pig grows by lateral branching, and that the suppression of the left eparterial bronchus and the development of the right infracardiac bronchus are correlated with the position of the aortic arch and heart respectively.
of the authorities consulted in its preparation. The short bibliographies appended to each chapter make no pretensions to completeness, but are merely indications of some of the more important
works, especially those of recent date, which consider the questions
discussed. For a very full bibliography of all works treating of human embryology up to 1893 reference may be made to Minot's  
Bibliography of Vertebrate Embryology, published in the "Memoirs
of the Boston Society of Natural History," volume iv, 1893. It is
fitting, however, to acknowledge an especial indebtedness, shared
by all writers on human embryology, to the classic papers of His,
chief among which is his Anatomie menschlicher Embryonen, and  
grateful acknowledgments are also due to the admirable text-books of Minot, O. Hertwig, and Kollmann.  


Anatomical Laboratory,  
Tandler's record of the pronephros in human embryos up to 20 mm. is noted, and the development of the ovary and testis liavc been rewritten with reference to Allen's work. Presumably McMurrich hesitates to accept the determination of the large cells appearing in the entoderm and supposedly migrating into the sexual glands, as germ cells; he does not refer to them.


University of Michigan.  
The suprarenal glands liave been redescribed, with a figui'e and references to Wiesel's studios. Tliey are placed in a chapter witli the carotid glands and Zuckerkandl's organs.


Under the nervous system the bearing of Harrison's experiments on the interpretation of sheath cells and on the neurone theory is recorded. Streeter's reconstruction of the otocyst of a 20-mm. human embryo replaces that of His of a similar stage. Fuch's observation in the rabbit that the stapes is at first separate from the second branchial cartilage is regarded as an ontogenetic condition and not of general significance.


The large number of changes in this edition of McMurrich's book reflect the progress of embryology during the last three years. Students will like the small size of the volume, and teachers will appreciate Professor McMurrich's estimate of the value of recent researches.


Frederic T. Lewis.
|}
==Contents==
==Contents==


Introduction 1
Introduction


PART I. - GENERAL DEVELOPMENT.  
Part I. - General Development.


CHAPTER I.


The Spermatozoon and Spermatogenesis; the Ovum and Its Maturation and Fertilization 1 1
[[McMurrich1914 Chapter 1|CHAPTER I. The Spermatozoon and Spermatogenesis; the Ovum and Its Maturation and Fertilization]]


CHAPTER II.  
[[McMurrich1914 Chapter 2|CHAPTER II. The Segmentation of the Ovum and the Formation of the Germ Layers]]


The Segmentation of the Ovum and the Formation of the Germ
[[McMurrich1914 Chapter 3|CHAPTER III. The Medullary Groove, Notochord, and Mesodermic Somites]]


Layers 3&
[[McMurrich1914 Chapter 4|CHAPTER IV. The Development of the External Form of the Human Embryo]]


CHAPTER III.  
[[McMurrich1914 Chapter 5|CHAPTER V. The Yolk-stalk, Belly-stalk, and Fetal Membranes]]


The Medullary Groove, Notochord, and Mesodermic Somites ... 64


CHAPTER IV.  
Part II. -  Organogeny.
The Development of the External Form of the Human Embryo ... 86


CHAPTER V.


The Yolk-stalk, Belly-stalk, and Fetal Membranes 107
[[McMurrich1914 Chapter 6|CHAPTER VI. The Development of the Integumentary System]]


PART II.— ORGANOGENY.  
[[McMurrich1914 Chapter 7|CHAPTER VII. The Development of the Connective Tissues and Skeleton]]


CHAPTER VI.  
[[McMurrich1914 Chapter 8|CHAPTER VIII. The Development of the Muscular System]]
The Development of the Integumentary System 141


CHAPTER VII.  
[[McMurrich1914 Chapter 9|CHAPTER IX. The Development of the Circulatory and Lymphatic Systems]]


The Development of the Connective Tissues and Skeleton . . . 153
[[McMurrich1914 Chapter 10|CHAPTER X. The Development of the Digestive Tract and Glands]]


CHAPTER VIII.  
[[McMurrich1914 Chapter 11|CHAPTER XI. The Development of the Pericardium, the Pleuro-peritoneum, and the Diaphragm]]


The Development of the Muscular System 193
[[McMurrich1914 Chapter 12|CHAPTER XII. The Development of the Organs of Respiration]]


[[McMurrich1914 Chapter 13|CHAPTER XIII. The Development of the Urinogenital System]]


CHAPTER IX.  
[[McMurrich1914 Chapter 14|CHAPTER XIV. The Suprarenal System of Organs]]


The Development of the Circulatory and Lymphatic Systems .. . . 221
[[McMurrich1914 Chapter 15|CHAPTER XV. The Development of the Nervous System]]


CHAPTER X.  
[[McMurrich1914 Chapter 16|CHAPTER XVI. The Development of the Organs of Special Sense]]
The Development of the Digestive Tract and Glands 280


CHAPTER XL
[[McMurrich1914 Chapter 17|CHAPTER XVII. Post-natal Development]]


The Development of the Pericardium, the Pleuro-peritoneum, and


the Diaphragm 316
==Preface to the Fourth Edition==


CHAPTER XII.  
The increasing interest in human and mammalian embryology which has characterized the last few years has resulted in many additions to our knowledge of these branches of science, and has necessitated not a few corrections of ideas formerly held. In this fourth edition of this book the attempt has been made to incorporate the results of all important recent contributions upon the topics discussed, and, at the same time, to avoid any considerable increase in the bulk of the volume. Several chapters have, therefore, been almost entirely recast, and the subject matter has been thoroughly revised throughout, so that it is hoped that the book forms an accurate statement of our present knowledge of the development of the human body.
The Development of the Organs of Respiration 331


CHAPTER XIII.
The Development of the Urinogenital System 338


CHAPTER XIV.  
To several colleagues the author is indebted for valuable suggestions, and in this connection he desires especially to thank Dr. J. C. Watt for much generous assistance in the revision of the manuscript and for undertaking the correction of the proof-sheets.
The Suprarenal System of Organs 370


CHAPTER XV.
The Development of the Nervous System 377


CHAPTER XVI.
In addition to the works mentioned in the preface to the first edition as of special value to the student of Embryology, mention should be made of the Handbuch der vergleichenden mid experimentellen Entwickhmgslehre der Wirbeltiere edited by Professor Oscar Hertwig and especially of the Manual of Human Embryology edited by Professors F. Keibel and F. P. Mall. University of Toronto.
The Development of the Organs of Special Sense 427


CHAPTER XVII.


Post-natal Development 47°
==Preface to the First Edition==


The assimilation of the enormous mass of facts which constitute what is usually known as descriptive anatomy has always been a difficult task for the student. Part of the difficulty has been due to a lack of information regarding the causes which have determined the structure and relations of the parts of the body, for without some knowledge of the why things are so, the facts of anatomy stand as so many isolated items, while with such knowledge they become bound together to a continuous whole and their study assumes the dignity of a science.




==Introduction==
The great key to the significance of the structure and relations of organs is their development, recognizing by that term the historical as well as the individual development, and the following pages constitute an attempt to present a concise statement of the development of the human body and a foundation for the proper understanding of the facts of anatomy. Naturally, the individual development claims the major share of attention, since its processes are the more immediate forces at work in determining the conditions in the adult, but where the embryological record fails to afford the required data, whether from its actual imperfection or from the incompleteness of our knowledge concerning it, recourse has been had to the facts of comparative anatomy as affording indications of the historical development or evolution of the parts under consideration.


Somewhat more than seventy years ago (1839) one of the fundamental principles of biology was established by Schleiden and
Schwann as the cell theory. According to this, all organisms are
composed of one or more structural units termed cells, each of which,
in multicellular organisms, maintains an individual existence and
yet contributes with its fellows to the general existence of the individual. Viewed in the light of this theory, the human body is a
community, an aggregate of many individual units, each of which
leads to a certain extent an independent existence and yet both
contributes to and shares in the general welfare of the community.


To the founders of the theory the structural units were vesicles
It has not seemed feasible to include in the book a complete list of the authorities consulted in its preparation. The short bibliographies appended to each chapter make no pretensions to completeness, but are merely indications of some of the more important works, especially those of recent date, which consider the questions discussed. For a very full bibliography of all works treating of human embryology up to 1893 reference may be made to Minot's Bibliography of Vertebrate Embryology, published in the "Memoirs of the Boston Society of Natural History," volume iv, 1893. It is fitting, however, to acknowledge an especial indebtedness, shared by all writers on human embryology, to the classic papers of His, chief among which is his Anatomie menschlicher Embryonen, and grateful acknowledgments are also due to the admirable text-books of Minot, O. Hertwig, and Kollmann.
with definite walls, and little attention was paid to their contents.
Hence the use of the term "cell" in connection with them. Long
before the establishment of the cell theory, however, the existence
of organisms composed of a gelatinous substance showing no indications of a definite limiting membrane had been noted, and in 1835 a
French naturalist, Dujardin, had described the gelatinous material
of which certain marine organisms (Rhizopoda) were composed,  
terming it sarcode and maintaining it to be the material substratum
which conditioned the various vital phenomena exhibited by the
organisms. Later, in 1846, a botanist, von Mohl, observed that
living plant cells contained a similar substance, upon which he




Anatomical Laboratory,
University of Michigan.


2 INTRODUCTION


believed the existence of the cell as a vital structure was dependent,
and he bestowed upon this substance the name protoplasm, by which
it is now universally known.


By these discoveries the importance originally attributed to the
cell-wall was greatly lessened, and in 1864 Max Schultze reformulated the cell theory, defining the cell as a mass of protoplasm, the
presence or absence of a limiting membrane or cell-wall being
immaterial. At the same time the spontaneous origination of cells
from an undifferentiated matrix, believed to occur by the older
authors, was shown to have no existence, every cell originating by
the division of a preexisting cell, a fact concisely expressed in the
aphorism of Virchow — omnis cellula a cellula.


Interpreted in the light of these results, the human body is an
==Introduction==
aggregate of myriads of cells,* — i. e., of masses of protoplasm, each
of which owes its origin to the division of a preexistent cell and all of
which may be traced back to a single parent cell— a fertilized ovum.
All these cells are not alike, however, but just as in a social community
one group of individuals devotes itself to the performance of one of
the duties requisite to the well-being of the community and another
group devotes itself to the performance of another duty, so too,
in the body, one group of cells takes upon itself one special
function and another another. There is, in other words, in
the cell-community a physiological division of labor. Indeed,
the comparison of the cell-community to the social community may
be carried still further, for just as gradations of individuality may be
recognized in the individual, the municipality, and the state, so too
in the cell-community there are cells; tissues, each of which is an
aggregate of similar cells; organs, which are aggregates of tissues, one,
however, predominating and determining the character of the organ;
and systems, which are aggregates of organs having correlated
functions.


It is the province of embryology to study the mode of division of  
Somewhat more than seventy years ago (1839) one of the fundamental principles of biology was established by Schleiden and Schwann as the cell theory. According to this, all organisms are composed of one or more structural units termed cells, each of which, in multicellular organisms, maintains an individual existence and yet contributes with its fellows to the general existence of the individual. Viewed in the light of this theory, the human body is a community, an aggregate of many individual units, each of which leads to a certain extent an independent existence and yet both contributes to and shares in the general welfare of the community.


* It has been estimated that the number of cells entering into the composition of
the body of an adult human being is about twenty-six million five hundred thousand
millions!


To the founders of the theory the structural units were vesicles with definite walls, and little attention was paid to their contents. Hence the use of the term "cell" in connection with them. Long before the establishment of the cell theory, however, the existence of organisms composed of a gelatinous substance showing no indications of a definite limiting membrane had been noted, and in 1835 a French naturalist, Dujardin, had described the gelatinous material of which certain marine organisms (Rhizopoda) were composed, terming it sarcode and maintaining it to be the material substratum which conditioned the various vital phenomena exhibited by the organisms. Later, in 1846, a botanist, von Mohl, observed that living plant cells contained a similar substance, upon which he believed the existence of the cell as a vital structure was dependent, and he bestowed upon this substance the name protoplasm, by which it is now universally known.




By these discoveries the importance originally attributed to the cell-wall was greatly lessened, and in 1864 Max Schultze reformulated the cell theory, defining the cell as a mass of protoplasm, the presence or absence of a limiting membrane or cell-wall being immaterial. At the same time the spontaneous origination of cells from an undifferentiated matrix, believed to occur by the older authors, was shown to have no existence, every cell originating by the division of a preexisting cell, a fact concisely expressed in the aphorism of Virchow  -  ''omnis cellula a cellula''.


INTRODUCTION 3


the fertilized ovum and the progressive differentiation of the resulting
Interpreted in the light of these results, the human body is an aggregate of myriads of cells,*  -  i. e., of masses of protoplasm, each of which owes its origin to the division of a preexistent cell and all of which may be traced back to a single parent cell -  a fertilized ovum. All these cells are not alike, however, but just as in a social community one group of individuals devotes itself to the performance of one of the duties requisite to the well-being of the community and another group devotes itself to the performance of another duty, so too, in the body, one group of cells takes upon itself one special function and another another. There is, in other words, in the cell-community a physiological division of labor. Indeed, the comparison of the cell-community to the social community may be carried still further, for just as gradations of individuality may be recognized in the individual, the municipality, and the state, so too in the cell-community there are cells; tissues, each of which is an aggregate of similar cells; organs, which are aggregates of tissues, one, however, predominating and determining the character of the organ; and systems, which are aggregates of organs having correlated functions.
cells to form the tissues, organs, and systems. But before considering these phenomena as seen in the human body it will be well to get
some general idea of the structure of an animal cell.  


This (Fig. i), as has been already stated, is a mass of protoplasm,
* It has been estimated that the number of cells entering into the composition of the body of an adult human being is about twenty-six million five hundred thousand millions!
a substance which in the living condition is a viscous fluid resembling
in many of its peculiarities egg-albumen, and like this being coagulated when heated or when exposed to
the action of various chemical reagents.
As to the structure of living protoplasm
little is yet known, since the application
of the reagents necessary for its accurate
study and analysis results in its disintegration or coagulation. But even in
the living cell it can be seen that the Fig. i.— Ovum of New-born
protoplasm is not a simple homogeneous ?^ IL r ? WI n TH Follicle - cells ~
substance. What is termed a nucleus is


usually clearly discernible as a more or less spherical body of a
greater refractive index than the surrounding protoplasm, and since
this is a permanent organ of the .cell it is convenient to distinguish
the surrounding protoplasm as the cytoplasm from the nuclear
protoplasm or karyoplasm.


The study of protoplasm coagulated by reagents seems to indicate that it is a mixture of substances rather than a simple chemical
It is the province of embryology to study the mode of division of the fertilized ovum and the progressive differentiation of the resulting cells to form the tissues, organs, and systems. But before considering these phenomena as seen in the human body it will be well to get some general idea of the structure of an animal cell.
compound. Both the cytoplasm and the karyoplasm consist of a
more solid substance, the reticulum, which forms a network or feltwork, in the interstices of which is a more fluid material, the enchylema* The karyoplasm, in addition, has scattered along the fibers
of its reticulum a peculiar material termed chromatin and usually
contains embedded in its substance one or more spherical bodies


* It has been observed that certain coagulable substances and gelatin, when subjected to the reagents usually employed for "fixing" protoplasm, present a structure
similar to that of protoplasm, and it has been held that protoplasm in the uncoagulated
condition is, like these substances, a more or less homogeneous material. On the
other hand, Biitschli maintains that living protoplasm has a foam-structure and is,
in other words, an emulsion.




This (Fig. i), as has been already stated, is a mass of protoplasm, a substance which in the living condition is a viscous fluid resembling in many of its peculiarities egg-albumen, and like this being coagulated when heated or when exposed to the action of various chemical reagents. As to the structure of living protoplasm little is yet known, since the application of the reagents necessary for its accurate study and analysis results in its disintegration or coagulation. But even in the living cell it can be seen that the Fig. i. -  Ovum of New-born protoplasm is not a simple homogeneous ?^ IL r ? WI n TH Follicle - cells ~ substance. What is termed a nucleus is
usually clearly discernible as a more or less spherical body of a greater refractive index than the surrounding protoplasm, and since this is a permanent organ of the .cell it is convenient to distinguish the surrounding protoplasm as the cytoplasm from the nuclear protoplasm or karyoplasm.


4 INTRODUCTION


termed nucleoli, which may be simply larger masses of chromatin or  
The study of protoplasm coagulated by reagents seems to indicate that it is a mixture of substances rather than a simple chemical compound. Both the cytoplasm and the karyoplasm consist of a more solid substance, the reticulum, which forms a network or feltwork, in the interstices of which is a more fluid material, the enchylema* The karyoplasm, in addition, has scattered along the fibers of its reticulum a peculiar material termed chromatin and usually contains embedded in its substance one or more spherical bodies termed nucleoli, which may be simply larger masses of chromatin or bodies of special chemical composition. And, finally, in all actively growing cells there is differentiated in the cytoplasm a peculiar body known as the archo plasm sphere, in the center of which there is usually a minute spherical body known as the centrosome.
bodies of special chemical composition. And, finally, in all actively  
growing cells there is differentiated in the cytoplasm a peculiar body  
known as the archo plasm sphere, in the center of which there is  
usually a minute spherical body known as the centrosome.  


It has been already stated that new cells arise by the division of
* It has been observed that certain coagulable substances and gelatin, when subjected to the reagents usually employed for "fixing" protoplasm, present a structure similar to that of protoplasm, and it has been held that protoplasm in the uncoagulated condition is, like these substances, a more or less homogeneous material. On the other hand, Biitschli maintains that living protoplasm has a foam-structure and is, in other words, an emulsion.
preexisting ones, and this process is associated with a series of complicated phenomena which have great significance in connection with
some of the problems of embryology. When such a cell as has been
described above is about to divide, the fibers of the reticulum in
the neighborhood of the archoplasm sphere arrange themselves so as
to form fibrils radiating in all directions from the sphere as a center,
and the archoplasm with its contained centrosome gradually elongates and finally divides, each portion retaining its share of the radiating fibrils, so that two asters, as the aggregate of centrosome, sphere
and fibrils is termed, are now to be found in the cytoplasm (Fig. 2, A) .
Gradually the two asters separate from one another and eventually
come to rest at opposite sides of the nucleus (Fig. 2, C). In this
structure important changes have been taking place in the meantime. The chromatin, originally scattered irregularly along the
reticulum, has gradually aggregated to form a continuous thread
(Fig. 2, A), and later this thread breaks up into a definite number
of pieces termed chromosomes (Fig. 2, B), the number of these being
practically constant for each species of animal. In man the number
has been placed at twenty-four (Flemming, Duesberg) , but the recent
observations of Guyer indicate that it is probably twenty-four in the
female and twenty-two in the male. The significance of this difference in the two sexes will be considered in connection with the
fertilization of the ovum (p. 32).  


As soon as the asters have taken up their position on opposite
sides of the nucleus, the nuclear reticulum begins to be converted
into a spindle-shaped bundle of fibrils which associate themselves
with the astral rays and have lying scattered among them the chromosomes (Fig. 2, C). To the figure so formed the term amphiaster is
applied, and soon after its formation the chromosomes arrange


It has been already stated that new cells arise by the division of preexisting ones, and this process is associated with a series of complicated phenomena which have great significance in connection with some of the problems of embryology. When such a cell as has been described above is about to divide, the fibers of the reticulum in the neighborhood of the archoplasm sphere arrange themselves so as to form fibrils radiating in all directions from the sphere as a center, and the archoplasm with its contained centrosome gradually elongates and finally divides, each portion retaining its share of the radiating fibrils, so that two asters, as the aggregate of centrosome, sphere and fibrils is termed, are now to be found in the cytoplasm (Fig. 2, A). Gradually the two asters separate from one another and eventually come to rest at opposite sides of the nucleus (Fig. 2, C). In this structure important changes have been taking place in the meantime. The chromatin, originally scattered irregularly along the reticulum, has gradually aggregated to form a continuous thread (Fig. 2, A), and later this thread breaks up into a definite number of pieces termed chromosomes (Fig. 2, B), the number of these being practically constant for each species of animal. In man the number has been placed at twenty-four (Flemming, Duesberg) , but the recent observations of Guyer indicate that it is probably twenty-four in the female and twenty-two in the male. The significance of this difference in the two sexes will be considered in connection with the fertilization of the ovum (p. 32).




INTRODUCTION
As soon as the asters have taken up their position on opposite sides of the nucleus, the nuclear reticulum begins to be converted into a spindle-shaped bundle of fibrils which associate themselves with the astral rays and have lying scattered among them the chromosomes (Fig. 2, C). To the figure so formed the term amphiaster is applied, and soon after its formation the chromosomes arrange themselves in a circle or plane at the equator of the spindle (Fig. 2, D) and the stages preparatory to the actual division, the prophases, are completed.




The next stage, the metaphase (Fig. 3, A), consists of the division, usually longitudinally, of each chromosome, so that the cell now contains twice as many chromosomes as it did previously. As soon as this division is completed the anaphases are inaugurated by the halves of each chromosome separating from one another and approaching one of the asters (Fig. 3, B), and a group of chromosomes, containing half the total number formed in the metaphase, comes to lie in close proximity to each archoplasm sphere (Fig. 3, C). The spindle and astral fibers gradually resolve themselves again into the reticulum and the chromosomes of each group become irregular in shape and gradually spread out upon the nuclear reticulum so that •two nuclei, each similar to the one from which the process started,


5


Fig. 2.  -  Diagrams Illustrating the Prophases of Mitosis.  -  (Adapted from E. B. Wilson.)




themselves in a circle or plane at the equator of the spindle (Fig. 2, D)
and the stages preparatory to the actual division, the prophases, are
completed.


The next stage, the metaphase (Fig. 3, A), consists of the division,  
Fig. 3.  -  Diagrams Illustrating the Metaphase and Anaphases of Mitosis.:  -  (Adapted from E. B. Wilson.)
usually longitudinally, of each chromosome, so that the cell now
are formed (Fig. 3, D). Before all these changes are accomplished, however, a constriction makes its appearance at the surface of the cytoplasm (Fig. 3, C) and, gradually deepening, divides the cytoplasm in a plane passing through the equator of the amphiaster and gives rise to two separate cells (Fig. 3, D).




This complicated process, which is known as karyokinesis or mitosis, is the one usually observed in dividing cells, but occasionally a cell divides by the nucleus becoming constricted and dividing into two parts without any development of chromosomes, spindle, etc., the division of the cell following that of the nucleus. This amitotic method of division is, however, rare, and in many cases, though not always, its occurrence seems to be associated with an impairment of the reproductive activities of the cells. In actively reproducing cells the mitotic method of division may be regarded as the rule.




 
Since the process of development consists of the multiplication of a single original cell and the differentiation of the cell aggregate so formed, it follows that the starting-point of each line of individual development is to be found in a cell which forms part of an individual of the preceding generation. In other words, each individual represents one generation in esse and the succeeding generation in posse. This idea may perhaps be made clear by the following considerations. As a result of the division of a fertilized ovum there is produced an aggregate of cells, which, by the physiological division of labor, specialize themselves for various functions. Some assume the duty of perpetuating the species and are known as the sexual or germ cells, while the remaining ones divide among themselves the various functions necessary for the maintenance of the individual, and may be termed the somatic cells. The germ cells represent potentially the next generation, while the somatic cells constitute the present one. The idea may be represented schematically thus:  
Fig. 2. — Diagrams Illustrating the Prophases of Mitosis. — {Adapted from
 
E. B. Wilson.)
 
 
 
contains twice as many chromosomes as it did previously. As soon
as this division is completed the anaphases are inaugurated by the
halves of each chromosome separating from one another and approaching one of the asters (Fig. 3, B), and a group of chromosomes,
containing half the total number formed in the metaphase, comes to
 
 
 
6 INTRODUCTION
 
lie in close proximity to each archoplasm sphere (Fig. 3, C). The
spindle and astral fibers gradually resolve themselves again into the
reticulum and the chromosomes of each group become irregular in
shape and gradually spread out upon the nuclear reticulum so that
•two nuclei, each similar to the one from which the process started,
 
 
 
 
 
 
 
Fig. 3. — Diagrams Illustrating the Metaphase and Anaphases of Mitosis.: —
{Adapted from E. B. Wilson.)
 
are formed (Fig. 3, D). Before all these changes are accomplished,
however, a constriction makes its appearance at the surface of the
cytoplasm (Fig. 3, C) and, gradually deepening, divides the cytoplasm in a plane passing through the equator of the amphiaster and
gives rise to two separate cells (Fig. 3, D).
 
 
 
INTRODUCTION 7
 
This complicated process, which is known as karyokinesis or
mitosis, is the one usually observed in dividing cells, but occasionally
a cell divides by the nucleus becoming constricted and dividing into
two parts without any development of chromosomes, spindle, etc.,
the division of the cell following that of the nucleus. This amitotic method of division is, however, rare, and in many cases, though
not always, its occurrence seems to be associated with an impairment
of the reproductive activities of the cells. In actively reproducing
cells the mitotic method of division may be regarded as the rule.
 
Since the process of development consists of the multiplication of  
a single original cell and the differentiation of the cell aggregate so  
formed, it follows that the starting-point of each line of individual  
development is to be found in a cell which forms part of an individual  
of the preceding generation. In other words, each individual  
represents one generation in esse and the succeeding generation in  
posse. This idea may perhaps be made clear by the following considerations. As a result of the division of a fertilized ovum there is  
produced an aggregate of cells, which, by the physiological division of  
labor, specialize themselves for various functions. Some assume  
the duty of perpetuating the species and are known as the sexual  
or germ cells, while the remaining ones divide among themselves the  
various functions necessary for the maintenance of the individual,  
and may be termed the somatic cells. The germ cells represent  
potentially the next generation, while the somatic cells constitute the  
present one. The idea may be represented schematically thus:  


First generation  
First generation  
Line 402: Line 211:
Somatic cells + germ cells  
Somatic cells + germ cells  


II  
II Second generation  
Second generation  


Somatic cells + germ cells  
Somatic cells + germ cells  


II  
II Third generation  
Third generation  
 
 
 
Somatic cells + germ cells, etc.
 
It is evident, then, while the somatic cells of each generation die
at their appointed time and are differentiated anew for each genera
 
 
8 INTRODUCTION
 
tion from the germ cells, the latter, which may be termed collectively
the germ-plasm, are handed on from generation to generation without
interruption, and it may be supposed that this has been the case ab
initio. This is the doctrine of the continuity of the germ-plasm, a
doctrine of fundamental importance on account of its bearings on
the phenomena of heredity.
 
It is necessary, however, to fix upon some link in the continuous
chain of the germ-plasm as the starting-point of the development
of each individual, and this link is the fertilized ovum. By this is
meant a germ cell produced by the fusion of two units of the germplasm. In many of the lower forms of life (e.g., Hydra and certain
turbellarian worms) reproduction may be accomplished by a division
of the entire organism into two parts or by the separation of a portion
of the body from the parent individual. Such a method of reproduction is termed non-sexual. Furthermore in a number of forms
(e. g., bees, Phylloxera, water-fleas) the germ cells are able to undergo
development without previously being fertilized, this constituting
a method of reproduction known as parthenogenesis. But in all
these cases sexual reproduction also occurs, and in all the more highly
organized animals it is the only method that normally occurs; in it a
germ cell develops only after complete fusion with another germ cell.
In the simpler forms of this process little difference exists between
the two combining cells, but since it is, as a rule, of advantage that
a certain amount of nutrition should be stored up in the germ cells
for the support of the developing embryo until it is able to secure food
for itself, while at the same time it is also advantageous that the cells
which unite shall come from different individuals (cross-fertilization),
and hence that the cells should retain their motility, a division of
labor has resulted. Certain germ cells store up more or less food
yolk, their motility becoming thereby impaired, and form what are
termed the female cells or ova, while otners discard all pretensions of
storing up nutrition, are especially motile and can seek and penetrate the inert ova; these latter cells constitute the male cells or
spermatozoa. In many animals both kinds of cells are produced by
the same individual, but in all the vertebrates (with rare exceptions
 
 
 
INTRODUCTION 9
 
in some of the lower orders) each individual produces only ova or
spermatozoa, or, as it is generally stated, the sexes are distinct.
It is of importance, then, that the peculiarities of the two forms
of germ cells, as they occur in the human species, should be considered.
 
LITERATURE.
 
E. B. Wilson: "The Cell in Development and Inheritance." Third edition. New
 
York, 1900.
O. Hertwig: "Die Zelle und die Gewebe." Jena, 1893.
 
 
 
PART I.
 
GENERAL DEVELOPMENT.
 
 
 
CHAPTER I.
 
 
 
THE SPERMATOZOON AND SPERMATOGENESIS; THE
 
OVUM AND ITS MATURATION AND
 
FERTILIZATION.
 
The Spermatozoon. — The human spermatozoon (Figs. 4 and 5)
is a minute and greatly elongated cell, measuring about 0.05 mm. in
length. It consists of an anterior broader portion or head (Fig. 5, H) ,
which measures about 0.005 mm - i n length and, when viewed from
one surface (Fig. 4, 1), has an oval outline, though since it is somewhat flattened or concave toward the tip, it has a pyriform shape
when seen in profile (Fig. 4, 2). Covering the flattened portion of
the head and fitting closely to it is a delicate cap-like membrane,
the head-cap (Fig. 5, He), whose apex is a sharp edge, this structure
corresponding to a pointed prolongation of the cap found in the
spermatozoon of many of the lower vertebrates and known as the
perforatorium. Immediately behind the head is a short portion
known as the neck (Fig. 5, N), which consists of an upper more
refractive body, the anterior nodule, and a lower clearer portion.
To this succeeds the connecting or middle-piece (Figs. 4 and 5, m)
which begins with a posterior nodule, from the center of which there
passes back through the axis of the piece an axial filament, enclosed
within a sheath, this latter having wrapped around it a spiral filament. At the lower end of the middle-piece this spiral filament
terminates in the annulus, through which the axial filament and its
sheath passes into the jiagellum or tail (Fig. 4,/). This portion,
 
 
 
12
 
 
 
THE SRERMATOZOON
 
 
 
which constitutes about four-fifths of the total length of the spermatozoon is composed simply of the axial filament and its sheath,
this latter gradually thinning out as it passes backward and ceasing
altogether a short distance above the end of the axial filament.
 
 
 
 
H. {
 
 
 
N.
 
 
 
M.
 
 
 
 
Fig. 4. — Human Spermatozoon.
1, Front view; 2, side view of the
head; e, terminal filament; k, head;
/, tail; m, middle-piece. — (After
Retzius.)
 
 
 
Fig. 5. — Diagram Showing the Structure
of a Human Spermatozoon.
 
Af, Axial filament; Ann, annulus; H, head;
He, lower border of head -cap; m, middle- piece;
N, neck; Na and Np, anterior and posterior
nodule; S, sheath of axial filament; Spf, spiral
filament. — (Bonnet, after Meves.)
 
 
 
The filament thus projects somewhat beyond the actual end of the
tail, forming what is known as the terminal filament or end-piece
(Fig. 4, e).
 
To understand the significance of the Various parts entering into
the composition of the spermatozoon a study of their development
is necessary, and since the various processes of spermatogenesis have
been much more accurately observed in such mammalia as the rat
 
 
 
SPERMATOGENESIS
 
 
 
13
 
 
 
and guinea-pig than in man, the description which follows will be
based on what has been described as occurring in these forms.
From what is known of the spermatogenesis in man it seems certain
that it closely resembles that of these mammals so far as its essential
features are concerned.
 
Spermatogenesis. — The spermatozoa are developed from the
cells which line the interior of the seminiferous tubules of the testis.
The various stages of development cannot all be seen at any one
part of a tubule, but the formation of the spermatozoa seems to pass
 
 
 
 
Fig. 6. — Diagram showing Stages of Spermatogenesis as seen in Different
Sectors of a Seminiferous Tubule of a Rat.
s, Sertoli cell; sc l , spermatocyte of the first order; sc 2 , spermatocyte of the second
order; sg, spermatogone; sp, spermatid; sz, spermatozoon. — (Modified from von
Lenhossek.)
 
 
 
along each tubule in a wave-like manner and the appearances presented at different points of the wave may be represented diagrammatically as in Fig. 6.
 
In the first section of this figure four different generations of
cells are represented; above are mature spermatozoa lying in the
lumen of the tubule, while next the basement membrane is a series
of cells from which a new generation of spermatozoa is about to
develop. The cells of this series are of two kinds; the larger one (s)
 
 
 
14 SPERMATOGENESIS
 
will develop into a structure known as a Sertoli cell, while the others
are parent cells of spermatozoa and are termed spermatogonia (sg).
In the next section the Sertoli cell is seen to have become considerably enlarged, its cytoplasm projecting toward the lumen of the tubule,
and in the third section the enlargement has increased to such an
extent that the spermatogonia are forced away from the basement
membrane, with which the Sertoli cell alone is in contact. In the
fourth section ("he spermatogonia are seen in process of division;
one of the cells so formed will persist as a spermatogone, while the
other forms what is termed a primary spermatocyte (sc 1 ). The
results of the division are seen in the last section, where four spermatogonia are seen again in contact with the basement membrane
and above them are four primary spermatocytes. Returning now
to the first and second sections, the layer of primary spermatocytes
may still be seen, indications of an approaching division being
furnished by the arrangement of the chromatin in those of the
second section, and in the third section the division is seen in progress, the two cells which result from it being termed secondary
spermatocytes (sc 2 ). These cells almost immediately undergo
division, as shown in the fourth section, each giving rise to two
spermatids (sp), each of which becomes later on directly transformed into a spermatozoon (sz). From each primary spermatocyte
there have been formed, therefore, as the result of two mitoses, four
cells, each of which represents a spermatozoon.
 
During these divisions important departures from the typical
method of mitosis occur, these departures leading to a reduction of
the chromosomes in each spermatid to one-half the number occurring
in the somatic cells. The general plan by which this is accomplished
may be described as follows: In the division of the spermatogonia
the number of chromosomes that appears is identical with that found
in the somatic cells, so that in a form whose somatic number is eight,
eight chromosomes appear in each spermatogonium, and divide so
that eight pass to each of the resulting primary spermatocytes.
When these cells divide, however, the number of chromosomes that
appears is only one-half the somatic number, namely, four in the
 
 
 
SPERMATOGENESIS
 
 
 
15
 
 
 
supposed case that is being described (Fig. 7, sc 1 ). The further
history of these chromosomes indicates that each is composed of
four elements more or less closely united to form a tetrad, and during
mitosis each tetrad divides into two dyads, four of which will therefore pass into each secondary spermatocyte. These cells (Fig. 7, sc 2 )
 
 
 
 
Fig. 7. — Diagram Illustrating the Reduction of the Chromosomes During
 
Spermatogenesis.
sc 1 , Spermatocyte of the first order; sc 2 , spermatocyte of the second order; sp,
 
spermatid.
 
 
 
undergo division without the usual reconstruction of the nucleus and
each of the dyads which they contain is halved, so that each spermatid receives a number of single chromosomes equal to half the
number characteristic for the species (Fig. 7, sp).
 
This account of the behavior of the chromosomes during sper
 
 
1 6 SPERMATOGENESIS
 
matogenesis assumes that all the chromosomes of the primary
spermatocytes are of equal value and behave similarly during
mitosis. It has been found, however, that in a number of forms
(insects, spiders, birds, etc.,) this is not the case and recent observations by Guyer indicate that in man certain of the spermatocytic
chromosomes differ decidedly from their fellows. At the division
of the primary spermatocytes twelve chromosomes make their
appearance, but two of these differ from the rest in that they do not
divide, but pass directly to one of the poles of the mitotic spindle
(Fig. 8). When the division is completed, accordingly, one of the
two daughter secondary spermatocytes will have received two
undivided or accessory chromosomes plus ten ordinary chromosomes,
resulting from the division of ten of the primary spermatocytic
chromosomes; the other daughter cell, on the other hand, will have
received only ten ordinary chromosomes in all, so that two classes of
secondary spermatocytes are formed, in one of which the cells
possess twelve chromosomes and in the other only ten.
 
In this respect, then, the spermatogenesis in man differs from the
general plan described above and the division of the secondary
spermatocytes reveals a second difference. For in these mitoses
instead of twelve and ten chromosomes, seven and five, respectively,
make their appearance. This may be explained on the supposition
that the ten ordinary chromosomes, present in each class of secondary
spermatocytes, have united to form five bivalent chromosomes,
while the two accessory chromosomes, present in one of the classes
have remained distinct. During the mitosis the accessory chromosomes divide just as do the ordinary ones, so that from each spermatocyte of one class two spermatids are formed, each containing seven
chromosomes, while from each spermatocyte of the other class two
spermatids, each containing five chromosomes, result (Fig. 8).
Since the spermatids are directly transformed into spermatozoa,
half of these latter will have received seven chromosomes, and the
remaining half will have received five, or, since the five ordinary
chromosomes are bivalent and the two accessories are univalent, the
spermatozoa of one class will each have received the equivalent of
 
 
 
SPERMATOGENESIS 1 7
 
ten plus two, i. e., twelve univalent chromosomes, while those of
the other class will have received the equivalent of only ten.*
 
The transformation of the spermatids into spermatozoa takes
place while they are in intimate association with the Sertoli cells,
a number of them fusing with the cytoplasm of an enlarged Sertoli
cell, as shown in Fig. 6, s, and probably receiving nutrition from it.
In each spermatid there is present in addition to the nucleus, an
 
 
 
 
Fig. 8. — Diagram Illustrating the Behavior of the Chromosomes in Human
 
Spermatogenesis.
The upper figure shows the mitotic spindle of a primary spermatocyte with the two
accessory chromosomes passing to one pole. The two figures in the second row represent the chromosomes of such a spindle in an anaphase ; seen from either pole, and the
figures of the last row represent spermatids derived from the two classes of secondary
spermatocytes. — (Based on Guyer.)
 
archoplasm sphere and two centrosom.es that have migrated from
the archoplasm and lie free in the cytoplasm. The centrosomes
and the archoplasm sphere take up their position at opposite poles
of the nucleus, the archoplasm eventually forming the head-cap of the
spermatozoon, and from one of the centrosomes a slender axial
 
* Doubt has been thrown upon the accuracy of these observations by Gutherz, who,
while he finds a structure in the human spermatocyte which he identifies as an accessor)'
chromosome, claims that it divides similarly to the other chromosomes. He does not
find, therefore, any numerical difference in the chromosomes of the spermatids dividing
them into two classes, although there may be qualitative differences indistinguishable
by our present technique.
 
 
 
15 SPEEMATOGENESIS
 
filament grows out and soon projects beyond the limits of the cytoplasm (Fig. g, A). The other centrosome becomes a rod-shaped
structure which applies itself closely to the posterior pole of the
nucleus, becoming the anterior nodule, while the lower one, from
which the filament arises, becomes at first pyramidal in shape
(Fig. 9, B) and later separates into a rod-like portion to which the
filament is attached and a ring, through which the filament passes
(Fig. 9, C). The rod-like portion becomes the posterior nodule,
 
 
 
 
 
 
ABC
 
Fig. g. — Stages in the Transformation of a Spermatid into a
Spermatozoon. — (After Meves.)
 
 
 
and the ring separates from it to form the annulus (Fig. g,D). The
nucleus becomes the head of the spermatozoon, the cytoplasm surrounding it becoming reduced to an exceedingly delicate layer, so
that the head is composed almost entirely of nuclear substance, if
the head-cap be left out of consideration. The spiral filament of
the middle-piece is, however, a derivative of the cytoplasm and
according to some authors this portion of the spermatid also furnishes the material for the sheath of the axial filament, though
this has been denied (Meves), the sheath being regarded as a differentiation of the axial filament. Each spermatozoon is, then, one
of four equivalent cells, produced by two successive divisions of a
primary spermatocyte and containing one-half the number of chromosomes characteristic for the species.
 
 
 
THE OVUM 19
 
The number of spermatozoa produced during the lifetime of a
single individual is very large. It has been found that 1 cu. mm. of
human ejaculate contains 60,876 spermatozoa, a single ejaculate,
therefore, containing over 200,000,000. This would indicate that
during his lifetime a man may produce 340 billion spermatozoa
(Lode).
 
The Ovum. — The human ovum is a spherical cell measuring
about 0.2 mm. in diameter and is contained within a cavity situated
 
 
 
 
 
 
 
 
 
 
-dp
 
.0
 
 
 
mgr — — %
 
 
 
Fig. 10. — Section through Portion of an Ovary of an Opossum {Didephys vir
giniana) showing Ova and Follicles in Various Stages of Development.
b, Blood-vessel; dp, discus proligerus; mg, stratum granulosum; o, ovum; s, stroma;
 
th, theca folliculi.
 
near or at the surface of the ovary and termed a Graafian follicle.
This follicle is surrounded by a capsule composed of two layers, an
outer one, the theca externa, consisting of fibrous tissue resembling
that found in the ovarian stroma, and an inner one, the theca interna,
composed of numerous spherical and fusiform cells. Both the
 
 
 
20 THE OVUM
 
thecse are richly supplied with blood-vessels, the theca interna
especially being the seat of a very rich capillary network. Internal
to the theca interna there is a transparent, thin, and structureless
hyaline membrane, within which is the follicle proper, whose wall is
formed by a layer of cells termed the stratum granulosum (Fig. 10, mg)
and inclosing a cavity filled with an albuminous fluid, the liquor
 
,^ - ■'• — ■: '■':.):- i--. ■.,■■-■'■' - -.-€>) / Z P
 
 
 
V 1
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. ii. — Ovum from Ovary of a Woman Thirty Years of Age.
 
cr, Corona radiata; n, nucleus; p, protoplasmic zone of ovum; ps, perivitelline space;
 
y, yolk; zp, zona pellucida. — (Nagel.)
 
folliculi. At one point, usually on the surface nearest the center
of the ovary, the stratum granulosum is greatly thickened to form a
mass of cells, the discus proligerus {dp), which projects into the
cavity of the follicle and encloses the ovum (0) . Usually but a single
ovum is contained in any discus, though occasionally two or even
three may occur.
 
 
 
OVULATION AND THE CORPUS LUTEUM 21
 
The cells of the discus proligerus are for the most part more or
less spherical or ovoid in shape and are arranged irregularly. In
the immediate vicinity of the ovum, however, they are more columnar
in form and are arranged in about two concentric rows, thus giving
a somewhat radiated appearance to this portion of the discus, which
is termed the corona radiata (Fig. u, cr). Immediately within the
corona is a transparent membrane, the zona pellucida (Fig. n, zp),
about as thick as one of the cell rows of the corona (0.02 to 0.024 mm.) ,
and presenting a very fine radial striation which has been held to be
due to minute pores traversing the membrane and containing delicate
prolongations of the cells of the corona radiata. Within the zona
pellucida is the ovum proper, whose cytoplasm is more or less clearly
differentiated into an outer more purely protoplasmic portion
(Fig. n, p) and an inner mass (y) which contains numerous fine
granules of fatty and albuminous natures. These granules represent
the food yolk or deutoplasm, which is usually much more abundant
in the ova of other mammals and forms a mass of relatively enormous
size in the ova of birds and reptiles. The nucleus (n) is situated
somewhat excentrically in the deutoplasmic portion of the ovum and
contains a single, well-defined nucleolus.
 
A follicle with the structure described above and containing a
fully grown ovum may measure anywhere from five to twelve millimeters in diameter, and is said to be "mature," having reached its
full development and being ready to burst and set free the ovum.
This, however, is not yet mature; it is not ready for fertilization, but
must first undergo certain changes similar to those through which
the spermatocyte passes, the so-called ovum at this stage being more
properly a primary oocyte. But before describing the phenomena of
maturation of the ovum it will be well to consider the extrusion of
the ovum and the changes which the follicle subsequently undergoes.
 
Ovulation and the Corpus Luteum.— As a rule, but a single
follicle near maturity is found in either the one or the other ovary
at any given time. In the early stages of its development a follicle
is situated somewhat deeply in the stroma of the ovary, but during
its growth it approaches the surface and eventually forms a marked
 
 
 
22
 
 
 
OVULATION AND THE CORPUS LUTEUM
 
 
 
prominence, only an exceedingly thin membrane separating the
cavity of the follicle from the abdominal cavity. This thin membrane finally ruptures, and the liquor folliculi, which is apparently
under some pressure while contained within the follicle, rushes out
through the rupture, carrying with it the ovum surrounded by some
of the cells of the discus proligerus.
 
The immediate cause of the bursting of the follicle is not yet
clearly understood. It has been suggested that a gradual increase
of the liquor folliculi under pressure must in itself finally lead to a
rupture, and it has also been pointed out that just before the maturation of the follicle the theca interna undergoes an exceedingly rapid
development and vascularization which may play an important part
in the phenomenon.
 
Normally the ovum when expelled from its follicle is received at
once into the Fallopian tube, and so makes its way to the uterus, in
 
whose cavity it undergoes its development. Occasionally, however, this normal course may be
interfered with, the ovum coming
to rest in the tube and there
undergoing its development and
producing a tubal pregnancy;
or, again, the ovum may not find
its way into the Fallopian tube,
but may fall from the follicle
into the abdominal cavity,
where, if it has been fertilized,
it will undergo development,
producing an abdominal pregnancy; and, finally, and still more rarely, the ovum may not be
expelled when the Graafian follicle ruptures and yet may be
fertilized and undergo its development within the follicle, bringing
about what is termed an ovarian pregnancy. All these varieties
of extra-uterine pregnancy are, of course, exceedingly serious, since
in none of them is the fetus viable.
 
 
 
 
Fig. 12. — Ovary of a Woman Nineteen Years of Age, Eight Days after
Menstruation.
 
d, Blood-clot; /, Graaffian follicle; th,
theca. — (Kollmann.)
 
 
 
OVULATION AND THE CORPUS LUTEUM
 
 
 
2 3
 
 
 
With the setting free of the ovum the usefulness of the Graafian
follicle is at an end, and it begins at once to undergo retrogressive
changes which result primarily in the formation of a structure
known as the corpus luteiim (Fig. 12). On the rupture of the follicle
 
 
 
 
Fig. 13. — Section through the Corpus Luteum of a Rabbit, Seventy Hours
 
post coitum.
The cavity of the follicle is almost completely filled with lutein cells among which
is a certain amount of connective tissue, g, Blood-vessels; ke, ovarial epithelium. —
(Sobotta.)
 
a considerable portion of the stratum granulosum remains in place,
and usually there is an effusion of a greater or less amount of blood
from the vessels of the theca interna into the follicular cavity. The
split in the wall of the follicle through which the ovum escaped soon
closes over and the cavity becomes filled with cells separated into
groups by trabecular of connective tissue containing blood-vessels
(Fig. 13). These cells contain a considerable amount of a peculiar
 
 
 
24 OVULATION AND THE CORPUS LUTEUM
 
yellow pigment known as lutein, the color imparted to the follicle
by this substance having suggested the name corpus luteum which
is now applied to it.
 
In later stages there is a gradual increase in the amount of connective tissue present and a corresponding diminution of the lutein
cells, the corpus luteum gradually losing its yellow color and becoming converted into a whitish, fibrous, scar-like body, the corpus
albicans, which may eventually almost completely disappear. These
various changes occur in every ruptured follicle, whether or not the
ovum which was contained in it be fertilized. But the rapidity
with which the various stages of retrogression ensue differs greatly
according to whether pregnancy occurs or not, and it is customary
to distinguish the corpora lutea which are associated with pregnancy
as corpora lutea vera from those whose ova fail to be fertilized and
which form corpora lutea spuria. In the latter the retrogression of
the follicle is completed usually in about five or six weeks, while the
corpora vera persist throughout the entire duration of the pregnancy
and complete their retrogression after the birth of the child.
 
Two very different views are held as to the origin of the lutein
cells. According to one, which may be termed von Baer's view,
the cells of the stratum granulosum remaining in the follicle rapidly
undergo degeneration and completely disappear, and the lutein cells
and connective-tissue trabecular are formed entirely from the cells of
the theca interna, which increase rapidly both in size and number.
The other view was first advanced by Bischoff and may be known
by his name. It is to the effect that the granulosa cells do not disintegrate, but, on the contrary, increase rapidly in number and become converted into the lutein cells, only the connective tissue and
the blood-vessels being derived from the theca interna.
 
Which of these two views is correct is at present uncertain.
The majority of those who have within recent years studied the
formation of the human corpus luteum have expressed themselves
in favor of von Baer's theory. Sobotta has, however, made a
thorough study of the phenomena in a perfect series of mice ovaries
and has demonstrated that in that form the lutein cells are derived
 
 
 
OVULATION AND THE CORPUS LUTEUM 25
 
from the granulosa cells. It would be strange if the lutein cells had
a different origin in two different mammals, and the observations on
mice are so thorough that one is tempted to regard different results
as being due to imperfections in the series of ovaries studied,
important steps in the development of the corpora lutea being thus
overlooked. This temptation is, moreover, greatly increased by the
fact that Sobotta's observations have been confirmed in the cases of
several other animals, such, for instance, as the rabbit (Sobotta,
Honore, Cohn), certain bats (van der Stricht), the sheep (Marshall),
the marsupial dasyurus (Sandes), the spermophile (Volker), and
the guinea-pig (Sobotta). The weight of evidence is at the present
time strongly in favor of Bischoff's view, but until the adverse
results obtained by Clarke and others from the study of the human
corpus luteum and those obtained by Jankowski fiom the pig have
been shown to be incorrect, the question as to the invariable derivation of the lutein cells from the stratum granulosum must be left
open. Since it is held that both the granulosa and theca cells are
derivatives of the embryonic ovarial epithelium the essential differences between the two origins that have been ascribed to the lutein
cells may not be so great as has been supposed. Indeed, it is possible
that both the follicular and thecal cells may in some cases contribute to the formation of the corpus luteum.
 
The persistence of the corpus luteum throughout the entire
period of pregnancy and its disappearance within a few weeks if
pregnancy does not supervene, have suggested the probability of its
being related to the changes that take place in the uterus in connection with the implantation of the ovum in its wall. Experimental
removal of the corpus luteum in rabbits either before or shortly
after the implantation of the ovum produces a failure of pregnancy
(Fraenkel), and similar results have been obtained in mice and
bitches (Marshall and Jolly). It has accordingly been held that
the corpus luteum is an organ of internal secretion directly concerned in the production and maintenance of the modifications of
the uterus necessary for the implantation and further development
of the ovum.
 
 
 
26 THE RELATION OF OVULATION TO MENSTRUATION
 
The Relation of Ovulation to Menstruation. — It was long
believed that ovulation was coincident with certain periodic changes
of the uterus which constitute what is termed menstruation. This
phenomenon makes its appearance at the time of puberty, the exact
age at which it appears being determined by individual and racial
peculiarities and by climate and other factors, and after it has once
appeared it normally recurs at definite intervals more or less closely
corresponding with lunar months ii. e., at intervals of about twentyeight days) until somewhere in the neighborhood of the fortieth or
forty-fifth year, when it ceases.
 
In each menstrual cycle four stages may be recognized, one of
which, the intermenstrual, greatly exceeds the others in its duration,
occupying about one-half the entire period. During this stage the
mucous membrane of the uterus is practically at rest, but toward
its close the membrane gradually begins to thicken and the second
stage, the premenstrual stage, then supervenes. This lasts for six or
seven days and is characterized by a . marked proliferation and
swelling of the uterine mucosa, the subjacent tissue becoming at
the same time highly vascular and eventually congested. The
walls of the blood-vessels situated beneath the mucosa then degenerate and permit the escape of blood here and there beneath the
mucous membrane, this leading to the third, or menstrual, stage in
which the mucous membrane diminishes in thickness, those portions
of it that overlie the effused blood undergoing fatty degeneration
and desquamation, so that the stage is characterized by more or
less extensive hsemorrhage. The duration of this stage is from
three to five days and then ensues the postmenstrual stage, lasting
from four to six days, during which the mucous membrane is regenerated and again returns to the intermenstrual condition.
 
It seems but natural to regard these changes as the expression
of a periodic attempt to prepare the uterus for the reception of the
fertilized ovum, this preparation being completed during the
premenstrual stage, the succeeding menstrual and postmenstrual
being merely the return of the uterine mucosa to the resting intermenstrual stage, pregnancy not having occurred. If this be the
 
 
 
THE RELATION OF OVULATION TO MENSTRUATION 27
 
real significance of the menstrual cycle, one would expect to find
ovulation occurring at a more or less definite portion of the cycle,
at such a time that the ovum, if fertilized would be able to make
use of the premenstrual preparation for its reception.
 
Attempts to determine the relation of ovulation to menstruation
have been made by estimating the age of the corpora lutea occurring
in ovaries removed in the course of operation from patients, the date
of whose last menstruation was known. The results obtained by
this method have, however, proved somewhat discordant. Thus,
Fraenkel records out of eighty-five cases ten in which the operation
was performed immediately before or after menstruation, and in
none of these was any corpus luteum present; further, in twenty
cases a newly formed corpus luteum was found and in these cases
the last menstruation had occurred on the average nineteen (13-27)
days previously. Villemin, too, reached a similar result, concluding
that ovulation took place about fifteen days after menstruation.
On the other hand, Leopold and Ravano found that in ninety-five
cases ovulation coincided with menstruation in fifty-nine, while in
the remaining thirty-six it occurred during other stages of the cycle.
 
If any conclusion may be drawn from these contradictory results
it would seem to be that in the human species ovulation may take
place at any stage of the menstrual cycle. Indeed, it may also be
said that ovulation may take place independently of the menstrual
cycle, since cases are on record of pregnancy having occurred in
girls who had not yet menstruated. In other words, it seems
probable that ovulation does not depend upon the condition of the
uterine mucous membrane, but upon some other factor as yet
undetermined.
 
' The conditions in lower animals seem also to point in this direction.
In these ovulation is, as a rule, associated with a certain condition known
as oestrus or "heat," this being preceded by certain phenomena constituting what is termed the procestrum and corresponding essentially to
menstruation. In several forms, such as the dog and the pig, ovulation
appears to occur regularly in association with "heat," but in others, such
as the cat, the mouse and probably the rabbit, it occurs at this time only
if copulation also occurs. Furthermore, it has been observed that
 
 
 
28 THE MATURATION OF THE OVUM
 
although female monkeys menstruate regularly throughout the year,
nevertheless there is but one annual cestral period when ovulation takes
place (Heape).
 
The Maturation of the Ovum. — Returning now to the ovum,
it has been shown that at the time of its extrusion from the Graafian
follicle it is not equivalent to a spermatozoon but to a primary
spermatocyte, and it may be remembered that such a spermatocyte
 
 
 
 
Fig. 14. — Ovum of a Mouse Showing the Maturation Spindle.
 
The ovum is enclosed by the zona pellucida (z.p), to which the cells of the corona radiata
 
are still attached. — (Sobotta.)
 
becomes converted into a spermatozoon only after it has undergone
two divisions, during which there is a reduction of the number of the
chromosomes to practically one-half the number characteristic for
the species.
 
Similar divisions and a similar reduction of the chromosomes
occur in the case of the ovum, constituting what is termed its
maturation. The phenomena have not as yet been observed in
 
 
 
THE MATURATION OF THE OVUM
 
 
 
20
 
 
 
human ova, and, indeed, among mammals only with any approach
to completeness in comparatively few forms (rat, mouse, guineapig, bat and cat); but they have been observed in so many other
forms, both vertebrate and invertebrate, and present in all cases so
 
 
 
 
Fig. 15. — Diagram Illustrating the Reduction of~the Chromosomes during
 
the Maturation of the Ovum.
0, Ovum; oc l , oocyte of the first generation; oc 2 , oocyte of the second generation;
 
p, polar globule.
 
much uniformity in their general features, that there can be little
question as to their occurrence in the human ovum.
 
In typical cases the ovum (the primary oocyte) undergoes a
division in the prophases of which the chromatin aggregates to form
half as many tetrads as there are chromosomes in the somatic cells
 
 
 
30 THE MATURATION OF THE OVUM
 
(Fig. 15, oc 1 ) and at the metaphase a dyad from each tetrad passes
into each of the two cells that are formed. These two cells (secondary oocytes) are not, however, of the same size; one of them is
almost as large as the original primary oocyte and continues to be
called an ovum (oc 2 ), while the other is very small and is termed a
polar globule (ft). A second division of the ovum quickly succeeds
the first (Fig. 15, oc 2 ), and each dyad gives a single chromosome to
each of the two cells which result, so that each of these cells possesses
half the number of chromosomes characteristic for the species.
The second division, like the first, is unequal, one of the cells being
relatively very large and constituting the mature ovum, while the
other is small and is the second polar globule. Frequently the first
polar globule divides during the formation of the second one, a
reduction of its dyads to single chromosomes taking place, so that
as the final result of the maturation four cells are formed (Fig. 15),
the mature ovum (o),and three polar globules (ft), each of which
contains half the number of chromosomes characteristic for the
species.
 
The similarity of the maturation phenomena to those of spermatogenesis may be perceived trom the following diagram:
 
n/"~N Spermato
( J cyte I
 
 
 
Spermatocyte II
 
 
 
Ovum
 
 
 
 
Oocyte II O O OO
 
 
 
 
OO OO Spermatids
 
 
 
Polar globules
 
 
 
In both processes the number of cells produced is the same and in
both there is a similar reduction of the chromosomes. But while
each of the four spermatids is functional, the three polar globules
are non-functional, and are to be regarded as abortive ova, formed
 
 
 
THE FERTILIZATION OF THE OVUM 3 1
 
during the process of reduction of the chromosomes only to undergo
degeneration. In other words, three out of every four potential
ova sacrifice themselves in order that the fourth may have the bulk,
that is to say, the amount of nutritive material and cytoplasm necessary for efficient development.
 
The Fertilization of the Ovum. — It is perfectly clear that the
reduction of the chromosomes in the germ cells cannot very long be
repeated in successive generations unless a restoration of the original
number takes place occasionally, and, as a matter of fact, such a
restoration occurs at the very beginning of the development of each
individual, being brought about by the union of a spermatozoon
with an ovum. This union constitutes what is known as the
fertilization of the ovum.
 
The fertilization of the human ovum has not yet been observed,
but the phenomenon has been repeatedly studied in lower forms,
and a thorough study of the process has been made on the mouse by
Sobotta, whose observations are taken as a basis for the following
account.
 
The maturation of the ovum is quite independent of fertilization,
but in many forms the penetration of the spermatozoon into the
ovum takes place before the maturation phenomena are completed.
This is the case with the mouse. A spermatozoon makes its way
through the zona pellucida and becomes embedded in the cytoplasm
of the ovum and its tail is quickly absorbed by the cytoplasm while
its nucleus and probably the middle-piece persist as distinct structures. As soon as the maturation divisions are completed the nucleus
of the ovum, now termed the female pronucleus (Fig. 16, ek), migrates
toward the center of the ovum, and is now destitute of an archoplasm sphere and centrosome, these structures having disappeared
after the completion of the maturation divisions. The spermatozoon
nucleus, which, after it has penetrated the ovum, is termed the male
pronucleus (spk), may lie at first at almost any point in the peripheral
part of the cytoplasm, and it now begins to approach the female
pronucleus, preceded by the middle-piece, which becomes an archoplasm sphere with its contained centrosome and is surrounded by
 
 
 
32 THE FERTILIZATION OF THE OVUM
 
astral rays. The two pronuclei finally come into contact near the
center of the ovum, forming what is termed the segmentation
nucleus (Fig. 16), and the archoplasm sphere and centrosome which
have been introduced with the spermatozoon undergo division and
the two archoplasm spheres so formed migrate to opposite poles of
the segmentation nucleus, an amphiaster forms and the compound
nucleus passes through the various prophases of mitosis. Since,
in the mouse, the male and female pronuclei have each contributed
twelve chromosomes, the equatorial plate of the mitosis is composed
of twenty-four chromosomes, the number characteristic for the
species being thus restored.
 
In describing the spermatogenesis it was shown (p. 16) that
two classes of spermatozoa were formed, those of one class containing the equivalent of twelve chromosomes, while those of the
other class contained only ten. A similar separation of the ovum
into two classes probably does not occur, the accessory chromosomes
in the oocytes dividing just as do the ordinary ones, so that each
ovum possesses twelve chromosomes. When, therefore, the union
of the male and female pronuclei takes place in fertilization, those
ova that are fertilized by a spermatozoon with twelve chromosomes
will possess twenty-four of these bodies, while in those in which the
fertilization is accomplished by a spermatozoon with ten chromosomes, only twenty-two will occur. The number of chromosomes
in the fertilized ovum determines the number in the somatic cells
of the embryo that develops from it and hence there will be two
classes of embryos, one in which the somatic cells possess twentyfour chromosomes and another in which there are twenty-two.
 
That this condition occurs in the human species is at present
merely a conjecture based partly on what occurs during spermatogenesis and partly on what has been shown to occur in a number
of invertebrates (insects). In these, two classes of spermatozoa
have been found to occur as in man, and two classes of individuals,
differing in the number of chromosomes in their somatic cells,
develop from the fertilized ova; and it has been further found that
in these forms those with the greater number of chromosomes
 
 
 
THE FERTILIZATION OF THE OVUM
 
 
 
33
 
 
 
 
 
 
ek
 
 
 
 
 
 
 
 
 
 
 
Fig. 16. — Six Stages in the Process of Fertilization of the Ovum of a Mouse.
After the first stage figured it is impossible to determine which of the two nuclei
represents the male or female pronucleus, ek, Female pronucleus; rk l and rk 2 , polar
globules; spk, male pronucleus. — (Sobotia.)
 
3
 
 
 
34 THE FERTILIZATION OF THE OVUM
 
become females and those with the smaller number males. If, as
seems probable, this condition also obtains in the human species,
it is evident that the sex of the future individual is determined at
the fertilization of the ovum and is correlated with the number of
chromosomes present in the ovum at that stage.
 
It seems to be a rule that but one spermatozoon penetrates the
ovum. Many, of course, come into contact with it and endeavor to
penetrate it, but so soon as one has been successful in its endeavor
no further penetration of others occurs. The reasons for this are
in most cases obscure; experiments on the ova of invertebrates have
shown that the subjection of the ova to abnormal conditions which
impair their vitality favors the penetration of more than a single
spermatozoon {polsypermy), and, indeed, it appears that in some
forms, such as the common newt {Diemyctylus) , polyspermy is the
rule, only one of the spermatozoa, however, which have penetrated
uniting with the female pronucleus, the rest being absorbed by the
cytoplasm of the ovum.
 
Fertilization marks the beginning of development, and it is
therefore important that something should be known as to where
and when it occurs. It seems probable that in the human species the
spermatozoa usually come into contact with the ovum and fertilize
it in the upper part of the Fallopian tubes, and the occurrence of
extra-uterine pregnancy (see p. 22) seems to indicate that occasionally the ovum may be fertilized even before it has been received into
the tube.
 
It is evident, then, that when fertilization is accomplished the
spermatozoon must have traveled a distance of about twenty-four
centimeters, the length of the upper part of the vagina being taken
to be about 5 cm., that of the uterus as 7 cm., and that of the tube
as 12 cm. A considerable interval of time is required for the completion of this journey, even though the movement of the spermatozoon be tolerably rapid. The observations of Henle and Hensen
indicate that a spermatozoon may progress in a straight line at about
the rate of from 1.2 to 2.7 mm. per minule, while Lott finds the rate
to be as high as 3.6 mm. Assuming the rate of progress to be about
 
 
 
THE FERTILIZATION OF THE OVUM 35
 
2.5 mm. per minute, the time required by the spermatozoon to
travel from the upper part of the vagina to the upper part of a
Fallopian tube will be about one and a half hours (Strassmann).
This, however, assumes that there are no obstacles in the way of the
rapid progress of the spermatozoon, which is not the case, since, in
the first place, the irregularities and folds of the lining membrane
of the tube render the path of the spermatozoon a labyrinthine one,
and, secondly, the action of the cilia of the epithelium of the tube
and uterus being from the ostium of the tube toward the os uteri, it
will greatly retard the progress; furthermore, it is presumable that
the rapidity of movement of the spermatozoon diminishes after a
certain interval of time. It seems probable, therefore, that fertilization does not occur for some hours after coition, even providing
an ovum is in the tube awaiting the approach of the spermatozoon.
 
But this condition is not necessarily present, and consequently
the question of the duration of the vitality of the sperm cell becomes
of importance. Ahlfeld has found that, when kept at a proper
temperature, a spermatozoon will retain its vitality outside the body
for eight days, and Diihrssen reports a case in which living spermatozoa were found in a Fallopian tube removed from a patient who
had last been in coitu about three and a half weeks previously.
As regards the duration of the vitality of the ovum less accurate data
are available. Hyrtl found an apparently normal ovum in the
uterine portion of the left tube of a female who died three days after
the occurrence of her second menstruation, and Issmer estimates
the duration of the capacity for fertilization of an ovum to be about
sixteen days.
 
It is evident, then, that even when the exact date of the coitus
which led to the fertilization is known, the actual moment of the
latter process can only be approximated, and in the immense majority of cases it is necessary to rely upon the date of the last menstruation for an estimation of the probable date of parturition.
And by this method the possibilities for error are much greater,
since, as been pointed out, ovulation is not necessarily associated
with menstruation. The duration of pregnancy is normally ten
 
 
 
36 LITERATURE
 
lunar or about nine calendar months and it is customary to estimate
the probable date of parturition as nine months and seven days
from the last menstruation. From what has been said, it is clear
that any such estimation can be depended upon only as an approximation, the possible variation from it being considerable.
 
Superf etation. — The occasional occurrence of twin fetuses in different
stages of development has suggested the possibility of the fertilization of
a second ovum as the result of a coition at an appreciable interval of time
after the first ovum has started upon its development. There seems to
be good reason for believing that many of the cases of supposed superfetation, as this phenomenon is termed, are instances of the simultaneous
fertilization of two ova, one of which, for some cause concerned with
the supply of nutrition, has later failed to develop as rapidly as the other.
At the same time, however, even although the phenomenon may be of
rare occurrence, it is by no means impossible, for occasionally a second
Graafian follicle, either in the same or the other ovary, may be so near
maturity that its ovum is extruded soon after the first one, and if the
development of the latter and the incidental changes in the uterine mucous
membrane have not proceeded so far as to prevent the access of the
spermatozoon to the ovum, its fertilization and development may ensue.
The changes, however, which prevent the passage of the spermatozoon
are completed early in development and the difference between the
normally developed embryo and that due to superfetation will be comparatively small, and will become less and less evident as development
proceeds, provided that the supply of nutrition to both embryos is equal.
 
LITERATURE.
 
E. Ballowitz: " Untersuchungen iiber die Struktur der Spermatozoen," No. 4.
 
Zeitschr. fiir wissensch. Zool., lii, 189 1.
K. VON Bardeleben: "Beitrage zur Histologic des Hodens und zur Spermatogenese
 
beim Menschen," Archiv fur Anat. und Physiol., Anat. Abth., Supplement, 1897.
Th. Boveri: "Befruchtung," Ergebnisse der Anat. und Entwicklungsgesch., I, 1892.
J. G. Clark: "Ursprung, Wachsthum und Ende des Corpus luteum nach Beobach
tungen am Ovarium des Schweines und des Menschen," Archiv filr Anat. und
 
Physiol., Anat. Abth., 1898.
L. Fraenkel: "Neue Experimente zur Function des Corpus luteum," Arch, fiir
 
Gynaek., xci, 1910.
L. Gerlach: "Ueber die Bildung der Richtungskorper bei Mus museums," Wiesbaden, 1906.
S. Gutherz: "Ueber ein bemerkenswertes Strukturelement (Heterochromosome) in
 
der Spermiogenese des Menschen," Arch.f. Mikr. Anat., lxxix, 1912. •
M. F, Guyer: "Accessory Chromosomes in Man," Biol. Bull., xix, 1910.
W. Heape: "The Sexual Season of Mammals and the Relation of the Procestrum to
 
 
 
LITERATURE 37
 
Menstruation," Quart. Journ. Micros. Sci., N. S., xliv, 1901 (contains very full
bibliography) .
O. Hertwig: "Vergleich der Ei- und Samenbildung bei Nematoden," Archiv filr
mikrosk. Anat., xxxvr, 1890.
 
F. Hitschmann and L. Adler: "Der Bau der Uterusschleimhaut des geschlechts
reifen Weibes, mit besonderer Beriicksichtigung der Menstruation," Monatsschr.
 
filr Geburtsk. und Gynaek., xxxn, 1908.
J. Jankowski: "Beitrag zur Entstehung des Corpus luteum der Saugetiere," Arch. f.
 
mikr. Anat., lxiv, 1904.
W. B. Klrkham: "The Maturation of the Mouse Egg," Biol. Bulletin, xii, 1907.
H. Lams and J. Doorme: "Nouvelles recherches sur la maturation et la fecondation
 
de 1'oeuf de mammiferes," Arch, de Biol., xxiii, 1907.
M. von Lenhossek: " Untersuchungen iiber Spermatogenese," Archiv fiir mikrosk.
 
Anat., LI, 1898.
 
G. Leopold and A. Rovano: "Neuer Beitrag zur Lehre von der Menstruation und
 
Ovulation," Arch, fur Gynaek., Lxxxm, 1907.
W. H. Longley: "The Maturation of the Egg and Ovulation in the Domestic Cat,"
 
Amer. Journ. Anat., xn, 191 1.
F. H. A. Marshall: "The (Estrus Cycle and the Formation of the Corpus luteum in
 
the Sheep," Philos. Trans., Ser. B, cxcvi, 1904.
F. H. A. Marshall: "The Development of the Corpus luteum: a Review," Quart.
 
Journ. Micros. Sci., N. S., xlix, 1906.
 
F. Meves: "Ueber Struktur und Histogenese der Samenfaden des Meerschweinchens,"
 
Archiv fiir mikrosk. Anat., liv, 1899.
T. H. Montgomery: "Differentiation of the human Cells of Sertoli," Biolog. Bull.,
 
xxi, 1911.
W. Nagel: "Das menschliche Ei," Archiv fiir mikrosk. Anat., xxxi, 1888.
 
G. Niessing: " Die Betheiligung der Centralkorper und Sphare am Aufbau des Samen
fadens bei Saugethieren," Archiv fiir mikrosk. Anat., XLvni, 1896.
G. Retzixjs: "Die Spermien des Menschen," Biolog. Untersuch., xrv, 1909.
W. Rubaschkin: "Ueber die Reifungs- und Befruchtungs-processe des Meerschwein
cheneies," Anat. Hefte, xxix, 1905.
J. Sobotta: "Die Befruchtung und Furchung des Eies der Maus," Archiv fiir mikrosk.
 
Anat., xxy, 1895.
J. Sobotta: " Ueber die Bildung des Corpus luteum bei der Maus," Archiv fiir mikrosk.
 
Anat., XL vn, 1897.
J. Sobotta: "Ueber die Bildung des Corpus luteum beim Meerschweinchen,'M«a<.
 
Hefte, xxxii, 1906.
J. Sobotta and G. Burckhard: "Reifung und Befruchtung der Eier des weissen
 
Ratte," Anat. Hefte, xlii, 1910.
P. Strassmann: "Beitrage zur Lehre von der Ovulation, Menstruation und Conception," Archiv fiir Gynaekol., lii, 1896.
F. Villemin: "Le corps jaune considere comme glande a secretion interne," Paris,
 
1908.
W. Waldeyer: "Eierstock und Ei," Leipzig, 1870.
 
 
 
CHAPTER II.
 
THE SEGMENTATION OF THE OVUM AND THE FORMATION
OF THE GERM LAYERS.
 
Segmentation. — The union of the male and female pronuclei
has already been described as being accompanied by the formation
of a mitotic spindle which produces a division of the ovum into two
cells. This first division is succeeded at more or less regular
intervals by others, until a mass of cells is produced in which a
differentiation eventually appears. These divisions of the ovum
constitute what is termed its segmentation.
 
The mammalian ovum has behind it a long line of evolution,
and even at early stages in its development it exhibits peculiarities
which can only be reasonably explained as an inheritance of past
conditions. One of the most potent factors in modifying the
character of the segmentation of the ovum is the amount of food
yolk which it contains, and it seems to be certain that the immediate
ancestors of the mammalia were forms whose ova contained a considerable amount of yolk, many of the peculiarities resulting from
its presence being still clearly indicated in the early development of
the almost yolkless mammalian ovum. To give some idea of the
peculiarities which result from the presence of considerable amounts
of yolk it will be well to compare the processes of segmentation and
differentiation seen in ova with different amounts of it.
 
A little below the scale of the vertebrates proper is a form,
Amphioxus, which possesses an almost yolkless ovum, presenting a
simple process of development. The fertilized ovum of Amphioxus
in its first division separates into two similar and equal cells, and
these are made four (Fig. 17, A) by a second plane of division
which cuts the previous one at right angles. A third plane at
 
38
 
 
 
THE SEGMENTATION OF THE OVUM
 
 
 
39
 
 
 
right angles to both the preceding ones brings about an eight-celled
stage (Fig. 17, B), and further divisions result in the formation
of a large number of cells which arrange themselves in the form
of a hollow sphere which is known as a blastula (Fig. 17, E).
 
The minute amount of yolk which is present in the ovum of
Amphioxus collects at an early stage of the segmentation at one pole
of the ovum, the cells containing it being somewhat larger than those
of the other pole (Fig. 17, B), and in the blastula the cells of one pole
are larger and more richly laden with yolk than those of the other
pole (Fig. 17, F). If, now, the segmenting ovum of an Amphibian
be examined, it will be found that a very much greater amount of
 
 
 
 
Fig. 17. — Stages in the Segmentation of Amphioxus.
 
A, Four-celled stage; B, eight-celled stage; C, sixteen-celled stage; D, early blastula; E>
 
blastula; F, section of blastula.- — (Hatschek.)
 
 
 
yolk is present and, as in Amphioxus, it is located especially at one
pole of the ovum. The first three planes of segmentation have the
same relative positions as in Amphioxus (Fig. 17), but one of the
tiers of cells of the eight-celled stage is very much smaller than the
other (Fig. 18, B). In the subsequent stages of segmentation the
small cells of the upper pole divide more rapidly than the larger ones
of the lower pole, the activity of the latter seeming to be retarded by
the accumulation of the yolk, and the resulting blastula (Fig. 18,
 
 
 
40 THE SEGMENTATION OF THE OVUM
 
D) shows a very decided difference in the size of the cells of the two
poles.
 
In the ova of reptiles and birds the amount of yolk stored up in
the ovum is very much greater even than in the amphibia, and it is
aggregated at one pole of the ovum, of which it forms the principal
mass, the yolkless protoplasm appearing as a small disk upon the
 
 
 
 
C D
 
Fig. 18. — Stages in the Segmentation or Amblystoma. — (Eycleshymer.)
 
surface of a relatively huge mass of yolk. The inertia of this mass of
nutritive material is so great that the segmentation is confined to the
small yolkless disk of protoplasm and affects consequently only a
portion of the entire ovum. To distinguish this form of segmentation from that which affects the entire ovum it is termed meroblastic
segmentation, the other form being known as holoblastic.
 
In the ovum of a turtle or a bird the first plane of segmentation
crosses the protoplasmic disk, dividing it into two practically equal
 
 
 
THE SEGMENTATION OF THE OVUM
 
 
 
41
 
 
 
halves, and the second plane forms at approximately right angles
to the first one, dividing the disk into four quadrants (Fig. 19, A).
The third division, like the two which precede it, is radial in position,
while the fourth is circular and cuts off the inner ends of the six
cells previously formed (Fig. 19, C). The disk now consists of
six central smaller cells surrounded by six larger peripheral ones.
 
 
 
 
 
 
 
Fig. 19. — Four Stages in the Segmentation
 
Chick. — (Coste.)
 
 
 
of the Blastoderm of the
 
 
 
Beyond this period no regularity can be discerned in the appearance
of the segmentation planes; but radial and circular divisions continuing to form, the disk becomes divided into a large number of
cells, those at the center being much smaller than those at the periphery. In the meantime, however, the smaller central cells have
begun to divide in planes parallel to the surface of the disk, which,
from being a simple plate of cells, thus becomes a discoidal cellmass.
 
 
 
42 THE SEGMENTATION OF THE OVUM
 
During the segmentation of the disk it has increased materially
in size, extending further and further over the surface of the yolk,
into the substance of which some of the lower cells of the discoidal
cell-mass have penetrated. A comparison of the diagram (Fig.
20) of the ovum of a reptile at about this stage of development with
the figure of the amphibian blastula (Fig. 18, D) will indicate the
similarity between the two, the large yolk-mass ( Y) of the reptile with
the scattered cells which it contains corresponding to the lower pole
 
 
 
 
Fig. 20.— Diagram Illustrating a Section of the Ovum of a Reptile at a Stage
Corresponding to the Blastula of an Amphibian.
bl, Blastoderm; Y, yolk-mass,
 
cells of the amphibian blastula, the central cavity of which is practically suppressed in the reptile. Beyond this stage, however, the
similarity becomes more obscured. The peripheral cells of the disk
continue to extend over the surface of the yolk and finally completely enclose it, forming an enveloping layer which is completed at the
upper pole of the egg by the discoidal cell-mass, or, as it is usually
termed, the blastoderm.
 
Turning now to the mammalia,* it will be found that the ovum
in the great majority is almost or quite as destitute of food yolk as is
 
* The segmentation of the human ovum has not yet been observed; what follows
is based on what occurs in the ovum of the rabbit, mole, and especially of a bat (Van
Beneden).
 
 
 
THE SEGMENTATION OF THE OVUM
 
 
 
43
 
 
 
the ovum of Amphioxus, with the result that the segmentation is of
the total or holoblastic type. It does not, however, proceed with
that regularity which marks the segmentation of Amphioxus or an
amphibian, but while at first it divides into two slightly unequal
cells (Fig. 21), thereafter the divisions become irregular, three-celled,
 
 
 
 
 
 
Fig. 21. — Four Stages in the Segmentation of the Ovum of a Mouse.
X, Polar globule.— {Sobolta.)
 
four-celled, five-celled, and six-celled stages having been observed
in various instances. Nor is the result of the final segmentation a
hollow vesicle or blastula, but a solid mass of cells, termed a morula,
is formed. This structure is not, however, comparable to the blastula of the lower forms, but corresponds to a stage of reptilian development a little later than that shown in Fig. 20, since, as will be
shown directly, the cells corresponding to the blastoderm and the
 
 
 
44 THE SEGMENTATION OF THE OVUM
 
enveloping layer are already present. There is, then, no blastula
stage in the mammalian development.
 
Differentiation now begins by the peripheral cells of the morula
becoming less spherical in shape and later forming a layer of flattened cells, the enveloping layer, surrounding the more spherical
central cells (Fig. 22, A). In the latter vacuoles now make their
appearance, especially in those cells which are nearest what may be
regarded as the lower pole of the ovum (Fig. 22, C), and these
vacuoles, gradually increasing in size, eventually become confluent,
the condition represented in Fig. 22, D, being produced. At this
stage the ovum consists of an enveloping layer, enclosing a cavity
which is equivalent to the yolk-mass of the reptilian ovum, the
vacuolization of the inner cells of the morula representing a belated
formation of yolk. On the inner surface of the enveloping layer,
at what may be termed the upper pole of the ovum, is a mass of cells
projecting into the yolk- cavity and forming what is known as the
inner cell-mass, a structure comparable to the blastoderm of the
reptile. In one respect, however, a difference obtains, the inner
cell-mass being completely enclosed within the enveloping cells,
which is not the case with the blastoderm of the reptile. That
portion of the enveloping layer which covers the cell-mass has been
termed Rauber^s covering layer, and probably owes its existence to the
precocity of the formation of the enveloping layer.
 
It is clear, then, that an explanation of the early stages of
development of the mammalian ovum is to be obtained by a comparison, not with a yolkless ovum such as that of Amphioxus, but
with an ovum richly laden with yolk, such as the meroblastic
ovum of a reptile or bird. In these forms the nutrition necessary
for the growth of the embryo and for the complicated processes
of development is provided for by the storing up of a quantity
of yolk in the ovum, the embryo being thus independent of external
sources for food. The same is true also of the lowest mammalia,
the Monotremes, which are egg-laying forms, producing ova
resembling greatly those of a reptile. When, however, in the
higher mammals the nutrition of the embryo became provided
 
 
 
THE SEGMENTATION OE THE OVUM
 
 
 
45
 
 
 
 
 
 
$ ^T?> ...
 
 
 
 
 
 
 
<l
 
 
 
 
 
 
•SI
 
 
 
 
 
 
 
 
 
 
 
 
V-"A
 
 
 
 
 
 
 
 
 
 
 
'1.
 
 
v â–  â–  â–  , .
 
 
 
D
 
 
 
Fig. 22. — Later Stages in the Segmentation of the Ovum of a Bat.
A, C, and D are sections, B a surface view. — (Van Beneden.)
 
 
 
46 TWIN DEVELOPMENT AND DOUBLE MONSTERS
 
for by the attachment of the embryo to the walls of the uterus
of the parent so that it could be nourished directly by the parent,
the storing up of yolk in the ovum was unnecessary and it became a
holoblastic ovum, although many peculiarities dependent on the
original meroblastic condition persisted in its development.
 
Twin Development. — As a rule, in the human species but one embryo
develops at a time, but the occurrence of twins is by no means infrequent,
and triplets and even quadruplets occasionally are developed. The
occurrence of twins may be due to two causes, either to the simultaneous
ripening and fertilization of two ova, either from one or from both
ovaries, or to the separation of a single fertilized ovum into two independent parts during the early stages of development. That twins may be
produced by this latter process has been abundantly shown by experimentation upon developing ova of lower forms, each of the two cells of an
Amphioxus ovum in that stage of development, if mechanically separated,
completing its development and producing an embryo of about half the
normal size.
 
Double Monsters and the Duplication of Parts. — The occasional
occurrence of double monsters is explained by an imperfect separation
into two parts of an originally single embryo, the extent of the separation,
and probably also the stage of development at which it occurs, determining
the amount of fusion of the two individuals constituting the monster. All
gradations of separation occur, from almost complete separation, as seen
in such cases as the Siamese twins, to forms in which the two individuals
are united throughout the entire length of their bodies. The separation
may also affect only a portion of the embryo, producing, for instance,
double-faced or double-headed monsters or various forms of so-called
parasitic monsters; and, finally, it may affect only a group of cells destined
to form a special organ, producing an excess of parts, such as supernumerary digits or accessory spleens.
 
It has been observed in the case of double monsters that one of the
two fused individuals always has the position of its various organs reversed,
it being, as it were, the looking-glass image of its fellow. Cases of a
similar situs inversus viscerum, as it is called, have not infrequently been
observed in single individuals, and a plausible explanation of such cases
regards them as one of a pair of twins formed by the division of a single
embryo, the other individual having ceased to develop and either having
undergone degeneration or, if the separation was an incomplete one,
being included within the body of the apparently single individual.
Another explanation of situs inversus has been advanced (Conklin) on
the basis of what has been observed in certain invertebrates. In some
species of snails situs inversus is a normal condition and it has been found
that the inversion may be traced back in the development even to the
 
 
 
FORMATION OF THE GERM LAYERS 47
 
earliest segmentation stages. The conclusion is thereby indicated that
its primary cause may reside in an inversion of the polarity of the ovum,
evidence being forthcoming in favor of the view that even in the ovum
of these and other forms there is probably a distinct polar differentiation.
How far this view may be applicable to the mammalian ovum is uncertain,
but if it be applicable it explains the phenomenon of inversion without
complicating it with the question of twin-formation.
 
The Formation of the Germ Layers. — During the stages
which have been described as belonging to the segmentation period
of development there has been but little differentiation of the cells.
In Amphioxus and the amphibians the cells at one pole of the blastula
are larger and more yolk-laden than those at the other pole, and in
the mammals an inner cell-mass can be distinguished from the
enveloping cells, this latter differentiation having been anticipated in
the reptiles and being a differentiation of a portion of the ovum from
which alone the embryo will develop from a portion which will give
 
 
 
 
 
A B
 
Fig. 23. — Two Stages in the Gastrulation of Amphioxus. — {Morgan and Hazen.)
 
rise to accessory structures. In later stages a differentiation of the
inner cell-mass occurs, resulting first of all in the formation of a twolayered or diploblastic and later of a three-layered or triploblastic
stage.
 
Just as the segmentation has been shown to be profoundly
modified by the amount of yolk present in the ovum and by its secondary reduction, so, too, the formation of the three primitive layers
 
 
 
4S
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
is much modified by the same cause, and to get a clear understanding
of the formation of the triploblastic condition of the mammal it will
be necessary to describe briefly its development in lower forms.
 
In Amphioxus the diploblastic condition results from the flattening
of the large-celled pole of the blastula (Fig. 23, A), and finally from
the invagination of this portion of the vesicle within the other portion
(Fig. 23 , B) . The original single-walled blastula in this way becomes
converted into a double-walled sac termed a gastrula, the outer layer
of which is known as the ectoderm or epiblast and the inner layer as
the endoderm or hypoblast. The cavity bounded by the endoderm is
the primitive gut or archenteron, and the opening by which this
communicates with the exterior is the blastopore. This last structure
is at first a very wide opening, but as development proceeds it
 
becomes smaller, and finally is a
relatively small opening situated at
the posterior extremity of what
will be the dorsal surface of the
embryo.
 
As the oval embryo continues
to elongate in its later development
the third layer or mesoderm makes
its appearance. It arises as a
lateral fold imp) of the dorsal surface of the endoderm (en) on each
side of the middle line as indicated
in the transverse section shown in
Fig. 24. This fold eventually becomes completely constricted off
from the endoderm and forms a
hollow plate occupying the space between the ectoderm and endoderm, the cavity which it contains being the body-cavity or coelom.
 
In the amphibia, where the amount of yolk is very much greater
than in Amphioxus, the gastrulation becomes considerably modified.
On the line where the large- and small-celled portions of the blastula
become continuous a crescentic groove appears and, deepening,
 
 
 
 
Fig. 24. — Transverse Section of
A mphioxus Embryo with Five Mesoderms Pouches.
 
Ch, Notochord; d, digestive cavity;
ec, ectoderm; en, endoderm; m, medullary plate; mp, mesodermic pouch. —
(Halschek.)
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
49
 
 
 
forms an invagination (Fig. 25, gc), the roof of which is composed
of relatively small yolk-containing cells while its floor is formed by
the large cells of the lower pole of the blastula. The cavity of the
blastula is not sufficiently large to allow of the typical invagination
of all these large cells, so that they become enclosed by the rapid
growth of the ectoderm cells of the upper pole of the ovum over
 
 
 
 
Fig. 25. — Section through a Gastrula of Amblystoma.
 
dl, Dorsal lip of blastopore; gc, digestive cavity; gr, area of mesoderm formation; mes,
 
mesoderm. — (Eycleshymer.)
 
them. Before this growth takes place the blastopore corresponds
to the entire area occupied by the large yolk cells, but later, as the
growth of the smaller cells gradually encloses the larger ones, it
becomes smaller and is finally represented by a small opening
situated at what will be the hind end of the embryo.
 
Soon after the archenteron has been formed a solid plate of cells,
eventually splitting into two layers, arises from its roof on each side
of the median line and grows out into the space between the ectoderm and endoderm (Fig. 26, mk l and mk 2 ),. evidently corresponding
to the hollow plates formed in the same situations in Amphioxus.
4
 
 
 
50 FORMATION OF THE GERM LAYERS
 
This is not, however, the only source of the mesoderm in the amphibia, for while the blastopore is still quite large there may be
found surrounding it, between the endoderm and ectoderm, a ring of
mesodermal tissue (Fig. 25, mes). As the blastopore diminishes in
size and its lips come together and unite, the ring of mesoderm
forms first an oval and then a band lying beneath the line of closure
of the blastopore and united with both the superjacent ectoderm
and the subjacent endoderm. This line of fusion of the three germ
 
 
 
 
Fig. 26. — Section through an Embryo Amphibian (Triton) of 2% Days, showing
the Formation of the Gastral Mesoderm.
ok, Ectoderm; ch, chorda endoderm; dk, digestive cavity; ik, endoderm; mk 1 and
mk 2 , somatic and splanchnic layers of the mesoderm. D, dorsal and V, ventral. —
(Herlwig.)
 
layers is known as the primitive streak. It is convenient to distinguish the mesoderm of the primitive streak from that formed from
the dorsal wall of the archenteron by speaking of the former as the
prostomial and the latter as the gastral mesoderm, though it must be
understood that the two are continuous immediately in front of the
definitive blastopore.
 
In the reptilia still greater modifications are found in the method
of formation of the germ layers. Before the enveloping cells have
completely surrounded the yolk-mass, a crescentic groove, resembling
that occurring in amphibia, appears near the posterior edge of the
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
51
 
 
 
blastoderm, the cells of which, in front of the groove, arrange themselves in a superficial layer one cell thick, which may be regarded as
the ectoderm (Fig. 27, ec), and a subjacent mass of somewhat
scattered cells. Later the lowermost cells of this subjacent mass
arrange themselves in a continuous layer, constituting what is termed
the primary endoderm (en 1 ), while the remaining cells, aggregated
 
 
 
 
 
prm
 
 
 
...
 
 
 
 
Fig. 27. — Longitudinal Sections through Blastoderms of the Gecko, showing
 
â–  Gastrulation.
ec, Ectoderm; en, secondary endoderm; en', primary endoderm; prm, prostomial mesoderm.— (Will.)
 
especially in the region of the crescentic groove, form the prostomial
mesoderm (prm). In the region enclosed by the groove a distinct
delimitation of the various layers does not occur, and this region
forms the primitive streak. The groove now begins to deepen,
forming an invagination of secondary endoderm, the extent of this
invagination being, however, very different in different species.
In the gecko (Will) it pushes forward between the ectoderm and
primary endoderm almost to the anterior edge of the blastoderm
(Fig. 27, B), but later the cells forming its floor, together with those
 
 
 
52
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
of the primary endoderm immediately below, undergo a degeneration, the roof cells at the tip and lateral margins of the invagination
becoming continuous with the persisting portions of the primary
endoderm (Figs. 27,0 and 28, B) . This layer, following the enveloping cells in their growth over the yolk-mass, gradually surrounds
that structure so that it comes to lie within the archenteron. In
some turtles, on the other hand, the disappearance of the floor of the
invagination takes place at a very early stage of the infolding, the
 
 
 
en
 
 
 
 
Fig. 28. — Diagrams Illustrating the Formation of the Gastral Mesoderm in
 
the Gecko.
 
ce, Chorda endoderm; ec, ectoderm; en, secondary endoderm; en 1 , primary endoderm;
 
gm, gastral mesoderm. — (Will.)
 
roof cells only persisting to grow forward to form the dorsal wall of
the archenteron. This interesting abbreviation of the process
occurring in the gecko indicates the mode of development which is
found in the mammalia.
 
The existence of a prostomial mesoderm in connection with the
primitive streak has already been noted, and when the invagination
takes place it is carried forward as a narrow band of cells on each
side of the sac of secondary endoderm. After the absorption of the
ventral wall of the invagination a folding or turning in of the margins
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
53
 
 
 
of the secondary endoderrn occurs (Fig. 28), whereby its lumen
becomes reduced in size and it passes off on each side into a double
plate of cells which constitute the gastral mesoderm. Later these
 
 
 
 
 
Fig. 29. — Sections of Ova of a Bat showing (A) the Formation of the Endoderm and (B and C) of the Amniotic Cavity. — {Van Beneden.)
 
plates separate from the archenteron as in the lower forms. All the
prostomial mesoderm does not, however, arise from the primitive
 
 
 
54 FORMATION OF THE GERM LAYERS
 
streak region, but a considerable amount also has its origin from
the ectoderm covering the yolk outside the limits of the blastoderm
proper, a mode of origin which serves to explain the phenomena later
to be described for the mammalia.
 
In comparison with the amphibians and Amphioxus, the reptilia
present a subordination of the process of invagination in the formation of the endoderm, a primary endoderm making its appearance
independently of an invagination, and, in association with this
subordination, there is an early appearance of the primitive streak,
which, from analogy with what occurs in the amphibia, may be
assumed to represent a portion of the blastopore which is closed
from the very beginning.
 
Turning now to the mammalia, it will be found that these
peculiarities become still more emphasized. The inner cell-mass
of these forms corresponds to the blastoderm of the reptilian ovum,
and the first differentiation which appears in it concerns the cells
situated next the cavity of the vesicle, these cells differentiating to
form a distinct layer which gradually extends so as to form a complete lining to the inner surface of the enveloping cells (Fig. 29, A).
The layer so formed is endodermal and corresponds to the primary
endoderm of the reptiles.
 
Before the extension of the endoderm is completed, however,
cavities begin to appear in the cells constituting the remainder of the
inner mass, especially in those immediately beneath Rauber's cells
(Fig. 29, B), and these cavities in time coalesce to form a single
large cavity bounded above by cells of the enveloping layer and
below by a thick plate of cells, the embryonic disk (Fig. 29, C). The
cavity so formed is the amniotic cavity, whose further history will be
considered in a subsequent chapter.
 
It may be stated that this cavity varies greatly in its development in
different mammals, being entirely absent in the rabbit at this stage of
development and reaching an excessive development in such forms as
the rat, mouse, and guinea-pig. The condition here described is that
which occurs in the bat and the mole, and it seems probable, from what
occurs in the youngest human embryos hitherto observed, that the processes in man are closely similar.
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
55
 
 
 
While these changes have been taking place a splitting of the
enveloping layer has occurred, so that the wall of the ovum is now
formed of three layers, an outer one which may be termed the
trophoblast, a middle one which probably is transformed into the
extra-embryonic mesoderm of later stages, though its significance
is at present somewhat obscure, and an inner one which is the
 
 
 
 
 
Fig. 30. — A, Side View of Ovum of Rabbit Seven Days Old {Kdlliker); B,
Embryonic Disk of a Mole (Heape); C, Embryonic Disk of a Dog's Ovum of
about Fifteen Days (Bonnet) .
 
ed, Embryonic disk; hn, Hensen's node; mg, medullary groove; ps, primitive streak;
 
va, vascular area.
 
 
 
primary endoderm. In the bat, of whose ovum Fig. 29, C, represents a section, that portion of the middle layer which forms the
roof of the amniotic cavity disappears, only the trophoblast persisting in this region, but in another form this is not the case, the
roof of the cavity being composed of both the trophoblast and the
middle layer.
 
A rabbit's ovum in which there is yet no amniotic cavity and no
splitting of the enveloping layer shows, when viewed from above,
 
 
 
56 FORMATION OF THE GERM LAYERS
 
a relatively small dark area on the surface, which is the embryonic
disk. But if it be looked at from the side (Fig. 30, A), it will be seen
that the upper half of the ovum, that half in which the embryonic
disk occurs, is somewhat darker than the lower half, the line of
separation of the two shades corresponding with the edge of the
primary endoderm which has extended so far in its growth around
the inner surface of the enveloping layer. A little later a dark area
appears at one end of the embryonic disk, produced by a proliferation of cells in this region and having a somewhat crescentic form.
As the embryonic disk increases in size a longitudinal band makes
its appearance, extending forward in the median line nearly to the
center of the disk, and represents the primitive streak (Fig. 30, B),
a slight groove along its median line forming what is termed the
primitive groove. In slightly later stages an especially dark spot
may be seen at the front end of the primitive streak and is termed
Hensen's node (Fig. 30, C, hn), while still later a dark streak may
be observed extending forward from this in the median line and is
termed the head-process of the primitive streak.
 
 
 
 
Fig. 31. — Posterior Portion of a Longitudinal Section through the Embryonic
 
Disk of a Mole.
bl, Blastopore, ec, ectoderm; en, endoderm; prm, prostomial mesoderm. — {After Heape.)
 
To understand the meaning of these various dark areas recourse
must be had to the study of sections. A longitudinal section through
the embryonic disk of a mole ovum at the time when the crescentic
area makes its appearance is shown in Fig. 31. Here there is to be
seen near the hinder edge of the disk what is potentially an opening
(bl) , in front of which the ectoderm (ec) and primary endoderm (en)
can be clearly distinguished, while behind it no such distinction of
 
 
 
FORMATION OF THE GERM LAYERS 57
 
the two layers is visible. This stage may be regarded as comparable to a stage immediately preceding the invagination stage of
the reptilian ovum, and the region behind the blastopore will
correspond to the reptilian primitive streak. The later forward
extension of the primitive streak is due to the mode of growth of the
embryonic disk. Between the stages represented in Figs. 31 and
30, B, the disk has enlarged considerably and the primitive streak
has shared in its elongation. Since the blastopore of the earlier
stage is situated immediately in front of the anterior extremity of
the primitive streak, the point corresponding to it in the older disk
is occupied by Hensen's node, this structure, therefore, representing
a proliferation of cells from the region formerly occupied by the
blastopore.
 
■ — .
 
 
 
 
 
 
§f#
 
 
 
M
 
 
 
WM§\ . :
 
 
 
 
 
 
Fig. 32. — Transverse Section of the Embryonic Area of a Dog's Ovum at about
 
the Stage of Development shown in Fig. 29, C.
 
The section passes through the head process (Chp); M, mesoderm. — (Bonnet.)
 
As regards the head process, it is at first a solid cord of cells
which grows forward in the median line from Hensen's node, lying
between the ectoderm and the primary endoderm. Later a lumen
appears in the center of the cord, forming what has been termed the
chorda canal, and, in some forms, including man, the canal opens to
the surface at the center of Hensen's node. The cord then fuses
with the subjacent primary endoderm and then opens out along the
line of fusion, becoming thus transformed into a flat plate of cells
continuous at either side with the primary endoderm (Fig. 32, Chp).
The portion of the chorda canal which traverses Hensen's node now
 
 
 
58
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
opens below into what will be the primitive digestive tract and is
termed the neurenteric canal (Fig. t>Z, nc); it eventually closes completely, being merely a transitory structure. The similarity of the
head process to the invagination which in the reptilia forms the
secondary endoderm seems clear, the only essential difference being
that in the mammalia the head process arises as a solid cord which
subsequently becomes hollow, instead of as an actual invagination.
The difference accounts for the occurrence of Hensen's node and
also for the mode of formation of the neurenteric canal, and cannot
be considered as of great moment since the development of what are
eventually tubular structures (e. g., glands) as solid cords of cells
which subsequently hollow out is of common occurrence in the
mammalia. It should be stated that in some mammals apparently
the most anterior portion of the roof of the archenteron is formed
directly from the cells of the primary endoderm, which in this region
are not replaced by the head process, but aggregate to form a compact
plate of cells with which the anterior extremity of the head process
 
 
 
 
Fig. 33. — Diagram of a Longitudinal Section through the Embryonic Disk of
 
a Mole.
am, Amnion; ce chorda endoderm; ec, ectoderm; nc, neurenteric canal; ps, primitive
 
streak. — (Heape.)
 
 
 
unites. Such a condition would represent a further modification of
the original condition.
 
As regards the formation of the mesoderm it is possible to recognize both the prostomial and gastral mesoderm in the mammalian
ovum, though the two parts are not so clearly distinguishable as in
lower forms. A mass of prostomial mesoderm is formed from the
primitive streak, and when the head process grows forward it carries
 
 
 
FORMATION OF THE GERM LAYERS
 
 
 
59
 
 
 
with it some of this tissue. But, in addition to this, a contribution to
the mesoderm is also apparently furnished by the cells of the head
process, in the form of lateral plates situated on each side of the
middle line. These plates are at first solid (Fig. 34, gm), but their
 
 
 
 
gm%
 
 
 
Fig. 34. — Transverse Section through the Embryonic Disk of a Rabbit.
ch, Chorda endoderm; ee, ectoderm; en, endoderm; gm, gastral mesoderm. — {After van
 
Beneden.)
 
 
 
 
Fig. 35.- — Diagrams Illustrating the Relations of the Chick Embryo to the
Primitive Streak at Different Stages of Development. — (Peebles.)
 
cells quickly arrange themselves in two layers, between which a
ccelomic space later appears.
 
Furthermore, as has already been pointed out, the layer of
 
 
 
60 SIGNIFICANCE OF THE GERM LAYERS
 
enveloping cells splits into two concentric layers, the inner of which
seems to be mesodermal in its nature and forms a layer lining the
interior of the trophoblast and lying between this and the
primary endoderm. This layer is by no means so evident in the
lower forms, but is perhaps represented in the reptilian ovum by the
cells which underlie the ectoderm in the regions peripheral to the
blastoderm proper (see p. 54).
 
It has been experimentally determined (Assheton, Peebles) that in
the chick, whose embryonic disk presents many features similar to those
of the mammalian ovum, the central point of the unincubated disk corresponds to the anterior end of the primitive streak and to the point situated
immediately behind the heart of the later embryo and immediately in
front of the first mesodermic somite (see p. 77), as shown in Fig. 35. If
these results be regarded as applicable to the human embryo, then it
may be supposed that in this the head region is developed from the
portion of the embryonic disk situated in front of Hensen's node, while
the entire trunk is a product of the region occupied by the node.
 
The Significance of the Germ Layers. — The formation of
the three germ layers is a process of fundamental importance, since
it is a differentiation of the cell units of the ovum into tissues which
have definite tasks to fulfil. As has been seen, the first stage in the
development of the layers is the formation of the ectoderm and
endoderm, or, if the physiological nature of the layers be considered,
it is the differentiation of a layer, the endoderm, which has principally nutritive functions. In certain of the lower invertebrates, the
class Ccelentera, the differentiation does not proceed beyond this
diploblastic stage, but in all higher forms the intermediate layer is
also developed, and with its appearance a further division of the
functions of the organism supervenes, the ectoderm, situated upon
the outside of the body, assuming the relational functions, the
endoderm becoming still more exclusively nutritive, while the remaining functions, supportive, excretory, locomotor, reproductive, etc.,
are assumed by the mesoderm.
 
The manifold adaptations of development obscure in certain
cases the fundamental relations of the three layers, certain portions
of the mesoderm, for instance, failing to differentiate simultaneously
 
 
 
SIGNIFICANCE OF THE GERM LAYERS 6 1
 
with the rest of the layer and appearing therefore to be a portion of
either the ectoderm or endoderm. But, as a rule, the layers are
structural units of a higher order than the cells, and since each
assumes definite physiological functions, definite structures have
their origin from each.
 
Thus from the ectoderm there develop:
i. The epidermis and its appendages, hairs, nails, epidermal
glands, and the enamel of the teeth.
 
2. The epithelium lining the mouth and the nasal cavities, as
well as that lining the lower part of the rectum.
 
3. The nervous system and the nervous elements of the senseorgans, together with the lens of the eye.
 
From the endoderm develop :
 
1. The epithelium lining the digestive tract in general, together
with that of the various glands associated with it, such as the liver
and pancreas.
 
2. The lining epithelium of the larynx, trachea, and lungs.
 
3. The epithelium of the bladder and urethra (in part).
From the mesoderm there are formed:
 
1. The various connective tissues, including bone and the teeth
(except the enamel).
 
2. The muscles, both striated and non-striated.
 
3. The circulatory system, including the blood itself and the
lymphatic system.
 
4. The lining membrane of the serous cavities of the body.
 
5. The kidneys and ureters.
 
6. The internal organs of reproduction.
 
From this list it will be seen that the products of the mesoderm
are more varied than those of either of the other layers. Among
its products are organs in which in either the embryonic or adult
condition the cells are arranged in a definite layer, while in other
structures its cells are scattered in a matrix of non-cellular material,
as, for example, in the connective tissue, bone, cartilage, and the
blood and lymph. It has been proposed to distinguish these two
forms of mesoderm as mesothelium and mesenchyme respectively,
 
 
 
62 LITERATURE
 
a distinction which is undoubtedly convenient, though probably devoid of the fundamental importance which has been attributed to it
by some embryologists.
 
LITERATURE.
 
R. Assheton: "The Reinvestigation into the Early Stages of the Development of
 
the Rabbit," Quarterly Journ. of Microsc. Science, xxxvn, 1894.
R. Assheton: "The Development of the Pig During the First Ten Days," Quarterly
 
Journ. of Microsc. Science, xli, 1898.
R. Assheton: "The Segmentation of the Ovum of the- Sheep, with Observations on
 
the Hypothesis of a Hypoblastic Origin for the Trophoblast," Quarterly Journ.
 
of Microsc. Science, xli, 1898.
E. van Beneden: "Recherches sur les premiers stades du developpement du Murin
 
(Vespertilio murinus)," Anatom. Anzeiger, xvi, 1899.
R. Bonnet: "Beitrage zur Embryologie der Wiederkauer gewonnen am Schafei,"
 
Archivfiir Anat. und Physiol., Anat. Abth., 1884 and 1889.
R. Bonnet: "Beitrage zur Embryologie des Hundes," Anat. Hefte, ix, 1897.
G. Born: "Erste Entwickelungsvorgange," Ergebnisse der Anat. und Entwicklungs
gesch., 1, 1892.
 
E. G. Conklin: "The Cause of Inverse Symmetry," Anatom. Anzeiger, xxm, 1903.
 
A. C. Eycleshymer: "The Early Development of Amblystoma with Observations
 
on Some Other Vertebrates," Journ. of Morphol., x, 1895.
 
B. Hatschek: "Studien uber Entwicklung des Amphioxus," Arbeiten aus dem zoolog.
 
Ins tit. zu Wien, rv, 1881.
W. Heape: "The Development of the Mole (Talpa europaea)," Quarterly Journ. of
 
Microsc. Science, xxm, 1883.
A. A. W. Hubrecht: "Studies on Mammalian Embryology II: The Development
 
of the Germinal Layers of Sorex vulgaris," Quarterly Journ. of Microsc. Science,
 
xxxi, 1890.
 
F. Keibel: "Studien zur Entwicklungsgeschichte des Schweines," Morpholog.
 
Arbeiten, in, 1893.
F. Keibel: "Die Gastrulation und die Keimblattbildung der Wirbeltiere," Ergebnisse
 
der Anat. und Entwicklungsgesch., x, 1901.
M. KunsemVJller: "Die Eifurchung des Igels (Erinaceus europasus L.)," Zeitschr.
 
fiir wissensch. Zool., lxxxv, 1906.
K. Mitsukuri and C. Ishikawa: "On the Formation of the Germinal Layers in
 
Chelonia," Quarterly Journ. of Microsc. Science, xxvn, 1887.
F. Peebles: "The Location of the Chick embryo upon the Blastoderm," Journ. of
 
Exper. Zool., 1, 1904.
E. Selenka: " Studien uber Entwickelungsgeschichte der Thiere," 4tes Heft, 1886-87;
 
5tes Heft, 1891-92.
J. Sobotta: "DieBefruchtungundFurchungdesEies der Maus," Archivfiir mikrosk.
 
Anat., xlv, 1895.
 
 
 
LITERATURE 63
 
J. Sobotta: " Die Furchung des Wirbelthiereies," Ergebnisse der Anal, unci Entwicke
lungsgeschichte, vi, 1897.
J. Sobotta: "Neuere Auschauungen iiber die Entstehung der Doppel (miss) bild
ungen, mit besonderer Beriicksichtigung der menschlichen Zwillingsgeburten,"
 
Wiirzburger Abhandl., I, 1901.
H. H. Wilder: "Duplicate Twins and Double Monsters," Amer. Jour, of Anal.,
 
in, 1904.
L. Will: "Beitrage zur Entwicklungsgeschichte der Reptilien," Zoolog. Jahrbilcher
 
Abth.fur Anal., vi, 1893.
 
 
 
CHAPTER III.
 
THE MEDULLARY GROOVE, NOTOCHORD, AND MESODERMS SOMITES.
 
In the preceding chapter the development of the mammalian
ovum has been described up to and including the formation of the
three germinal layers. The earlier stages of development there
described are practically unknown in the human ovum, but for the
stages subsequent to the establishment of the germinal layers
human material is available, and it will, therefore, now be convenient to consider the structure of the younger human ova at
present known and to trace in them the appearance and development of such structures as the primitive streak, the head process and
the gastral mesoderm.
 
The youngest human ovum at present known is that described
by Bryce and Teacher, but, unfortunately, it presents certain
features that are evidently abnormal, so that it becomes doubtful
how far it may be accepted as representing the typical condition.
The trophoblast, which was very thick and clearly differentiated
into two layers, enclosed a space whose diameter was about 0.63
mm. and which was largely occupied by a loose syncytial tissue,
presumably mesoderm. Toward the center of this was an irregular
cavity in which were two vesicles, quite separate from one another
and probably together representing the embryo, the smaller one
being the amniotic cavity and the larger one the yolk-sac (Fig. 36).
The separation of these two structures is apparently an abnormality
and it is possible that the cavity in which they lie is, as Bryce and
Teacher suggest, an artefact produced by contraction of the syncytial
mesoderm during the preservation of the ovum.
 
If comparison of this ovum with those of other mammals is
warranted, it may be likened to that of the bat as shown in Fig. 29,
 
64
 
 
 
THE MEDULLARY GROOVE
 
 
 
65
 
 
 
C, with the difference that the mesoderm that lines the trophoblast
in that ovum has become much more voluminous and forms the
syncytial mass in which the ovum is supposed to have been imbedded,
a condition that may be "represented diagrammatically as in Fig.
38, A.
 
Somewhat older are the ova described by Peters, Fetzer, Jung
and Herzog. The Peters ovum was taken from the uterus of a
 
 
 
 
Fig. 36.
 
 
 
-From a Reconstruction of the Bryce-Teacher Ovum. —
(Bryce-Teacher .)
 
 
 
woman who had committed suicide one calendar month after the
last menstruation, and it measured about 1 mm. in diameter. The
entire inner surface of the trophoblast (Fig. 37, ce) was lined by a
layer of mesoderm {cm), which, on the surface furthest away from
the uterine cavity, was considerably thicker than elsewhere, forming
an area of attachment of the embryo to the wall of the ovum. In
the substance of this thickening was the amniotic cavity (am),
whose roof was formed by flattened cells, which, at the sides, became
continuous with a layer of columnar cells forming the floor of the
cavity and constituting the embryonic ectoderm (ec). Immediately
5
 
 
 
66
 
 
 
THE MEDULLARY GROOVE
 
 
 
below this was a layer of mesoderm (m) which split at the edge of
the embryonic disk into two layers, one of which became continuous
with the mesodermic thickening and so with the layer of mesoderm
lining the interior of the trophoblast, while the other enclosed a sac
lined by a layer of endodermal cells and forming the yolk-sac (ys).
The total length of the embryo was 0.19 mm., and so far as its
ectoderm and mesoderm are concerned it might be described as a
 
 
 
 
cm
 
 
<r \
 
 
 
 
*
 
 
 
 
 
 
 
 
 
*
 
 
 
1 *-5SC§* ^k « m
 
 
 
Fig. 37. — Section of Embryo and Adjacent Portion of an Ovum of i mm.
 
am, Amniotic cavity; ce, chorionic ectoderm; cm, chorionic mesoderm; ec, embryonic
 
ectoderm; en, endoderm; m, embryonic mesoderm; ys, yolk-sack. — (Peters.)
 
 
 
flat disk resting on the surface of the yolk-sac, though it must be
understood that the yolk-sac also to a certain extent forms part of
the embryo.
 
This embryo seems to be in an early stage of the primitive streak
formation, before the development of the head process. On comparing it with the stage of development represented in Fig. 38, A,
it will be seen to present some important advances. The cavity
(Fig. 38, B, C) into which the yolk-sac projects is unrepresented in
 
 
 
THE MEDULLARY GROOVE
 
 
 
6 7
 
 
 
Fig. 38, A. How this cavity is formed can only be conjectured, but
it seems probable that it arises by the splitting of the layer of cells
which lines the interior of the trophoblast in the earlier stage (or
perhaps by the vacuolization of the central cells of this layer) and
the subsequent accumulation of fluid between the two mesodermal layers so formed. However that may be, it seems clear that
the size of the human ovum is due mainly to the rapid growth of
this cavity, which, as future stages show, is the extra-embryonic
portion of the body-cavity, the splitting or vacuolization of the
 
 
 
 
Fig. 38. — Diagrams to show the Probable Relationships of the Parts in the
Embryos Represented in Figs. 29, C, and 37.
Ac, Amniotic cavity; C, extra-embryonic body-cavity; Me, (in figure to the left)
mesoderm, (in figure to the right) somatic mesoderm; Me, splanchnic mesoderm; D,
digestive tract; En, endoderm; T, trophoblast. The broken line in the mesoderm of the
figure to the left indicates the line along which the splitting of the mesoderm occurs.
 
 
 
mesoderm by which it is probably formed being the precocious
appearance of the typical splitting of the mesoderm to form the
embryonic body-cavity which, as will be seen in a subsequent chapter, takes place only at a later stage of development. From now on
the trophoblast and the layer of mesoderm lining it may together
be spoken of as the chorion, the mesoderm layer being termed the
chorionic mesoderm.
 
A little older again than the Peters and Herzog ova are those
described by Strahl and Beneke and by von Spee (Embryo v. H.),
the chorionic cavity of the former two having an average diameter
 
 
 
68
 
 
 
THE MEDULLARY GROOVE
 
 
 
of about 2.4 mm., while the corresponding size of the latter two was
somewhat less than 4.0 mm. Notwithstanding the considerable
increase in the size of these older ova, due to the continued increase
in the size of the extra-embryonic ccelom, the embryos are but
 
 
 
 
Fig. 39. — The Embryo v. H. of von Spee. The Left Half of theT Chorion has
 
been Removed to show the Embryo.
a, Amniotic cavity; b, belly-stalk; ch, chorion; d, yolk-sac; e, extra-embryonic ccelom;
k y embryonic disk; 2, chorionic villus. — {von Spee.)
 
little advanced beyond the stage shown by the Peters embryo.
The thickening of the chorionic mesoderm that encloses the amniotic cavity has increased in size and now forms a pedicle, known as
the belly-stalk (Fig. 39, 6), at the extremity of which is the yolk-sac
 
 
 
 
Fig. 40. — Embryo from the Beneke Ovum, the Roof of the Amniotic Cavity
 
having been Removed.
From a model, b, Belly-stalk; p.g., primitive groove; y, yolk-sac — {Strahl and Beneke.)
 
(d). Furthermore, the amniotic cavity (a) now lies somewhat excentrically in this pedicle, being near what may be termed its anterior
surface, and the entire embryo projects like a papilla from the inner
surface of the chorion into the extra-embryonic ccelom. Fig. 40 is
 
 
 
THE MEDULLARY GROOVE 69
 
from a model of the Beneke embryo, detached from the chorion by
cutting through the belly-stalk, and with the roof of the amniotic
cavity removed. The dorsal surface of the embryo, thus exposed,
is an oval disk, resting, as it were, on the yolk-sac, and quite smooth
except for a slight longitudinal groove upon its posterior portion.
This is the primitive groove and sections passing through it show the
primitive streak, consisting of a sheet of mesoderm interposed
between the ectoderm and endoderm, as in the Peters embryo, and
but poorly defined from the other two layers. From its anterior
edge a median process extends forward for a short distance and is
the head process (see p. 56). In front and to the sides of this there
is as yet no mesoderm intervening between the ectoderm and
endoderm.
 
 
 
 
Fig. 41. — Embryo from the Frassi Ovum, the Roof of the Amniotic Cavity
 
having been removed.
From a model, b, belly-stalk; p.g., primitive groove; mg, medullary groove; n, neuren
teric canal. — (Frassi.)
 
The embryonic disk of the Beneke embryo measured 0.75 mm.
in length. That of an embryo described by Frassi (Fig. 41) was
1. 1 7 mm. in length, and in correspondence with its greater size, it
presents some advances in structure that are of interest. As in
the younger embryo one sees a distinct primitive groove on the
posterior portion of the embryonic disk, but the groove terminates
anteriorly at a distinct pore (w) , which perforates the disk and opens
ventrally into the yolk-sac. This is the neurenteric canal (see p. 58)
and in front of it a groove extends forward in the median line almost
to the anterior edge of the embryonic disk and is evidently the first
 
 
 
7°
 
 
 
THE MEDULLARY GROOVE
 
 
 
indication of the medullary groove, whose walls are destined to give
rise to the central nervous system. Sections passing through the
region of the medullary groove show, lying beneath it, the head
process (Fig. 42, hp), already fused with the endoderm (compare
p. 57), and on each side of the process is a plate of mesoderm (gm),
representing the gastral mesoderm of lower forms (see Figs. 28
and 34) , but not as yet showing any indications of splitting into the
two layers that bound the embryonic ccelom (see p. 59).
 
 
 
am
 
 
 
 
Fig. 42. — Section through the Frassi Embryo just in Front of the Neuren
teric Canal.
am, Amniotic cavity; gm, gastral mesoderm; hp, head process; mp, medullary plate; ys>
 
yolk-sac. — (Frassi.)
 
 
 
This is just beginning to appear in an embryo, also described by
von Spee and known as embryo Gle. It measured 1.54 mm. in
length and is closely similar, in general appearance, to an embryo
described by Eternod and measuring 1.34 mm. in length (Fig. 43).
It differs from the Frassi embryo most markedly in that the posterior
portion of the embryonic disk, that is to say the primitive streak
region, is bent ventrally so. as to lie almost at a right angle with the
anterior portion. As a result the belly-stalk arises from the ventral
surface of the embryo instead of from its posterior extremity, near
which the opening of the neurenteric canal (Fig. 43, nc) is now situated, almost the whole length of the surface seen in dorsal view
being occupied by the medullary groove (m), which, in the embryo
Gle, is bounded laterally by distinct ridges, the medullary folds.
 
 
 
THE MEDULLARY GROOVE
 
 
 
71
 
 
 
 
Fig. 43. — Embryo 1.34 mm. Long.
 
al Allantois; am, amnion; bs, belly-stalk; h, heart; m, medullary groove; tic neuren
 
tenc canal; pc, caudal protuberance; ps, primitive streak; ys, yolk-stalk.— (Eternod.)
 
 
 
7 2
 
 
 
THE MEDULLARY FOLDS
 
 
 
In the Kromer embryo Klb (Fig. 44), measuring i.8 mm. in
length, a new feature has made its appearance. The medullary folds
have become quite high, and lateral to them there is on each side
a series of five or six oblong elevations, which represent what are
termed mesodermic somites and are due to divisions of the underlying mesoderm.
 
 
 
 
Fig. 44. — Model of the Kromer Embryo Klb seen from the Dorsal Surface, the
Roof of the Amniotic Cavity having been Removed. — (Keibel and Elze.)
 
Instead of proceeding with a description of the external form of
still older embryos it will be convenient to consider the further
development of certain structures whose appearance has already
been noted, namely, the head process, the medullary folds and the
mesodermic somites, and first of all • the medullary folds may be
considered.
 
The Medullary Folds. — The two folds are continuous anteriorly,
but behind they are at first separate, the anterior portion of the primitive streak lying between them. In forms, such as the Reptilia,
which possess a distinct blastopore, this opening lies in the interval
between the two, and consequently is in the floor of the medullary
groove, and in the mammalia, even though no well-defined blastopore
is formed, yet at the time of the formation of the medullary fold an
opening breaks through at the anterior end of the primitive streak
in the region of Hensen's node, and places the cavity lying below
the endoderm in communication with the space bounded by the
medullary folds. The canal so formed is termed the neurenteric
 
 
 
THE MEDULLARY FOLDS
 
 
 
73
 
 
 
canal (Figs. 43 and 45, nc) and is so called because it unites what
will later become the central canal of the nervous system with the
intestine (enteron). The significance of this canal has already been
discussed (p. 58) ; it is of very brief persistence, closing at an early
stage of development so as to leave no trace of its existence.
 
 
 
 
Fig. 45. — Diagram of a Longitudinal Section through the Embryo Gle, Measuring 1.54 mm. in Length.
al, Allantois; am, amnion; B, belly-stalk; ch, chorion; h, heart; nc, neurenteric canal; V,
chorionic villi; Y, yolk-sac. — (vonSpee.)
 
 
 
As development proceeds the medullary folds increase in height
and at the same time incline toward one another (Fig. 44), so that
their edges finally come into contact and later fuse, the two ectodermal layers forming the one uniting with the corresponding layers
of the other (Fig. 46). By this process the medullary groove becomes converted into a medullary canal which later becomes the
 
 
 
74 THE NOTOCHORD
 
central canal of the spinal cord and the ventricles of the brain, the
ectodermal walls of the canal thickening to give rise to the central
nervous system. The closure of the groove does not, however, take
place simultaneously along its entire length, but begins in what
corresponds to the neck region of the adult and thence proceeds both
 
 
 
 
 
 
Fig. 46. — Diagrams showing the Manner of the Closure of the Medullary
 
Groove.
 
anteriorly and posteriorly, the extension of the fusion taking place
rather slowly, however, especially anteriorly, so that an anterior
opening into the otherwise closed canal can be distinguished for a
considerable period (Fig. 53).
 
The Noto chord. — While these changes have been taking place in
the ectoderm of the median line of the embryonic disk, modifications
of the subjacent endoderm have also occurred. This endoderm,
it will be remembered, was formed by the head process of the primitive streak, and was a plate of cells continuous at the sides with the
primary endoderm and extending forward as far as what will eventually be the anterior part of the pharynx. Along the line of its
junction with the primary endoderm it gives rise to the plates of
gastral mesoderm (Fig. 28), while the remainder of it produces an
 
 
 
THE NOTOCHORD
 
 
 
75
 
 
 
important embryonic organ known as the notochord or chorda dorsalis
and on this account is sometimes termed the chorda endoderm.
 
After the separation of the plates of gastral mesoderm the chorda
endoderm, which is at first a flat band, becomes somewhat curved
(Fig. 47, A), so that it is concave on its under surface, and, the curvature increasing, the edges of the plate come into contact and finally
fuse together (Fig. 47, B), the edges of the primary endoderm at the
same time uniting beneath the chordal tube so formed, so that this
layer becomes a continuous sheet, as it was at its first appearance.
 
 
 
 
Fig. 47. — Transverse Sections through Mole Embryos, showing the Formation
 
of the Notochord.
ec, Ectoderm; en, endoderm; m, mesoderm; nc. notochord. — (Heape.)
 
 
 
The lumen which is at first present in the chordal tube is soon
obliterated by the enlargement of the cells which bound it, and
these cells later undergo a peculiar transformation whereby the
chordal tube is converted into a solid elastic rod surrounded by a
cuticular sheath secreted by the cells. The notochord lies at first
immediately beneath the median line of the medullary groove, between the ectoderm and the endoderm, and has on either side of it
the mesodermal plates. It is a temporary structure of which only
rudiments persist in the adult condition in man, but it is a structure
characteristic of all vertebrate embryos and persists to a more or
less perfect extent in many of the fishes, being indeed the only axial
 
 
 
j6 THE MESODERMIC SOMITES
 
skeleton possessed by Amphioxus. In the higher vertebrates it is
almost completely replaced by the vertebral column, which develops
around it in a manner to be described later.
 
The Mesodermic Somites. — Turning now to the middle
germinal layer, it will be found that in it also important changes take
place during the early stages of development. The probable mode
of development of the extra-embryonic mesoderm and body-cavity
has already been described (p. 67) and attention may now be directed
toward what occurs in the embryonic mesoderm. In both the
Peters embryo and the embryo v.H described by von Spee this
portion of the mesoderm is represented by a plate of cells lying
between the ectoderm and endoderm and becoming continuous at
the edges of the embryonic area with both the layer which surrounds
the yolk-sac and, through the mesoderm of the belly-stalk, with the
chorionic mesoderm (Fig. 37). It seems probable, since there is in
these embryos no indication as yet of the formation of the chorda
endoderm, that this plate of mesoderm corresponds to the prostomial
mesoderm of lower forms. In older embryos, such as the embryo
Gle of Graf Spee and the younger embryo described by Eternod
(Fig. 43), the mesoderm no longer forms a continuous sheet extending completely across the embryonic disk, but is divided into two
lateral plates, in the interval between which the ectoderm of the
floor of the medullary groove and the chorda endoderm are in close
contact (Fig. 48). These lateral plates represent the gastral mesoderm, whose origin has already been described (p. 59), and which
apparently supplants the original prostomial mesoderm, whose
fate in the human embryo is at present unknown. The changes
which now occur have not as yet been observed in the human embryo,
though they probably resemble those described in other mammalian
embryos, and the phenomena which occur in the sheep may serve
to illustrate their probable nature.
 
It has been seen that in the stage represented by the Frassi
embryo a plate of mesoderm has formed on either side of the chorda
endoderm, and that in a later stage, represented by the Kromer
embryo Klb, a differentiation occurs in these plates leading to the
 
 
 
THE MESODERMIC SOMITES 77
 
formation of mesodermic somites. These make their appearance
in what will later be the cervical region of the embryo and their
formation proceeds backward as the body of the embryo increases
in length. A longitudinal groove appears on the dorsal surface of
each lateral plate of mesoderm, marking off the more median thicker
portion from the lateral parts (Fig. 48), which from this stage
onward may be termed the ventral mesoderm. The median or dorsal
portions then become divided transversely into a number of more
or less cubical masses which are termed the protoverlebrce or, better,
 
 
 
 
Fig. 48. — Transverse Section through the Second Mesodermic Somite of a
Sheep Embryo 3 mm. Long.
am, Amnion; en, endoderm; I, intermediate cell-mass; mg, medullary groove; ms,
mesodermic somite; so, somatic and sp, splanchnic layers of the ventral mesoderm. —
(Bonnet.)
 
mesodermic somites (Fig. 48, ms). The cells of the somites and of
the ventral mesoderm, are at first stellate in form, but later become
more spindle-shaped, and those near the center of each somite and
those of the ventral mesoderm arrange themselves in regular layers
so as to enclose cavities which appear in these regions (Fig. 48).
Each original lateral plate of gastral mesoderm thus becomes
divided longitudinally into three areas, a more median area composed of mesodermic somites, lateral to this a narrow area underlying the original longitudinal groove which separated the somite
area from the ventral mesoderm and which from its position is
termed the intermediate cell-mass (Fig. 48, 1) , and, finally, the ventral
mesoderm. This last portion is now divided into two layers, the
 
 
 
78 THE MESODERMIC SOMITES
 
dorsal of which is termed the somatic mesoderm, while the ventral one
is known as the splanchnic mesoderm (Fig. 48, so and sp; and Fig. 49) ,
the cavity which separates these two layers being the embryonic
body-cavity or pleuroperitoneal cavity (coslom) , which will eventually
give rise to the pleural, pericardial and peritoneal cavities of the adult
as well as the cavity of each tunica vaginalis testis.
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 49. — Transverse Section of an Embryo of 2.5 mm. (See Fig. 53) showing
on either side of the medullary canal a mesodermic somite, the interMEDIATE Cell-mass, and the Ventral Mesoderm. — (vonLenhossek.)
 
Beginning in the neck region, the formation of the mesodermic
somites proceeds posteriorly until finally there are present in the
human embryo thirty-eight pairs in the neck and trunk regions of
the body, and, in addition, a certain number are developed in what
is later the occipital region of the head. Exactly how many of these
occipital somites are developed is not known, but in the cow four
have been observed, and there are reasons for believing that the
same number occurs in the human embryo.
 
In the lower vertebrates a number of cavities arranged in pairs occur
in the more anterior portions of the head and have been homologized with
mesodermic somities. Whether this homology be perfectly correct or not,
 
 
 
THE MESODERMIC SOMITES
 
 
 
79
 
 
 
these head-cavities, as they are termed, indicate the existence of a division
of the head mesoderm into somites, and although practically nothing
is known as to their existence in the human embryo, yet, from the relations
in which they stand to the cranial nerves and musculature in the lower
forms, there is reason to suppose that they are not entirely unrepresented
 
 
 
\\'W^; — M
 
 
 
 
.*$'$te$&\& P
 
 
 
 
 
 
 
 
 
1 • ;
 
Vu
 
 
 
1 - -4 ;
 
 
 
Fig. 50. — Transverse Section of an Embryo of 4.25 mm. at the Level of the Arm
 
Rudiment.
A, Axial mesoderm of arm; Am, amnion; il, inner lamella of myotome; M, myotome;
me, splanchnic mesoderm; ol, outer lamella of myotome; Pn, place of origin of pronephros;^ sclerotome; S 1 , defect in wall of myotome due to separation of the sclerotome;
st, stomach ; Vu, umbilical vein. — (Kollmann.)
 
 
 
The mesodermic somites in the earliest human embryos in
which they have been observed contain a completely closed cavity,
and this is true of the majority of the somites in such a form as the
sheep. In the four first-formed somites in this species, however,
the somite cavity is at first continuous with the pleuroperitoneal
 
 
 
3o THE MESODERMIC SOMITES
 
cavity and only later becomes separated from it, and in lower vertebrates this continuity of the somite cavities with the general bodycavity is the rule. The somite cavities are consequently to be
regarded as portions of the general pleuroperitoneal cavity which
have secondarily been separated off. They are, however, of but
short duration and early become filled up by spindle-shaped cells
derived from the walls of the somites, which themselves undergo a
differentiation into distinct portions. The cells of that portion of the
wall of each somite which is opposite the notochord become spindleshaped and grow inward toward the median line to surround the
notochord and central nervous system, and give rise eventually to
the lateral half of the body of a vertebra and the corresponding
portion of a vertebral arch. This portion of the somite is termed a
sclerotome (Fig. 50, S), and the remainder forms a muscle plate or
myotome (M) which is destined to give rise to a portion of the voluntary musculature of the body. The outer wall of the somite has
been generally believed to take part in the formation of the cutis
layer of the integument and hence has been termed the cutis plate
or dermatome, but it seems probable that it becomes entirely transformed into muscular tissue.
 
The intermediate cell-mass in the human embryo, as in lower
forms, partakes of the transverse divisions which separate the individual mesodermic somites. From one portion of the tissue in most of
the somites (Fig. 50, Pri) the provisional kidneys or Wolffian bodies
develop, this portion of each mass being termed a nephrotome, while
the remaining portion gives rise to a mass of cells showing no tendency to arrange themselves in definite layers and constituting that
form of mesoderm which has been termed mesenchyme (see p. 61).
These mesenchymatous masses become converted into connective
tissues and blood-vessels.
 
The ventral mesoderm in the neck and trunk regions never
becomes divided transversely into segments corresponding to the
mesodermic somites, differing in this respect from the other portions
of the gastral mesoderm. In the head, however, that portion
of the middle layer which corresponds to the ventral mesoderm of
 
 
 
THE MESODERMIC SOMITES 8 1
 
the trunk does undergo a division into segments in connection with
the development of the branchial arches and clefts (see p. 90). A
consideration of these segments, which are known as the branchiomeres, may conveniently be postponed until the chapters dealing
with the development of the cranial muscles and nerves, and in what
follows here attention will be confined to what occurs in the ventral
mesoderm of the neck and trunk.
 
Its splanchnic layer (Fig. 51, vm), applies itself closely to the
endodermal digestive tract, which is constricted off from the dorsal
portion of the yolk-sac, and becomes converted into mesenchyme
out of which the muscular coats of the digestive tract develop.
The cells which line the pleuroperitoneal cavity, however, retain
their arrangement in a layer and form a part of the serous lining of
the peritoneal and other serous cavities, the remainder of the lining
being formed by the corresponding cells of the somatic layer; and
in the abdominal region the superficial cells, situated near the line
where the splanchnic layer passes into the somatic, and in close
proximity to the nephrotome of the intermediate cell-mass, become
columnar in shape and are converted into reproductive cells.
 
The somatic layer, if traced peripherally, becomes continuous
at the sides with the layer of mesoderm which lines the outer surface
of the amnion (Fig. 50) and posteriorly with the mesoderm of the
belly-stalk. That portion of it which lies within the body of the
embryo, in addition to giving rise to the serous lining of the parietal
layer of the pleuroperitoneum, becomes converted into mesenchyme,
which for a considerable length of time is clearly differentiated into
two zones, a more compact dorsal one which may be termed the
somatic layer proper, and a thinner, more ventral vascular zone
which is termed the membrana reuniens (Fig. 51). In the earlier
stages the somatic layer proper does not extend ventrally beyond
the line which passes through the limb buds and it grows out into
these buds to form an axial core for them, in which later the skeleton
of the limb forms. The remainder of the mesoderm lining the sides
and ventral portions of the body-wall is at first formed from the
membrana reuniens, but as development proceeds the somatic
6
 
 
 
82
 
 
 
THE MESODERMIC SOMITES
 
 
 
layer gradually extends more ventrally and displaces, or, more
properly speaking, assimilates into itself, the membrana reuniens
until finally the latter has completely disappeared.
 
It is to be noted that no part of the voluntary musculature
of the lateral and ventral walls of the neck and trunk is derived
from the somatic layer; it is formed entirely from the myotomes
which gradually extend ventrally (Fig. 51) and finally come into
contact with their fellows of the opposite side in the mid-ventral line.
 
 
 
 
Fig. 51. — Diagrams Illustrating the History of the Gastral Mesoderm.
 
dM, dorsal portion of myotome; gr, genital ridge; I, intestine; M, myotome, mr,
membrana reuniens; N, nervous system; SC, sclerotome; Sm, somatic mesoderm;
vm, splanchnic mesoderm; vM, ventral portion of myotome; Wd, Wolffian duct.
 
Whether the voluntary musculature of the limbs is also derived
from the myotomes is at present doubtful. It has been very generally
believed that the myotomes in their growth ventrally sent prolongations into the limb buds which invested the axial core of mesenchyme and eventually gave rise to the voluntary muscles. The
actual existence of the prolongations of the myotomes and their
conversion into the limb musculature has, however, not yet been
observed and it is quite possible that the limb musculature may be
derived from the axial core of somatic mesoderm from which the
limb skeleton develops.
 
The appearance of the mesodermic somites is an important
 
 
 
THE MESODERMIC SOMITES 8$
 
phenomenon in the development of the embryo, since it influences
fundamentally the future structure of the organism. If each pair
of mesodermic somites be regarded as a structural unit and termed
a metamere or segment, then it may be said that the body is composed of a series of metameres, each more or less closely resembling
its fellows, and succeeding one another at regular intervals. Each
somite differentiates, as has been stated, into a sclerotome and a
myotome, and, accordingly, there will primarily be as many vertebra? and muscle segments as there are mesodermic somites, or, in
other words, the axial skeleton and the voluntary muscles of the
trunk are primarily metameric. Nor is this all. Since each
metamere is a distinct unit, it must possess its own supply of nutrition, and hence the primary arrangement of the blood-vessels is also
metameric, a branch passing off on either side from the main longitudinal arteries and veins to each metamere. And, further, each
pair of muscle segments receives its own nerves, so that the arrangement of the nerves, again, is distinctly metameric.
 
It is to be noted that this metamerism is essentially resident in
the dorsal mesoderm, the segmentation shown by structures derived
from other embryonic tissues being secondary and associated with
the relations of these structures to the mesodermic somites. The
metamerism is most distinct in the neck and trunk regions, and at
first only in the dorsal portions of these regions, the ventral portions
showing metamerism only after the extension into them of the myotomes. But there is clear evidence that the arrangement extends
also into the head, and that a portion of its mesoderm is to be regarded
as composed of metameres. It has been seen that in the notochordal region of the head of lower vertebrates mesodermic somites
are present, while anteriorly in the prechordal region there are headcavities which resemble closely the mesodermic somites, and are
probably directly comparable to the somites of the trunk. There is
reason, therefore, for believing that the fundamental arrangement
of the dorsal mesoderm in all parts of the body is metameric, but
though this arrangement is clearly defined in early embryos, it
loses distinctness in later periods of development. But even in the
 
 
 
84 LITERATURE
 
adult the original metamerism is clearly indicated in the arrangement of the nerves and of parts of the axial skeleton, and careful
study frequently reveals indications of it in highly modified muscles
and blood-vessels.
 
In the head the development of the branchial arches and clefts
produces a series of parts presenting many of the peculiarities of
metameres, and, indeed, it has been a very general custom to regard
them as expressions of the general metamerism which prevails
throughout the body. It is to be noted, however, that they are produced by the segmentation of the ventral mesoderm, a structure
which in the neck and trunk regions does not share in the general
metamerism, and, furthermore, recent observations on the cranial
nerves seem to indicate that these branchiomeres cannot be regarded
as portions of the head metameres or even as structures comparable to these. They represent, more probably, a second metamerism
superposed upon the more general one, or, indeed, possibly more
primitive than it, but whose relations can only be properly understood in connection with a study of the cranial nerves.
 
LITERATURE.
 
In addition to many of the papers cited in the list at the close of Chapter II, the
following may be mentioned:
C. R. Bardeen: " The Development of the Musculature of the Body Wall in the Pig,
 
etc.," Johns Hopkins Hosp. Rep., ix, 1900.
T. H. Bryce and J. H. Teacher: " Contributions to the Study of the Early Development and Imbedding of the Human Ovum," Glasgow, 1908.
A. C. F. Eternod: "Communication sur un ceuf humain avec embryon excessive
ment jeune," Arch. Ital. de Biologie, xxn, 1895.
A. C. F. Eternod: "II y a un canal notochordal dans l'embryon humain," Anat.
 
Anzeiger, xvi, 1899.
Fetzer: "Ueber ein durch Operation gewonnenes menschliches Ei das in seiner
 
Entwickelung etwa dem Peterssehen Ei entspricht," Verh. Anat. Gesellschaft,
 
xxiv, 1910.
L. Frassi: "Weitere Ergebnisse des Studiums eines jungen menschlichen Eies in
 
situ," Arch.f. mikr. Anat., lxxi, 1908.
W. Heape: "The Development of the Mole (Talpa Europaea)," Quarterly Journ.
 
Microsc. Science, xxvn, 1887.
M. Herzog: "A Contribution to our Knowledge of the Earliest Known Stages of
 
Placentation and Embryonic Development in Man," Amer. Journ. Anat., ix, 1909.
 
 
 
LITERATURE 85
 
F. Keibel: "Zur Entwickelungsgeschichte der Chorda bei Saugern (Meerschwein
chen und Kaninchen)," Archiv fur Anat. und Physiol., Anat. Abth., 1889.
S. Kaestner: "Ueber die Bildung von animalen Muskelfasern aus dem Urwirbel,"
 
Arch, filr Anat. und Phys., Anat. Abth., Suppl., 1890.
J. Kollmann: "Die Rumpfsegmente menschlicher Embryonen von 13 bis 35 Unvir
beln," Archiv filr Anat. und Physiol., Anat. Abth., 1891.
H. Peters: "Ueber die Einbettung des menschlichen Eies und das friiheste bisher
 
bekannte menschliche Placentarstadium," Leipzig und Wien, 1899.
F. Graf von Spee: " Beobachtungen an einer menschlichen Keimscheibe mit
 
offener Medullarrinne und Canalis neurentericus," Arch.f. Anat. u. Phys., Anat.
 
Abth., 1889.
F. Graf von Spee: "Ueber friihe Entwicklungsstufen des menschlichen Eies,"
 
Arch.f. Anat. u. Phys., Anat. Abth., 1896.
H. Strahl and R. Beneke: "Ein junger menschlicher Embryo," Wiesbaden, 1910.
J. W. VAN Wijhe: "Ueber die Mesodermsegmente des Rumpfes und die Entwick
lung des Excretionsystems bei Selachiern," Archiv fur mikrosk. Anat., xxxin,
 
1889.
K. W. Zimmermann: " Ueber Kopfhohlenrudimente beim Menschen," Archiv filr
 
mikrosk. Anat., liii, 1898.
 
 
 
CHAPTER IV.
 
THE DEVELOPMENT OF THE EXTERNAL FORM OF THE
HUMAN EMBRYO.
 
In the preceding chapter descriptions have been given of human
embryos representing the earlier known stages and the development
of the general form of the human embryo has been traced up to the
time when the mesodermic somites have made their appearance.
It will now be convenient to continue the history of the general
development up to the stage when the embryo becomes a fetus.
 
In the earlier stages, that is to say up to that represented by the
Eternod embryo (Fig. 43), the embryonic disk may be described as
floating upon the surface of the yolk-sac, and while this description
still holds good for the Eternod embryo a distinct groove may be seen
in that embryo between the peripheral portions of the embryonic
disk and the upper part of the sac. This groove marks the beginning
of the separation or constriction of the embryo from the yolk-sac,
the result of which is the transformation of the discoidal embryonic
portion of the embryonic disk into a cylindrical structure. Primarily this depends upon the deepening of the furrow which surrounds the embryonic area, the edges of this area being thus bent in
on all sides toward the yolk-sac. This bending in proceeds most
rapidly at the anterior end of the body, as shown in the diagrams
(Fig. 52), and less rapidly at the posterior end where the bellystalk is situated, and produces a constriction of the yolk-sac, the
portion of this structure nearest the embryonic disk becoming enclosed within the body of the embryo to form the digestive tract,
while the remainder is converted into a pedicle-like portion, the
yolk-stalk, ' at the extremity of which is the yolk-vesicle. The
further continuance of the folding in of the edges of the embryonic
area leads to an almost complete closing in of the embryonic ccelom
 
86
 
 
 
DEVELOPMENT OF EXTERNAL FORM
 
 
 
87
 
 
 
and reduces the opening through which the yolk-stalk and bellystalk communicate with the embryonic tissues to a small area known
as the umbilicus.
 
In the Kromer embryo Klb (Fig. 44) this separation of the embryo proper from the yolk-sac has proceeded to such an extent that
both extremities of the embryonic disk are free from the yolk-sac,
and the anterior extremity is bent ventrally almost at a right angle to
 
 
 
 
 
 
 
Fig. 52. — Diagrams Illustrating the Constriction of the Embryo from the
 
Yolk-sac.
A and C are longitudinal, and B and D transverse sections. B is drawn to a larger scale
 
than the other figures.
 
 
 
the rest of the disk, producing what is termed the vertex bend, a
feature characteristic of all later embryos. The marked development in this embryo of the medullary folds and the occurrence of
mesodermic somites have already been mentioned (p. 72).
 
Somewhat more advanced is the Bulle embryo described by
Kollmann and shown from the side and dorsally in Fig. 53, the
greater part of the yolk-sac having been removed as well as the most
of the amnion. The embryo measured about 2.5 mm. in length and
showed a considerable increase in the number of mesodermic
somites, there being about fourteen of them on either side. Pos
 
 
88 DEVELOPMENT OF EXTERNAL FORM
 
teriorly the medullary groove has become converted into a medullary canal by the medullary folds meeting over it and fusing, but
anteriorly it is still open. The vertex bend is well marked and
 
 
 
am-\
 
 
 
 
Om
 
 
 
M^' L rX. j
 
 
 
 
Y.
 
 
 
-am
 
 
 
Fig. 53. — Embryo 2.5 mm. Long.
 
om, Amnion; B, belly-stalk; h, heart; M, closed, and M', still open portions of the
 
medullary groove; Om, vitelline vein; OS, oral fossa; Y, yolk-sac. — {Kallmann.)
 
 
 
immediately behind the tip of the head, on the ventral surface of the
body, there may be seen a well-marked depression, the oral fossa,
between which and the anterior surface of the yolk-sac is a rounded
 
 
 
DKVELOPMENT OF EXTERNAL FORM
 
 
 
8 9
 
 
 
 
Fig. 54. — Embryo Lr, 4.2 mm. Long.
 
am, Amnion; au, auditory capsule; B, belly-stalk; h, heart; LI, lower, and Ul, upper
 
limb; Y, yolk-sac. — (His.)
 
 
 
90 DEVELOPMENT OF EXTERNAL FORM
 
elevation due to the formation of the heart. Attention may be
called to the fact that the position of this organ is far forward of that
which it will eventually occupy, so that it must undergo a marked
retrogression during later development.
 
As an example of a later stage. of development the embryo Lr of
His, measuring 4.2 mm. in length, may be taken (Fig. 54). In this
the constriction of the yolk-sac has progressed so far that its proximal portion may now be spoken of as the yolk-stalk. The mesodermic somites have undergone a further increase and have almost
reached their final number, the vertex bend has become still more
pronounced and the medullary groove, throughout its entire length,
has been converted into the medullary canal, which, anteriorly, shows
distinct enlargements and constrictions which foreshadow various
portions of the future brain. The auditory organ, which made its
appearance in earlier stages, has now become quite distinct, and a
lateral bulging of the most anterior portion of the head indicates the
position of the future eye.
 
In addition certain other important features have now appeared.
Thus, about opposite the head a second bend, the nape bend, is
becoming visible on the dorsal surface of the body and toward the
posterior end a distinct sacral bend is evident. Secondly, a little
posterior to the level of the nape bend a slight elevation is to be seen
on the side of the body; this is the limb bud for the upper limb and
a corresponding, though smaller, elevation in the region of the sacral
bend represents the lower limb.
 
Thirdly, three grooves having a dorso-ventral direction have
appeared on the sides of what will be the future pharyngeal region.
These are representatives of a series of branchial clefts, structures
that are of great morphological importance in the further development inasmuch as they determine to a large extent the arrangement
of various organs of the head region. They represent the clefts
which exist in the walls of the pharynx in fishes, through which
water, taken in at the mouth, passes to the exterior, bathing on its
way the gill filaments attached to the bars or arches, as they are
termed, which separate successive clefts. Hence the name "bran
 
 
DEVELOPMENT OF EXTERNAL FORM
 
 
 
9 1
 
 
 
 
Fig. 55. — Floor of the Pharynx
of Embryo B, 7 mm. Long.
Ep, Epiglottis; Sp, sinus prsecervicalis; t 1 , tuberculum impar; t 2 ,
posterior portions of the tongue;
I, II, III, and IV, branchial arches.
—(His.)
 
 
 
chial" which is applied to them, though in the mammals they never
have respiratory functions to perform, but, appearing, persist for
a time and then either disappear or are applied to some entirely different purpose. Indeed, in man they are never really clefts but
merely grooves, and corresponding to
each groove in the ectoderm there is
also one in the subjacent endoderm
of what will eventually be the pharyngeal region of the digestive tract, so
that in the region of each cleft the
ectoderm and endoderm are in close
relation, being separated only by a
very thin layer of mesoderm. In
the intervals between successive clefts
a more considerable amount of mesoderm is present (Fig. 55).
 
In the human embryo four clefts
and five branchial arches develop
on each side of the body, the last arch lying posteriorly to the fourth
cleft and not being very sharply denned along its posterior margin.
 
As just stated, the clefts are normally merely grooves, and in later
development either disappear or are converted into special structures.
Occasionally, however, a cleft may persist and the thin membrane which
forms its floor may become perforated so that an opening from the exterior
into the pharynx occurs at the side of the neck, forming what is termed a
branchial fistula. Such an abnormality is most frequently developed
from the lower (ventral) part of the first cleft; normally this disappears,
the upper portion of the cleft persisting, however, to form the external
auditory meatus and tympanic cavity.
 
A further stage in the differentiation of these clefts and arches
is shown by the embryo represented in Fig. 56. The nape bend
has now increased to such an extent that the whole anterior part of
the body is bent at a right angle to the middle part and the entire
embryo is coiled in a spiral manner. The limb buds are much more
distinct than in the previous stage and four branchial arches are
now present; the second and third have become more defined and
 
 
 
92 DEVELOPMENT OF EXTERNAL FORM
 
a strong process has developed from the dorsal part of the anterior
border of the first one, which has thus become somewhat <3 -shaped.
The anterior limb of each V is destined to give rise to the upper jaw,
and hence is known as the maxillary process, while the posterior
limb represents the future lower jaw and is termed the mandibular
process.
 
 
 
 
M— — — I—
 
Fig. 56.— Embryo Backer, 7.3 mm. in Length. X5. — (Keibefand Ehe.)
 
In the stage represented by this embryo the closing in of the
embryonic ccelom has progressed to such a degree that only a small
opening is left in the ventral body-wall of the embryo through which
the yolk-stalk and its accompanying vessels and the belly-stalk pass.
Indeed the margins of the umbilicus may have begun to be prolonged outward over these structures, enclosing them in a cylindrical
investment, the first stage of what will later be the umbilical cord
being thus established.
 
 
 
DEVELOPMENT OF EXTERNAL FORM
 
 
 
93
 
 
 
Leaving aside for the present all consideration of the further
development of the limbs and branchial arches, the further evolution
of the general form of the body may be rapidly sketched. In an
embryo (Fig. 57) from Ruge's collection, described and figured by
His and measuring 9.1 mm. in length,* the prolongation of the
 
 
 
 
_W
 
 
 
L-LI
 
 
 
Fig. 57. — Embryo 9.1 mm. Long.
LI, Lower limb; U, umbilical cord; Ul, upper limb; Y, yolk-sac. — (His.)
 
margins of the umbilicus has increased until more than half the
yolk-stalk has become enclosed within the umbilical cord. The
nape and sacral bends are still very pronounced, although the embryo
is beginning to straighten out and is not quite so much coiled as in
the preceding stage. At the posterior end of the body there has
 
* This measurement is taken in a straight line from the most anterior portion of the
nape bend to the middle point of the sacral bend and does not follow the curvature
of the embryo. It may be spoken of as the nape-rump length and is convenient for use
during the stages when the embryo is coiled upon itself.
 
 
 
94 DEVELOPMENT OF EXTERNAL FORM
 
developed a rather abruptly conical tail filament, in the place of the
blunt and gradually tapering termination seen in earlier stages,
and a well-marked rotundity of the abdomen, due to the rapidly
increasing size of the liver, begins to become evident.
 
In later stages the enclosure of the yolk- and belly-stalks within
the umbilical cord proceeds until finally the cord is complete through
the entire interval between the embryo and the wall of the ovum.
At the same time the straightening out of the embryo continues, as
may be seen in Fig. 58 representing the embryo xlv (Br 2 ) of His,
which shows also, both in front of and behind the neck bend, a
 
 
 
 
Fig. 58.' — Embryo B r 2 , 13.6 mm. Long. — (His.)
 
distinct depression, the more anterior being the occipital and the more
posterior the nape depression; both these depressions are the indications of changes taking place in the central nervous system. The
tail filament has become more marked, and in the head region a slight
ridge surrounding the eyeball and marking out the conjunctival area
has appeared; a depression anterior to the nasal fossae marks off the
nose from the forehead; and the external ear, whose development
will be considered later on, has become quite distinct. This embryo
had a nape-rump length of 13.6 mm.
 
 
 
DEVELOPMENT OF EXTERNAL FORM
 
 
 
95
 
 
 
In the embryos xxxv (S 2 ) and xcix (L 3 ) (Fig. 59, A and B) of
His' collection the straightening out of the nape bend is proceeding,
and indeed is almost completed in embryo xcix, which begins to
resemble closely the fully formed fetus. The tail filament, somewhat reduced in size, still persists and the rotundity of the abdomen
continues to be well marked. The neck region is beginning to be
distinguishable in embryo S 2 and in embryo L 3 the eyelids have
appeared as slight folds surrounding the conjunctival area. The
 
 
 
 
 
Fig. 59. — A, Embryo S 2 , 15 mm. Long (showing Ectopia of the Heart); B, Embryo
L 3 , 17.5 mm. Long. — (His.)
 
nose and forehead are clearly defined by the greater development
of the nasal groove and the nose has also become raised above the
general surface of the face, while the external ear has almost acquired
its final fetal form. These embryos measure respectively about
15 and 17.5 mm. in length.*
 
Finally, an embryo — again one of those described by His,
 
 
 
* The embryo S 2 presents a slight abnormality [in the great projection of the
heart, but otherwise it appears to be normal.
 
 
 
9 6
 
 
 
DEVELOPMENT OF EXTERNAL EORM
 
 
 
namely, his lxxvti (Wt), having a length of 23 mm. — may be
figured (Fig. 60) as representing the practical acquisition of the
fetal form. This embryo dates from about the end of the second
month of pregnancy, and from this period onward it is proper to
use the term fetus rather than that of embryo. The changes which
 
 
 
 
Fig. 60. — Embryo Wt, 23 mm. Long. — (His.)
 
 
 
have been described in preceding stages are now complete and it
remains only to be mentioned that the caudal filament, which is still
prominent, gradually disappears in later stages, becoming, as it
were, submerged and concealed beneath adjacent parts by the
development of the buttocks. The incompleteness of the development of these regions in embryo Wt is manifest, not only from the
 
 
 
DEVELOPMENT OF THE BRANCHIAL ARCHES
 
 
 
97
 
 
 
projection of the tail filament, but also from the external genitalia
being still largely visible in a side view of the embryo, a condition
which will disappear in later stages.
 
The Later Development of the Branchial Arches, and the
Development of the Face. — In the embryo shown in Fig. 56, the
four branchial clefts and five arches which develop in the human
embryo are visible in surface views, but in the Ruge embryo (Fig. 57)
it will be noticed that only the first two arches, the first with a welldeveloped maxillary process, and the cleft separating them can be
 
 
 
 
Fig. 61. — Head of Embryo of 6.9 mm.
na, Nasal pit; ps, precervical sinus.— (His.)
 
distinguished. This is due to a sinking inward of the region occupied by the three posterior arches so that a triangular depression,
the sinus pracervicalis, is formed on each side of what will later
become the anterior part of the neck region. This is well shown in
an embryo (Br 3 ) described by His which measured 6.9 mm. in
length and of which the anterior portion is shown in Fig. 61. The
anterior boundary of the sinus {ps) is formed by the posterior edge
7
 
 
 
98 DEVELOPMENT OF THE BRANCHIAL ARCHES
 
of the second arch and its posterior boundary by the thoracic wall,
and in later stages these two boundaries gradually approach one
another so as first of all to diminish the opening into the sinus and
later to completely obliterate it by fusing together, the sinus thus
becoming converted into a completely closed cavity whose floor is
formed by the ectoderm covering the three posterior arches and the
clefts separating these. This cavity eventually undergoes degeneration, no traces of it occurring normally in the adult, although
 
 
 
 
Fig. 62. — Face of Embryo of 8 mm.
mxp, Maxillary process; np, nasal pit; os, oral fossa; pg, processus globularis.— (His.)
 
certain cysts occasionally observed in the sides of the neck may
represent persisting portions of it.
 
A somewhat similar process results in the closure of the ventral
portion of the first cleft,* a fold growing backward from the posterior
edge of the first arch and fusing with the ventral part of the anterior
border of the second arch. The upper part of the cleft persists,
however, and, as already stated, forms the external auditory meatus,
the pinna of the ear being developed from the adjacent parts of
the first and second arches (Figs. 58 and 59).
 
* See page 91, small type.
 
 
 
DEVELOPMENT OF THE FACE
 
 
 
99
 
 
 
The region immediately in front of the first arch is occupied by
a rather deep depression, the oral fossa, whose early development
has already been noticed. In an embryo measuring 8 mm. in
length (Fig. 62) the fossa (os) has assumed a somewhat irregular
quadrilateral form. Its posterior boundary is formed by the
mandibular processes of the first arch, while laterally it is bounded
by the maxillary processes (mxp) and anteriorly by the free edge of
a median plate, termed the nasal process, which on either side of the
 
 
 
 
Fig. 63. — Face of Embryo after the Completion of the Upper Jaw. — (His.)
 
median line is elevated to form a marked protuberance, the processus
globular is (pg). The ventral ends of the maxillary processes are
widely separated, the nasal process and the processus globulares
intervening between them, and they are also separated from the
globular processes by a deep and rather wide groove which anteriorly
opens into a circular depression, the nasal pit (np).
 
 
 
IOO DEVELOPMENT OF THE LIMBS
 
Later on the maxillary and globular processes unite, obliterating
the groove and cutting off the nasal pits — which have by this time
deepened to form the nasal fossae — from direct communication
with the mouth, with which, however, they later make new communications behind the maxillary processes, an indication of the
anterior and posterior nares being thus produced.
 
Occasionally the maxillary and globular processes fail to unite on one
or both sides, producing a condition popularly known as "harelip."
 
At the time when this fusion occurs the nasal fossa? are widely
separated by the broad nasal process (Fig. 63), but during later
development this process narrows to form the nasal septum and is
gradually elevated above the general surface of the face as shown
in Figs. 58-60. By the narrowing of the nasal process the globular
processes are brought nearer together and form the portions of the
upper jaw immediately on each side of the median line, the rest
of the jaw being formed by the maxillary processes. In the meantime a furrow has appeared upon the mandibular process, running
parallel with its borders (Fig. 59); the portion of the process in front
of this furrow gives rise to the lower lip and is known as the lip
ridge, while the portion behind the furrow becomes the lower jaw
proper and is termed the chin ridge.
 
The Development of the Limbs. — As has been already pointed
out, the limbs make their appearance in an embryo measuring about
4 mm. in length (Fig. 54) and are at first bud-like in form. As they
increase in length they at first have their long axes directed parallel
to the longitudinal axis of the body and become somewhat flattened
at their free ends, remaining cylindrical in their proximal portions.
A furrow or constriction appears at the junction of the flattened and
cylindrical portions (Fig. 57), and later a second constriction divides
the cylindrical portion into a proximal and distal moiety, the three
segments of each limb — the arm, forearm, and hand in the upper
limb, and the thigh, leg, and foot in the lower — being thus marked
out. The digits are first indicated by the development of four
radiating shallow grooves upon the hand and foot regions (Fig. 58),
 
 
 
DEVELOPMENT OF THE LIMBS IOI
 
and a transverse furrow uniting the proximal ends of the digital
furrows indicates the junction of the digital and palmar regions of
the hand or of the toes and body of the foot. After this stage is
reached the development of the upper limb proceeds more rapidly
than that of the lower, although the processes are essentially the
same in both limbs. The digits begin to project slightly, but are at
first to a very considerable extent united together by a web, whose
further growth, however, does not keep pace with that of the digits,
these thus coming to project more and more in later stages. Even
in comparatively early stages the thumb, and to a somewhat slighter
extent the great toe, is widely separated from the second digit
(Figs. 59 and 60).
 
While these changes have been taking place the entire limbs
have altered their position with reference to the axis of the body,
being in stages later than that shown in Fig. 57 directed ventrally
so that their longitudinal axes are at right angles to that of the body.
From the figures of later stages it may be seen that it is the thumb
(radial) side of the arm and the great toe (tibial) side of the leg
which are directed forward; the plantar and palmar surfaces of
the feet and hands are turned toward the body and the elbow is
directed outward and slightly backward, while the knee looks
outward and slightly forward. It seems proper to conclude that
the radial side of the arm is homologous with the tibial side of the
leg, the palmar surface of the hand with the plantar surface of the
foot, and the elbow with the knee.
 
The limbs are, however, still in the quadrupedal condition, and
they must later undergo a second alteration in position so that their
long axes again become parallel with that of the body. This is accomplished by a rotation of the limbs around axes passing through the
shoulders and hip-joints, together with a rotation about their longitudinal axes through an angle of 90 degrees. This axial rotation of
the upper limb is, however, in exactly the opposite direction to that
of the lower limb of the corresponding side, so that the homologous
surfaces of the two limbs have entirely different relations, the radial
side of the arm, for instance, being the outer side while the tibial side
 
 
 
102 AGE OF EMBRYO AT DIFFERENT STAGES
 
of the leg is the inner side, and whereas the palmar surface of the
hand looks ventrally, the plantar surface of the foot looks dorsally.
In making these statements no account is taken of the secondaryposition which the hand may assume as the result of its pronation;
the positions given are those assumed by the limbs when both the
bones of their middle segment are parallel to one another.
 
It may be pointed out that the prevalent use of the physiological
terms flexor and extensor to describe the surfaces of the limbs has a
tendency to obscure their true morphological relationships. Thus if,
as is usual, the dorsal surface of the arm be termed its extensor surface,
then the same term should be applied to the entire ventral surface of the
leg, and all movements of the lower limb ventrally should be spoken of as
movements of extension and any movement dorsally as movements of
flexion. And yet a ventral movement of the thigh is generally spoken of
as a flexion of the hip-joint, while a straightening out of the foot upon
the leg — that is to say, a movement of it dorsally — is termed its extension.
 
The Age of the Embryo at Different Stages. — The age of an
 
embryo must be dated from the moment of fertilization and from
what has been said in preceding pages (pp. 27, 34) it is evident that
it must be difficult to determine the exact date of this event from
that of the cessation of the menses, or even when the date of the
coition that resulted in pregnancy is known. And, furthermore,
not only is the actual date of the beginning of development uncertain,
but in the majority of known early human embryos the time of the
cessation of development is also more or less uncertain, since so
many of these embryos are abortions and their expulsion need not
necessarily have immediately succeeded their death.
 
These various sources of uncertainty are of especial importance
in the cases of embryos in the early stages of development, when a
day more or less means much, and it seems probable that many of the
estimated ages given for young embryos, based on the date of the
last menstruation, are too low. This certainly is the case with the
ages assigned to such embryos by His, who estimated embryos of
2.2 to 3.0 mm. to be two to two and one-half weeks old, those of
5.0 to 6.0 mm. to be about three and one-half weeks and those of
10.0 to 11.0 mm. to be about four and one-half weeks.
 
 
 
AGE OF EMBRYO AT DIFFERENT STAGES
 
 
 
103
 
 
 
There are on record, however, a few cases in which the date of the
fruitful coition is definitely known, and from these, few though they
be, somewhat more definite information may be obtained. Thus
it is fairly certain that the Bryce-Teacher ovum, with an embryo
measuring about 0.15 mm. in length, was the result of a coition
which took place sixteen days before the ovum was aborted, and one
cannot be far astray in assuming the embryo to be about two weeks
old. Similarly, an embryo described by Eternod and measuring
1.3 mm. in length was the result of a single coition occurring twentyone days previously and its age may be set at approximately three
weeks or better at eighteen or nineteen days. A later embryo in
which the nape bend and the coiling of the body had appeared and
which measured 8.8 mm. in vertex-breech length, resulted from a
single coitus that took place thirty-eight days before the abortion,
so that the embryo may be regarded as having been somewhat more
than five weeks old. These and two other similar cases may be
combined into a table thus:
 
 
 
Length of embryo
 
 
Days intervening
 
 
Probable age in
 
 
Authority
 
 
in mm.
 
 
between coition
 
 
days
 
 
 
 
and abortion
 
 
 
 
 
 
About 0.15
 
 
16J
 
 
i3- J 4
 
 
Bryce-Teacher.
 
 
i-3
 
 
21
 
 
18-19
 
 
Eternod
 
 
V. B. 8.8
 
 
38
 
 
37
 
 
Tandler.
 
 
V. B. 14.0
 
 
47
 
 
44-45
 
 
Rabl.
 
 
V. B. 25.0
 
 
56
 
 
53-54
 
 
Mall.
 
 
 
If, on the basis of these figures, one may venture to estimate the
age of embryos of other lengths those of 2.0 to 3.0 mm. may be
supposed to belong to the fourth week of development, those of
5.0 to 6.0 vertex-breech length to the latter part of the fifth week,
those of 10.0 mm. to the end of the sixth week and those of 25.0 to 28.0
mm. which are just passing into the fetus stage, to the end of the
eighth week. As regards the later periods of development, the
 
 
 
104 LITERATURE
 
limits of error for any date become of less importance. Schroder
gives the following measurements as the average:
 
3d lunar month 70-90 mm.
 
4th lunar month ' 100-170 mm.
 
5th lunar month 180-270 mm.
 
6th lunar month 280-340 mm.
 
7th lunar month 350-380 mm.
 
8th lunar month 425 mm.
 
9th lunar month 467 mm.
 
10th lunar month 490-500 mm.
 
The data concerning the weight of embryos of different ages are
as yet very insufficient, and it is well known that the weights of newborn children may vary greatly, the authenticated extremes being,
according to Vierordt, 717 grams and 6123 grams. It is probable
that considerable variations in weight occur also during fetal life.
So far as embryos of the first two months are concerned, the data are
too imperfect for tabulation; for later periods Fehling gives the
following as average weights:
 
3d month 20 grams.
 
4th month 120 grams.
 
5th month 285 grams.
 
6th month 635 grams.
 
7th month 1220 grams.
 
8th month 1700 grams.
 
9th month 2240 grams.
 
10th month 3 2 5° grams.
 
and the results obtained by Jackson are essentially similar.
 
LITERATURE.
 
In addition to the papers of Bryce and Teacher, Eternod, Fetzer, Frassi, Herzog,
Peters, Von Spee and Strahl and Beneke cited in the preceding chapter, the following
may be mentioned:
 
Bremer: "Description of a 4 mm. Human Embryo," Amer. Journ. Anal., v, 1906.
J. Broman: "Beobachtung eines menschlichen Embryos von beinahe 3 mm. Lange
 
mit specieller Bemerkung uber die bei demselben befindlichen Hirnfalten,"
 
Morpholog. Arbeiten, v, 1895.
A. J. P. van den Broek: "Zur Kasuistik junger menschlicher Embryonen," Anal,.
 
Hefte, xliv, 191 1.
 
 
 
LITERATURE 105
 
J. M. Coste: " Histoire generale et particuliere du developpement des corps organises,"
 
Paris, 1847-1859.
W. E. Dandy: "A Human Embryo with Seven Pairs of Somites, Measuring about
 
2 mm. in Length," Amer. Joiirn. Anal., x, 1910.
A. Ecker: "Beitrage zur Kenntniss der ausserer Formen jiingster menschlichen
 
Embryonen," Archiv fur Anat. und Physiol., Anat. Abth., 18S0.
C. Elze: " Beschreibung eines menschlichen Embryos von zirka 7 mm. grosster Lange,"
 
Anat. Hefte, xxxv, 1907.
C. Giacomini: "Un ceuf humain de 11 jours," Archives Hal. de Biologie, xxix, 1898.
V. Hensen: "Beitrag zur Morphologie der Korperform und des Gehirns des
 
menschlichen Embryos," Archiv fur Anat. und Physiol., Anat. Abth., 1877.
W. His: "Anatomie menschlicher Embryonen," Leipzig, 1880.
F. Hochstetter: "Bilder der ausseren Korperform einiger menschlicher Embryonen
 
aus den beiden Ersten Monaten der Entwicklung," Munich, 1907.
N. W. Ingalls: "Beschreibung eines menschlichen Embryos von 4.9 mm.," Arch.
 
fiir mikr. Anat., lxx, 1907.
C. M. Jackson: " On the Prenatal Growth of the Human Body and the Relative Growth
 
of the Various Organs and Parts," Amer. Journ. Anat., ix, 1909.
J. Janosik: "Zwei junge menschliche Embryonen," Archiv fiir mikrosk. Anat., xxx,
 
1887.
H. E Jordan: "Description of a 5 mm. Human Embryo," Anat. Record, ill, 1909.
P. Jung: "Beitrage zur friihesten Ei-einbettung beim menschlichen Weibe," Berlin,
 
1908.
F. Keibel: "Ein sehr junges menschliches Ei," Archiv fiir Anat. und Physiol., Anat.
 
Abth., 1890.
F. Keibel: "Ueber einen menschlichen Embryo von 6.8 mm. grosster Lange,"
 
Verhandl. Anatom. Gesellsch., xiii, 1899.
F. Keibel and C. Elze: " Normentafeln zur Entwicklungsgeschichte der Wirbeltiere,"
 
Heft viii, 1 90S.
J. Kollmann: "Die Korperform menschlicher normaler und pathologischer Embryonen," Archiv fur Anat. und Physiol., Anat Abth., Supplement, 18S9.
A. Low: "Description of a Human Embryo of 13-14 Mesodermic Somites," Journ.
 
Anat. and Phys., xlii, 1908.
F. P. Mall: "A Human Embryo Twenty-six Days Old," Journ. of Morphology, V,
 
1891.
F. P. Mall: "A Human Embryo of the Second Week," Anat. Anzeiger, viii, 1893.
F. P. Mall: "Early Human Embryos and the Mode of their Preservation," Bulletin of
 
the Johns Hopkins Hospital, XV, 1S94.
C. S. Minot: "Human Embryology," New York, 1892.
J. Muller: " Zergliederungen menschlicher Embryonen aus friiherer Zeit," Archiv
 
fiir Anat. und Physiol., 1830.
C. Phisalix: "Etude d'un Embryon humain de 11 millimeters," Archives de zoolog.
 
experimentale et generale, Ser. 2, vi, 1888.
H. Piper: "Ein menschlicher Embryo von 6.8 mm. Nackenlinie," Archiv fiir Anat.
 
und Physiol., Anat. Abth,, 1898.
 
 
 
106 LITERATURE
 
C. Rabl: "Die Entwicklung des Gesichtes, Heft i, Das Gesicht der Saugetiere,
 
Leipzig, 1902.
G. Retzitts: "Zur Kenntniss der Entwicklung der Korperformen des Menschen
 
wahrend der fotalen Lebensstufen," Biolog. Untersuch., xi, 1904.
J. Tandler: "Ueber einen menschlichen Embryo von 38 Tage," Anat. Anzeiger,
 
xxxi, 1907.
Allen Thompson: "Contributions to the History of the Structure of the Human
 
Ovum and Embryo before the Third Week after Conception, with a Description
 
of Some Early Ova," Edinburgh Med. and Surg. Journal, in, 1839. (See also
 
Froriep's Neue Notizen, xiu, 1840.)
P. Thompson: "Description of a human embryo of twenty-three paired somites,"
 
Journ. Anat. and Phys., xli, 1907.
 
 
 
CHAPTER V.
 
THE YOLK -STALK, BELLY-STALK, AND FETAL
MEMBRANES.
 
The conditions to which the embryos and larvse of the majority
of animals must adapt themselves are so different from those under
which the adult organisms exist that in the early stages of development special organs are very frequently developed which are of use
only during the embryonic or larval period and are discarded when
more advanced stages of development have been reached. This
remark applies with especial force to the human embryo which leads
for a period of nine months what may be termed a parasitic existence,
drawing its nutrition from and yielding up its waste products to the
blood of the parent. In- order that this may be accomplished certain special organs are developed by the embryo, by means of which
it forms an intimate connection with the walls of the uterus, which,
on its part, becomes greatly modified, the combination of embryonic
and maternal structures producing what are termed the deciduce,
owing to their being discarded at birth when the parasitic mode of
life is given up.
 
Furthermore, it has already been seen that many peculiar modifications of development in the human embryo result from the inheritance of structures from more or less remote ancestors, and among
the embryonic adnexes are found structures which represent in a
more or less modified condition organs of considerable functional
importance in lower forms. Such structures are the yolk-stalk and
vesicle, the amnion, and the allantois, and for their proper understanding it will be well to consider briefly their development in some
lower form, such as the chick.
 
At the time when the embryo of the chick begins to be constricted off from the surface of the large yolk-mass, a fold, consisting
 
107
 
 
 
io8
 
 
 
YOLK-STALK AND FETAL MEMBRANES
 
 
 
of ectoderm and somatic mesoderm, arises just outside the embryonic
area, which it completely surrounds. As development proceeds the
fold becomes higher and its edges gradually draw nearer together
over the dorsal surface of the embryo (Fig. 64, A, Af), and finally
meet and fuse (Fig. 64, B and C), so that the embryo becomes
enclosed within a sac, which is termed the amnion and is formed by
the fusion of the layers which constituted the inner wall of the fold.
The layers of the outer wall of the fold after fusion form part of the
 
 
 
 
Fig. 64. — Diagrams Illustrating the Formation of the Amnion and Allantois
 
in the Chick.
Af, Amnion folds; Al, allantois; Am, amniotic cavity; Ds, yolk-sac. — (Cegenbaur.)
 
 
 
general ectoderm and somatic mesoderm which make up the outer
wall of the ovum and together are known as the serosa, corresponding to the chorion of the mammalian embryo. The space which
occurs between the amnion and the serosa is a portion of the extraembryonic ccelom and is continuous with the embryonic pleuroperitoneal cavity.
 
In the ovum of the chick, as in that of the reptile, the protoplasmic material is limited to one pole and rests upon the large yolk
 
 
THE AMNION IO9
 
mass. As development proceeds the germ layers gradually extend
around the yolk-mass and eventually completely enclose it, the yolkmass coming to lie within the endodermal layer, which, together
with the splanchnic mesoderm which lines it, forms what is termed
the yolk-sac. As the embryo separates from the yolk-mass the yolksac is constricted in its proximal portion and so differentiated into a
yolk-stalk and a yolk-sac, the contents of the latter being gradually
absorbed by the embryo during its growth, its walls and those of the
stalk being converted into a portion of the embryonic digestive tract.
 
In the meantime, however, from the posterior portion of the
digestive tract, behind the point of attachment of the yolk-sac, a
diverticulum has begun to form (Fig. 64, A, Al). This increases in
size, projecting into the extra-embryonic portion of the pleuroperitoneal cavity and pushing before it the splanchnic mesoderm which
lines the endoderm (Fig. 64, B and C) . This is the allantois, which,
reaching a very considerable size in the chick and applying itself
closely to the inside of the serosa, serves as a respiratory and excretory organ for the embryo, for which purpose its walls are richly
supplied with blood-vessels, the allantoic arteries and veins.
 
Toward the end of the incubation period both the amnion and
allantois begin to undergo retrogressive changes, and just before
the hatching of the young chick they become completely dried up
and closely adherent to the egg-shell, at the same time separating
from their point of attachment to the body of the young chick, so
that when the chick leaves the egg-shell it bursts through the driedup membranes and leaves them behind as useless structures.
 
The Amnion. — Turning now to the human embryo, it will be
found that the same organs are present, though somewhat modified
either in the mode or the extent of their development. A welldeveloped amnion occurs, arising, however, in a very different manner from what it does in the chick; a large yolk-sac occurs even
though it contains no yolk; and an allantois which has no respiratory
or excretory functions is present, though in a somewhat degenerated
condition. It has been seen from the description of the earliest
stages of development that the processes which occur in the lowe
 
 
 
IIO THE AMNION
 
forms are greatly abbreviated in the human embryo. The enveloping layer, instead of gradually extending from one pole to enclose
the entire ovum, develops in situ during the stages immediately
succeeding segmentation, and the extra-embryonic mesoderm,
instead of growing out from the embryo to enclose the yolk-sac,
splits off directly from the enveloping layer. The earliest stages in
the development of the amnion are not yet known for the human
embryo, but from the condition in which it is found in the Peters
embryo (Fig. 37) and in the embryo v.H. of von Spee (Fig. 39) it
is probable that it arises, not by the fusion of the edges of a fold, as
in the chick, but by a vacuolization of a portion of the inner cellmass, as has been described as occurring in the bat (p. 54). It is,
then, a closed cavity from the very beginning, the floor of the cavity
being formed by the embryonic disk, its posterior wall by the
anterior surface of the belly-stalk, while its roof and sides are thin
and composed of a single layer of flattened ectodermal cells lined
on the outside by a layer of mesoderm continuous with the somatic
mesoderm of the embryo and the mesoderm of the belly-stalk
(Fig. 65, A).
 
When the bending downward of the peripheral portions of the
embryonic disk to close in the ventral surface of the embryo occurs,
the line of attachment of the amnion to the disk is also carried
ventrally (Fig. 65, B), so that when the constriction off of the embryo
is practically completed, the amnion is attached anteriorly to the
margin of the umbilicus and posteriorly to the extremity of the band
of ectoderm lining what may now be considered the posterior
surface of the belly-stalk, while at the sides it is attached along an
oblique line joining these two points (Fig. 65, B and C, in which the
attachment of the amnion is indicated by the broken line).
 
Leaving aside for the present the changes which occur in the
attachment of the amnion to the embryo (see p. 116), it may be
said that during the later growth of the embryo the amniotic cavity
increases in size until finally its wall comes into contact with the
chorion, the extra-embryonic body-cavity being thus practically
obliterated (Fig. 65, D), though no actual fusion of amnion and
 
 
 
THE AMNION
 
 
 
III
 
 
 
chorion occurs. Suspended by the umbilical cord, which has by
this time developed, the embryo floats freely in the amniotic cavity,
which is filled by a fluid, the liquor amnii, whose origin is involved
in doubt, some authors maintaining that it infiltrates into the cavity
from the maternal tissues, while others hold that a certain amount
 
 
 
 
Fig. 65. — Diagrams Illustrating the Formation of the Umbilical Cord.
 
The heavy black line represents the embryonic ectoderm; the dotted line represents
the line of reflexion of the body ectoderm into that of the amnion. Ac, Amniotic cavity ;
Al, allantois; Be, extra-embryonic ccelom; Bs, belly-stalk; Ch, chorion; P, placenta; Uc,
umbilical cord; V, chorionic villi; Ys, yolk-sac.
 
 
 
of it at least is derived from the embryo. It is a fluid with a specific
gravity of about 1.003 an( ^ contains about 1 per cent, of solids,
principally albumin, grape-sugar, and urea, the last constituent
probably coming from the embryo. When present in greatest
quantity — that is to say, at about the beginning of the last month
 
 
 
112 THE YOLK-SAC
 
of pregnancy — it varies in amount between one-half and threefourths of a liter, but during the last month it diminishes to about
half that quantity. To protect the epidermis of the fetus from
maceration during its prolonged immersion in the liquor amnii, the
sebaceous glands of the skin at about the sixth month of development pour out upon the surface of the body a white fatty secretion
known as the vernix caseosa.
 
During parturition the amnion, as a rule, ruptures as the result
of the contraction of the uterine walls and the liquor amnii escapes
as the "waters," a phenomenon which normally precedes the
delivery of the child. As a rule, the rupture is sufficiently extensive
to allow the passage of the child, the amnion remaining behind in
the uterus, to be subsequently expelled along with the deciduae.
 
Occasionally it happens, however, that the amnion is sufficiently
strong to withstand the pressure exerted upon it by the uterine contractions
and the child is born still enveloped in the amnion, which, in such cases,
is popularly known as the "caul," the possession of which, according to
an old superstition, marks the child as a favorite of fortune.
 
As stated above, the liquor amnii varies considerably in amount in
different cases, and occasionally it may be present in excessive quantities,
producing a condition known as hydramnios. On the other hand, the
amount may fall considerably below the normal, in which case the amnion
may form abnormal unions with the embryo, sometimes producing
malformations. Occasionally also bands of a fibrous character traverse
the amniotic cavity and, tightening upon the embryo during its growth,
may produce various malformations, such as scars, splitting of the eyelids
or lips, or even amputation of a limb.
 
The Yolk-sac. — The probable mode of development of the
yolk-sac in the human embryo, and its differentiation into yolk-stalk
and yolk- vesicle have already been described (p. 86). When these
changes have been completed, the vesicle is a small pyriform structure
lying between the amnion and the chorionic mesoderm, some distance away from the extremity of the umbilical cord (Fig. 65, D),
and the stalk is a long slender column of cells extending from the
vesicle through the umbilical cord to unite with the intestinal
tract of the embryo. The vesicle persists until birth and may be
found among the decidual tissues as a small sac measuring from 3 to
 
 
 
THE ALLANTOIS AND BELLY-STALK II3
 
10 mm. in its longest diameter. The stalk, however, early undergoes degeneration, the lumen which it at first contains becoming
obliterated and its endoderm also disappearing as early as the end
of the second month of development. The portion of the stalk
which extends from the umbilicus to the intestine usually shares in
the degeneration and disappears, but in about 3 per cent, of cases it
persists, forming a more or less extensive diverticulum of the lower
part of the small intestine, sometimes only half an inch or so in
length and sometimes much larger. It may or may not retain connection with the abdominal wall at the umbilicus, and is known as
Meckel's diverticulum.
 
This embryonic rudiment is of no little importance, since, when
present, it is apt to undergo invagination into the lumen of the small
intestine and so occlude it. How frequently this happens relatively to
the occurrence of the diverticulum may be judged from the fact that out
of one hundred cases of occlusion of the small intestine six were due to an
invagination of the diverticulum.
 
In the reptiles and birds the yolk-sac is abundantly supplied with
blood-vessels by means of which the absorption of the yolk is carried
on, and even although the functional importance of the yolk-sac as
an organ of nutrition is almost nil in the human embryo, yet it
still retains a well-developed blood-supply, the walls of the vesicle,
especially possessing a rich network of vessels. The future history
of these vessels, which are known as the vitelline vessels, will be
described later on.
 
The Allantois and Belly-stalk. — It has been seen that in
reptilian and avian embryos the allantois reaches a high degree of
development and functions as a respiratory and excretory organ by
coming into contact with what is comparable to the chorion of the
mammalian embryo. In man it is very much modified both in its
mode of development and in its relations to other parts, so that its
resemblance to the avian organ is somewhat obscured. The differences depend partly upon the remarkable abbreviation manifested
in the early development of the human embryo and partly upon the
fact that the allantois serves to place the embryo in relation with the
8
 
 
 
U4
 
 
 
THE . ALLANTOIS AND BELLY-STALK
 
 
 
maternal blood, instead of with the external atmosphere, as is the
case in the egg-laying forms. Thus, the endodermal portion of the
allantois, instead of arising from the intestine and pushing before
it a layer of splanchnic mesoderm to form a large sac lying freely in
the extra-embryonic portion of the body-cavity, appears in the human
embryo before the intestine has differentiated from the yolk-sac and
pushes its way into the solid mass of mesoderm which forms the
belly-stalk (Fig. 65, A). To understand the significance of this process it is necessary to recall the abbreviation in the human embryo of
the development of the extra-embryonic mesoderm and body-cavity.
Instead of growing out from the embryonic area, as it does in the
lower forms, this mesoderm develops in situ by splitting off from
the layer of enveloping cells and, furthermore, the extra-embryonic
 
body-cavity arises by a splitting of the
mesoderm so formed before there is any
trace of a splitting of the embryonic
mesoderm (Fig. 38). The belly-stalk,
whose development from a portion of
the inner cell-mass has already been
traced (p. 68), is to be regarded as a
portion of the body of the embryo,
since the ectoderm which covers one
surface of it resembles exactly that of
the embryonic disk and shows an extension backward of the medullary
groove upon its surface (Fig. 66). The
mesoderm, therefore, of the belly-stalk
is to be regarded as a portion of the embryonic mesoderm which has
not yet undergone a splitting into somatic and splanchnic layers,
and, indeed, it never does undergo such a splitting, so that there is
no body-cavity into which the endodermal allantoic diverticulum
can grow.
 
But this does not account for all the peculiarities of the human
allantois. In the birds, and indeed in the lower oviparous mammals,
the endodermal portion of the allantois is equally developed with
 
 
 
 
Fig. 66. — Transverse Section THROUGH THE BELLY-STALK
 
of an Embryo of 2.15 mm.
 
Aa, Umbilical (allantoic)
artery; All, allantois; am, amnion; Va, umbilical (allantoic)
vein. — (His.)
 
 
 
THE ALLANTOIS AND BELLY-STALK 115
 
the mesodermal portion, the allantois being an extensive sac whose
cavity is rilled with fluid, and this is also true of such mammals as
the marsupials, the rabbit, and the ruminants. In man, however,
the endodermal diverticulum never becomes a sac-like structure,
but is a slender tube extending from the intestine to the chorion and
lying in the substance of the mesoderm of the belly-stalk (Fig. 65,
D), the greater portion of which is to be regarded as homologous
with the relatively thin layer of splanchnic mesoderm covering the
endodermal diverticulum of the chick. An explanation of this
disparity in the development of the mesodermal and endodermal
portions of the human allantois is perhaps to be found in the altered
conditions under which the respiration and secretion take place.
In all forms, the lower as well as the higher, it is the mesoderm which
is the more important constituent of the allantois, since in it the
blood-vessels, upon whose presence the physiological functions
depend, arise and are embedded. In the birds and oviparous
mammals there are no means by which excreted material can be
passed to the exterior of the ovum, and it is, therefore, stored up
within the cavity of the allantois, the allantoic fluid containing
considerable quantities of nitrogen, indicating the presence of urea.
In the higher mammals the intimate relations which develop between
the chorion and the uterine walls allow of the passage of excreted
fluids into the maternal blood; and the more intimate these relations,
the less necessity there is for an allantoic cavity in which excreted
fluid may be stored up. The difference in the development of the
cavity in the ruminants, for example, and man depends probably
upon the greater intimacy of the union between ovum and uterus
in the latter, the arrangement for the passage of the excreted material
into the maternal blood being so perfect that there is practically no
need for the development of an allantoic cavity.
 
The portion of the endodermal diverticulum which is enclosed
within the umbilical cord persists until birth in a more or less
rudimentary condition, but the intra-embryonic portion extending
from the apex of the bladder to the umbilicus becomes converted
into a solid cord of fibrous tissue termed the urachus.
 
 
 
Il6 THE UMBILICAL CORD
 
Occasionally a lumen persists in the urachal portion of the allantois
and may open to the exterior at the umbilicus, in which case urine from
the bladder may escape at the umbilicus.
 
Since the allantois in the human embryo, as well as in the lower
forms, is responsible for respiration and excretion, its blood-vessels
are well developed. They are represented in the belly-stalk by
two veins and two arteries (Fig. 66), known in human embryology
as the umbilical veins and arteries. These extend from the body of
the embryo out to the chorion, there branching repeatedly to enter
the numerous chorionic villi by which the embryonic tissues are
placed in relation with the maternal.
 
The Umbilical Cord. — During the process of closing in of the
ventral surface of the embryo a stage is reached in which the embryonic and extra-embryonic portions of the body-cavity are
completely separated except for a small area, the umbilicus, through
which the yolk-stalk passes out (Fig. 65, B). At the edges of this
area in front and at the sides the embryonic ectoderm and somatic
mesoderm become continuous with the corresponding layers of the
amnion, but posteriorly the line of attachment of the amnion passes
up upon the sides of the belly-stalk (Fig. 65, B), so that the whole of
the ventral surface of the stalk is entirely uncovered by ectoderm,
this layer being limited to its dorsal surface (Fig. 66). In subsequent stages the embryonic ectoderm and somatic mesoderm at
the edges of the umbilicus grow out ventrally, carrying with them
the line of attachment of the amnion and forming a tube which
encloses the proximal part of the yolk-stalk. The ectoderm of the
belly-stalk at the same time extending more laterally, the condition
represented in Fig. 65, C, is produced, and, these processes continuing, the entire belly-stalk, together with the yolk-stalk, becomes
enclosed within a cylindrical cord extending from the ventral
surface of the body to the chorion and forming the umbilical cord
(Fig. 65, D).
 
From this mode of development it is evident that the cord is,
strictly speaking, a portion of the embryo, its surfaces being completely covered by embryonic ectoderm, the amnion being carried
 
 
 
THE UMBILICAL CORD
 
 
 
117
 
 
 
 
-uv
 
 
 
ua.
 
 
 
 
lev
 
 
 
Fig. 67. — -Transverse Sections of the Umbilical Cord of Embryos of (A) 1.8 cm.
 
and (B) 25 cm.
al, Allantois; c, coelom; ua, umbilical artery; uv, umbilical vein; ys, yolk-stalk.
 
 
 
Il8 THE CHORION
 
during its formation further and further from the umbilicus until
finally it is attached around the distal extremity of the cord.
 
In enclosing the yolk-stalk the umbilical cord encloses also a
small portion of what was originally the extra-embryonic bodycavity surrounding the yolk-stalk. A section of the cord in an early
stage of its development (Fig. 67, A) will show a thick mass of
mesoderm occupying its dorsal region; this represents the mesoderm
of the belly-stalk and contains the allantois and the umbilical
arteries and vein (the two veins originally present in the belly-stalk
having fused), while toward the ventral surface there will be seen a
distinct cavity in which lies the yolk-stalk with its accompanying
blood-vessels. The portion of this ccelom nearest the body of the
embryo becomes much enlarged, and during the second month of
development contains some coils of the small intestine, but later the
entire cavity becomes more and more encroached upon by the
growth of the mesoderm, and at about the fourth month is entirely
obliterated. A section of the cord subsequent to that period of
development will show a solid mass of mesoderm in which are
embedded the umbilical arteries and vein, the allantois, and the
rudiments of the yolk-stalk (Fig. 67, B).
 
When fully formed, the umbilical cord measures on the average
55 cm. in length, though it varies considerably in different cases, and
has a diameter of about 1.5 cm. It presents the appearance of being
spirally twisted, an appearance largely due, however, to the spiral
course pursued by the umbilical arteries, though the entire cord may
undergo a certain amount of torsion from the movements of the
embryo in the later stages of development and may even be knotted.
The greater part of its substance is formed by the mesoderm, the
cells of which become stellate and form a recticulum, the meshes
of which are occupied by connective-tissue fibrils and a mucous fluid
which gives to the tissue a jelly-like consistence, whence it has received the name of Wharton's jelly.
 
The Chorion. — To understand the developmental changes
which the chorion undergoes it will be of advantage to obtain some
insight into the manner in which the ovum becomes implanted in
 
 
 
THE CHORION
 
 
 
II 9
 
 
 
the wall of the uterus. Nothing is known as to how this implantation is effected in the case of the human ovum; it has already been
accomplished in the youngest ovum at present known. But the
process has been observed in other mammals, and what takes place
in Spermophilus, for example, may be supposed to give a clue to
what occurs in the human ovum. In the spermophile the ovum
lies free in the uterine cavity up to a stage at which the vacuolization
 
 
 
 
* I
 
 
 
 
 
 
 
 
 
Fig. 68. — Successive Stages in the Implantation of the Ovum of the Spermophile .
a, syncytial knob; k, inner cell-mass. — (Rejsek.)
 
of the central cells is almost completed (Fig. 68, A). At one region
of the covering layer the cells become thicker and later form a syncytial projection or knob which comes into contact with the uterine
mucosa (Fig. 68, B), and at the point of contact the mucosa cells
undergo degeneration, allowing the knob to come into relation with
the deeper tissues of the uterus (Fig. 68, C), the process apparently
being one in which the mucosa cells are eroded by the syncytial knob.
It seems probable that in the human ovum the process is at first
of a similar nature and that as the covering layer cells come into
 
 
 
120
 
 
 
THE CHORION
 
 
 
 
c
 
Fig. 69. — Diagrams Illustrating the Implantation of the Ovum.
ac, amniotic cavity; bs, belly-stalk; cf, chorion frondosum; cl, chorion laeve;Jc,
decidua capsularis; ic, inner cell-mass; s, space surrounding ovum which becomes the
intervillous space; um, uterine mucosa; v, chorionic villus; ys, yolk-sac.
 
 
 
THE CHORION 121
 
contact with the deeper layers of the uterus, these too are eroded, and,
the uterine blood-vessels being included in the erosion process, an
extravasation of blood plasma and corpuscles occurs in the vicinity
of the burrowing ovum. In the meantime the ovum has increased
considerably in size, its growth in these early stages being especially
rapid, and the area of contact consequently increases in size, entailing
continued erosion of the uterine mucosa. At the same time, too,
the uterine tissues surrounding the ovum grow up around it, forming
at first as it were a circular wall (Fig. 69, A), and eventually com
Sch.
 
 
 
â– -K
 
 
 
 
 
 
 
 
^,^^f^>r%^^ c y
 
 
 
iM^fe^
 
 
 
X,#'
 
 
 
 
 
 
 
 
 
 
 
 
w
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 70. — Section of an Ovum of i mm. A Section of the Embryo Lies in the
 
Lower Part of the Cavity of the Ovum.
 
D, Decidua; E.U., uterine epithelium; Sch, blood-clot closing the aperture left by
 
the sinking of the ovum into the uterine mucosa. — (From Strahl, after Peters.)
 
pletely enclose it, forming an envelope known as the decidua capsularis or rejiexa. The blood extravasation is now contained within
a closed space bounded on the one hand by the uterine tissues and
on the other by the wall of the ovum (Fig. 69, B).
 
The youngest known human ova have already reached approxi
 
 
122 THE CHORION
 
mately this stage. Thus, the Peters ovum (Fig. 70) had already
sunk deeply into the uterine mucosa, the point of entrance being
indicated by a gap in the decidua capsularis, closed in this case by a
patch of coagulated blood (Sch). The uterine tissues in the immediate vicinity of the ovum were much swollen and apparently somewhat necrotic and their blood-vessels could be seen to communicate
with the space between the wall of the ovum and the maternal tissues.
This space, however, was converted into an irregular network of
blood lacunae by anastomosing cords of cells, which arose from the
wall of the ovum and extended through the space to the maternal
tissues ; these cords of cells are represented in Fig. 70 by the darker
masses projecting from the wall of the ovum and scattered among
the paler blood lacunae. This stage of implantation of the ovum is
shown diagrammatically in Fig. 69, B, where, for simplicity's sake,
the cell cords are represented merely as processes radiating from
the ovum without reaching the maternal tissues.
 
The cell cords are derivatives of the trophoblast and are, therefore, of embryonic origin. If examined under a higher magnification than that shown in Fig. 70 they will be seen to be composed of an
axial core of cells with distinct outlines, enclosed within a layer of
protoplasm which lacks all traces of cell boundaries, although it
contains numerous nuclei, being what is termed a syncytium or
Plasmodium. The original trophoblast has thus become differentiated into two distinct tissues, a cellular one, which has been termed
the cyto-trophoblast, and a plasmodial one, which, similarly, is
known as the plasmodi-trophoblast and is the tissue that comes into
contact with the maternal blood contained in the lacunar spaces and
with the maternal tissues, in connection with these latter sometimes
developing into masses of considerable extent. To this plasmoditrophoblast may be ascribed the active part in the destruction of
the maternal tissues and probably also the absorption of the products
of the destruction for the nutrition of the growing ovum. For up to
this stage the ovum has been playing the role of a parasite thriving
upon the tissues of^ its host.
 
The food material that the ovum thus obtains may conveniently
 
 
 
THE CHORION
 
 
 
123
 
 
 
be termed the embryotroph and the type of placentation which obtains
up to this stage and for some time longer may be termed the embryotrophic type. But even in the Peters ovum the preparation for
another type has begun. In earlier stages the cell cords were entirely
trophoblastic, but in this ovum (Fig. 70) processes from the chorionic
mesoderm may be seen projecting into the bases of the cell cords,
and in later stages these processes extend farther and farther into the
axis of each cord, the anastomoses of the cords disappear and the
cords themselves become converted into branching processes, the
 
 
 
 
Fig. 71. — Entire Ovum Aborted at about the Beginning of the Second
Month. Xi 1/2. — (Grosser.)
 
 
 
chorionic villi, which project from the entire surface of the ovum
(Fig. 71) into the surrounding space, which may now be termed the
intervillous space, and are bathed by the maternal blood which it
contains. Toward the maternal surface of the space some masses of
the trophoblast still persist, uniting the extremities of certain of the
villi to the enclosing uterine wall, such villi being termed fixation
villi to distinguish them from the majority, which project freely into
the intervillous space. Later, when the embryonic blood-vessels
 
 
 
124
 
 
 
THE CHORION
 
 
 
develop, those associated with the allantois extend outward into
the chorionic mesoderm and thence send branches into each villus.
The second type of placentation, the hcemotrophic type, is thus established, the fetal blood contained in the vessels of the villi receiving
nutrition through the walls of the villi from the maternal blood
contained in the intervillous space, and, similarly, transferring
waste products to it.
 
At first, as stated above, the villi usually cover the entire surface
of the ovum, but later, as the ovum increases in size, those villi
which are remote from the attachment of the belly-stalk to the chorion
are placed at a disadvantage so far as their blood supply is concerned
 
 
 
 
Fig. 72. — Two Villi prom the Chorion of an Embryo of 7 mm.
 
and gradually disappear, and this process continues until, finally,
only those villi are retained which are in the immediate region of
the belly-stalk (Fig. 69, C), these persisting to form the fetal portion
of the placenta. By these changes the chorion becomes differentiated into two regions (Fig. 69, C), one of which is destitute of villi and
is termed the chorion lave, while the other provided with them, is
known as the chorion frondosum.
 
 
 
THE CHORION
 
 
 
1^5
 
 
 
 
 
Fig. 73. — Transverse Sections through Chorionic Villi in (4) the Fifth
and (B) the Seventh Month of Development.
 
cf, Canalized fibrin; Ic, Langhans cells; s, syncytium. — (A which is more highly
magnified than B, from Szymonowicz; B from Minot.)
 
 
 
126 THE CHORION
 
Occasionally one or more patches of villi may persist in the area that
normally becomes the chorion lseve and thus accessory placenta {-placenta
succenturiatce) , varying in number and size, may be formed.
 
The villi when fully formed are processes of the chorion, branching profusely and irregularly (Fig. 72), and each consists of a core of
mesoderm, containing blood-vessels, enclosed within a double
layer of trophoblastic tissue (Fig. 73, A). The inner layer consists
of a sheet of well defined cells arranged in a single series; it is
derived from the cyto-trophoblast and forms what is known as the
layer of Langhans cells. The outer layer is syncytial in structure
and is formed from the plasmodi-trophoblast.
 
 
 
ck
 
 
 
Fig. 74. — Mature Placenta after Separation from the Uterus.
c, Cotyledons; eh, chorion, amnion, and decidua vera; urn, umbilical cord. — (Kollmann.)
 
As development proceeds the villi, which are at first distributed
evenly over the chorion frondosum, become separated into groups
termed cotyledons (Fig. 74) by the growth into the intervillous space
of trabecular from the walls of the uterus, the fixation villi becoming
connected with these septa as well as with the general uterine wall.
The ectoderm of the villi also undergoes certain changes with advancing growth, the layer of Langhans cells disappearing except in
small areas scattered irregularly in the villi, and the syncytium,
 
 
 
THE CHORION
 
 
 
127
 
 
 
though persisting, undergoes local thickenings which become
replaced, more or less extensively, by depositions of fibrin
(Fig. 73, B, cf).
 
The changes which occur during the later stages of development
in the chorion are very similar to those described for the villi.
 
 
 
 
 
 
 
 
 
TTIES
 
 
 
y"B*3??r^^Bi">
 
 
 
 
Fig. 75. — Section through the Placental Chorion of an Embryo of Seven
 
Months.
c, Cell layer; ep, remnants of epithelium; fb, fibrin layer; mes, mesoderm. — {Minot.)
 
Thus, the mesoderm thickens, its outermost layers becoming
exceedingly fibrillar in structure, while the ectoderm differentiates
into two layers, the outer of which is syncytial while the inner is
cellular, and later still, as in the villi, the syncytial layer is replaced
 
 
 
128 THE DECIDED
 
in irregular patches by a peculiar form of fibrin which is traversed
by flattened anastomosing spaces and to which the name canalized
fibrin or fibrinoid has been applied (Fig. 75).
 
The Deciduae. — It has been pointed out (p. 26) that in connection with the phenomenon of menstruation periodic alterations
occur in the mucous membrane of the uterus. If during one of
these periods a fertilized ovum reaches the uterus, the desquamation
 
 
 
 
Fig. 76. — Diagram showing the Relations of the Fetal Membranes.
 
Am, Amnion; Ch, chorion; M, muscular wall of uterus; C, decidua capsularis; B,
 
decidua basalis; V, decidua vera; F, yolk-stalk.
 
of portions of the epithelium does not occur nor is there any appreciable hemorrhage into the cavity of the uterus; the uterine mucosa
remains in what is practically the ante-menstrual condition until the
conclusion of pregnancy, when, after the birth of the fetus, a considerable portion of its thickness is expelled from the uterus, forming
what is termed the decidua. In other words, the sloughing of the
 
 
 
THE DECIDILE
 
 
 
12$
 
 
 
uterine tissue which concludes the process of menstruation is postponed until the close of pregnancy, and then takes place simultaneously over the whole extent of the uterus. Of course, the changes
in the uterine tissues are somewhat more extensive during pregnancy
than during menstruation, but there is an undoubted fundamental
similarity in the changes during the two processes.
 
 
 
 
Fig. 77. — Surface View op Half of the Decldua Vera at the End of the Third
 
Week of Gestation.
 
d, Mucous membrane of the Fallopian tubes; ds, prolongation of the vera toward the
 
cervix uteri; pp., papillae; rf, marginal furrow. (Kollmann.)
 
 
 
The human ovum comes into direct apposition with only a small
portion of the uterine wall, and the changes which this portion of the
wall undergoes differ somewhat from those occurring elsewhere.
Consequently it becomes possible to divide the deciduae into (1) a
portion which is not in direct contact with the ovum, the decidua vera
(Fig. 76, V) and (2) a portion which is. The latter portion is again
9
 
 
 
13°
 
 
 
THE DECIDUA VERA
 
 
 
capable of division. The ovum becomes completely embedded in
the mucosa, but, as has been pointed out, the chorionic villi reach
their full development only over that portion of the chorion to which
the belly-stalk is attached. The decidua which is in relation to this
chorion frondosum undergoes much more extensive modifications
than that in relation to the chorion laeve, and
to it the name of decidua basalts (decidua
serotina) (Fig. 76, B) is applied, while the
rest of the decidua which encloses the ovum
is termed the decidua capsularis (decidua
rejlexa) (C).
 
The changes which give rise to the decidua
vera may first be described and those occurring in the others considered in succession.
 
(a) Decidua vera. — On opening a uterus
during the fourth or fifth month of pregnancy,
when the decidua vera is at the height of its
development, the surface of the mucosa presents a corrugated appearance and is traversed
 
 
 
 
 
 
 
 
 
Fig. 78. — Diagrammatic Sections of the Uterine Mucosa, A, in the Nonpregnant Uterus, and B, at the Beginning of Pregnancy.
c, Stratum compactum; gl, the deepest portions of the glands; m, muscular layer;
sp, stratum spongiosum. — (Kundrat and Engelmann.)
 
 
 
by irregular and rather deep grooves (Fig. 77). This appearance
ceases at the internal orifice, the mucous membrane of the cervix
uteri not forming a decidua, and the deciduae of the two surfaces of
the uterus are separated by a distinct furrow known as the marginal
groove.
 
 
 
THE DECIDUA CAPSULARIS 131
 
In sections the mucosa is found to have become greatly thickened, frequently measuring i cm. in thickness, and its glands have
undergone very considerable modification. Normally almost
straight (Fig. 78, A), they increase in length, not only keeping pace
with the thickening of the mucosa, but surpassing its growth, so that
they become very much contorted and are, in addition, considerably
dilated (Fig. 78, B). Near their mouths they are dilated, but not
very much contorted, while lower down the reverse is the case, and
it is possible to recognize three layers in the decidua, (1) a stratum
compactum nearest the lumen of the uterus, containing the straight
but dilated portions of the glands; (2) a stratum spongiosum, so called
from the appearance which it presents in sections owing to the dilated
and contorted portions of the glands being cut in various planes;
and (3) next the muscular coat of the uterus a layer containing the
contorted but not dilated extremities of the glands is found. Only
in the last layer does the epithelium of the glands retain its normal
columnar form; elsewhere the cells, separated from the walls of the
glands, become enlarged and irregular in shape and eventually
degenerate.
 
In addition to these changes, the epithelium of the mucosa disappears completely during the first month of pregnancy, and the
tissue between the glands in the stratum compactum becomes packed
with large, often multinucleated cells, which are termed the decidual
cells and are probably derived from the connective tissue cells of the
mucosa.
 
After the end of the fifth month the increasing size of the embryo
and its membranes exerts a certain amount of pressure on the decidua,
and it begins to diminish'in thickness. The portions of the glands
which lie in the stratum compactum become more and more compressed and finally disappear, while in the spongiosum the spaces
become much flattened and the vascularity of the whole decidua,
at first so pronounced, diminishes greatly.
 
(b) Decidua capsularis. — The decidua capsularis has also been
termed the decidua reflexa, on the supposition that it was formed as a
fold of the uterine mucosa reflected over the ovum after this had
 
 
 
132 THE DECIDUA BASALIS
 
attached itself to the uterine wall. Since, however, the attachment
of the ovum is to be regarded as a process of burrowing into the
uterine tissues (see p. 119), the necessity for an upgrowth of a fold is
limited to an elevation of the uterine tissues in the neighborhood of
the ovum to keep pace with its increasing size. Since it is part of the
area of contact with the ovum it possesses no epithelium upon the
surface turned toward the ovum, although in the earlier stages its
surface is covered by an epithelium continuous with that of the
decidua vera, and between it and the chorion there is a portion of
the blood extravasation in which the villi formed from the chorion
laeve float. Glands and blood-vessels also occur in its walls in the
earlier stages of development.
 
As the ovum continues to increase in size the capsularis begins
to show signs of degeneration, these appearing first over the pole
of the ovum opposite the point of fixation. Here, even in the case
of the ovum described by Rossi Doria, the cavity of which measured
6X5 mm. in diameter, it has become reduced to a thin membrane
destitute of either blood-vessels or glands, and the degeneration
gradually extends throughout the entire capsule, the portion of the
blood space which it encloses also disappearing. At about the fifth
month the growth of the ovum has brought the capsularis in contact
throughout its whole extent with the vera, and it then appears as a
whitish transparent membrane with ho trace of either glands or
blood-vessels, and it eventually disappears by fusing with the vera.
 
(c) Decidua basalis. — The structure of the decidua basalis, also
known as the decidua serotina, is practically the same as that of the
vera up to about the fifth month. It differs only in that, being part of
the area of contact of the ovum, it loses its epithelium much earlier
and is also the seat of extensive blood extravasations, due to the
erosion of its vessels by the chorionic trophoblast. Its glands,
however, undergo the same changes as those of the vera, so that in
it also a compactum and a spongiosum may be recognized. Beyond
the fifth month, however, there is a great difference between it and
the vera, in that, being concerned with the nutrition of the embryo,
it does not partake of the degeneration noticeable in the other deciduae,
 
 
 
THE PLACENTA 133
 
but persists until birth, forming a part of the structure termed the
placenta.
 
The Placenta. — This organ, which forms the connection between
the embryo and the maternal tissues, is composed of two parts,
separated by the intervillous space. One of these parts is of embryonic origin, being the chorion frondosum, while the other belongs to
the maternal tissues and is the decidua- basalis. Hence the terms
placenta fetalis and placenta uterina frequently applied to the two
parts. The fully formed placenta is a more or less discoidal structure, convex on the surface next the uterine muscularis and concave
on that turned toward the embryo, the umbilical cord being continuous with it near the center of the latter surface. It averages about
3.5 cm. in thickness, thinning out somewhat toward the edges, and
has a diameter of 15 to 20 cm., and a weight varying between 500
and 1250 grams. It is situated on one of the surfaces of the uterus,
the posterior more frequently than the anterior, and usually much
nearer the fundus than the internal orifice. It develops, in fact,
wherever the ovum happens to become attached to the uterine walls,
and occasionally this attachment is not accomplished until the ovum
has descended nearly to the internal orifice, in which case the
placenta may completely close this opening and form what is termed
a placenta prcevia.
 
If a section of a placenta in a somewhat advanced stage of development be made, the following structures may be distinguished: On
the inner surface there will be a delicate layer representing the amnion
(Fig. 79, Am), and next to this a somewhat thicker one which is the
chorion (Cho), in which the degenerative changes already mentioned
may be observed. Succeeding this comes a much broader area composed of the large intervillous blood space in which lie sections of
the villi (vi) cut in various directions. Then follows the stratum
compactum of the basalis, next the stratum spongiosum (Z)')>
next the outermost layer of the mucosa (D"), in which the
uterine glands retain their epithelium, and, finally, the muscularis
uteri (Mc)
 
These various structures have, for the most part, been already
 
 
 
134
 
 
 
THE PLACENTA
 
 
 
 
Fig. 79. — Section through a Placenta of Seven Months' Development.
 
Am, Amnion; cho, chorion; D, layer of decidua containing the uterine glands ;{Mc,
muscular coat of the uterus; Ve, maternal blood-vessel; Vi, stalk of a villus; vi, villi
in section. — (Minoi.)
 
 
 
THE PLACENTA 135
 
described and it remains here only to say a few words concerning the
special structure of the basal compactum and concerning certain
changes that take place in the intervillous space.
 
The stratum compactum of the basal decidua forms what is
termed the basal plate of the placenta, closing the intervillous space
on the uterine side and being traversed by the maternal blood-vessels
that open into the space. The formation of canalized fibrin, already
mentioned in connection with the decidua vera and the syncytium of
the villi, also occurs in the basal portion of the decidua, a definite
layer of it, known as NitabucJi's fibrin stria, being a characteristic
constituent of the basal plate and patches of greater or less extent
also occur upon the surface of the plate. Leucocytes also occur in
considerable abundance in the plate and their presence has been
taken to indicate an attempt on the part of the maternal tissues to
resist the erosive action of the parasitic ovum. From the surface
of the basal plate processes, termed placental septa, project into the
intervillous space, grouping the villi into cotyledons and giving
attachment to some of the fixation villi (Fig. 80). Throughout the
greater extent of the placenta the septa do not reach the surface of
the chorion, but at the periphery, throughout a narrow zone, they
do come into contact with the chorion and unite beneath it to form a
membrane which has been termed the closing plate. Beneath this lies
the peripheral portion of the intervillous space, which, owing to the
arrangement of the septa in this region, appears to be imperfectly
separated from the rest of the space and forms what is termed the
marginal sinus (Fig. 80).
 
Attention has already been called to the formation of canalized
fibrin or fibrinoid in connection with the syncytium of the villi. In
the later stages of pregnancy there may be produced by this process
masses of fibrinoid of considerable size, lying in the intervillous space;
these, on account of their color, are termed white infarcts and may
frequently be observed as whitish or grayish patches through the
walls of the placenta after its expulsion. Red infarcts produced by
the clotting of the blood, also occurs, but with much less regularity
and frequency.
 
 
 
136
 
 
 
THE PLACENTA
 
 
 
 
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SEPARATION OF TEE DECTDtLE 137
 
The Separation of the Deciduae at Birth. — At parturition,
after the rupture of the amnion and the expulsion of the fetus, there
still remain in the uterine cavity the deciduae and the amnion, which
is in contact but not fused with the deciduae. A continuance of the
uterine contractions, producing what are termed the "after-pains,"
results in the separation of the placenta from the uterine walls, the
separation taking place in the deep layers of the spongiosum, so
that the portion of the mucosum which contains the undegenerated
glands remains behind. As soon as the placenta has separated,
the separation of the decidua vera takes place gradually though
rapidly, the line of separation again being in the deeper layers of the
stratum spongiosum, and the whole of the deciduae, together with
the amnion, is expelled from the uterus, forming what is known as
the "after-birth."
 
Hemorrhage from the uterine vessels during and after the separation of the deciduae is prevented by the contractions of the uterine
walls, assisted, according to some authors, by a preliminary blocking
of the mouths of the uterine vessels by certain large polynuclear
decidual cells found during the later months of pregnancy in the outer
layers of the decidua basalis. The regeneration of the uterine mucosa
after parturition has its starting-point from the epithelium of the
undegenerated glands which persist, this epithelium rapidly evolving
a complete mucosa over the entire surface of the uterus.
 
The complicated arrangement of the human placenta is, of course,
the culmination of a long series of specializations, the path along which
these have proceeded being probably indicated by the conditions obtaining
in some of the lower mammals. The Monotremes resemble the reptiles
in being oviparous and in this group of forms there is no relation of the
ovum to the maternal tissues such as occurs in the formation of a placenta.
In the other mammals viviparity is the rule and this condition does
demand some sort of connection between the fetal and maternal tissues.
One of the simplest of such connections is that seen in the pig, where the
chorionic villi of the ovum fit into corresponding depressions in the
uterine mucosa, this tissue, however, undergoing no destruction, and at
birth the villi simply withdraw from the depressions of the mucosa,
leaving it intact. This type of placentation is an embryo trophic one, and
since there is no separation of deciduae from the uterine wall after pregnancy it is also of the indeciduate type. In the sheep the placentation is
 
 
 
138 LITERATURE
 
also embryotrophic and indeciduate, but destruction of the maternal
mucosa does take place, the villi penetrating deeply into it and coming into
relation with the connective tissue surrounding the maternal blood-vessels.
Another step in advance is shown by the dog, in which even the connective tissue around the maternal vessels in the placental area undergoes
almost complete destruction so that the chorionic villi are separated
from the maternal blood practically only by the endothelial lining of the
maternal vessels. In this case the mucosa undergoes so much alteration
that the undestroyed portions if it are sloughed off after birth as a decidua,
so that the placentation, like that in man, is of the deciduate type. It
still represents, however, an embryotrophic type, although closely approximating to the haemotrophic one found in man, in which, as described above,
the destruction of the maternal tissues proceeds so far as to open into the
maternal blood-vessels, so that the fetal villi are in direct contact with the
maternal blood.
 
If these various stages may be taken to represent steps by which
the conditions obtaining in the human placenta have been evolved, the
entire process may be regarded as the result of a progressive activity of a
parasitic ovum. In the simplest stage the pabulum supplied by the
uterus was sufficient for the nutrition of the parasite, but gradually the
ovum, by means of its plasmodi-trophoblast, began to attack the tissues
of its host, thus obtaining increased nutrition, until finally, breaking
through into the maternal blood-vessels, it achieved for itself still more
favorable nutrition, by coming into direct contact with the maternal
blood.
 
LITERATURE.
 
In addition to the papers by Beneke and Strahl, Bryce and Teacher, Frassi, Jung,
and Herzog, cited in Chapter III, the following may be mentioned:
 
E. Cova: " Ueber ein menschliches Ei der zweiten Woche," Arch, fur Gynaek., lxxxiii,
 
1907.
L. Frassi: "Ueber ein junges menschliches Ei in situ," Arch, fiir mikr. Anal., lxx,
 
1907.
O. Grosser: "Vergleichende Anatomic und Entwicklungsgeschichte der Eihaute
 
und der Placenta mit besonderer Berticksichtigung des Menschen," Wien, 1909.
H. Happe: "Beobachtungen an Eihauten junger menschlicher Eier," Anat. Hefte,
 
xxxii, 1906.
W. His: "Die Umschliessung der menschlichen Frucht wahrend der friihesten Zeit.
 
des Schwangerschafts," Archiv fiir Anat. und Physiol., Anat. Abth., 1897.
M. Hofmeier: "Die menschliche Placenta," Wiesbaden, 1890.
 
F. Keibel: "Zur Entwickelungsgeschichte der Placenta," Anat. Anzeiger, iv, 1889.
 
J. Kollmann: "Die menschlichen Eier von 6 mm. Grosse," Archiv fiir Anat. und
Physiol., Anat. Abth., 1879.
 
G. Leopold: "Ueber ein sehr junges menschliches Ei in situ," Arb. aus der
 
Frauenklinik in Dresden, rv, 1906.
 
 
 
LITERATURE 139
 
F. Marchand: "Beobachtungen an jungen menschlichen Eiern," Anat.Hefte, xxi,
 
1903.
J. Merttens: "Beitrage zur normalen und pathologischen Anatomie der mensch
lichen Placenta," Zeitschrift fiir Geburtshiilfe und Gynaekol., xxx and xxxi, 1894.
C. S. Minot: "Uterus and Embryo," Journal of Morphol., n, 1889.
 
G. Paladino: "Sur la genese des espaces intervilleux du placenta humain et de leur
 
premier contenu, comparativement a la meme partie chez quelques mammiferes,"
 
Archives Ital. de Biolog., xxxi and xxxn, 1899.
H. Peters: "Ueber die Einbettung des menschlichen Eies und das friiheste bisher
 
bekannte menschliche Placentationsstadium," Leipzig und Wien, 1899.
J. Rejsek: "Anheftung (Implantation) des Sangetiereies an die Uteruswand, insbe
sondere des Eies von Spermophilus citellus," Arch, fiir mikrosk. Anat., lxiii, 1964.
T. Rossi Doria: "Ueber die Einbettung des menschlichen Eies, studirt an einem
 
kleinen Eie der zweiten Woche," Arch, fiir Gynaek., lxxvi. 1905.
C. Ruge: "Ueber die menschliche Placentation," Zeitschrift fur Geburtshiilfe und
 
Gynaekol., xxxix, 1898.
Siegenbeek van Hetjkelom: "Ueber die menschliche Placentation," Arch. f. Anat.
 
undPhys., Anat. Abth., 1898.
F. Graf Spee: "Ueber die menschliche Eikammer und Decidua reflexa," Verhandl.
 
des Anat. Gesellsch., xii, 1898.
H. Strahl: "Die menschliche Placenta," Ergebn der Anat. und Enlwickl., II, 1893.
 
"Neues uber den Bau der Placenta," ibid, vi, 1897.
 
"Placentaranatomie," ibid., viii, 1899.
R. Todyo: "Ein junges menschliches Ei," Arch, fiir Gynaek., xcv, 1912.
Van Cauwenberghe : "Recherches sur la role du Syncytium dans la nutrition
 
embryonnaire de la femme," Arch, de Biol., xxiii, 1907.
J. C. Webster: "Human Placentation," Chicago, 1901.
E. Wormser: "Die Regeneration der Uterusschleimhaut nach der Geburt," Arch.
 
fiir Gynaek., lxix, 1903.
 
 
 
PART II.
ORGANOGENY.
 
 
 
CHAPTER VI.
THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM.
 
The Development of the Skin. — The skin is composed of two
embryologically distinct portions, the outer epidermal layer being
developed from the ectoderm, while the dermal layer is mesenchymatous in its origin.
 
The ectoderm covering the general surface of the body is, in the
earliest stages of development, a single layer of cells, but at the end
of the first month it is composed of two layers, an outer one, the
epitrichium, consisting of slightly flattened cells, and a lower one
whose cells are larger and which will give rise to the epidermis
(Fig. 81, A). During the second month the differences between
the two layers become more pronounced, the epitrichial cells assuming a characteristic domed form and becoming vesicular in structure
(Fig. 81, B). These cells persist until about the sixth month of
development, but after that they are cast off, and, becoming mixed
with the secretion of sebaceous glands which have appeared by this
time, form a constituent of the vernix caseosa.
 
In the meantime changes have been taking place in the epidermal
layer which result in its becoming several layers thick (Fig. 81, B),
the innermost layer being composed of cells rich in protoplasm,
while those of the outer layers are irregular in shape and have clearer
contents. As development proceeds the number of layers increases
and the superficial ones, undergoing a horny degeneration, give rise
to the stratum corneum, while the deeper ones become the stratum
 
141
 
 
 
142
 
 
 
DEVELOPMENT OF THE SKIN
 
 
 
Malpighii. At about the fourth month ridges develop on the under
surface of the epidermis, projecting downward into the dermis,
and later secondary ridges appear in the intervals between the
primary ones, while on the palms and soles ridges appear upon the
outer surface of the epidermis, corresponding in position to the
primary ridges of the under surface.
 
The mesenchyme which gives rise to the dermis grows in from
all sides between the epidermis and the outer layer of the myotomes,
 
 
 
 
 
si^j^mmw**
 
 
 
 
 
 
 
Fig. 81. — A, Section of Skin from the Dorsum of Finger of an Embryo of 4.5 cm.;
 
B, from the Plantar Surface of the Foot of an Embryo of 10.2 cm
 
et, Epitrichium; ep, epidermis.
 
 
 
which are at first in contact, and forms a continuous layer underlying the epidermis and showing no indications of a segmental
arrangement. It becomes converted "principally into fibrous connective tissue, the outer layers of which are relatively compact,
while the deeper ones are looser, forming the subcutaneous areolar
tissue. Some of the mesenchymal cells, however, become converted
into non-striated muscle-fibers, which for the most part are few in
number and associated with the hair follicles, though in certain
regions, such as the skin of the scrotum, they are very numerous and
 
 
 
DEVELOPMENT OF THE SKIN
 
 
 
form a distinct layer known as the dartos.
Some cells also arrange themselves in groups
and undergo a fatty degeneration, well-defined
masses of adipose tissue embedded in the
lower layers of the dermis being thus formed
at about the sixth month.
 
Although the dermal mesenchyme is unsegmental in character, yet the nerves which send
branches to it are segmental, and it might be
expected that indications of this condition would
be retained by the cutaneous nerves even in the
adult. A study of the cutaneous nerve-supply in
the adult realizes to a very considerable extent
this expectation, the areas supplied by the various
nerves forming more or less distinct zones, and
being therefore segmental (Fig. 82). But a considerable commingling of adjacent areas has also
occurred. Thus, while the distribution of the
cutaneous branches of the fourth thoracic nerve,
as determined experimentally in the monkey
(Macacus), is distinctly zonal or segmental, the
nipple lying practically in the middle line of the
zone, the upper half of its area is also supplied or
overlapped by fibers of the third nerve and the
lower half by fibers of the fifth (Fig. 83), so that
any area of skin in the zone is innervated by fibers
coming from at least two segmental nerves (Sherrington). And, furthermore, the distribution of
each nerve crosses the mid-ventral line of the body,
forming a more or less extensive crossed overlap.
 
And not only is there a confusion of adjacent
areas but an area may shift its position relatively
to the deeper structures supplied by the same
nerve, so that the skin over a certain muscle is not
necessarily supplied by fibers from the nerve
which supplies the muscle. Thus, in the lower
half of the abdomen, the skin at any point will
be supplied by fibers from higher nerves than
those supplying the underlying muscles (Sherrington), and the skin of the limbs may receive twigs
from nerves which are not represented at all in
the muscle-supply (second and third thoracic and
third sacral).
 
 
 
Ti
 
 
 
'Ts 7i\
 
 
 
^
 
 
 
Ts
 
 
 
Te
 
 
 
Ja
 
 
 
Ts
 
 
 
r,o
 
 
 
S2
 
 
 
Lj
 
 
 
IJ
 
 
 
Sj
 
Fig. 82. — Diagram
showing the cutaneOUS Distribution of the
Spinal Nerves— (Head.)
 
 
 
144
 
 
 
DEVELOPMENT OF THE NAILS
 
 
 
The Development of the Nails.— The earliest indications of
the development of the nails have been described by Zander in
embryos of about nine weeks as slight thickenings of the epidermis
 
 
 
 
Fig. 83. — Diagram showing the Overlap of the III, IV, and V Intercostal
Nerves of a Monkey. — (Sherrington.)
 
 
 
 
Fig. 84. — Longitudinal Section through the Terminal Joint of the IndexFinger of an Embryo of 4.5 cm.
e, Epidermis; ep, epitrichium; nf, nail fold; Ph, terminal phalanx; sp, sole plate.
 
of the tips of the digits, these thickenings being separated from the
neighboring tissue by a faint groove. Later the nail areas migrate
to the dorsal surfaces of the terminal phalanges (Fig. 84) and the
 
 
 
DEVELOPMENT OF THE NAILS
 
 
 
145
 
 
 
sp
 
 
sc
 
 
 
ep
 
 
 
grooves surrounding the areas deepen, especially at their proximal
edges, where they form the nail-folds (nf) , while distally thickenings
of the epidermis occur to form what have been
termed sole-plates (sp), structures quite rudimentary in man, but largely developed in the
lower animals, in which they form a considerable
portion of the claws.
 
The actual nail substance does not form,
however, until the embryo has reached a length
of about 17 cm. By this time the epidermis has
become several layers thick and its outer layers,
over the nail areas as well as elsewhere, have
become transformed into the stratum corneum
(Fig. 85, sc), and it is in the deep layers of this
(the stratum lucidum) that keratin granules develop in cells which degenerate to give rise to
the nail substance (n). At its first formation,
accordingly, the nail is covered by the outer layers
of the stratum corneum as well as by the epitrichium, the two together forming what has
been termed the eponychium (Fig. 85, ep). The
epitrichium soon disappears, however, leaving
only the outer layers of the stratum corneum as
a covering, and this also later disappears with the
exception of a narrow band surrounding the base
of the nail which persists as the perionyx.
 
The formation of the nail begins in the more
proximal portion of the nail area and its further
growth is by the addition of new keratinized
cells to its proximal edge and lower surface,
these cells being formed only in the proximal part
of the nail bed in a region marked by its whitish
color and termed the lunula.
 
 
 
 
The first appearance of the nail-areas at the tips
of the digits as described by Zander has not yet been
 
 
 
Fig. 85. — Longitudinal Section
through the nail
Area in an Embryo
 
OF 17 CM.
 
ep, Eponychium;
n, nail substance; nb,
nail bed; sc, stratum
corneum; sp, sole
plate. — (Okamura.)
 
 
 
146
 
 
 
DEVELOPMENT OF THE HAIRS
 
 
 
confirmed by later observers, but the migration of the areas to the dorsal
surface necessitated by such a location of the primary differentiation affords
an explanation of the otherwise anomalous cutaneous nerve-supply of the
nail-areas in the adult, this being from the palmar (plantar) nerves.
 
The Development of the Hairs. — The hairs begin to develop
at about the third month and continue to be formed during the
remaining portions of fetal life. They arise as solid cylindrical
downgrowths, projecting obliquely into the subjacent dermis from
 
 
 
to -wl^lvfi
 
 
 
p ...
 
 
 
A
 
 
 
 
Fig. 86. — The Development of a Hair.
c, Cylindrical cells of stratum mucosum; hf, wall of hair follicle; m, mesoderm;
mu, stratum mucosum of epidermis; p, hair papilla; r, root of hair; s, sebaceous gland.
— (Kollmann.)
 
the lower surface of the epidermis. As these downgrowths continue
to elongate, they assume a somewhat club-shaped form (Fig. 86, A),
and later the extremity of each club moulds itself over the summit of
a small papilla which develops from the dermis (Fig. 86, B). Even
before the dermal papilla has made its appearance, however, a
 
 
 
DEVELOPMENT OF THE HAIRS 147
 
differentiation of the cells of the downgrowth becomes evident, the
central cells becoming at first spindle-shaped and then undergoing
a keratinization to form the hair shaft, while the more peripheral
ones assume a cuboidal form and constitute the lining of the hair
follicle. The further growth of the hair takes place by the addition to its basal portion of new keratinized cells, probably produced
by the multiplication of the epidermal cells which envelop the
papilla.
 
From the cells which form the lining of each follicle an outgrowth
takes place into the surrounding dermis to form a sebaceous gland,
which is at first solid and club-shaped, though later it becomes
lobed. The central cells of the outgrowth separate from the peripheral and from one another, and, their protoplasm undergoing a
fatty degeneration, they finally pass out into the space between the
follicle walls and the hair and so reach the surface, the peripheral
cells later giving rise by division to new generations of central cells.
During fetal life the fatty material thus poured out upon the surface
of the body becomes mingled with the cast-off epitrichial cells and
constitutes the white oleaginous substance, the vernix caseosa, which
covers the surface of the new-born child. The muscles, arrectores
pilorum, connected with the hair follicles arise from the mesenchyme cells of the surrounding dermis.
 
The first growth of hairs forms a dense covering over the entire
surface of the fetus, the hairs which compose it being exceedingly fine
and silky and constituting what is termed the lanugo. This growth
is cast off soon after birth, except over the face, where it is hardly
noticeable on account of its extreme fineness and lack of coloration.
The coarser hairs which replace it in certain regions of the body
probably arise from new follicles, since the formation of follicles takes
place throughout the later periods of fetal life and possibly after
birth. But even these later formed hairs do not individually persist
for any great length of time, but are continually being shed, new or
secondary hairs normally developing in their places. The shedding
of a hair is preceded by a cessation of the proliferation of the cells
covering the dermal papilla and by a shrinkage of the papilla,
 
 
 
148
 
 
 
DEVELOPMENT OF THE SUDORIPAROUS GLANDS
 
 
 
~h
 
 
 
whereby it becomes detached from the hair, and the replacing hair
arises from a papilla which is probably budded off from the older
one before its degeneration and carries with it a cap of epidermal
cells.
 
It is uncertain whether the cases of excessive development of hair
over the face and upper part of the body which occasionally occur are
due to an excessive development of the later hair follicles (hypertrichosis)
or to a persistence and continued growth of the lanugo.
 
The Development of the Sudoriparous Glands. — The sudoriparous glands arise during the fifth month as solid cylindrical outgrowths from the primary ridges
of the epidermis (Fig. 87), and
at first project vertically downward into the subjacent dermis.
Later, however, the lower end of
each downgrowth is thrown into
coils, and at the same time a
lumen appears in the center.
Since, however, the cylinders are
formed from the deeper layers
of the epidermis, their lumina do
not at first open upon the surface, but gradually approach it
as the cells of the deeper layers
of the epidermis replace those which are continually being cast off
from the surface of the stratum corneum. The final opening to
the surface occurs during the seventh month of development.
 
The Development of the Mammary Glands. — In the majority
of the lower mammals a number of mammary glands occur, arranged in two longitudinal rows, and it has been observed that in the
pig the first indication of their development is seen in a thickening
of the epidermis along a line situated at the junction of the abdominal walls with the membrana reuniens (Schulze). This thickening
subsequently becomes a pronounced ridge, the milk ridge, from
which, at certain points, the mammary glands develop, the ridge
 
 
 
 
Fig. 87. — Lower Surface of a Detached Portion of Epidermis from
the Dorsum of the Hand.
h, Hair follicle; s, sudoriparous gland. —
(Blaschko.)
 
 
 
DEVELOPMENT OF THE MAMMARY GLANDS
 
 
 
149
 
 
 
disappearing in the intervals. In a human embryo 4 mm. in length
an epidermal thickening has been observed which extended from
just below the axilla to the inguinal region (Fig. 88) and was apparently equivalent to the milk line of the pig, and in embryos of 14 or
15 mm. the upper end of the line had become a pronounced ridge,
while more posteriorly the thickening had disappeared.
 
The further history of the ridge has not, however, been yet
traced in human embryos, and the next stage of the development of
the glands which has been observed is one in which they are
represented by a circular thickening of the epidermis which
projects downward into the
dermis (Fig. 89, A). Later
the thickening becomes lobed
(Fig. 89, B), and its superficial
and central cells become cornified and are cast off, so that the
gland area appears as a depression of the surface of the skin.
During the fifth and sixth
months the lobes elongate into
solid cylindrical columns of cells
(Fig. 90) resembling not a little the cylinders which become converted into sudoriparous glands, and each column becomes slightly
enlarged at its lower end, from which outgrowths begin to develop to
form the acini. A lumen first appears in the lower ends of the columns and is formed by the separation and breaking down of the
central cells, the peripheral cells persisting as the lining of the acini
and ducts.
 
The elevation of the gland area above the surface to form the
nipple appears to occur at different periods in different embryos and
frequently does not take place until after birth. In the region around
the nipple sudoriparous and sebaceous glands develop, the latter
also occurring within the nipple area and frequently opening into
 
 
 
 
Fig. 88. — Milk Ridge (mr) in a Human
Embryo. — (Kallius.)
 
 
 
150 DEVELOPMENT OF THE MAMMARY GLANDS
 
the extremities of the lacteal ducts. In the areola, as the area surrounding the nipple is termed, other glands known as Montgomery' 's
glands, also appear, their development resembling that of the mammary gland so closely as to render it probable that they are really
rudimentary mammary glands.
 
 
 
 
 
 
 
 
 
 
 
 
J
 
 
 
 
 
 
K»'
 
 
 
" B
 
Fig. 89. — Sections through the Epidermal Thickenings which Represent the
Mammary Gland in Embryos (A) of 6 cm. and (B) or 10.2 cm.
 
 
 
The further development of the glands, consisting of an increase
in the length of the ducts and the development from them of additional acini, continues slowly up to the time of puberty in both sexes,
but at that period further growth ceases in the male, while in females
it continues for a time and the subjacent dermal tissues, especially
the adipose tissue, undergo a rapid development.
 
 
 
LITERATURE 151
 
The occurrence of a milk ridge has not yet been observed in a sufficient
number of embryos to determine whether it is a normal development or
is associated with the formation of supernumerary glands {polymastia).
This is by no means an infrequent anomaly; it has been observed in 19
per cent, of over 100,000 soldiers of the German army who were examined,
and occurs in 47 per cent, of individuals in certain regions of Germany
The extent to which the anomaly is developed varies from the occurrence
of well-developed accessory glands to that of rudimentary accessory
nipples {Jiy perihelia), these latter sometimes occurring in the areolar area
of a normal gland and being possibly due in such cases to an hypertrophy
of one or more of Montgomery's glands.
 
 
 
 
c€; * .-/'-* ., ° '>_,
 
Fig. 90. — Section through the Mammary Gland of an Embryo of 25 cm.
1, Stroma of the gland. — {From Nagel, after Basch.)
 
Although the mammary glands are typically functional only in
females in the period immediately succeeding pregnancy, cases are not
unknown in which the glands have been well developed and functional in
males {gynecomastia). Furthermore, a functional activity of the glands
normally occurs immediately after birth, infants of both sexes yielding a
few drops of a milky fluid, the so-called witch-milk (Hexenmilch) , when
the glands are subjected to pressure.
 
LITERATURE.
 
J. T. Bowen: "The Epitrichial Layer of the Human Epidermis," Anat. Anzeiger, rv
 
1889.
Brouha: •' Recherches sur les diverses phases du developpement et de l'activite dela
 
mammelle," Arch, de Biol., xxi, 1905.
G. Burckhard: "Ueber embryonale Hypermastie und Hyperthelie," Anat. Hefte
 
viii, 1897.
H. Head: "On Disturbances of Sensation with Special Reference to the Pain of
 
Visceral Disease," Brain, xvi, 1892; xvn, 1894; and xix, 1896.
E. Kallius: "Ein Fall von Milchleiste bei einem menschlichen Embryo," Anat.
 
Hefte, viii, 1897.
 
 
 
152 LITERATURE
 
T. Okamura: "Ueber die Entwicklung des Nagels beim Menschen," Archiv fur
 
Dermatol, und Syphilol., xxv, 1900.
H. Schmidt: "Ueber normale Hyperthelie menschlicher Embryonen und uber die
 
erste Anlage der menschlichen Milchdriisen iiberhaupt," Morphol. Arbeiten, xvil,
 
1897.
C. S. Sherrington: "Experiments in Examination of the Peripheral Distribution of
 
the Fibres of the Posterior Roots of some Spinal Nerves," Philos. Trans. Royal
 
Soc, clxxxiv, 1893, and cxc, 1898.
P. Stohr: " Entwickelungsgeschichte des menschlichen Wollhaares," Anat. Hefte,
 
xxiii, 1903.
H. Strahl: "Die erste Entwicklung der Mammarorgane beim Menschen," Verhandl.
 
Anat. Gesellsch., xii, 1898.
 
 
 
CHAPTER VII.
 
THE DEVELOPMENT OF THE CONNECTIVE TISSUES
AND SKELETON.
 
It has been seen that the cells of a very considerable portion of
the somatic and splanchnic mesoderm, as well as of parts of the
mesodermic somites, become converted into mesenchyme. A
very considerable portion of this becomes converted into what are
termed connective or supporting tissues, characterized by consisting
of a non-cellular matrix in which more or less scattered cells are
embedded. These tissues enter to a greater or less extent into the
formation of all the organs of the body, with the exception of those
forming the central nervous system, and constitute a network which
holds together and supports the elements of which the organs are
composed; in addition, they take the form of definite membranes
(serous membranes, fasciae), cords (tendons, ligaments), or solid
masses (cartilage), or form looser masses or layers of a somewhat
spongy texture (areolar tissue). The intermediate substance is
somewhat varied in character, being composed sometimes of white,
non-branching, non-elastic fibers, sometimes of yellow, branching,
elastic fibers, of white, branching, but inelastic fibers which form
a reticulum, or of a soft gelatinous substance containing considerable
quantities of mucin, as in the tissue which constitutes the Whartonian
jelly of the umbilical cord. Again, in cartilage the matrix is compact and homogeneous, or, in other cases, more or less fibrous,
passing over into ordinary fibrous tissue, and, finally, in bone the
organic matrix is largely impregnated with salts of lime.
 
Two views exist as to the mode of formation of the matrix, some
authors maintaining that in the fibrous tissues it is produced by the
actual transformation of the mesenchyme cells into fibers, while
others claim that it is manufactured by the cells but does not directly
 
i53
 
 
 
154 DEVELOPMENT OF CONNECTIVE TISSUE
 
represent the cells themselves. Fibrils and material out of which
fibrils could be formed have undoubtedly been observed in connective-tissue cells, but whether or not these are later passed to the
exterior of the cell to form a connective-tissue fiber is not yet certain,
and on this hangs mainly the difference between the theories.
Recently it has been held (Mall) that the mesenchyme of the embryo
is really a syncytium in and from the protoplasm of which the matrix
 
 
 
 
W/^ii
 
 
 
 
 
 
\%jfL
 
 
 
m^mmF%
 
 
 
â– a fmm0m^&
 
 
 
â–  J
 
 
 
>
 
 
 
Fig. 91. — Portion of the Center of Ossification of the Parietal Bone of a
 
Human Embryo.
 
 
 
forms; if this be correct, the distinction which the older views make
between the intercellular and intracellular origin of the matrix
becomes of little importance.
 
Bone differs from the other varieties of connective tissue in that
it is never a primary formation, but is always developed either in
fibrous tissue or cartilage; and according as it is associated with the
one or the other, it is spoken of as membrane bone or cartilage bone.
In the development of membrane bone some of the connective-tissue
cells, which in consequence become known as osteoblasts, deposit
lime salts in the matrix in the form of bony spicules which increase
in size and soon unite to form a network (Fig. 91). The trabecular
of the network continue to thicken, while, at the same time, the formation of spicules extends further out into the connective-tissue membrane, radiating in all directions from the region in which it first
 
 
 
DEVELOPMENT OF BONE
 
 
 
155
 
 
 
developed. Later the connective tissue which lies upon either surface of the reticular plate of bone thus produced condenses to form
a stout membrane, the periosteum, between which and the osseous
plate osteoblasts arrange themselves in a more or less definite layer
and deposit upon the surface of the plate a lamella of compact bone.
A membrane bone, such as one of the flat bones of the skull, thus
comes to be composed of two plates
of compact bone, the inner and
outer tables, enclosing and united
to a middle plate of spongy bone
which constitutes the diploe.
 
With bones formed from cartilage the process is somewhat different. In the center of the
cartilage the intercellular matrix
becomes increased so that the cells
appear to be more scattered and
a calcareous deposit forms in it.
All around this region of calcification the cells arrange themselves
in rows (Fig. 92) and the process
of calcification extends into the
trabecular of matrix which separate
these rows. While these processes
have been taking place the mesenchyme surrounding the cartilage
has become converted into a
periosteum (po), similar to that of membrane bone, and its osteoblasts deposit a layer of bone (p) upon the surface of the cartilage.
The cartilage cells now disappear from the intervals between the
trabeculae of calcified matrix, which form a fine network into which
masses of mesenchyme (Fig. 93, pi), containing blood-vessels and
osteoblasts, here and there penetrate from the periosteum, after
having broken through the layer of periosteal bone. These masses
absorb a portion of the fine calcified network and so transform it
 
 
 
 
po
 
 
 
P
 
 
 
Fig 92. — Longitudinal section of
Phalanx of a Finger of an Embryo
of 3 1/2 Months.
 
c, Cartilage trabeculae; p, periosteal
bone; po, periosteum; x, ossification
center. — (Szymonowicz.)
 
 
 
i56
 
 
 
DEVELOPMENT OF BONE
 
 
 
po
 
 
 
pi
 
 
 
C ^ ;
 
 
 
 
into a coarse network, the meshes of which they occupy to form
the bone maigow (m), and the osteoblasts which they contain arrange
themselves on the surface of the persisting trabeculse and deposit
layers of bone upon their surfaces. In the meantime the calcification of the cartilage matrix has been extending, and as fast as the
 
network of calcified trabeculse is
formed it is invaded by the mesenchyme, until finally the cartilage
becomes entirely converted into a
mass of spongy bone enclosed
within a layer of more compact
periosteal bone.
 
As a rule, each cartilage bone
is developed from a single center
of ossification, and when it is found
that a bone of the skull, for instance, develops by several centers, it is to be regarded as formed
by the fusion of several primarily
distinct bones, a conclusion which
may generally be confirmed by a
comparison of the bone in question with its homologues in the
lower vertebrates. Exceptions to
this rule occur in bones situated in the median line of the body,
these occasionally developing from two centers lying one on either
side of the median line, but such centers are usually to be regarded
as a double center rather than as two distinct centers, and are
merely an expression of the fundamental bilaterality which exists
even in median structures.
 
More striking exceptions are to be found in the long bones in
which one or both extremities develop from special centers which
give rise to the epiphyses (Fig. 94, ep, ep'), the shaft or diaphysis (d)
being formed from the primary center. Similar secondary centers
appear in marked prominences on bones to which powerful muscles
 
 
 
Fig. 93. — The Ossification Center
of Fig. 92 More Highly Magnified.
c, Ossifying trabeculse; cc, cavity of
cartilage network; m, marrow cells; p,
periosteal bone; pi, irruption of periosteal tissue; po, periosteum. — (Szymonowicz.)
 
 
 
DEVELOPMENT OF BONE
 
 
 
157
 
 
 
 
are attached (Fig. 94, a and b), but these, as well as the epiphysial
centers, can readily be recognized as secondary from the fact that
they do not appear until much later than the primary centers of the
bones to which they belong. These secondary
centers give the necessary firmness required
for articular surfaces and for the attachment
of muscles and, at the same time, make provision for the growth in length of the bone,
since a plate of cartilage always intervenes
between the epiphyses and the diaphysis.
This cartilage continues to be transformed
into bone on both its surfaces by the extension
of both the epiphysial and diaphysial ossification into it, and, at the same time, it grows
in thickness with equal rapidity until the
bone reaches its required length, whereupon
the rapidity of the growth of the cartilage
diminishes and it gradually becomes completely ossified, uniting together the epiphysis
and diaphysis.
 
The growth in thickness of the long bones
is, however, an entirely different process, and
is due to the formation of new layers of periosteal bone on the outside of those already
present. But in connection with this process'
an absorption of bone also takes place. A
section through the middle of the shaft of a
humerus, for example, at an early stage of
development would show a peripheral zone of
compact bone surrounding a core of spongy bone, the meshes of the
latter being occupied by the marrow tissue. A similar section of
an adult bone, on the other hand, would show only the peripheral
compact bone, much thicker than before and enclosing a large
marrow cavity in which no trace of spongy bone might remain.
The difference depends on the fact that as the periosteal bone
 
 
 
 
Fig. 94. — The Ossification Centers of
the Femur.
 
a, and b, Secondary
centers for the great and
lesser trochanters; d,
diaphysis; ep, upper and
ep', lower epiphysis. —
(Testut.)
 
 
 
158 DEVELOPMENT OF BONE
 
increases in thickness, there is a gradual absorption of the spongy
bone and also of the earlier layers of periosteal bone, this absorption
being carried on by large multinucleated cells, termed osteoclasts,
derived from the marrow mesenchyme. By their action the bone
is enabled to reach its requisite diameter and strength, without
becoming an almost solid and unwieldy mass of compact bone.
 
During the ossification of the cartilaginous trabeculse osteoblasts
become enclosed by the bony substance, the cavities in which they
lie forming the lacuna and processes radiating out from them, the
 
 
 
 
 
Fig. 95. — A, Transverse Section of the Femur of a Pig Killed after
Having Been fed with Madder for Four Weeks; B, the Same of a Pig Killed
Two Months after the Cessation of the Madder Feeding.
The heavy black line represents the portion of bone stained by the madder. — (After
 
Flourens.)
 
canaliculi, so characteristic of bone tissue. In the growth of periosteal bone not only do osteoblasts become enclosed, but bloodvessels also, the Haversian canals being formed in this way, and
around these lamellae of bone are deposited by the enclosed osteoblasts to form Haversian systems.
 
That the absorption of periosteal bone takes place during growth
can be demonstrated by taking advantage of the fact that the coloring
substance madder, when consumed with food, tinges the bone being
formed at the time a distinct red. In pigs fed with madder for a time
and then killed a section of the femur shows a superficial band of red bone
(Fig. 95, A), but if the animals be allowed to live for one or two months
after the cessation of the madder feeding, the red band will be found to be
covered by a layer of white bone varying in thickness according to the
interval elapsed since the cessation of feeding (Fig. 95, B); and if this
 
 
 
DEVELOPMENT OF THE SKELETON
 
 
 
159
 
 
 
interval amount to four months, it will be found that the thickness of the
uncolored bone between the red bone and the marrow cavity will have
greatly diminished (Flourens).
 
The Development of the Skeleton. — Embryologically considered, the skeleton is composed of two portions, the axial skeleton,
consisting of the skull, the vertebrae, ribs, and sternum, developing
 
 
 
 
fin* ' \m. :
 
 
 
SCcr
 
 
 
tsa
 
 
 
 
ri>
 
 
 
<*r
 
 
 
Fig. 96. — Frontal Section through Mesodermic Somites of a Calf Embryo.
 
isa, Intersegmental artery; my, myotome; n, central nervous system; nc, notochord;
 
sea and scp, anterior and posterior portions of sclerotomes.
 
from the sclerotomes of the mesodermal somites, and the appendicular skeleton, which includes the pectoral and pelvic girdles and
the bones of the limbs, and which arises from the mesenchyme of
the somatic mesoderm. It will be convenient to consider first the
development of the axial skeleton, and of this the differentiation of
the vertebral column and ribs may first be discussed.
 
 
 
i6o
 
 
 
DEVELOPMENT OP THE VERTEBRAE
 
 
 
The Development of the Vertebrae and Ribs. — The mesenchyme formed from the sclerotome of each mesodermic somite
grows inward toward the median line and forms a mass lying
between the notochord and the myotome, separated from the
similar mass in front and behind by some loose tissue in which lies
an intersegmental artery. Toward the end of the third week of
 
development the cells of the
posterior portion of each sclerotome condense to a tissue more
compact than that of the anterior
portion (Fig. 96), and a little
later the two portions become
separated by a cleft. At about
the same time the posterior portion sends a process medially, to
enclose the notochord by uniting
with a corresponding process
from the sclerotome of the other
side, and it also sends a prolongation dorsally between the
myotome and the spinal cord to
form the vertebral arch, and a
third process laterally and ventrally along the distal border of
the myotome to form a costal process (Fig. 97). The looser tissue
of the anterior half of the sclerotome also grows medially to surround the notochord, filling up the intervals between successive
denser portions, and it forms too a membrane extending between
successive vertebral arches. Later the tissue surrounding the notochord, which is derived from the anterior half of the sclerotome,
associates itself with the posterior portion of the preceding sclerotome
to form what will later be a vertebra, the tissue occupying and
adjacent to the line of division between the anterior and posterior
portions of the sclerotomes condensing to form intervertebral
fibrocartilages. Consequently each vertebra is formed by parts
 
 
 
 
Fig. 97. — Transverse Section
through the intervertebral plate
of the First Cervical Vertebra of a
Calf Embryo of 8.8 mm.
 
be 1 , Intervertebral plate; m i , fourth
myotome; s, hypochordal bar; XI, spinal
accessory nerve. — (Froriep.)
 
 
 
DEVELOPMENT OF THE VERTEBRA l6l
 
from two sclerotomes, the original intersegmental artery passes over
the body of a vertebra, and the vertebrae themselves alternate with
the myotomes. With this differentiation the first or blastemic stage
of the development of the vertebras closes.
 
In the second or cartilaginous stage, portions of the sclerotomic
mesenchyme become transformed into cartilage. In the posterior
portion of each vertebral body, that is to say in the portion formed
from the anterior halves of the more posterior of the two pairs of
sclerotomes entering into its formation, two centers of chondrification appear, one on each side of the median line, and these eventually
unite to form a single cartilaginous body, the chondrification probably also extending to some extent into the denser anterior portion
of the body. A center also appears in each half of the vertebral
arch and in each costal process, the cartilages formed in the costal
processes of the anterior cervical region uniting across the median
line below the notochord, to form what has been termed a hypochordal bar (Figs. 97 and 98). These bars are for the most part
but transitory, recalling structures occurring in the lower vertebrates;
in the mammalia they degenerate before the close of the cartilaginous
stage of development, except in the case of the atlas, whose development will be described later. As development proceeds the cartilages of the vertebral arches and costal processes increase in length
and come into contact with the cartilaginous bodies, with which
they eventually fuse, and from the vertebral arches processes grow
out which represent the future transverse and articular processes.
 
The fusion of the cartilage of the costal process with the body of
the vertebra does not, however, persist. Later a solution of the
junction occurs and the process becomes a rib cartilage, the mesenchyme surrounding the area of solution forming the costo-vertebral
ligaments. At first the rib cartilage is separated by a distinct
interval from the transverse process of the vertebral arch, but later
it develops a process, the tubercle, which bridges the gap and forms
an articulation with the transverse process.
 
The mesenchyme which extends between successive vertebral
arches does not chondrify, but later becomes transformed into the
 
 
 
162 DEVELOPMENT OF THE VERTEBRAE
 
interspinous ligaments and the ligamenta ftava, while the anterior
and posterior longitudinal ligaments are formed from unchondrified
portions of the tissue surrounding the vertebral bodies.
 
As was pointed out, the mesenchyme in the region of the cleft
separating the anterior and posterior portions of a sclerotome becomes an intervertebral fibrocartilage, and, as the cartilaginous
bodies develop, the portions of the notochord enclosed by them
become constricted, while at the same time the portions in the
intervertebral regions increase in size. Finally the notochord disappears from the vertebral regions, although a canal, representing
its former position, traverses each body for a considerable time, but
in the intervertebral regions it persists as relatively large flat disks
forming the pulpy nuclei of the fibrocartilages.
 
The mode of development described above applies to the great
majority of the vertebrae, but some departures from it occur, and
these may be conveniently considered before passing on to an
account of the ossification of the cartilages. The variations affect
principally the extremes of the series. Thus the posterior vertebrae
present a reduction of the vertebral arches, those of the last sacral
vertebrae being but feebly developed, while in the coccygeal vertebrae
they are indicated only in the first. In the first cervical vertebra,
the atlas, the reverse is the case, for the entire adult vertebra is
formed from the posterior portion of a sclerotome, its lateral masses
and posterior arch being the vertebral arches, while its anterior arch
is the hypochordal bar, which persists in this vertebra only. A welldeveloped centrum is also formed, however (Fig. 98), but it does not
unite with the parts derived from the preceding sclerotome, but
during its ossification unites with the centrum of the epistropheus
(axis), forming the odontoid process of that vertebra. The epistropheus consequently is formed by one and a half sclerotomes, while
but half a one constitutes the atlas.
 
The extent to which the ribs are developed in connection with
the various vertebrae also varies considerably. Throughout the cervical region they are short, the upper five or six being no longer than
the transverse processes, with the tips of which their extremities
 
 
 
DEVELOPMENT OF THE VERTEBRA
 
 
 
163
 
 
 
unite at an early stage. In the upper five or six vertebrae a relatively
large interval persists between the rib and the transverse process,
forming the costo-transverse foramen, through which the vertebral
vessels pass, but in the seventh vertebra the fusion is more extensive
and the foramen is very small and hardly noticeable. In the thoracic
region the ribs reach their greatest development, the upper eight or
 
 
 
â–  \;-^Z ... --â– 
 
 
 
 
Fig. 9,8. — Longitudinal Section through the Occipital Region and Upper
 
Cervical Vertebrae of a Calf Embryo of 18.5 mm.
 
has, Basilar artery; ch, notochord; Kc l ~ 4 , vertebral centra; lc 2 ~ 4 , intervertebral
 
disks; occ, basioccipital; Sc x ~ 4 , hypochordal bars. — (Froriep.)
 
 
 
nine extending almost to the mid-ventral line, where their extremities
unite to form a longitudinal cartilaginous bar from which the sternum
develops (see p. 166). The lower three or four thoracic ribs are
successively shorter, however, and lead to the condition found in
the lumbar vertebras, where they are again greatly reduced and
firmly united with the transverse processes, the union being so close
that only the tips of the latter can be distinguished, forming what
are known as the accessory tubercles. In the sacral region the ribs
 
 
 
164
 
 
 
DEVELOPMENT OF THE VERTEBRA
 
 
 
are reduced to short flat plates, which unite together to form the
lateral masses of the sacrum, and, finally, in the coccygeal region the
blastemic costal processes of the first vertebra unite with the transverse processes to form the transverse processes of the adult vertebra,
but no indications of them are to be found in the other vertebrae
beyond the blastemic stage.
 
The third stage in the development of the axial skeleton begins
with the ossification of the cartilages, and in each vertebra there are
typically as many primary centers of ossification as there were
originally cartilages, except that but a single center appears in the
body. Thus, to take a thoracic vertebra as a type, a center appears
in each half of each vertebral arch at the base of the transverse process
and gradually extends to form the bony lamina, pedicle, and the
greater portion of the transverse and spinous processes; a single
center gives rise to the body of the vertebra; and each rib ossifies
 
 
 
 
 
Fig. 99. — A, A Vertebra at Birth; B, Lumbar Vertebra showing Secondary
Centers of Ossification.
a, Center for the articular process; c, body; el, lower epiphysial plate; en, upper
epiphysial plate; na, vertebral arch; s, center for spinous process; t, center for transverse
process. — (Sappey.)
 
 
 
from a single center. These various centers appear early in embryonic life, but the complete transformation of the cartilages into bone
does not occur until some time after birth, each vertebra at that
period consisting of three parts, a body and two halves of an arch,
separated by unossified cartilage (Fig. 99, A). At about puberty
secondary centers make their appearance; one appears in the cartilage which still covers the anterior and posterior surfaces of the
vertebral body, producing disks of bone in these situations (Fig. 99,
 
 
 
DEVELOPMENT OF THE VERTEBRA
 
 
 
l6 5
 
 
 
B, en and el), another appears at the tip of each spinous and transverse process (Fig. 99, B), and in the lumbar vertebrae others appear
at the tips of the articulating processes. The epiphyses so formed
remain separate until growth is completed and between the sixteenth
and twenty-fifth years unite with the bones formed from the primary
centers, which have fused by this time, to form a single vertebra.
 
Each rib ossifies from a single primary center situated near the
angle, secondary centers appearing for the capitulum and tuberosity.
 
In some of the vertebras modifications of this typical mode of
ossification occur. Thus, in the upper five cervical vertebrae the
centers for the rudimentary ribs are suppressed, ossification extending into them from the vertebral arch centers, and a similar suppression of the costal centers occurs in the lower lumbar vertebrae, the
first only developing a separate rib center. Furthermore, in the
 
 
 
 
 
Fig. ioo.—A, Upper Surface of the First Sacral Vertebra, and B, Ventral
 
View of the Sacrum showing Primary Centers of Ossification.
 
c, Body; na, vertebral arch; r, rib center. — (Sappey.)
 
atlas a double center appears in the persisting hypochordal bar, and
the body which corresponds to the atlas, after developing the terminal epiphysial disks, fuses with the body of the epistropheus (axis)
to form its odontoid process, this vertebra consequently possessing,
in addition to the typical centers, one (double) other primary and two
secondary centers. In the sacral region the typical centers appear
 
 
 
1 66 DEVELOPMENT OF THE STERNUM
 
in all five vertebrae, with the exception of rib centers for the last one
or two (Fig. ioo) and two additional secondary centers give rise to
plate-like epiphyses on each side, the upper plates forming the
articular surface for the ilium. At about the twenty-fifth year all
the sacral vertebrae unite to form a single bone, and a similar fusion
occurs also in the rudimentary vertebrae of the coccyx.
 
The majority of the anomalies seen in the vertebral column are due
to the imperfect development of one or more cartilages or of the centers of
ossification. Thus, a failure of an arch to unite with the body or even the
complete absence of an arch or half an arch may occur, and in cases of
spina bifida the two halves of the arches fail to unite dorsally. Occasionally the two parts of the double cartilaginous center for the body fail to
unite, a double body resulting; or one of the two parts may entirely fail,
the result being the formation of only one-half of the body of the vertebra.
Other anomalies result from the excessive development of parts. Thus,
the rib of the seventh cervical vertebra may sometimes remain distinct and
be long enough to reach the sternum, and the first lumbar rib may also
fail to unite with Its vertebra. On the other hand, the first thoracic rib is
occasionally found to be imperfect.
 
The Development of the Sternum. — Longitudinal bars, which
are formed by the fusion of the ventral ends of the anterior eight or
nine cartilaginous thoracic ribs, represent the future sternum. At
an early period the two bars come into contact anteriorly and fuse
together (Fig. 101), and at this anterior end two usually indistinctly
separated masses of cartilage are to be observed at the vicinity of the
points where the ventral ends of the cartilaginous clavicles articulate.
These are the episternal cartilages {em), which later normally unite
with the longitudinal bars and form part of the manubrium sterni,
though occasionally they persist and ossify to form the ossa suprasternal. The fusion of the longitudinal bars gradually extends backward until a single elongated plate of cartilage results, with which the
seven anterior ribs are united, one or two of the more posterior ribs
which originally took part in the formation of each bar having
separated. The portions of the bars formed by these posterior ribs
constitute the xiphoid process.
 
The ossification of the sternum (Fig. 102) partakes to a certain
extent of the original bilateral segmental origin of the cartilage,
 
 
 
DEVELOPMENT OF THE STERNUM 167
 
but there is a marked condensation of the centers of ossification and
considerable variation in their number also occurs. In the portion of
the cartilage which lies below the junction of the third costal cartilages
a series of pairs of centers appears just about birth, each center
 
 
 
 
 
 
 
 
 
 
Fig. ioi. — Formation of the Sternum in an Embryo of About 3 cm.
el, Clavicle; em, episternal cartilage. — (Ruge.)
 
 
 
probably representing an epiphysial center of a corresponding rib.
Later the centers of each pair fuse and the single centers so formed,
extending through the cartilage, eventually unite to form the greater
part of the body of the bone. In each of the two uppermost segments, however, but a single center appears, that of the second
segment uniting with the more posterior centers and forming the
upper part of the body, while the uppermost center gives rise to the
manubrium, which frequently persists as a distinct bone united to the
body by a hinge-joint.
 
 
 
i68
 
 
 
DEVELOPMENT OF THE SKULL
 
 
 
A failure of the cartilaginous bars to fuse produces the condition
known as cleft sternum, or if the failure to fuse affects only a portion of the
bars there results a perforated sternum. A perforation or notching of the
xiphoid cartilage is of frequent occurrence owing to this being the region
where the fusion of the bars takes place last.
 
 
 
 
 
Fig. i 02. — Sternum of
New-born Child, showing
Centers of Ossification.
I to VII, Costal cartilages. —
(Gegenbaur.)
 
 
 
Fig. 103. — Reconstruction of the Chondrocranium of an embryo of 14 mm.
as, Alisphenoid; bo, basioccipital; bs, basisphenoid; eo, exoccipital; m, Meckel's cartilage;
os, orbitosphenoid; p, periotic; ps, presphenoid;
so, sella turcica; s, supraoccipital. — {Levi.)
 
 
 
The suprasternal bones are the rudiments of a bone or cartilage, the
omosternum, situated in front of the manubrium in many of the lower
mammalia. It furnishes the articular surfaces for the clavicles and is
possibly formed by a fusion of the ventral ends of the cartilages which
represent those bones; hence its appearance as a pair of bones in the rudimentary condition.
 
The Development of the Skull. — In its earliest stages the
human skull is represented by a continuous mass of mesenchyme
which invests the anterior portion of the notochord and extends
forward beyond its extremity into the nasal region, forming a core
for the nasal process (see p. 99). From each side of this basal mass
a wing projects dorsally to enclose the anterior portion of the medullary canal which will later become the cerebral part of the central
nervous system. No indications of a segmental origin are to be
 
 
 
DEVELOPMENT OF THE SKULL 169
 
found in this mesenchyme; as stated, it is a continuous mass, and
this is likewise true of the cartilage which later develops in it.
 
The chondrihcation occurs first along the median line in what
will be the occipital and sphenoidal regions of the skull (Fig. 103)
and thence gradually extends forward into the ethmoidal region and
to a certain extent dorsally at the sides and behind into the regions
later occupied by the wings of the sphenoid (as and os) and the
squamous portion of the occipital (s). No cartilage develops,
however, in the rest of the sides or in the roof of the skull, but the
mesenchyme of these regions becomes converted into a dense membrane of connective tissue. While the chondrification is proceeding
in the regions mentioned, the mesenchyme which encloses the
internal ear becomes converted into cartilage, forming a mass, the
periotic capsule (Fig. 103, p), wedged in on either side between the
occipital and sphenoidal regions, with which it eventually unites to
form a continuous chondro cranium, perforated by foramina for the
passage of nerves and vessels.
 
The posterior part of the basilar portion of the occipital cartilage
presents certain peculiarities of development. In calf embryos
there are in this region, in very early stages, four separate condensations of mesoderm corresponding to as many mesodermic somites
and to the three roots of the hypoglossal nerve together with the first
cervical or suboccipital nerve (Froriep) (Fig. 104). These mesenchymal masses in their general characters and relations resemble
vertebral bodies, and there are good reasons for believing that they
represent four vertebrae which, in later stages, are taken up into the
skull region and fuse with the primitive chondrocranium. In the
human embfyo they are less distinct than in lower mammals, but since
a three-rooted hypoglossal and a suboccipital nerve also occur in man
it is probable that the corresponding vertebrae are also represented.
Indeed, confirmation of their existence may be found in the fact
that during the cartilaginous stage of the skull the hypoglossal foramina are divided into three portions by two cartilaginous partitions
which separate the three roots of the hypoglossal nerve. It seems
certain from the evidence derived from embryology and comparative
 
 
 
170
 
 
 
DEVELOPMENT OF THE SKULL
 
 
 
anatomy that the human skull is composed of a primitive unsegmental chondrocranium plus four vertebrae, the latter being added
 
to and incorporated with the occipital portion of the chondrocranium.
Emphasis must be laid upon
the fact that the cartilaginous portion of the skull forms only the
base and lower portions of the sides
of the cranium, its entire roof, as
well as the face region, showing no
indication of cartilage, the mesenchyme in these regions being converted into fibrous connective tissue,
which, especially in the cranial region, assumes the form of a dense
membrane.
 
But in addition to the chondrocranium and the vertebras incorporated with it, other cartilaginous elements enter into the composition of
the skull. The mesenchyme which
occupies the axis of each branchial
arch undergoes more or less complete chondrification, cartilaginous
bars being so formed, certain of
which enter into very close relations with the skull. It has been
seen (p. 92) that each half of the
first arch gives rise to a maxillary
process which grows forward and
ventrally to form the anterior
boundary of the mouth, while the
remaining portion of the arch forms
the mandibular process. The
whole of the axis of the mandib
 
 
 
J-^V : y:
 
 
 
 
Fig. 104. — Frontal Section
through the occipital and upper
Cervical Regions of a Calf Embryo
of 8.7 MM.
 
ai and ai 1 , Intervertebral arteries;
be 1 , first cervical intervertebral plate;
bo, suboccipital intervertebral plate;
c 1— 2 , cervical nerves; eh, notochord;
K, vertebral centrum; m l — 3 , occipital
myotomes; m 4— 5 , cervical myotomes;
1— 3 , roots of hypoglossal nerve; vj,
jugular vein; x and xi, vagus and spinal
accessory nerves. — (Froriep.)
 
 
 
DEVELOPMENT OF THE SKULL
 
 
 
171
 
 
 
 
ular process becomes chondrified, forming a rod known as Meckel's
cartilage, and this, at its dorsal end, comes into relation with the
periotic capsule, as does also the dorsal end of the cartilage of
the second arch. In the remaining three arches cartilage forms
only in the ventral portions, so that their rods do not come into
relation with the skull, though it will be convenient to consider
their further history together with that of the other branchial arch
cartilages. The arrangement
of these cartilages is shown diagrammatically in Fig. 105.
 
By the ossification of these
various parts three categories of
bones are formed: (1) cartilage
bones formed in the chondrocranium, (2) membrane bones,
and (3) cartilage bones developing from the cartilages of the
branchial arches. The bones
belonging to each of these categories are primarily quite distinct from one another and from
those of the other groups, but in the human skull a very considerable amount of fusion of the primary bones takes place, and
elements belonging to two or even to all three categories may unite
to form a single bone of the adult skull. In a certain region of the
chondrocranium also and in one of the branchial arches the original
cartilage bone becomes ensheathed by membrane bone and eventually disappears completely, so that the adult bone, although represented by a cartilage, is really a membrane bone. And, indeed,
this process has proceeded so far in certain portions of the branchial
arch skeleton that the original cartilaginous representatives are
no longer developed, but the bones are deposited directly in connective tissue. These various modifications interfere greatly with the
precise application to the human skull of the classification of bones
into the three categories given above, and indeed the true significance
of certain of the skull bones can only be perceived by comparative
 
 
 
Fig. 105. — Diagram showing the Five
 
Branchial Cartilages, I to'F.
 
At, Atlas; Ax, epistropheus; 3, third
 
cervical vertebra.
 
 
 
172
 
 
 
OSSIFICATION OF THE CH0NDR0CRANIUM
 
 
 
studies. Nevertheless it seems advisable to retain the classification,
indicating, where necessary, the confusion of bones of the various
categories.
 
The Ossification of the Chondrocranium. — The ossification
of the cartilage of the occipital region results in the formation of
four distinct bones which even at birth are separated from one
 
another by bands of cartilage.
The portion of cartilage lying in
front of the foramen magnum
ossifies to form a basioccipital
bone (Fig. 106, bo), the portions
on either side of this give rise to
the two exoccipitals (eo), which
bear the condyles, and the portion above the foramen produces
a supraoccipital (so), which represents the part of the squamous
portion of the adult bone lying
below the superior nuchal line.
All that portion of the bone
which lies above that line is
composed of membrane bone
which owes its origin to the
fusion of two or sometimes four
centers of ossification, appearing
in the membranous roof of the embryonic skull. The bone so
formed (ip) represents the interparietal of lower vertebrates and, at
an early stage, unites with the supraoccipital, although even at
birth an indication of the line of union of the two parts is to be seen
in two deep incisions at the sides of the bone. The union of the
exoccipitals and supraoccipital takes place in the course of the first
or second year after birth, but the basioccipital does not fuse with
the rest of the bone until the sixth or eighth year. It will be noticed
that no special centers occur for the four occipital vertebrae, these
structures having become completely incorporated in the chondro
 
 
 
Fig. 106. — Occipital Bone of a Fetus
 
at Term.
bo, Basioccipital; eo, exoccipital; ip, interparietal; so, supraoccipital.
 
 
 
OSSIFICATION OF THE CHONDROCRANIUM 173
 
cranium, and even the cartilaginous partitions which divide the
hypoglossal foramina usually disappear during the process of
ossification.
 
Two pairs of centers have been described for the interparietal
bone and it has been claimed that the deep lateral incisions divide
the lower pair, so that when the incisions meet and persist as the
sutura mendosa, separating the so-called inca bone from the rest of
the occipital, the division does not correspond to the line between
the supraoccipital and the interparietal, but a portion of the latter
bone remains in connection with the supraoccipital. Mall, however, in twenty preparations, found but a single pair of centers for
the interparietal.
 
Occasionally an additional pair of small centers appear for the
uppermost angle of the interparietal, and the bones formed from
them may remain distinct as what have been termed fontanelle
bones.
 
 
 
 
Fig. 107. — Sphenoid Bone from Embryo of 3^ to 4 Months.
The parts which are still cartilaginous are represented in black, as, Alisphenoid ;
b, basisphenoid; /, lingula; os, orbitosphenoid ; p, internal pterygoid plate. — (Sappey.)
 
In the sphenoidal region the number of distinct bones which
develop is much greater than in the occipital region. At the beginning of the second month a center appears in each of the cartilages
which represent the alisphenoids (great wings). These cartilages
do not, however, represent the entire extent of the great wings and
their ossification gives rise only to those portions of the bone in the
neighborhood of the foramina ovale and rotundum and to the
lateral pterygoid plates. The remaining portions of the wings, the
orbital and temporal portions, develop as membrane bone (Fawcett)
 
 
 
174 OSSIFICATION OF THE CHONDROCRANIUM
 
and early unite with the portions formed from the cartilage. At
the end of the second month a center appears in each orbito sphenoid
(lesser wing) cartilage (Fig. 107, os), and a little later a pair of
centers (b), placed side by side, are developed in the cartilage
representing the posterior portion of the body; together these form
what is known as the basisphenoid. Still later a center appears on
either side of the basisphenoids to form the UngulcB (I), and another
pair appears in the anterior part of the cartilage, between the orbitosphenoids, and represent the presphenoid.
 
In addition to these ten centers in cartilage and the membrane
portion of the alisphenoid, two other membrane bones are included
in the adult sphenoid. Thus, a little before the appearance of the
center for the alisphenoids an ossification is formed in the mesenchyme of each lateral wall of the posterior part of the nasal cavity
and gives rise to the medial lamina of the pterygoid process, the
mesenchyme at the tip of the ossification condensing to form a
cartilaginous hook-like structure over which the tendon of the tensor
veli palatini plays. This cartilage later ossifies to form the pterygoid
hamulus, the medial pterygoid lamina being thus a combination of
membrane and cartilage, the latter, however, being a secondary
development and quite independent of the chondrocranium.
 
By the sixth month the lingular have fused with the basisphenoid
and the orbitosphenoids with the presphenoid, and a little later the
basisphenoid and presphenoid unite. The alisphenoids and medial
pterygoid laminae remain separate, however, until after birth, fusing
with the remaining portions of the adult bone during the first year.
 
The cartilage of the ethmoidal region of the chondrocranium
forms somewhat later than the other portions and consists at first
of a stout median mass projecting downward and forward into the
nasal process (Fig. 108, Ip), and two lateral masses {lm), situated one
on either side in the mesenchyme on the outer side of each olfactory
pit. Ossification of the lateral masses or ectethmoids begins relatively early, but it appears in the upper part of the median cartilage
only after birth, producing the crista galli and the perpendicular
plate, which together form what is termed the mesethmoid. When
 
 
 
OSSIFICATION OF THE CHONDROCRANIUM
 
 
 
175
 
 
 
 
first formed, the three cartilages are quite separate from one another,
the olfactory and nasal nerves passing down between them to the
olfactory pit, but later trabecular begin to extend across from
the mesethmoid to the upper part of the ectethmoids and eventually
form a fenestrated horizontal lamella which ossifies to form the
cribriform plate.
 
The lower part of the median cartilage does not ossify, but a
center appears on each side of the median line in the mesenchyme
behind and below its posterior or lower
border. From these centers two vertical bony plates develop which unite
by their median surfaces below, and
above invest the lower border of the
cartilage and form the vomer. The
portion of the cartilage which is thus
invested undergoes resorption, but the
more anterior portions persist to form
the cartilaginous septum of the nose.
The vomer, consequently, is not really
a portion of the chondrocranium, but
is a membrane bone; its intimate
relations with the median ethmoidal
cartilage, however, make it convenient
to consider it in this place.
 
When first formed, the ectethmoids are masses of spongy bone
and show no indication of the honeycombed appearance which they
present in the adult skull. This condition is produced by the
absorption of the bone of each mass by evaginations into it of the
mucous membrane lining the nasal cavity. This same process also
brings about the formation of the curved plates of bone which
project from the inner surfaces of the lateral masses and are known
as the superior and middle conchse (turbinated bones). The inferior
and sphenoidal conchae are developed from special centers, but
belong to the same category as the others, being formed from portions of the lateral ethmoidal cartilages which become almost
 
 
 
Fig. 108. — Anterior Portion
of the Base of the Skull of a
6 to 7 Months' Embryo.
 
The shaded parts represent
cartilage. cp, Cribriform plate;
hn, lateral mass of the ethmoid;
Ip, perpendicular plate; of optic
foramen; os, orbitosphenoid. —
(After von Spec.)
 
 
 
176
 
 
 
OSSIFICATION OF THE CHONDROCRANIUM
 
 
 
separated at an early stage before the ossification has made much
progress. Absorption of the body of the sphenoid bone to form
the sphenoidal cells, of the frontal to form the frontal sinuses, and
of the maxillaries to form the maxillary antra is also produced by
outgrowths of the nasal mucous membrane, all these cavities, as
well as the ethmoidal cells, being continuous with the nasal cavities
and lined with an epithelium which is continuous with the mucous
membrane of the nose.
 
In the lower mammalia the erosion of the mesial surface of the
ectethmoidal cartilages results, as a rule, in the formation of five conchae,
while in man but three are usually recognized. Not infrequently,
however, the human middle concha shows indications, more or less
marked, of a division into an upper and a lower portion, which correspond to the third and fourth bones of the typical mammalian arrangement. Furthermore, at the upper portion of the nasal wall, in front of
 
the superior concha, a slight elevation,
termed the agger nasi, is always observable, its lower edge being prolonged downward to form what is termed the uncinate
process of the ethmoid. This process
and the agger together represent the uppermost concha of the typical arrangement, to which, therefore, the human
arrangement may be reduced.
 
A number of centers of ossification — the exact number is yet uncertain — appear in the periotic capsule
during the later portions of the fifth
month, and during the sixth month
these unite together to form a single
center from which the complete ossification of the cartilage proceeds to form the petrous and mastoid
portions of the temporal bone (Fig. 109, p). The mastoid process
does not really form until several years after birth, being produced
by the hollowing and bulging out of a portion of the petrous bone
by out-growths from the lining membrane of the middle ear. The
cavities so formed are the mastoid cells, and their relations to the
middle-ear cavity are in all respects similar to those of the ethmoidal
 
 
 
 
Fig. 109. — The Temporal
 
Bone at Birth. The Styloid
 
Process and Auditory Ossicles
 
are not Represented.
 
p, Petrous bone; s, squamosal;
 
t, tympanic. — (Poirier.)
 
 
 
OSSIFICATION OF THE CHONDROCRANIUM
 
 
 
177
 
 
 
and sphenoidal cells to the nasal cavities. The remaining portions
of the temporal bone are partly formed by membrane bone and
partly from the branchial arch skeleton. An ossification appears at
the close of the eighth week in the membrane which forms the side
of the skull in the temporal region and gives rise to a squamosal
bone (s), which later unites with the petrous to form the squamosal
portion of the adult temporal, and another membrane bone, the
tympanic (/), develops from a center appearing in the mesenchyme
surrounding the external auditory meatus, and later also fuses with
the petrous to form the floor and sides of the external meatus, giving
attachment at its inner edge to the tympanic membrane. Finally,
the styloid process is developed from the upper part of the second
branchial arch, whose history will be considered later.
 
The various ossifications which form in the chondrocranium and
the portions of the adult skull which represent them are shown in the
following table:
 
 
 
Region of
Chondrocranium.
 
 
 
Ossification.
 
 
 
IBasioccipital
Exoccipitals
Supraoccipital
 
 
 
Sphenoidal
 
 
 
Ethmoidal .
 
 
 
Basisphenoid
 
Presphenoid
 
Lingulae
 
Alisphenoids
 
Orbitosphenoids
 
Mesethmoid
 
 
 
Ectethmoids
 
 
 
Parts of Adult Skull;
 
Basilar process.
Condyles.
 
Squamous portion below superior nuchal
line.
 
Body.
 
Greater wings (in part) .
Lesser wings.
Lamina perpendicularis.
Crista galli.
Nasal septum.
Lateral masses.
Superior concha.
Middle concha.
 
 
 
Inferior concha.
 
Sphenoidal concha.
 
,, . . f Mastoid.
 
Penolic capsule < _.
 
1 Petrous.
 
The Membrane Bones of the Skull.— In the membrane forming the sides and roof of the skull in the second stage of its develop
 
 
178 THE MEMBRANE BONES OF THE SKULL
 
ment ossifications appear, which give rise, in addition to the interparietal and squamosal bones already mentioned in connection with
the occipital and temporal, to the parietals and frontal. Each of the
former bones develops from a single center which appears at the
end of the eighth week, while the frontal is formed at about the same
time from two centers situated symmetrically on each side of the
median line and eventually fusing completely to form a single bone,
although more or less distinct indications of a median suture, the
metopic, are not infrequently present.
 
Furthermore, ossifications appear in the mesenchyme of the
facial region to form the nasal, lachrymal, and zygomatic bones, all
of which arise from single centers of ossification. In the case of each
zygomatic bone, however, three osseous thickenings appear on the
inner surface of the original ossification, which then disappears and
the thickenings unite to form the adult bone, though occasionally
one or more of their lines of union may persist, producing a bipartite
or tripartite zygomatic.
 
The vomer, which has already been described, belongs also
strictly to the category of membrane bones, as do also the maxillae
and the palatines; these latter, however, primarily belonging to the
branchial arch skeleton, with which they will be considered.
 
The purely membrane bones in the skull, are, then, the following:
 
Interparietals Part of squamous portion of occipital.
 
Pterygoids Medial pterygoid plates.
 
Squamosals Squamous portions of temporals.
 
Tympanies Tympanic plates of temporals.
 
Parietals.
 
Frontal.
 
Nasals.
 
Lachrymals.
 
Zygomatics.
 
Vomer.
 
The Ossification of the Branchial Arch Skeleton. — It has
 
been seen (p. 171) that a cartilaginous bar develops only in the
mandibular process of the first branchial arch. In the maxillary
process no cartilaginous skeleton forms, but two membrane bones,
 
 
 
OSSIFICATION OF BRANCHIAL ARCH SKELETON
 
 
 
179
 
 
 
 
Fig. i 10. — Diagram of the Ossifications of which the Maxilla
is Composed, as seen from the
Outer Surface. The Arrow
Passes through the Infraorbital Canal. — {From von Spee,
after Sappey.)
 
 
 
the palatine and maxilla, are developed in it, their cartilaginous
representatives, which are to be found in lower vertebrates, having
been suppressed by a condensation of the development. The
palatine bone develops from a single center of ossification, but for
each maxilla no less than five centers have been described (Fig. no).
One of these gives rise to so much of the alveolar border of the bone
as contains the bicuspid and molar teeth; a second forms the nasal
process and the part of the alveolar
border which contains the canine
tooth; a third the portion which contains the incisor teeth; while the
fourth and fifth centers lie above the
first and give rise to the inner and
outer portions of the orbital plate
and the body of the bone. The
first, second, fourth, and fifth portions early unite together, but the
third center, which really lies in the
ventral part of the nasal process, remains separate for some time,
forming what is termed the premaxilla, a bone which remains permanently distinct in the majority of the lower mammals.
 
The above is the generally accepted view as to the development of
the maxilla. Mall, however, maintains^ that it has but tw r o centers of
ossification, one giving rise to the premaxilla and the other to the rest of
the bone. The maxillary center makes its appearance about the middle
of the sixth week.
 
Since the condition known as hare-lip results from a failure of the
maxillary process to unite completely with the frontonasal process (see
p. 100), and since the premaxilla develops in the latter and the maxilla
in the former, the cleft may pass between these two bones and prevent
their union (see also p. 284).
 
The upper end of Meckel's cartilage passes between the tympanic
bone and the outer surface of the periotic capsule and thus comes
to lie apparently within the tympanic cavity of the ear; this portion
of the cartilage divides into two parts which ossify to form two of the
bones of the middle ear, the malleus and incus, a description of
 
 
 
i8o
 
 
 
OSSIFICATION OF BRANCHIAL ARCH SKELETON
 
 
 
whose further development may be postponed until a later chapter.
At about the middle of the sixth week of development a plate of
membrane bone appears to the outer side of the lower portion of the
cartilage and gradually extends to form the body and ramus of the
mandible.
 
In the region of the body the bone develops so as to enclose the
cartilage, together with the inferior alveolar (dental) nerve which
lies to the outer side of the cartilage, but in the region of the ramus
 
 
 
 
Z.ChT
 
 
 
Fig. hi.— Model of Right Half of Mandible of a Fetus 95 mm. in Length,
seen from the mesial surface.
C 1 and C 2 , Accessory cartilages; Ch. T., chorda tympanijO., cartilage for coronoid
process; Cy., cartilage for condyloid process; Mai., malleus; M.C., Meckel's cartilage;
N. Al., inferior alveolar nerve; N. Aur., auriculo-temporal nerve; N.L., lingual nerve;
N.Mh., mylo-hyoid nerve; N.T., trigeminal nerve; Sy., symphysis. — (Low.)
 
 
 
the bone remains entirely to the outer side of the cartilage and nerve,
whence the position of the mandibular foramen on the inner surface
of the adult bone. The anterior portion of Meckel's cartilage
becomes ossified by the extension of ossification from the membrane
bone into it, the portion corresponding to the body of the bone behind
the mental foramen disappears and the portion above the mandibular foramen is said to become transformed into fibrous connective
tissue and to persist as the spheno-mandibular ligament. At the
upper extremity of the ramus two nodules of cartilage develop, quite
independently, however, of Meckel's cartilage (Fig. in, Cr and Cy),
 
 
 
OSSIFICATION OF BRANCHIAL ARCH SKELETON
 
 
 
181
 
 
 
and ossification extends into these from the ramus to form the
coronoid and condyloid processes. And, finally, two other independent cartilages appear toward the anterior extremity of each half
 
 
 
 
Fig. 112. — Diagram showing the Categories to which the Bones of the Skull
. . Belong.
 
The unshaded bones are membrane bones, the heavily shaded represent the
chondrocranium, while the black represents the branchial arch elements. AS, Alisphenoid; ExO, exoccipital; F, frontal; Hy, hyoid; IP, interparietal; Z, zygomatic;
Mn, mandible; Mx, maxilla; NA, nasal; P, parietal; Pe, periotic; SO, supraoccipital;
Sg, squamosal; St, styloid process; Th, thyreoid cartilage; Ty, tympanic.
 
 
 
of the bone, one at the alveolar (C t ) and the other at the lower
border (C 2 ), and these, also are later incorporated into the bone
without developing special centers of ossification.
 
 
 
182 OSSIFICATION OF BRANCHIAL ARCH SKELETON
 
Each half of the mandible thus ossifies from a single center, and
is essentially a membrane bone replacing a cartilaginous precursor.
At birth the two halves are united at the symphysis by fibrous tissue,
into which ossification extends later, union occurring in the first
or second year.
 
The upper part of the cartilage of the second branchial arch also
comes into relation with the tympanic cavity and ossifies to form the
styloid process of the temporal bone. The succeeding moiety of the
cartilage undergoes degeneration to form the stylo-hyoid ligament,
while its most ventral portion ossifies as the lesser comu of trie hyoid
bone. The great variability which may be observed in the length
of the styloid processes and of the lesser cornua of the hyoid depends
upon the extent to which the ossification of the original cartilage
proceeds, the length of the stylo-hyoid ligaments being in inverse
ratio to the length of the processes or cornua. The greater cornua
of the hyoid are formed by the ossification of the cartilages of the
third arch, and the body of the bone is formed from a cartilaginous
plate, the copula, which unites the ventral ends of the two arches
concerned.
 
Finally, the cartilages of the fourth and fifth branchial arches
early fuse together to form a plate of cartilage, and the two plates
of opposite sides unite by their ventral edges to form the thyreoid
cartilage of the larynx.
 
The accompanying diagram (Fig. 112) shows the various structures derived from the branchial arch skeleton, as well as some of
the other elements of the skull, and a re'sume' of the fate of the branchial arches may be stated in tabular form as follows, the parts represented by cartilage which becomes replaced by membrane bone
being printed in italics, while membrane bones which have no
cartilaginous representatives are enclosed in brackets:
 
(Maxilla).
 
(Palatine) .
 
Malleus.
 
Incus.
 
Spheno-mandibular ligament.
 
Mandible.
 
 
 
1st arch.
 
 
 
DEVELOPMENT OF APPENDICULAR SKELETON 1 83
 
(Styloid process of the temporal.
Stylo-hyoid ligament.
Lesser cornu of hyoi< 1 .
 
3d arch Greater cornu of hyoid.
 
4th and 5th arches Thyreoid cartilage of larynx.
 
The Development of the Appendicular Skeleton. — While
the greater portion of the axial skeleton is formed from the sclerotomes of the mesodermic somites, the appendicular skeleton is
derived from the somatic mesenchyme, which is not divided into
metameres. This mesenchyme forms the core of the limb bud and
becomes converted into cartilage, by the ossification of which all the
bones of the limbs, with the possible exception of the clavicle, are
formed.
 
Of the bones of the pectoral girdle the clavicle requires further
study before it can be certain whether it is to be regarded as a purely
cartilage bone or as a combination of cartilage and membrane
ossification (Gegenbaur). It is the first bone of the skeleton to
ossify, two centers appearing for each bone at about the sixth week
of development. The tissue in which the ossifications form has
certain peculiar characters, and it is difficult to say whether it is to be
regarded as cartilage which, on account of the early differentiation
of the center, has not yet become thoroughly differentiated histologically, or as some other form of connective tissue. However that may
be, true cartilage develops on either side of the ossifying region, and
into this the ossification gradually extends, so that at least a portion
of the bone is preformed in cartilage.
 
The scapula is at first a single plate of cartilage in which two
centers of ossification appear. One of these gives rise to the body
and the spine, while the other produces the coracoid process (Fig.
113, co), the rudimentary representative of the coracoid bone which
extends between the scapula and sternum in the lower vertebrates.
The coracoid does not unite with the body until about the fifteenth
year, and secondary centers which give rise to the vertebral edge (b)
and inferior angle of the bone (a) and to the acromion process (c)
unite with the rest of the bone at about the twentieth year.
 
 
 
1 84
 
 
 
DEVELOPMENT OF APPENDICULAR SKELETON
 
 
 
The humerus and the bones of the forearm are typical long bones,
each of which develops from a primary center, which gives rise to
the shaft, and has, in addition, two or more epiphysial centers. In
the humerus an epiphysial center appears for the head, another for
the greater tuberosity, and usually a third for the lesser tuberosity,
while at the distal end there is a center for each condyle, one for the
trochlea and one for the capitulum, the fusion of these various
epiphyses with the shaft taking place between the seventeenth and
 
 
 
 
Fig. 113. — The Ossification Centers of the Scapula.
a, b, and c, Secondary centers for
the angle, vertebral border, and acromion; co, center for the coracoid process. — (Testut.)
 
 
 
 
Fig. 114. — Reconstruction of an
Embryonic Carpus.
 
c, Centrale; cu, triquetral; lu, lunate;
m, capitate; p, pisiform; sc, navicular; t,
greater multangular; tr, lesser multangular;
u, hamate.
 
 
 
twentieth years. The radius and ulna each possesses a single epiphysial center for each extremity in addition to the primary center
for the shaft, the proximal epiphysial center for the ulna giving
rise to the tip of the olecranon process.
 
The embryological development of the carpus is somewhat
complicated. A cartilage is found representing each of the bones
normally occurring in the adult (Fig. 114), and these are arranged
in two distinct rows: a proximal one consisting of three elements,
 
 
 
 
DEVELOPMENT OF APPENDICULAR SKELETON 185
 
named from their relation to the bones of the forearm, radiate,
intermedium, and ulnar e; and a distal on^composed of four elements,
termed carpalia. In addition, a cartilage, termed the pisiform, is
found on the ulnar side of the proximal row ^nd is generally j^g&rded
as a sesamoid cartilage developed in the /tendon of the flei
ulnaris, and furthermore a number of inconstant carti
been observed whose significance in the majority of cast
less obscure. These accessory cartilage^either disappc
stages of development or fuse with neighboring cartilages^
cases, ossify and form distinct elements of the carpus,
however, occurs so frequently as almdK to deserve^ classification as
a constant element; it \p known asvthje ceniraie (Fig. 114, c) and
occupies a position between the/car\ua!§;es of the proximal and distal
rows and apparently correspond ~r&. a cartilage typVally present
in lower forms and o^fying*to~f»rai a distinct bone. Iri tha human
carpus its fate varies, wfe it may\eitnfer disappear or unitp with other
cartilages, that with wpich it most usually fuses b'eing probably the
radiale. There is evraence also to sfrftw that another ofJthe accessory
cartilages unites/with the ulnar element of the distatsAw, representing the carpale v typically present in lower forms.
 
Each of the eleinents corresponding to an adu^t) bone ossifies
 
from a single centerwith the exception of carpale iv-Xwhich has two
centers, a furtherindication of its composite character. The relation of the cartrteg&s to the adult bones may be seen from the table
given on page loX^J \v_^
 
With regard toYhe metacarpals and phalanges; it need merely
be stated that each develops from a single primary center for the
shaft and one secondary epiphysial center. The" primary center
appears at about the middle of the shaft excepJ in the terminal
phalanges, in which it appears at the distal enfr of the cartilage.
The epiphyses for the metacarpals are at the distends of the bones,
except in the case of the metacarpal of the ihumb, which resembles
the phalanges in having its epiphysis at the proximal end.
 
Each innominate bone appears as a somewhat oval plate of
cartilage whose long axis is directed almost at right angles to the
 
 
 
i86
 
 
 
DEVELOPMENT OF APPENDICULAR SKELETON
 
 
 
vertebral column and which is in close relation with the fourth and
fifth sacral vertebrae. As development proceeds a rotation of the
cartilage, accompanied by a slight shifting of position, occurs, so
that eventually the plate has its long axis almost parallel with the
vertebral column and is in relation with the first three sacrals.
Ossification appears at three points in each cartilage, one in the
 
upper part to form the ilium (Fig.
115, il) and two in the lower part,
the anterior of these giving rise to
the pubis (p), while the posterior
produces the ischium (is). At
birth these three bones are still
separated from one another by a
Y-shaped piece of cartilage whose
three limbs meet at the bottom
of the acetabulum, but later a
secondary center appears in this
cartilage and unites the three
bones together. The central part
of the lower half of each original
cartilage plate does not undergo
complete chondrification, but remains membranous, constituting
the obturator membrane which
closes the obturator foramen.
In addition to the Y-shaped secondary center, other epiphysial
centers appear in the prominent portions of the cartilage, such as
the pubic crest (Fig. 115, c), the ischial tuberosity (d), the anterior
inferior spine (b) and the crest of the ilium (a), and unite with the
rest of the bone at about the twentieth year.
 
The femur, tibia, and fibula each develop from a single primary
center for the shaft and an upper and a lower epiphysial center, the
femur possessing, in addition, epiphysial centers for the greater
and lesser trochanters (Fig. 94). The patella does not belong to
the same category as the other bones, but resembles the pisiform
 
 
 
 
Fig. 115. — The Ossification Centers
of the os innominatum.
a, b, c, and d, Secondary centers for
the crest, anterior inferior spine, symphysis, and ischial tuberosity; il, ilium;
is, ischium; p, pubis. — (Testut.)
 
 
 
DEVELOPMENT OF APPENDICULAR SKELETON
 
 
 
l8 7
 
 
 
bone of the carpus in being a sesamoid bone, developed in the tendon
of the quadriceps extensor cruris. Its cartilage does not appear
until the fourth month of intrauterine life, when most of the primary
centers for other bones have already appeared, and its ossification
does not begin until the third year after birth.
 
The tarsus, like the carpus, consists of a proximal row of three
cartilages, termed the tibiale, the intermedium, and the fibulare, and
of a distal row of four tarsalia. Between these two rows a single
cartilage, the centrale, is interposed. Each of these cartilages ossifies
from a single center, that of the intermedium early fusing with the
tibiale, though it occasionally remains distinct as the os trigonum, and
from a comparison with lower forms it seems probable that the
fibular cartilage of the distal row really represents two separate
elements, there being, properly speaking, five tarsalia instead ot
four. The fibulare, in addition to its primary center, possesses also
an epiphysial center, which develops at the point of insertion of the
tendo Achillis.
 
A comparison of the carpal and tarsal cartilages and their
relations to the adult bones may be seen from the following table:
 
 
 
Carpus
 
 
Tarsus
 
 
Cartilages
 
 
Bones
 
 
Bones
 
 
Cartilages
 
 
Radiale
 
 
Navicular
 
 
Talus
 
 
f Tibiale
 
\ Intermedium
 
 
Intermedium
 
 
Lunate
 
 
Ulnare
 
 
Triquetral
 
 
Calcaneus
 
 
Fibulare
 
 
Sesamoid cartilage
 
 
Pisiform
 
 
 
 
 
— —
 
 
Centrale
 
 
Fuses with navicular
 
 
Navicular
 
 
Centrale
 
 
Carpale I
 
 
Gr. multangular
 
 
1 st Cuneiform
 
 
Tarsale I
 
 
Carpale II
 
 
Less, multangular
 
 
2d Cuneiform
 
 
Tarsale II
 
 
Carpale III
 
 
Capitate
 
 
3d Cuneiform
 
 
Tarsale III
 
 
Carpale IV 1
Carpale V J
 
 
Hamate
 
 
Cuboid
 
 
( Tarsale TV
I Tarsale V
 
 
 
1 88 DEVELOPMENT OF THE JOINTS
 
The development of the metatarsals and phalanges is exactly
similar to that of the corresponding bones of the hand (see p. 185).
 
The Development of the Joints. — The mesenchyme which
primarily represents each, vertebra, or the skull, or the skeleton of
a limb, is at first a continuous mass, and when it becomes converted
into cartilage this also may be continuous, as in the skull, or may
appear as a number of distinct parts united by unmodified portions
of the mesenchyme. In the former case the various ossifications
as they extend will come into contact with their neighbors and will
either fuse with them or will articulate with them directly, forming
a suture.
 
When, however, a portion of unmodified mesenchyme intervenes
between two cartilages, the mode of articulation of the bones formed
from these cartilages will vary. The intermediate mesenchyme
may in time undergo chondrification and unite the bones in an
almost immovable articulation known as a synchondrosis (e. g., the
articulation of the first rib with the sternum) ; or a cavity may appear
in the center of the intervening cartilage so that a slight amount of
movement of the two bones is possible, forming an amphiar thro sis
(e. g., the symphysis pubis); or, finally, the intermediate mesenchyme may not chondrify, but its peripheral portions may become
converted into a dense sheath of connective tissue (Fig. 116, c)
which surrounds the adjacent ends of the two bones like a sleeve,
forming the articular capsule, while the central portions degenerate
to form a cavity. The bones which enter into such an articulation
are more or less freely movable upon one another and the joint is
termed a diarthrosis (e. g., the knee- or shoulder-joint).
 
In a diarthrosis the connective-tissue cells near the inner surface
of the capsule arrange themselves in a layer to form a synovial
membrane for the joint, and portions of the capsule may thicken
to form special bands, the reinforcing ligaments, while other strong
fibrous bands, which may pass from one bone to the other, forming
accessory ligaments, are shown by comparative studies to be in many
cases degenerated portions of what were originally muscles.
 
In certain diarthroses, such as the temporo-mandibular and
 
 
 
DEVELOPMENT OF THE JOINTS 189
 
sternoclavicular, the whole of the central portions of the intermediate mesenchyme does not degenerate, but it is converted into a
fibro-cartilage, between each surface of which and the adjacent
bone there is a cavity. These interarticular cartilages seem, in the
sterno-clavicular joints, to represent the sternal ends of a bone
existing in lower vertebrates and known as the precoracoid, but it
seems doubtful if those of the temporo-mandibular and knee
 
 
 
Fig. 116.— Longitudinal Section through the Joint oe the Great Toe in an
 
Embryo of 4.5 cm.
c, Articular capsule; i, intermediate mesenchyme which has almost disappeared in the
center; p 1 and p 2 , cartilages of the first and second phalanges. — (Nicholas.)
 
joints have a similar significance, the most recent observations on
their development tending to derive them from the intermediate
mesenchyme.
 
From their mode of development it is evident that the cavities of
diarthrodial joints are completely closed and their walls, except where
they are formed by cartilage, are lined by a continuous layer of synovial
cells. Ligaments or tendons, which, at first sight, appear to traverse the
cavities of certain joints, are in reality excluded from them, being lined
by a sheath of synovial cells continuous with the layer fining the general
cavity. Thus, the tendon of the long head of the biceps, which seems to
traverse the shoulder-joint is, in the fetus, entirely outside the articular
capsule, upon which it rests. Later it sinks in toward the joint cavity,
pushing the articular capsule before it, so that it lies at first in a groove
in the capsule, which later on becomes converted into a canal and, finally,
separates from the rest of the capsule except at its two extremities,
 
 
 
190 LITERATURE
 
forming a cylindrical canal, open at either end, traversing the joint cavity
and containing the tendon of the biceps.
 
The ligamentum teres of the hip-joint is similarly excluded from the
joint cavity by a sheath of synovium, which extends outward around it
from the bottom of the acetabular fossa to the depression in the head of
the femur, and in the knee-joint the crucial ligaments are also excluded
from the cavity by a reflection of the synovium. This joint, indeed, is
in the fetus incompletely divided into two parts, one corresponding to
each femoral condyle, by a partition which extends backward from the
patellar ligament to the crucial ligaments, remains of this partition
persisting in the adult as the so-called ligamentum mucosum.
 
 
 
LITERATURE.
 
C. R. Bardeen: " The Development of the Thoracic Vertebrae in Man," Amer. Journ.
 
Anat., iv, 1905.
C. R. Bardeen: "Studies of the Development of the Human Skeleton," Amer
 
Journ. Anat. iv, 1905.
C. R. Bardeen: "Early Development of the Cervical Vertebra and the Base of the
 
Occipital Bone in Man," Amer. Journ. Anat., vm, 1908.
C. R. Bardeen: "Vertebral Regional Determination in Young Human Embryos,"
 
Anat. Record, 11, 1908.
E. T. Bell: "On the Histogenesis of the Adipose Tissue of the Ox," Amer. Journ.
 
Anat., ix, 1909.
A. Bernays: "Die Entwicklungsgeschichte des Kniegelenks des Menschen mit
 
Bemerkungen liber die Gelenke im Allgemeinen," Morpholog. Jahrbuch, TV, 1878.
E. Dtjrsy: "Zur Entwicklungsgeschichte des Kopfes des Menschen und der hoheren
 
Wirbelthiere," Tubingen, 1869.
E. Fawcett: "On the Development, Ossification and Growth of the Palate Bone,"
 
Journ. Anat. and Phys., XL, 1906.
E. Fawcett: "Notes on the Development of the Human Sphenoid," Journ. Anat.
 
and Phys., xliv, 1910.
E. Fawcett: "The Development of the Human Maxilla, Vomer and Paraseptal Cartilages," Journ. Anat. and Phys., xlv, 1911.
A. Froriep: "Zur Entwicklungsgeschichte der Wirbelsaule, insbesondere des Atlas
 
und Epistropheus und der Occipitalregion," Archiv fur Anat. und Physiol., Anat.
 
Abth., 1886.
E. Gaupp: "Alte Probleme und neuere Arbeiten iiber den Wirbeltierschadel," Ergeb.
 
der Anat. und Entwicklungsgesch., x, 1901.
C. Gegenbaur: "Ein Fall von erblichem Mangel der Pars acromialis Claviculae, mit
 
Bemerkungen iiber die Entwicklung der Clavicula," Jenaische Zeitschr.filr medic.
 
Wissensch., I, 1864.
J. Golowinski: "Zur Kenntnis der His.togenese der Bindegewebsfibrillen," Anat.
 
Hefte, xxxiii, 1907.
 
 
 
LITERATURE - 191
 
E. Grafenberg: "Die Entwirklung der Knochen, Muskeln unci Nerven der Hand und
 
der fur die Bewegungen der Hand bestimmten Muskeln des Unterarms," Anat.
 
Hefte, xxx, 1906.
Henkeand Reyher: "Studien liber die Entwickelung der Extremitaten des Menschen,
 
insbesondere der Gelenkflachen," Sitzungsberichte der kais. Akad. Wien, LXX, 1875.
M. Jakoby: "Beitrag zur Kenntnis des menschlichen Primordialcraniums," Archiv
 
fiir mikrosk. Anat., xliv, 1894.
K. Kjellberg: "Beitrage zur Entwicklungsgeschichte des Kiefergelenks," Morph.
 
Jahrbuch, xxxii, 1904.
H. Leboucq: "Recherches sur la morphologie du carpe chez les mammiferes,"
 
Archives de Biolog., V, 1884.
G. Levi: "Beitrag zum Studium der Entwickelung des knorpeligen Primordialcraniums des Menschen," Archiv fiir mikrosk. Anat., lv, 1900.
A. Linck: "Beitrage zur Kennlnis der menschlichen Chorda dorsalis in Hals- und
 
Kopfskelett, etc.," Anat. Hefte, xlii, 1911.
A. Low: "Further Observations on the Ossification of the Human Lower Jaw,"
 
Journ. Anat. and Phys., xliv, 1910.
M. Lucien: " Developpement de l'articulation du genou et formation du ligament
 
adipeux," Bibliogr. Anat., xiii, 1904.
 
F. P. Mall: "The Development of the Connective Tissues from the Connective-tissue
 
Syncytium," Amer. Jour. Anat., 1, 1902.
F. P. Mall: "On Ossification Centers in Human Embryos Less Than One Hundred
Days Old," Amer. Journ. Anat., V 1906.
 
F. Merkel: "Betrachtungen fiber die Entwicklung des Bindegewebes," Anat. Hefte,
 
xxxviii, 1909.
W. van Noorden: "Beitrag zur Anatomie der knorpeligen Schadelbasis menschlicher
 
Embryonen," Archiv fiir Anat. und Physiol., Anat. Abth., 1887.
A. M. Paterson: "The Human Sternum," Liverpool, 1904.
K. Peter: " Anlage und Homologie der Muscheln des Menschen und der Saugetiere,"
 
Arch, fur mikrosk. Anat., lx, 1902.
J. W. Pryor: "The Chronology and Order of Ossification of the Bones of the Human
 
Carpus," Bulletin State Univ., Lexington, Ky., 1908.
Rambaut et Renault: "Origine et developpement des Os," Paris, 1864.
E. Rosenberg: "Ueber die Entwickelung der Wirbelsaule und das Centrale carpi des
 
Menschen," Morpholog. Jahrbuch, 1, 1876.
H. and H. Rouviere: "Sur le developpement de l'antre mastoidien et les cellules
 
mastoidiennes," Bibliogr. Anat., xx, 1910.
 
G. Ruge: " Untersuchungen liber die Entwickelungsvorgange am Brustbein des
Menschen," Morpholog. Jahrbuch, VI, 1880.
 
J. P. Schaffer: "The Lateral Wall of the Cavum Nasi in Man, with Especial
Reference to the Various Developmental Stages," Journ. Morph., xxi, 1910.
 
J. P. Schaffer: "The Sinus Maxillaris and its Relations in the Embryo, Child and
Adult Man," Amer Journ. Anat., x, 1910.
 
G. Thilenius: "Untersuchungen iiber die morphologische Bedeutung accessorischer
Elemente am menschlichen Carpus (und Tarsus)," Morpholog. Arbeiten, V, 1896.
 
 
 
192 LITERATURE
 
K. Toldt Jr.: "Entwicklung und Struktur des menschlichen Jochbeines," Sitzungsber.
 
k. Acad. Wissensch. Wien, M ath.-naturwiss Kl., Cxi, 1902.
A. Vinogradoff: "Developpement de l'articulation temporo-maxillaire chez l'homme
 
dans la periode intrauterine," Internal. Monatsschr. Anat. Phys., xxvil, 1910.
R. H. Whitehead and J. A. Waddell: "The Early Development of the Mammalian
 
Sternum," Amer. Journ. Anat., xii, 191 1.
L. W. Williams: "The Later Development of the Notochord," Amer. Journ. Anat.,
 
vin, 1908.
E. Zuckerkandl: "Ueber den Jacobsonschen Knorpel und die Ossifikation des
 
Pflugscharbeines," Sitzb. Akad. Wiss. Wien., cxvn, 1908.
 
 
 
CHAPTER VIII.
THE DEVELOPMENT OF THE MUSCULAR SYSTEM.
 
Two forms of muscular tissue exist in the human body, the
striated tissue, which forms the skeletal muscles and is under the
control of the central nervous system, and the non-striated, which is
controlled by the sympathetic nervous system and is found in the
skin, in the walls of the digestive tract, the blood-vessels and lymphatics, and in connection with the genito-urinary apparatus. In
the walls of the heart a muscle tissue occurs which is frequently
regarded as a third form, characterized by being under control of
the sympathetic system and yet being striated; it is, however, in its
origin much more nearly allied to the non-striated than to the
striated form of tissue, and will be considered a variety of the former.
 
The Histogenesis of Non-striated Muscular Tissue. — With
the exception of the sphincter and dilator of the pupil and the muscles
of the sudoriparous glands, which are formed from the ectoderm,
all the non-striated muscle tissue of the body is formed by the conversion of mesenchyme cells into muscle-fibers. The details of
this process have been worked out by McGill for the musculature
of the digestive and respiratory tracts of the pig and are as follows:
The mesenchyme surrounding the mucosa in these tracts is at first
a loose syncytium (Fig. 117, m) and in the regions where the muscle
tissue is to form a condensation of the mesenchyme occurs followed
by an elongation of the mesenchyme cells and their nuclei, so that
the muscle layers become clearly distinguishable from the neighboring undifferentiated tissue (Fig. 117, mm). Fibrils of two kinds
then begin to appear in the cytoplasm of the muscle cells. Coarse
fibrils (f.c) make their appearance as rows of granules, which enlarge
and increase in number until they finally fuse to form homogeneous
13 i93
 
 
 
194 HYSTOGENESIS OF NON-STRIATED MUSCULAR TISSUE
 
 
 
 
mm.
 
 
 
 
 
 
7.nz.
 
 
 
 
Fig. 117. — Longitudinal Section of the Lower Part of the Oesophagus of a
Pig Embryo of 15 mm, Showing the Histogenesis of the Non-striated
Musculature.
 
b, Basement membrane; e, epithelium; /.c, coarse fibril;//., fine fibril; ga, ganglion
of Auerbach's plexus; gm, ganglion of Meissner's plexus; m, mesenchyne; mm,
muscularis mucosae; pb, protoplasmic bridge; vf, varicose fibril. — (McCill.)
 
 
 
HISTOGENESIS OF NON-STRIATED MUSCULAR TISSUE
 
 
 
J 95
 
 
 
fibrils that are at first varicose, but later become of uniform caliber.
Fine fibrils (/./) which are homogeneous from the first, make their
appearance after the coarse ones and in some cases seem to be
formed by the splitting of the latter. They are scattered uniformly
throughout the cytoplasm of the muscle cells and increase in number
as development proceeds, while the coarse fibrils diminish and may
be entirely wanting in the adult tissue.
 
Some of the mesenchyme cells in each muscle sheet fail to
undergo the differentiation just described and multiply to form the
interstitial connective tissue,
which usually divides the muscle cells into more or less distinct bundles. Traces of the
original syncytial nature of
the tissue are to be seen in
the intercellular bridges that
occur between the non-striated
muscle cells of many adult
forms.
 
The cells from which the
heart musculature develops
are at first of the usual well
defined embryonic type, but,
as development proceeds, they
become irregularly stellate in
form, the processes of neighboring cells fuse and, eventually,
there is formed a continuous
mass of protoplasm or syncytium in which all traces of cell boundaries are lacking (Fig. 118). While the individual cells, or myoblasts
as they are termed, are still recognizable, granules appear in their
cytoplasm, and these arrange themselves in rows and unite to form
slender fibrils, which at first do not extend beyond the limits of the
myoblasts in which they have appeared, but later, as the fusion of the
cells proceeds, are continued from one cell territory into the other
 
 
 
 
Fig. 118. — Section through the Heartwall of a Duck Embryo of Three Days.
— (M. Heidenhain.)
 
 
 
196
 
 
 
HISTOGENESIS OF NON-STRIATED MUSCULAR TISSUE
 
 
 
through considerable stretches of the syncytium, without regard to
the original cell areas.
 
The fibrils multiply, apparently by longitudinal division, and
arrange themselves in circles around areas of the syncytium (compare Fig. 119). As the multiplication of the fibrils continues those
newly formed arrange themselves around the interior of each of the
original circles and gradually occupy the entire cytoplasm, or sarcoplasm as it may now be termed, except immediately around the nuclei
where, even in the adult, a certain amount of undifferentiated sarcoplasm persists. The fibrils when first formed are apparently homo
 
 
 
Fig. 119. — Cross-section of a Muscle prom the Thigh of a Pig Embryo 75 mm.
 
Long.
A, Central nucleus; B, new peripheral nucleus. — (Macallum.)
 
 
 
geneous, but later they become differentiated into two distinct substances which alternate with one another throughout the length
of the fibril and produce the characteristic transverse striation of the
tissue. Finally stronger interrupted transverse bands of so-called
cement substance appear, dividing the tissue into areas which have
usually been regarded as representing the original myoblasts, but
are really devoid of significance as cells, the tissue remaining,
strictly speaking, a syncytium.
 
 
 
HISTOGENESIS OF STRIATED MUSCLE TISSUE 197
 
The Histogenesis of Striated Muscle Tissue.— The histogenesis of striated or voluntary muscle tissue resembles very closely
that which has just been described for the heart muscle. There is a
similar formation of a syncytium by the fusion of the cells of the
myotomes, an appearance of granules which unite to form fibrils,
an increase of the fibrils by longitudinal division and a primary
arrangement of the fibrils around the periphery of areas of sarcoplasm (Fig. 119), each of which represents a muscle fiber. In
addition there is an active proliferation of the nuclei of the original
myoblasts, the new nuclei arranging themselves more or less regularly in rows and later migrating from their original central position
to the periphery of the fibers, and, in the limb muscles, the development is further complicated by a process of degeneration which
affects groups of muscle fibers, so that bundles of normal fibers are
separated by strands of degenerated tissue in which the fibrils have
disappeared, the nuclei have become pale and the sarcoplasm vacuolated and homogeneous. Later the degenerated tissue seems to
disappear entirely and mesenchymatous connective tissue grows in
between the persisting fibers, grouping them into bundles and the
bundles into the individual muscles.
 
So long as the formation of new fibrils continues, the increase in
the thickness of a muscle is probably due to a certain extent to an
increase in the actual number of fibers, which results from the division of those already existing. Subsequently, however, this mode of
growth ceases, the further increase of the muscle depending upon an
increase in size of its constituent elements (Macallum).
 
The Development of the Skeletal Muscles. — It has already
been pointed out that all the skeletal muscles of the body, with the
exception of those connected with the branchial arches, are derived
from the myotomes of the mesodermic somites, even the limb
muscles possibly having such an origin, although the cells of the
tissue from which the muscles of the limb buds form lack an epithelial arrangement and are indistinguishable from the somatic mesenchyme which forms the axial cores of the limbs.
 
The various fibers of each myotome are at first loosely arranged,
 
 
 
I98 DEVELOPMENT OF SKELETAL MUSCLES
 
but later they become more compact and are arranged parallel with
one another, their long axes being directed antero-posteriorly.
This stage is also transitory, however, the fibers of each myotome
undergoing various modifications to produce the conditions existing
in the adult, in which the original segmental arrangement of the
fibers can be perceived in comparatively few muscles. The exact
nature of these modifications is almost unknown from direct observation, but since the relation between a nerve and the myotome
belonging to the same segment is established at a very early period
of development and persists throughout life, no matter what changes
of fusion, splitting, or migration the myotome may undergo, it is
possible to trace out more or less completely the history of the various
myotomes by determining their segmental innervation. It is known,
for example, that the latissimus dorsi arises from the seventh and
eighth* cervical myotomes, but later undergoes a migration, becoming attached to the lower thoracic and lumbar vertebrae and to the
crest of the ilium, far away from its place of origin (Mall), and yet
it retains its nerve-supply from the seventh and eighth cervical
nerves with which it was originally associated, its nerve-supply
consequently indicating the extent of its migration.
 
By following the indications thus afforded, it may be seen that
the changes which occur in the myotomes may be referred to one or
more of the following processes:
 
1. A longitudinal splitting into two or more portions, a process
well illustrated by the trapezius and sternomastoid, which have
differentiated by the longitudinal splitting of a single sheet and
contain therefore portions of the same myotomes. The sternohyoid and omohyoid have also differentiated by the same process,
and, indeed, it is of frequent occurrence.
 
2. A tangential splitting into two or more layers. Examples of
this are also abundant and are afforded by the muscles of the fourth,
fifth, and sixth layers of the back, as recognized in English text-books
 
* This enumeration is based on convenience in associating the myotomes with the
nerves which supply them. The myotomes mentioned are those which correspond to
the sixth and seventh cervical vertebrae.
 
 
 
DEVELOPMENT OF SKELETAL MUSCLES 1 99
 
of anatomy, by the two oblique and the transverse layers of the
abdominal walls, and by the intercostal muscles and the transversus
of the thorax.
 
3. A fusion of portions of successive myotomes to form a single
muscle, again a process of frequent occurrence, and well illustrated
by the rectus abdominis (which is formed by the fusion of the
ventral portions of the last six or seven thoracic myotomes) or by
the superficial portions of the sacro-spinalis.
 
4. A migration of parts of one or more myotomes over others.
An example of this process is to be found in the latissimus dorsi,
whose history has already been referred to, and it is also beautifully
shown by the serratus anterior and the trapezius, both of which have
extended far beyond the limits of the segments from which they are
derived.
 
5. A degeneration of portions or the whole of a myotome.
This process has played a very considerable part in the evolution
of the muscular system in the vertebrates. When a muscle normally degenerates, it becomes converted into connective tissue, and
many of the strong aponeurotic sheets which occur in the body owe
their origin to this process. Thus, for example, the aponeurosis
connecting the occipital and frontal portions of the occipito-frontalis
is formed in this process and is muscular in such forms as the lower
monkeys, and a good example is also to be found in the aponeurosis
which occupies the interval between the superior and inferior
serrati postici, these two muscles being continuous in lower forms.
The strong lumbar aponeurosis and the aponeuroses of the oblique
and transverse muscles of the abdomen are also good examples.
 
Indeed, in comparing one of the mammals with a member of
one of the lower classes of vertebrates, the greater amount of connective tissue compared with the amount of muscular tissue in the
former is very striking, the inference being that these connectivetissue structures (fasciae, aponeuroses, ligaments) represent portions
of the muscular tissue of the lower form (Bardeleben). Many of the
accessory ligaments occurring in connection with diarthrodial joints
apparently owe their origin to a degeneration of muscle tissue, the
 
 
 
200 THE TRUNK MUSCULATURE
 
fibular lateral ligament of the knee-joint, for instance, being probably
a degenerated portion of the peroneus longus, while the sacrotuberous ligament appears to stand in a similar relation to the long
head of the biceps femoris (Sutton).
 
6. Finally, there may be associated with any of the first four
processes a change in the direction of the muscle-fibers. The
original antero-posterior direction of the fibers is retained in comparatively few of the adult muscles and excellent examples of the
process here referred to are to be found in the intercostal muscles
and the muscles of the abdominal walls. In the musculature
associated with the branchial arches the alteration in the direction
of the fibers occurs even in the fishes, in which the original direction
of the muscle-fibers is very perfectly retained in other myotomes, the
branchial muscles, however, being arranged parallel with the
branchial cartilages or even passing dorso-ventrally between the
upper and lower portions of an arch, and so forming what may be
regarded as a constrictor of the arch. This alteration of direction
dates back so far that the constrictor arrangement may well be
taken as the primary condition in studying the changes which the
branchial musculature has undergone in the mammalia.
 
It would occupy too much space 'in a work of this kind to consider in detail the history of each one of the skeletal muscles of the
human body, but a statement of the general plan of their development will not be out of place. For convenience the entire system
may be divided into three portions — the cranial, trunk and limb
musculature; and of these, the trunk musculature may first be
considered.
 
The Trunk Musculature. — It has already been seen (p. 82)
that the myotomes at first occupy a dorsal position, becoming
prolonged ventrally as development proceeds so as to overlap the
somatic mesoderm, until those of opposite sides come into contact
in the mid-ventral line. Before this is accomplished, however, a
longitudinal splitting of each myotome occurs, whereby there is
separated off a dorsal portion which gives rise to a segment of the
dorsal musculature of the trunk and is supplied by the ramus dorsalis
 
 
 
THE TRUNK MUSCULATURE 201
 
of its corresponding spinal nerve. In the lower vertebrates this
separation of each of the trunk myotomes into a dorsal and ventral
portion is much more distinct in the adult than it is in man, the two
portions being separated by a horizontal plate of connective tissue
extending the entire length of the trunk and being attached by its
inner edge to the transverse processes of the vertebrae, while peripherally it becomes continuous with the connective tissue of the
 
 
 
 
Fig. 120. — Embryo of 13 mm. showing the Formation of the Rectus Muscle.—
 
{Mall.)
 
dermis along a line known as the lateral line. In man the dorsal
portion is also much smaller in proportion to the ventral portion
than in the lower vertebrates. From this dorsal portion of the
myotomes are derived the muscles belonging to the three deepest
layers of the dorsal musculature, the more superficial layers being
 
 
 
202 THE TRUNK MUSCULATURE
 
composed of muscles belonging to the limb system. Further
longitudinal and tangential divisions and a fusion of successive
myotomes bring about the conditions which obtain in the adult
dorsal musculature.
 
While the myotomes are still some distance from the mid-ventral
line another longitudinal division affects their ventral edges (Fig.
120), portions being thus separated which later fuse more or less
perfectly to form longitudinal bands of muscle, those of opposite
sides being brought into apposition along the mid-ventral line by
the continued growth ventrally of the myotomes. In this way are
formed the rectus and pyramidalis muscles of the abdomen and the
depressors of the hyoid bone, the genio-hyoid and genio-glossus*
in the neck region. In the thoracic region this rectus set of muscles,
as it may be termed, is not represented except as an anomaly, its
absence being probably correlated with the development of the
sternum in this region.
 
The lateral portions of the myotomes which intervene between
the dorsal and rectus muscles divide tangentially, producing from
their dorsal portions in the cervical and lumbar regions muscles,
such as the longus capitis and colli and the psoas, which lie beneath
the vertebral column and hence have been termed hyposkeletal
muscles (Huxley). More ventrally three sheets of muscles, lying
one above the other, are formed, the fibers of each sheet being
arranged in a definite direction differing from that found in the other
sheets. In the abdomen there are thus formed the two oblique and
the transverse muscles, in the thorax the intercostals and the transversa thoracis, while in the neck these portions of some of the myotomes disappear, those of the remainder giving rise to the scaleni
muscles, portions of the trapezius and sternomastoid (Bolk), and
possibly the hyoglossus and styloglossus. In the abdominal region,
and to a considerable extent in the neck also, the various portions of
myotomes fuse together, but in the thorax they retain in the intercostals their original distinctness, being separated by the ribs.
 
* This muscle is supplied by the hypoglossal nerve, but for the present purpose it is
convenient to regard this as a spinal nerve, as indeed it primarily is.
 
 
 
THE TRUNK MUSCULATURE
 
 
 
203
 
 
 
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204
 
 
 
THE TRUNK MUSCULATURE
 
 
 
The table on page 203 will show the relation of the various trunk
muscles to the portions of the myotomes.
 
The intimate association between the pelvic girdle and the axial
skeleton brings about extensive modifications of the posterior trunk
myotomes. So far as their dorsal portions are concerned probably
all these myotomes as far back as the fifth sacral are represented in
the sacro-spinalis, but the ventral portions from the first lumbar
myotome onward are greatly modified. The last myotome taking
part in the formation of the rectus abdominis is the twelfth thoracic
and the last to be represented in the lateral musculature of the
 
 
 
 
A B
 
Fig. 121. — Perineal Region of Embryos of (A) Two Months and (25) Four to
 
Five Months, showing the Development of the Perineal Muscles.
 
dc, Nervus dorsalis clitoridis; p, pudendal nerve; sa, sphincter ani; sc sphincter cloacae;
 
sv, sphincter vaginse. — {Popowsky.)
 
abdomen is the first lumbar, the ventral portions of the remaining
lumbar and of the first and second sacral myotomes either having
disappeared or being devoted to the formation of the musculature
of the lower limb.
 
The ventral portions of the third and fourth sacral myotomes are
represented, however, by the levator ani and coccygeus, and are the
last myotomes which persist as muscles in the human body, although
traces of still more posterior myotomes are to be found in muscles
such as the curvator coccygis sometimes developed in connection
with the coccygeal vertebrae.
 
The perineal muscles and the external sphincter ani are also
 
 
 
THE CRANIAL MUSCULATURE 205
 
developments of the third and fourth (and second) sacral myotomes.
At a time when the cloaca (see p. 280) is still present, a sheet of
muscles lying close beneath the integument forms a sphincter around
its opening (Fig. 121). On the development of the partition which
divides the cloaca into rectal and urinogenital portions, the sphincter
is also diyided, its more posterior portion persisting as the external
sphincter ani, while the anterior part gradually differentiates into the
various perineal muscles (Popowsky).
 
The Cranial Musculature. — As was pointed out in an earlier
chapter, the existence of distinct mesodermic somites has not yet
been completely demonstrated in the head of the human embryo,
but in lower forms, such as the elasmobranch fishes, they are clearly
distinguishable, and it may be supposed that their indistinctness in
man is a secondary condition. Exactly how many of these somites
are represented in the mammalian head it is impossible to say, but
it seems probable, from comparison with lower forms, that there is
a considerable number. The majority of them, however, early
undergo degeneration, and in the adult condition only three are
recognizable, two of which are prseoral in position and one postoral.
The myotomes of the anterior praeoral segment give rise to the
muscles of the eye supplied by the third cranial nerve, those of the
posterior one furnish the superior oblique muscles innervated by the
fourth nerve, while from the postoral myotomes the lateral recti,
supplied by the sixth nerve, are developed. The muscles supplied by the hypoglossal nerve are also derived from myotomes, but
they have already been considered in connection with the trunk
musculature.
 
The remaining muscles of the head differ from all other voluntary
muscles of the body in the fact that they are derived from the
branchiomeres formed by the segmentation of the cephalic ventral
mesoderm. These muscles, therefore, are not to be regarded as
equivalent to the myotomic muscles if their embryological origin is
to be taken as a criterion of equivalency, and in their case it would
seem, from the fact that they are innervated by nerves fundamentally
distinct from those which supply the myotomic muscles, that this
 
 
 
2o6 THE CRANIAL MUSCULATURE
 
criterion is a good one. They must be regarded, therefore, as
belonging to a special category, and may be termed branchiomeric
muscles to distinguish them from the myotomic set.
 
If their embryological origin be taken as a basis for homology, it is
clear that they should be regarded as equivalent to the muscles derived
from the ventral mesoderm of the trunk, and these, as has been seen,
are the non-striated muscles associated with the viscera, among which
may be included the striated heart muscle. At first sight this homology
seems decidedly strained, chiefly because long-continued custom has
regarded the histological and physiological peculiarities of striated and
non-striated muscle tissue as fundamental. It may be pointed out,
however, that the branchiomeric muscles are, strictly speaking, visceral
muscles, and indeed give rise to muscle sheets (the constrictors of the
pharynx) which surround the upper or pharyngeal portion of the digestive
tract. It is possible, then, that the homology is not so strained as might
appear, but further discussion of it may profitably be deferred until the
cranial nerves are under consideration.
 
The skeleton of the first branchial arch becomes converted partly
into the jaw apparatus and partly into auditory ossicles, and the
muscles derived from the corresponding branchiomere become
the muscles of mastication (the temporal, masseter, and pterygoids),
the mylohyoid, anterior belly of the digastric, the tensor veli palatini
and the tensor tympani. The nerve which corresponds to the first
branchial arch is the trigeminus or fifth, and consequently these
various muscles are supplied by it.
 
The second arch has corresponding to it the seventh nerve, and
its musculature is partly represented by the stylohyoid and posterior
belly of the digastric and by the stapedius muscle of the middle ear.
From the more superficial portions of the branchiomere, however, a
sheet of tissue arises which gradually extends upward and downward
to form a thin covering for the entire head and neck, its lower portion
giving rise to the platysma and the nuchal fascia which extends
backward from the dorsal border of this muscle, while its upper parts
become the occipito-frontalis and the superficial muscles of the face
(the muscles of expression), together with the fascia? which unite
the various muscles of this group. The extension of the
platysma sheet of muscles over the face is well shown by the
 
 
 
THE CRANIAL MUSCULATURE
 
 
 
207
 
 
 
 
 
Fig. 122. — Head of Embryos (.4) of Two Months and (B) of Three
Months showing the Extension of the Seventh Nerve upon the Face. —
(Popowsky.)
 
 
 
208 THE CRANIAL MUSCULATURE
 
development of the branches of the facial nerve which supply it
(Fig. 122).
 
The degeneration of the upper part of the third arch produces a
shifting forward of one of the muscles derived from its branchiomere,
the stylopharyngeus arising from the base of the styloid process.
The innervation of this muscle by the ninth nerve indicates, however,
its true significance, and since fibers of this nerve of the third arch
also pass to the constrictor muscles of the pharynx, a portion of
these must also be regarded as having arisen from the third
branchiomere.
 
The cartilages of the fourth and fifth arches enter into the formation of the larynx and the muscles of the corresponding branchiomeres constitute the muscles of the larynx, together with the remaining portions of the constrictors of the pharynx and the muscles of
the soft palate, with the exception of the tensor. Both these arches
have branches of the tenth nerve associated with them and hence
this nerve supplies the muscles named. In addition, two of the
extrinsic muscles of the tongue, the glosso-palatinus and chondroglossus, belong to the fourth or fifth branchiomere, although
the remaining muscles of this physiological set are myotomic in
origin.
 
Finally, portions of two other muscles should probably be
included in the list of branchiomeric muscles, these muscles being
the trapezius and sternomastoid. It has already been seen that
they are partly derived from the cervical myotomes, but they are
also innervated in part by the spinal accessory, and since the motor
fibers of this nerve are serially homologous with those of the vagus
it would seem that the muscles which they supply are probably
branchiomeric in origin. Observations on the development of
these muscles, determining their relations to the branchiomeres,
are necessary, however, before their morphological significance can
be regarded as definitely settled.
 
The table on page 209 shows the relations of the various cranial
muscles to the myotomes and branchiomeres, as well as to the motor
cranial nerves.
 
 
 
THE CRANIAL MUSCULATURE
 
 
 
209
 
 
 
Eleventh
 
 
 
 
 
 
Trapezius.
Sternomastoid.
 
 
 
 
Tenth
 
 
 
 
 
 
Constrictors of
pharynx
(in part).
Pharyngopalatinus.
Levator veli
palatini.
Musculus
 
uvulae.
Muscles of
the larynx.
Glosso-pal
atinus.
 
Chrondro
glossus.
 
 
â– 5
.S
 
 
 
 
 
 
Stylo-pha
ryngeus.
 
Constrictors
 
of pharynx
 
(in part).
 
 
6
 
>
<U
CO
 
 
 
 
 
 
Stylohyoid.
 
Digastric
 
(posterior
 
belly).
 
Stapedius.
Platysma.
Occipitofrontalis.
 
Muscles of
 
expression.
 
 
CO
 
 
a) 3
 
h-1 M
 
 
 
 
 
 
3
 
 
 
 
 
 
Temporal.
. Masseter.
 
Pterygoids.
 
Mylohyoid.
 
Digastric
 
(anterior
 
belly).
 
Tensor veli
 
palatini.
 
Tensor
 
tympani.
 
 
3
 
3
 
 
 
O u
CO O
 
 
 
 
 
 
 
 
•A <u
3
 
 
 
 
 
 
Superior
Inferior
Medial _
Inferior
 
 
 
 
>
 
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V
 
 
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14
 
 
 
2IO THE LIMB MUSCLES
 
The Limb Muscles. — It has been customary to regard the limb
muscles as derivatives of certain of the myotomes, these structures
in their growth vent rally in the trunk walls being supposed to pass
out upon the postaxial surface of the limb buds and loop back again
to the trunk along the praeaxial surface, each myotome thus giving
rise to a portion of both the dorsal and the ventral musculature of
the limb. This view has not, however, been verified by direct
observation of an actual looping of the myotomes over the axis of
the limb buds; indeed, on the contrary, the limb muscles have been
found to develop from the cores of mesenchyme which form the
axes of the limb buds and from which the limb skeleton is also
developed. This may be explained by supposing that the limb
muscles are primarily derivatives of the myotomes and that an
extensive concentration of their developmental history has taken
place, so that the axial mesenchyme actually represents myotomic
material even though no direct connection between it and the
myotomes can be discovered. Condensations of the developmental
history certainly occur and the fact that the muscles of the human
limbs, as they differentiate from the axial cores, present essentially
the same arrangement as in the adult seems to indicate that there is
actually an extensive condensation of the phylogenetic history of the
individual muscles, since comparative anatomy shows the arrangement of the muscles of the higher mammalian limbs to be the result
of a long series of progressive modifications from a primitive condition. However, even though this be the case, there is yet the
possibility that the limb musculature, like the limb skeleton, may
take its origin from the ventral mesoderm and consequently belong
to a different embryological category from the axial myotomic
muscles.
 
The strongest evidence in favor of the myotomic origin of the
limb muscles is that furnished by their nerve supply, this presenting
a distinctly segmental arrangement. This does not necessarily
imply, however, a corresponding primarily metameric arrangement
of the muscles, any more than the pronouncedly segmental arrangement of the cutaneous nerves implies a primary metamerism of the
 
 
 
THE LIMB MUSCLES
 
 
 
211
 
 
 
dermis (see p. 143). It may mean only that the nerves, being segmental, retain their segmental relations to one another even in their
distribution to non-metameric structures, and that, consequently,
the limb musculature is supplied in succession from one border of
the limb bud to the other from succeeding nerve roots.
 
But whether further observation may prove or disprove the
myotomic origin of the limb musculature, the fact remains that it
possesses a segmentally arranged innervation, and it is possible,
 
 
 
 
Fig. 123. — Diagram of a Segment of the Body and Limb.
bl, Axial blastema; dm, dorsal musculature of trunk; rl, nerve to limb; s, septum
between dorsal and ventral trunk musculature; str.d, dorsal layer of limb musculature;
tr.d and tr.v, dorsal and ventral divisions of a spinal nerve; vm, ventral musculature
of the trunk. — (Kollmann.)
 
therefore, to recognize in the limb buds a series of parallel bands of
muscle tissue, extending longitudinally along the bud and each
supplied by a definite segmental nerve. And furthermore, corresponding to each band upon the ventral (praeaxial) surface of the
limb bud, there is a band similarly innervated upon the dorsal (postaxial) surface, since the fibers which pass to the limb from each nerve
root sooner or later arrange themselves in praeaxial and postaxial
 
 
 
212
 
 
 
THE LIMB MUSCLES
 
 
 
groups as is shown in the diagram Fig. 123. The first nerve which
enters the limb bud lies along its anterior border, and consequently
the muscle bands which are supplied by it will, in the adult, lie along
 
 
 
 
Fig. 124. — External Surface of the Os Innominatum showing the Attachment
 
of Muscles and the Zones Supplied by the Various Nerves.
 
12, Twelfth thoracic nerve; I to V, lumbar nerves; 1 and 2, sacral nerves. — {Bolk.)
 
the outer side of the arm and along the inner side of the leg, in consequence of the rotation in opposite directions which the limbs undergo
during development (see p. 101).
 
 
 
THE LIMB MUSCLES
 
 
 
213
 
 
 
The first nerve which supplies the muscles attached to the dorsum
of the ilium is the second lumbar, and following that there come
successively from before backward the remaining lumbar and the
 
 
 
 
my
 
 
 
 
Fig. 125. — Sections through (A) the Thigh and (B) the Calf showing the
Zones Supplied by the Nerves. The Nerves are Numbered in Continuation
with the Thoracic Series. — (A, after Bolk.)
 
first and second sacral nerves. The arrangement of the muscle
bands supplied by these nerves and the muscles of the adult to which
they contribute may be seen from Fig. 124. What is shown there is
only the upper portions of the postaxial bands, their lower portions
 
 
 
214
 
 
 
THE LIMB MUSCLES
 
 
 
extending downward on the anterior surface of the leg. Only the
sacral bands, however, extend throughout the entire length of the
limb into the foot, the second lumbar band passing down only to
about the middle of the thigh, the third to about the knee, the fourth
to about the middle of the crus and the fifth as far as the base of the
fifth metatarsal bone, and the same is true of the corresponding
praeaxial bands, which descend from the ventral surface of the os
coxae (innominatum) along the inner and posterior surfaces of the
leg to the same points. The first and second sacral bands can be
traced into the foot, the first giving rise to the musculature of its
 
 
 
 
Fig. 126. — Section through the Upper Part of the Arm showing the Zones
Supplied by the Nerves.
 
$v to jv, Ventral branches; 5J to Sd, dorsal branches of the cervical nerves.— (Bolk.)
 
inner side and the second to that of its outer side, the praeaxial bands
forming the plantar musculature, while the postaxial ones are upon
the dorsum of the foot as a result of the rotation which the limb has
undergone.
 
In a transverse section through a limb at any level all the muscle
bands, both praeaxial and postaxial, which descend to that level
will be cut and will lie in a definite succession from one border of the
limb to the other, as is seen in Fig. 125. In the differentiation of the
individual muscles which proceeds as the nerves extend from the
trunk into the axial mesenchyme of the limb, the muscle bands
 
 
 
THE LIMB MUSCLES 215
 
undergo modifications similar to those already described as occurring
in the trunk myotomes. Thus, each of the muscles represented in
Fig. 125, B, is formed by the fusion of elements derived from two
or more bands; the soleus and gastrocnemius represent deep and
superficial layers formed from the same bands by a horizontal
(tangential) splitting, these same muscles contain a portion of the
second sacral band which overlaps muscles composed only of higher
myotomes, and the intermuscular septum between the peroneus
brevis and the flexor hallucis longus represents a portion of the third
sacral band which has degenerated into connective tissue.
 
A similar arrangement occurs in the bands which are to be recognized in the musculature of the upper limb. These are supplied by
the fourth, fifth, sixth, seventh and eighth cervical and the first
thoracic nerves, and only those supplied by the eighth cervical and
the first thoracic nerves extend as far as the tips of the fingers. The
arrangement of the bands in the upper part of the brachium may be
seen from Fig. 126, in connection with which it must be noted that
the fourth cervical band does not extend down to the level at which
the section is taken and that the praeaxial band of the eighth cervical
nerve and both the praeaxial and postaxial bands of the first thoracic
are represented only by connective tissue in this region.
 
In another sense than the longitudinal one there is a division
of the limb musculature into more or less definite areas, namely, in a
transverse direction in accordance with the jointing of the skeleton.
Thus, there may be recognized a group of muscles which pass from
the axial skeleton to the pectoral girdle, another from the limb
girdle to the brachium or thigh, another from the brachium or thigh
to the antibrachium or crus, another from the antibrachium or crus
to the carpus or tarsus, and another from the carpus or tarsus to the
digits. This transverse segmentation, if it may be so termed, is not,
however, perfectly definite, many muscles, even in the lower vertebrates, passing over more than one joint, and in the mammalia,
especially, it is further obscured by secondary migrations, by the
partial degeneration of muscles and by an end to end union of
primarily distinct muscles.
 
 
 
2l6 THE LIMB MUSCLES
 
The latissimus dorsi, serratus anterior and pectoral muscles are
all examples of a process of migration as is shown by their innervation
from cervical nerves, as well as by the actual migration which has
been traced in the developing embryo (Mall, Lewis). In the lower
limb evidences of migration may be seen in the femoral head of the
biceps, comparative anatomy showing this to be a derivative of the
gluteal set of muscles which has secondarily become attached to the
femur and has associated itself with a praeaxial muscle to form a
compound structure. An appearance of migration may also be
produced by a muscle making a secondary attachment below its
original origin or above the insertion and the upper or lower part,
as the case may be, then degenerating into connective tissue. This
has been the case with the peroneus longus, which, in the lower
mammals, has a femoral origin, but has in man a new origin from
the fibula, its upper portion being represented by the fibular lateral
ligament of the knee-joint. So too the pectoralis minor is primarily
inserted into the humerus, but it has made a secondary attachment
to the coracoid process, its distal portion forming a coraco-humeral
ligament.
 
The comparative study of the flexor muscles of the antibrachial
and crural regions has yielded abundant evidence of extensive
modifications in the differentiation of the limb muscles. In the
tailed amphibia these muscles are represented by a series of superposed layers, the most superficial of which arises from the humerus
or femur, while the remaining ones take their origin from the ulna
or fibula and are directed distally and laterally to be inserted either
into the palmar or plantar aponeurosis, or, in the case of the deeper
layers, into the radius (tibia) or carpus (tarsus). In the arm of the
lower mammalia the deepest layer becomes the pronator quadratus,
the lateral portions of the superficial layer are the flexor carpi ulnaris
and the flexor carpi radialis, while the intervening layers, together
with the median portion of the superficial one, assuming a more
directly longitudinal direction, fuse to form a common flexor mass
which acts on the digits through the palmar aponeurosis. From
this latter structure and from the carpal and metacarpal bones five
 
 
 
THE LIMB MUSCLES
 
 
 
217
 
 
 
layers of palmar muscles take origin. The radial and ulnar portions
of the most superficial of these become the flexor pollicis brevis and
abductor pollicis brevis and the abductor quinti digiti, while the rest
of the layer degenerates into connective tissue, forming tendons
 
 
 
 
 
Fig. 127. — Transverse sections through (A) the forearm and (B) the hand showing
the arrangement of the layers of the flexor muscles. The superficial layer is shaded
horizontally, the second layer vertically, the third obliquely to the left, the fourth
vertically, and the fifth obliquely to the right. AbM, abductor digiti quinti; AdP,
adductor pollicis; BR, brachio-radialis; ECD, extensor digitorum communis; ECU,
extensor carpi ulnaris;£Z, extensor indicis; EMD, extensor digiti quinti; EMP, abductor
pollicis longus; ERB, extensor carpi radialis brevis; FCR, flexor carpi radialis; FCU,
flexor carpi ulnaris; FLP, flexor pollicis longus; FM, flexor digiti quinti brevis; FP,
flexor digitorum profundus; FS, flexor digitorum sublimis; ID, interossei dorsales;
IV, interossei volares; L, lumbricales; OM, opponens digiti quinti; PL, palmaris
longus; PT, pronator teres; R, radius; U, ulna; II-V, second to fifth metacarpal.
 
which pass to the four ulnar digits. Gradually superficial portions
of the antibrachial flexor mass separate off, carrying with them the
layers of the palmar aponeurosis from which the tendons representing
 
 
 
2l8
 
 
 
THE LIMB MUSCLES
 
 
 
the superficial layer of the palmar muscles arise, and they form with
these the flexor digitorum sublimis. The deeper layers of the antibrachial flexor mass become the flexor digitorum profundus and
the flexor pollicis longus (Fig. 127, A), and retain their connection
with the deeper layers of the palmar aponeurosis which form
their tendons; and since the second layer of the palmar muscles
takes origin from this portion of the aponeurosis it becomes the
lumbrical muscles, arising from the profundus tendons (Fig. 127,
 
 
 
 
 
Fig. 128. — Transverse sections through (A) the crus and (B) the foot, showing the
arrangement of the layers of the flexor muscles. The shading has the same significance
as in the preceding figure. AbH, abductor hallucis; AbM, abductor minimi digiti;
AdH, adductor hallucis; ELD, extensor longus digitorum; F, fibula; FBD, flexor
brevis digitorium; FBH, flexor brevis hallucis; FBM, flexor brevis minimi digiti;
FLD, flexor longus digitorum; G, gastrocnemius; ID, interossei dorsalis; IV, interossei
ventrales; L, lumbricales; P, plantaris; Pe, peroneus longus; Po, popliteus; S, soleus;
T, tibia; TA, tibialis anticus; TP, tibialis posticus; I-V, first to fifth metatarsal.
 
B). The third layer of palmar muscles becomes the adductors
of the digits, reduced in man to the adductor pollicis, while from
the two deepest layers the interossei are developed. Of these
the fourth layer consists primarily of a pair of slips corresponding to each digit, while the fifth is represented by a series of muscles
which extend obliquely across between adjacent metacarpals.
With these last muscles certain of the fourth layer slips unite to form
the dorsal interossei, while the rest become the volar interossei.
j The modifications of the almost identical primary arrangement
in the crus and foot are somewhat different. The superficial layer
 
 
 
LITERATURE 210,
 
of the crural flexors becomes the gastrocnemius and plantaris (Fig.
128, A) and the deepest layer becomes the popliteus and the interosseous membrane. The second and third layers unite to form a
common mass which is inserted into the deeper layers of the plantar
aponeurosis and later differentiates into the soleus and the long
digital flexor, the former shifting its insertion from the plantar
aponeurosis to the os calcis, while the flexor retains its connection
with the deeper layers of the aponeurosis, these separating from the
superficial layer to form the long flexor tendons. The fourth layer
partly assumes a longitudinal direction and becomes the tibialis
posterior and the flexor hallucis longus and partly retains its original
cblique direction and its connection with the deep layers of the
plantar aponeurosis, becoming the quadratus plantse. In the foot
(Fig. 128, B) the superficial layer persists as muscular tissue, forming
the abductors, the flexor digitorum brevis and the medial head of the
flexor hallucis brevis, the second layer becomes the lumbricales, and
the third the lateral head of the flexor hallucis brevis and the adductor hallucis, while the fourth and fifth layers together form the ioterossei, as in the hand, the flexor quinti digiti brevis really belonging
to that group of muscles.
 
LITERATURE.
 
C. R. Bardeen and W. H. Lewis: "Development of the Limbs, Body-wall, and
 
Back in Man," The American Journal of Anat., 1, 1901.
K. Bardeleben: "Musk el und Fascia," Jenaische Zeitschr. fiir Naturwissensch.,
 
xv, 1882.
L. Bolk: "Beziehungen zwischen Skelett, Muskulatur und Nerven der Extremitaten,
 
dargelegt am Beckengurtel, an dessen Muskulatur sowie am Plexus lumbo
sacralis," Morphol. Jahrbuch, xxi, 1894.
L. Bolk: " Rekonstruktion der Segmentirung der Gliedmassenmuskulatur dargelegt
 
an den Muskeln des Oberschenkels und des Schultergurtels," Morphol. Jahrbuch,
 
xxii, 1895.
L. Bolk: "Die Sklerozonie des Humerus," Morphol. Jahrbuch, xxill, 1S96.
L. Bolk: "Die Segmentaldifferenzierung des menschlichen Rumpfes und seiner
 
Extremitaten," 1, Morphol. Jahrbuch, xxv, 1898.
R. Futamtjra: "Ueber die Entwickelung der Facialismuskulatur des Menschen,"
 
Anat. Hefte, xxx, 1906.
E. Godlewski: "Die Entwicklung des Skelet- und Herzmuskelgewebes der Sauge
thiere," Archiv fur mikr. Anat., lx, 1902.
 
 
 
220 LITERATURE
 
E. Grafenberg: "Die Entwicklung der menschlichen Beckenmuskulatur," Anal.
 
Hefte, xxiii, 1904.
W. P. Herringham: "The Minute Anatomy of the Brachial Plexus," Proceedings
 
of the Royal Soc. London, xli, 1886.
W. H. Lewis: " The Development of the Arm in Man," Amer. Jour, of Anat., 1, 1902
J. B. MacCallum: "On the Histology and Histogenesis of the Heart Muscle-cell,"
 
Anat. Anzeiger, xiil, 1897.
J. B. MacCallum: "On the Histogenesis of the Striated Muscle-fiber and the
 
Growth of the Human Sartorius Muscle," Johns Hopkins Hospital Bulletin, 1898
 
F. P. Mall: "Development of the Ventral Abdominal Walls in Man," Journ. of
 
Morphol., xiv, 1898.
Caroline McGill: "The Histogenesis of Smooth Muscle in the Alimentary Canal
 
and Respiratory Tract of the Pig," Internat. Monatschr. Anat. und Phys., xxiv,
 
1907.
J. P. McMurrich: "The Phylogeny of the Forearm Flexors," Amer. Journ, of Anat.,
 
11, 1903.
J. P. McMurrich: "The Phylogeny of the Palmar Musculature," Amer. Journ. of
 
Anat., 11, 1903.
J. P. McMurrich: "The Phylogeny of the Crural Flexors," Amer. Journ. of Anat.,
 
iv, 1904.
J. P. McMurrich: "The Phylogeny of the Plantar Musculature," Amer. Journ. of
 
Anat., vi, 1907.
 
A. Meek: "Preliminary Note on the Post-embryonal History of Striped Muscle-fibers
 
in Mammalia," Anat. Anzeiger, xiv, 1898. (See also Anat. Anzeiger, xv, 1899.)
 
B. Morpurgo: "Ueber die post-embryonale Entwickelung der quergestreiften Muskel
 
von weissen Ratten," Anat. Anzeiger, xv, 1899.
I. Popowsky: " Zur Entwicklungsgeschichte des N. facialis beim Menschen," Morphol.
 
Jahrbuch, xxiii, 1896.
I. Popowsky: " Zur Entwickelungsgeschichte der Dammmuskulatur beim Menschen,"
 
Anat. Hefte, xi, 1899.
L. Rethi: "Der peripheren Verlauf der motorischen Rachen- und Gaumennerven,"
 
Sitzungsber. der kais. Akad. Wissensch. Wien. Math.-Naturwiss. Classe, Cii, 1893.
 
C. S. Sherrington: " Notes on the Arrangement of Some Motor Fibers in the Lumbo
sacral Plexus," Journal of Physiol., xin, 1892.
J. B. Sutton: "Ligaments, their Nature and Morphology," London, 1897.
 
 
 
CHAPTER IX.
 
THE DEVELOPMENT OF THE CIRCULATORY AND LYMPHATIC SYSTEMS.
 
At present nothing is known as to the earliest stages of development of the circulatory system in the human embryo, but it may be
supposed that they resemble in their fundamental features what has
been observed in such forms as the rabbit and the chick. In both
these the system originates in two separate parts, one of which,
located in the embryonic mesoderm, gives rise to the heart, while the
other, arising in the extra-embryonic mesoderm, forms the first
blood-vessels. It will be convenient to consider these two parts
separately, and the formation of the blood-vessels may be first
described.
 
In the rabbit the extension of the mesoderm from the embryonic
region, where it first appears, over the yolk-sac is a gradual process,
and it is in the more peripheral portions of the layer that the bloodvessels first make their appearance. They can be distinguished
before the splitting of the mesoderm has been completed, but are
always developed in that portion of the layer which is most intimately
associated with the yolk-sac, and consequently becomes the splanchnic layer. They belong, indeed, to the deeper portion of that layer,
that nearest the endoderm of the yolk-sac, and so characteristic is
their origin from this portion of the layer that it has been termed the
angioblast and has been held to be derived from the endoderm
independently of the mesoderm proper. The first indication of
blood-vessels is the appearance in the peripheral portion of the
mesoderm of cords or minute patches of spherical cells (Fig. 129, .4).
These increase in size by the division and separation of the cells from
one another (Fig. 129, B), a clear fluid appearing in the intervals
which separate them. Soon the cells surrounding each cord arrange
 
 
 
222
 
 
 
DEVELOPMENT OF THE BLOOD-VESSELS
 
 
 
themselves to form an enclosing wall, and the cords, increasing in
size, unite together to form a network of vessels in which float the
spherical cells which may be known as mesamceboids (Minot).
Viewed from the surface at this stage a portion of the vascular area
of the mesoderm would have the appearance shown in Fig. 130,
revealing a dense network of canals in which, at intervals, are
groups of mesamaeboids adherent to the walls, constituting what have
been termed the blood-islands, while in the meshes of the network
unaltered mesoderm cells can be seen, forming the so-called substance-islands.
 
 
 
 
Fig. 129. — Transverse Section through the Area Vasculosa of Rabbit
Embryos showing the Transformation of Mesoderm cells into the Vascular
Cords.
 
Ec, Ectoderm; En, endoderm; Me, mesoderm. — {van der Stricht.)
 
At the periphery of the vascular area the vessels arrange themselves to form a sinus terminalis enclosing the entire area, and the
vascularization of the splanchnic mesoderm gradually extends
toward the embryo. Reaching it, the vessels penetrate the embryonic tissues and eventually come into connection with the heart,
which has already differentiated and has begun to beat before the
connection with the vessels is made, so that when it is made the
circulation is at once established. Before, however, the vascularization reaches the embryo some of the canals begin to enlarge (Fig.
 
 
 
DEVELOPMENT OF THE BLOOD-VESSELS
 
 
 
223
 
 
 
B£
 
 
 
131,-4), producing arteries and veins, the rest of the network forming
capillaries uniting these two sets of vessels, and, this process continuing, there are eventually differentiated a single vitelline artery and
two vitelline veins (Fig. 131, B).
 
In the human embryo the small size of the yolk-sac permits of the
extension of the vascular area over
its entire surface at an early period,
and this condition has already been
reached in the earliest stages known
and consequently no sinus terminalis such as occurs in the rabbit is
visible. Otherwise the conditions
are probably similar to what has
been described above, the first circulation developed being associated
with the yolk-sac.
 
It is to be noted that the capillary network of the area vasculosa
consists of relatively wide anastomosing spaces whose endothelial
lining rests directly upon the substance islands (Fig. 130). In certain of the embryonic organs, notably the liver, the metanephros
and the heart, the network has a
similar character, consisting of wide
anastomosing spaces bounded by
an endothelium which rests directly, or almost so, upon the parenchyma of the organ (the hepatic
cylinders, the mesonephric tubules, or the cardiac muscle trabecular)
(Figs. 132 and 190, B). To this form of capillary the term sinusoid
has been applied (Minot), and it appears to be formed by the expansion of the wall of a previously existing blood-vessel, which thus
moulds itself, as it were, over the parenchyma of the organ. The
 
 
 
 
Fig. 130. — Surface View of a
Portion of the Area Vasculosa of
a Chick.
 
The vascular network is represented
by the shaded portion. Bi, Bloodisland; Si, substance-island. — (Disse.)
 
 
 
224
 
 
 
THE FORMATION OF THE BLOOD
 
 
 
true capillaries, on the other hand, are more definitely tubular in
form, are usually imbedded in mesenchymatous connective tissue
and are developed in the same manner as the primary capillaries
of the area vasculosa, by the aggregation of vasifactive cells to form
cords, and the subsequent hollowing out of these. Whether these
vasifactive cells are new differentiations of the embryonic mesenchyme or are budded off from the walls of existing capillaries which
have grown in from extra-embryonic regions, is at present undecided.
The Formation of the Blood. — The mesamceboids, which are
 
 
 
gl
 
 
 
 
 
 
i i
 
 
 
 
A , \
 
Fig. 131. — The Vascular Areas of Rabbit Embryos. In B the Veins are
Represented by Black and the Network is Omitted. — (van Beneden and
Julin.)
 
 
 
the first formed blood-corpuscles are all nucleated and destitute or
nearly so of haemoglobin. They have been held by some observers
to be the only source of the various forms of corpuscles that are
found in the adult vessels, while others maintain that they give rise
only to the red corpuscles, the leukocytes arising in tissues external
to the blood-vessels and only secondarily making their way into
them. According to this latter view the red and white corpuscles
have a different origin and remain distinct throughout life.
 
 
 
THE FORMATION OF THE BLOOD 225
 
So long as the formation of blood-vessels is taking place in the
extra-embryonic mesoderm, so long are new mesamceboids being
differentiated from the mesoderm. But whether the formation of
blood-vessels within the embryo results from a differentiation of the
embryonic mesoderm in situ, or from the actual ingrowth of vessels
from the extra-embryonic regions (His), is as yet uncertain, and
hence it is also uncertain whether mesamceboids are differentiated
from the embryonic mesoderm or merely pass into the embryonic
region from the more peripheral areas. However this may be, it
is certain that they and the erythrocytes that are formed from them
increase by division in the interior of the embryo, and that there
are certain portions of the body in which these divisions take place
most abundantly, partly, perhaps, on account of the more favorable
conditions of nutrition which they present and partly because they are
regions where the circulation is sluggish and permits the accumulation of erythrocytes. These regions constitute what have been
termed the hematopoietic organs, and are especially noticeable in the
later stages of fetal life, diminishing in number and variety about the
time of birth. It must be remembered, however, that the life of
individual corpuscles is comparatively short, their death and disintegration taking place continually during the entire life of the
individual, so that there is a necessity for the formation of new
corpuscles and for the existence of haematopoietic organs at all
stages of life.
 
In the fetus mesamceboids in process of division may be found in
the general circulation and even in the heart itself, but they are much
more plentiful in places where the blood-pressure is diminished, as,
for instance, in the larger capillaries of the lower limbs and in the
capillaries of all the visceral organs and of the subcutaneous tissues.
Certain organs, however, such as the liver, the spleen, and the
bone-marrow, present especially favorable conditions for the multiplication of the blood-cells, and in these not only are the capillaries
enlarged so as to afford resting-places for the corpuscles, but gaps
appear in the walls of the vessels through which the blood-elements
may pass and so come into intimate relations with the actual tissues
is
 
 
 
226
 
 
 
THE FORMATION OF THE BLOOD
 
 
 
of the organs (Fig. 132). After birth the haematopoietic function of
the liver ceases and that of the spleen becomes limited to the formation of white corpuscles, though the complete function may be
re-established in cases of extreme anaemia. The bone-marrow,
however, retains the function completely, being throughout life the
seat of formation of both red and white corpuscles, the lymphatic
nodes and follicles, as well as the spleen, assisting in the formation
of the latter elements.
 
The mesamceboids early become converted into nucleated red
 
corpuscles or erythrocytes by
the development of haemoglobin in their cytoplasm, their
nuclei at the same time becoming granular. Up to a
stage at which the embryo has
a length of about 12 mm. these
are the only form of red corpuscle in the circulation, but
at this time (Minot) a new
form, characterized by its
smaller size and more deeply
staining nucleus, makes its appearance. These erythrocytes
have been termed normoblasts
(Ehrlich), although they are
merely transition stages leading to the formation of erythroplastids by the extrusion of their nuclei (Fig. 133). The cast-off
nuclei undergo degeneration and phagocytic absorption by the
leukocytes, and the masses of cytoplasm pass into the circulation,
becoming more and more numerous as development proceeds,
until finally they are the typical haemoglobin-containing elements
in the blood and form what are properly termed the red bloodcorpuscles.
 
It has already (p. 224) been pointed out that discrepant views
 
 
 
 
Fig. 132. — Section of a Portion or
the Liver of a Rabbit Embryo of 5 mm.
e, Erythrocytes in the liver substance and
in a capillary; h, hepatic cells. — {van der
Stricht)
 
 
 
THE FORMATION OF THE BLOOD 227
 
prevail as to the origin of the white blood-corpuscles. Indeed, three
distinct modes of origin have been assigned to them. According to
one view they have a common origin with the erythrocytes from the
mesamceboids (Weidenreich), according to another they are formed
from mesenchyme cells outside the cavities of the blood-vessels
(Maximo w), while according to a third view the first formed leukocytes take their origin from the endodermal epithelial cells of the
thymus gland (Prenant).
 
But whatever may be their origin in later stages the leukocytes
multiply by mitosis and there is a tendency for the dividing cells to
collect in the lymphoid tissues, such as
 
the lymph nodes, tonsils, etc., to form /||\ /^s 0$\ /^p. &
more or less definite groups which — ^^ v^ KjJ \Jj
 
have been termed germ-centers (Flem
. m , .. . . _ Fig. 133. — Stages in the
 
ming). The new cells when they first transformation of an Ery
pass into the circulation have a rel- throcyte into an _ Erythro
r plastid. — (van der Stricnt.)
 
atively large nucleus surrounded by a
 
small amount of cytoplasm without granules and, since they resemble
the cells found in the lymphatic vessels, are termed lymphocytes
(Fig. 134, a). In the circulation, however, other forms of leukocytes
also occur, which are believed to have their origin from cells with
much larger nuclei and more abundant cytoplasm, which occur
throughout life in the bone-marrow and have been termed myelocytes. Cells of a similar type, free in the circulation, constitute
what are termed the finely granular leukocytes (neutrophile cells of
Ehrlich) (Fig. 134, b), but whether these and the myelocytes are
derived from lymphocytes or have an independent origin is as yet
undetermined. Less abundant are the coarsely granular leukocytes
(eosinophile cells of Ehrlich) (Fig. 134, c), characterized by the coarseness and staining reactions of their cytoplasmic granules and by
their reniform or constricted nucleus. They are probably derivatives of the finely granular type and it has been maintained by
Weidenreich that their granules have been acquired by the phagocytosis of degenerated erythrocytes. Finally, a third type is formed
by the polymorphonuclear or polynuclear leukocytes (basophile cells
 
 
 
228
 
 
 
THE FORMATION OF THE BLOOD
 
 
 
of Ehrlich) (Fig. 134, d), which are to be regarded as leukocytes in
the process of degeneration and are characterized by their irregularly lobed or fragmented nuclei, as well as by their staining
peculiarities.
 
In the fetal haematopoietic organs and in the bone-marrow of the
adult large, so-called giant-cells are found, which, although they do
not enter into the general circulation, are yet associated with the
development of the blood-corpuscles. These giant-cells as they
 
 
 
 
 
Fig. 134. — Figures of the Different Forms of White Corpuscles occurring
 
in Human Blood.
 
a, Lymphocytes; b, finely granular (neutrophile) leukocyte; c, coarsely granular (eosino
phile) leukocyte; d, polymorphonuclear (basophile) leukocyte. — (Weidenreich.)
 
 
 
occur in the bone-marrow are of two kinds which seem to be quite
distinct, although both are probably formed from leukocytes. In
one kind the cytoplasm contains several nuclei, wherce they have
been termed polycaryocytes, and they seem to be the cells which have
already been mentioned as osteoclasts (p. 158). In the other kind
(Fig- I 35) tne nucleus is single, but it is large and irregular in shape,
frequently appearing as if it were producing buds. These megacaryocytes appear to be phagocytic cells, having as their function the
destruction of degenerated corpuscles and of the nuclei of the
erythrocytes.
 
 
 
THE FORMATION OF THE HEART 229
 
The blood-platelets have recently been shown by Wright to be
formed from the cytoplasm of the megacaryocytes, by the constriction and separation of portions of the slender processes to which
they give rise in their amoeboid movements (Fig. 135).
 
 
 
 
Fig. 135. — Megacaryocyte from a Kitten, which has Extended two
pseudopodial processes through the wall of blood-vessel and is budding
off blood-platelets.
 
bp, Blood-platelets; V, blood-vessel. — (J. H. Wright.)
 
The Formation of the Heart. — The heart makes its appearance
while the embryo is still spread out upon the surface of the yolk-sac,
and arises as two separate portions which only later come into contact in the median line. On each side of the body near the margins
of the embryonic area a fold of the splanchnopleure appears, projecting into the ccelomic cavity, and within this fold a very thinwalled sac is formed, probably by a splitting off of its innermost
cells (Fig. 136, .4). Each fold will produce a portion of the muscular
walls {myocardium) of the heart, and each sac part of its endothelium
{endocardium). As the constriction of the embryo from the yolk-sac
proceeds, the two folds are gradually brought nearer together (Fig.
136, B), until they meet in the mid-ventral line, when the myocardial
folds and endocardial sacs fuse together (Fig. 136, C) to form a
cylindrical heart lying in the mid-ventral line of the body, in front
of the anterior surface of the yolk-sac and in what will later be the
 
 
 
230
 
 
 
THE FORMATION OF THE HEART
 
 
 
cervical region of the body. At an early stage the various veins
which have already been formed, the vitellines, umbilicals, jugulars
 
 
 
 
en
 
 
 
Fig. 136. — Diagrams Illustrating the Formation or the Heart in the
 
Guinea-pig.
The mesoderm is represented in black and the endocardium by a broken line.
am, Amnion; en, endoderm; h, heart; i, digestive tract.- — {After Strahl and
Carius.)
 
and cardinals, unite together to open into a sac-like structure, the
sinus venosus, and this opens into the posterior end of the heart
cylinder. The anterior end of the cylinder tapers off to form the
 
 
 
THE FORMATION OF THE HEART
 
 
 
231
 
 
 
aortic bulb, which is continued forward on the ventral surface of the
pharyngeal region and carries the blood away from the heart. The
blood accordingly opens into the posterior end of the heart tube and
flows out from its anterior end.
 
The simple cylindrical form soon changes, however, the heart
tube in embryos of 2.15 mm. in length having become bent upon
itself into a somewhat S-shaped curve (Fig. 137). Dorsally and to
the left is the end into which the sinus venosus opens, and from this
 
 
 
 
Fig. 137. — Heart of EmbrycTof
2.15 mm., from a Reconstruction.
 
a, Atrium; ab, aortic bulb; d, diaphragm; dc, ductus Cuvieri; /, liver;
v, ventricle; vj, jugular vein; vu, umbilical vein. — (His.)
 
 
 
 
Fig. 138. — Heart of Embryo of
4.2 mm., seen from the Dorsal
Surface.
 
DC, Ductus Cuvieri; I A , left atrium
rA, right atrium; vf, jugular vein; VI,
left ventricle; vu, umbilical vein. —
{His.)
 
 
 
the heart tube ascends somewhat and then bends so as to pass at
first ventrally and then caudally and to the right, where it again
bends at first dorsally and then anteriorly to pass over into the aortic
bulb. The portion of the curve which lies dorsally and to the left
is destined to give rise to both atria, the portion which passes from
right to left represents the future left ventricle, while the succeeding
portion represents the right ventricle. In later stages (Fig. 138)
the left ventricular portion drops downward in front of the atrial
 
 
 
232
 
 
 
THE FORMATION OF THE HEART
 
 
 
portion, assuming a more horizontal position, while the portion
which represents the right ventricle is drawn forward so as to lie in
the same plane as the left.
 
At the same time two small out-pouchings develop from the
atrial part of the heart and form the first indications of the two
atria. As development progresses, these increase in size to form
large pouches opening into a common atrial canal (Fig. 139) which
is directly continuous with the left ventricle, and as the enlargement of the pouches continues their openings into the canal enlarge,
 
until finally the pouches become
continuous with one another,
forming a single large sac, and
the atrial canal becomes reduced
to a short tube which is slightly
invaginated into the ventricle
(Fig. 140).
 
In the meantime the sinus
venosus, which was originally an
oval sac and opened into the
atrial canal, has elongated transversely until it has assumed the
form of a crescent whose convexity is in contact with the walls of
the atria, and its opening into the
heart has verged toward the right, until it is situated entirely within the
area of the right atrium. As the enlargement of the atria continues,
the right horn and median portion of the crescent are gradually taken
up into their walls, so that the various veins which originally opened
into the sinus now open directly into the right atrium by a single
opening, guarded by a projecting fold which is continued upon the
roof of the atrium as a muscular ridge known as the septum spurium
(Fig. 140, sp). The left horn of the crescent is not taken up into
the atrial wall, but remains upon its posterior surface as an elongated
sac forming the coronary sinus.
 
The division of the now practically single atrial cavity into the
 
 
 
 
Fig. 139. — Heart of Embryo of 5
mm., Seen from in Front and slightly
from Above. — (His).
 
 
 
THE FORMATION OF THE HEART
 
 
 
233
 
 
 
permanent right and left atria begins with the formation of a falciform ridge running dorso-ventrally across the roof of the cavity.
This is the atrial septum or septum primum (Fig. 140, ss), and it
rapidly increases in size and thickens upon its free margin, which
reaches almost to the upper border of the short atrial canal (Fig. 142).
The continuity of the two atria is thus almost dissolved, but is soon
re-established by the formation in the dorsal part of the septum of
an opening which soon reaches a considerable size and is known as
 
 
 
 
 
 
 
Fig. 140. — Inner Surface of the Heart of an Embryo of 10 mm.
 
al, Atrio-ventricular thickening; sp, septum spurium; ss, septum primum; sv, septum
 
ventriculi; ve, Eustachian valve. — (His.)
 
the foramen ovale (Fig. 141, fo). Close to the atrial septum, and
parallel with it, a second ridge appears in the roof and ventral wall
of the right atrium. This septum secundum (Fig. 141, S 2 ) is of
relatively slight development in the human embryo, and its free
edge, arching around the ventral edge and floor of the foramen
ovale, becomes continuous with the left lip of the fold which guards
the opening of the sinus venosus and with this forms the annulus
of Vieussens of the adult heart.
 
 
 
234
 
 
 
THE FORMATION OF THE HEART
 
 
 
Si Sz
 
 
 
 
When the absorption of the sinus venosus into the wall of the
right atrium has proceeded so far that the veins communicate
directly with the atrium, the vena cava superior opens into it at the
upper part of the dorsal wall, the vena cava inferior more laterally,
and below this is the smaller opening of the coronary sinus. The
 
upper portion of the right lip of the fold
which originally surrounded the opening
of the sinus venosus, together with the
septum spurium, gradually disappears;
the lower portion persists, however, and
forms (i) the Eustachian valve (Fig. 141,
Ve), guarding the opening of the inferior
cava and directing the blood entering by
it toward the foramen ovale, and (2) the
Thebesian valve, which guards the opening of the coronary sinus. At first no
Fig. 141.— Heart of Embryo veins communicate with the left atrium,
 
OF I0.2 CM. FROM WHICH HALF , 111 c t i 1
 
of the Right Auricle has but on the development of the lungs and
been Removed. the establishment of their vessels, the
 
fo, Foramen ovale; pa, pul- , . .,,
 
monary artery; S u septum pri- pulmonary veins make connection with
 
mum; S 2 ,_ septum secundum; j t TwQ ye j ns arise from eac h l ung an( J
ba, systemic aorta; V, right ven- °
 
tricle; vd and vcs, inferior and as they pass toward the heart they unite
 
superior venae cavae; Ve, Eusta- ,-, i r „ j •
 
chfan valve. . in pairs, the two vessels so formed again
 
uniting to form a single short trunk which
opens into the upper part of the atrium (Fig. 142, Vep). As is the
case with the right atrium and the sinus venosus, the expansion of
the left atrium brings about the absorption of the short single trunk
into its walls, and, the expansion continuing, the two vessels are also
absorbed, so that eventually the four primary veins open independently into the atrium.
 
While the atrial septa have been developing there has appeared
on the dorsal wall of the atrial canal a tubercle-like thickening of
the endocardium, and a similar thickening also forms on the ventral
wall. These endocardial cushions increase in size and finally unite
together by their tips, forming a complete partition, dividing the
 
 
 
THE FORMATION OF THE HEART
 
 
 
2 35
 
 
 
atrial canal into a right and left half (Fig. 142). With the upper
edge of this partition the thickened lower edge of the atrial septum
unites, so that the separation of the atria would be complete were it
not for the foramen ovale.
 
 
 
 
SM
 
 
 
En.s
 
 
 
Fig. 142. — Section through a Reconstruction of the Heart of a Rabbit
 
Embryo of 10. i mm.
Ad and Ad u Right and As, left atrium; Bw x and Bw 2 , lower ends of the ridges
which divide the aortic bulb; En, endocardial cushion; En.r and En.s, thickenings
of the cushion; la, interatrial and Iv, interventricular communication; S v septum
primum; Sd, right and Ss, left horn of the sinus venosus; S.iv, ventricular septum;
SM, opening of the sinus venosus into the atrium; Vd, right and Vs, left ventricle;
Vej, jugular vein; Vep, pulmonary vein; Vvd and Vvs, right and left limbs of the
valve guarding the opening of the sinus venosus. — (Born.)
 
While these changes have been taking place in the atrial portion
of the heart, the separation of the right and left ventricles has also
been progressing, and in this two distinct septa take part. From
the floor of the ventricular cavity along the line of junction of the
 
 
 
236 THE FORMATION OF THE HEART
 
right and left portions a ridge, composed largely of muscular tissue,
arises (Figs. 140 and 142), and, growing more rapidly in its dorsal
than its ventral portion, it comes into contact and fuses with the
dorsal part of the partition of the atrial canal. Ventrally, however,
the ridge, known as the ventricular septum, fails to reach the ventral
part of the partition , so that an oval foramen, situated just below the
point where the aortic bulb arises, still remains between the two
ventricles. This opening is finally closed by what it termed the
aortic septum. This makes its appearance in the aortic bulb just at
the point where the first lateral branches which give origin to the
pulmonary arteries (see p. 243) arise, and is formed by the fusion
of the free edges of two endocardial ridges which develop on opposite
sides of the bulb. From its point of origin it gradually extends
down the bulb until it reaches the ventricle, where it fuses with
the free edge of the ventricular septum and so completes the separation of the two ventricles (Fig. 143). The bulb now consists of two
vessels lying side by side, and owing to the position of the partition
at its anterior end, one of these vessels, that which opens into the
right ventricle, is continuous with the pulmonary arteries, while the
other, which opens into the left ventricle, is continuous with the rest
of the vessels which arise from the forward continuation of the bulb.
As soon as the development of the partition is completed, two grooves,
corresponding in position to the lines of attachment of the partition
on the inside of the bulb, make their appearance on the outside and
gradually deepen until they finally meet and divide the bulb into two
separate vessels, one of which is the pulmonary aorta and the other
the systemic aorta.
 
In the early stages of the heart's development the muscle bundles
which compose the wall of the ventricle are very loosely arranged,
so that the ventricle is a somewhat spongy mass of muscular tissue
with a relatively small cavity. As development proceeds the bundles
nearest the outer surface come closer together and form a compact
layer, those on the inner surface, however, retaining their loose
arrangement for a longer time (Fig. 142). The lower edge of the
atrial canal becomes prolonged on the left side into one, and on the
 
 
 
THE FORMATION OF THE HEART
 
 
 
237
 
 
 
right side into two, flaps which project downward into the ventricular
cavity, and an additional flap arises on each side from the lower
 
 
 
 
S.ur
 
 
 
Eav.d
 
 
 
 
Sivr
 
 
 
Fig. 143. — Diagrams of Sections through the Heart of Embryo Rabbits
to Show the Mode of Division of the Ventricles and of the Atrio-ventricular
Orifice.
 
Ao, Aorta; Ar. p, pulmonary artery; B, aortic bulb; Bw 2 and *, one of the ridges
which divide the bulb; Eo, and Eu, upper and lower thickenings of the margins of
the atrio-ventricular orifice; F.av.c, the original atrio-ventricular orifice; F.av.d and
F.av.s, right and left atrio-ventricular orifices; Oi, interventricular communication;
S.iv, ventricular septum; Vd and Vs, right and left ventricles. — {Born.)
 
edge of the partition of the atrial canal, so that three flaps occur in
the right atrio-ventricular opening and two in the left. To the
 
 
 
2 3 8
 
 
 
THE FORMATION OF THE HEART
 
 
 
under surfaces of these flaps the loosely arranged muscular trabecular of the ventricle are attached, and muscular tissue also occurs
in the flaps. This condition is transitory, however; the muscular
tissue of the flaps degenerates to form a dense layer of connective
tissue, and at the same time the muscular trabecular undergo a
condensation. Some of them separate from the flaps, which represent the atrio-ventricular valves, and form muscle bundles which
may fuse throughout their entire length with the more compact
portions of the ventricular walls, or else may be attached only by
their ends, forming loops; these two varieties of muscle bundles
constitute the trabecule carnece of the adult heart. Other bundles
 
 
 
 
 
Fig. 144. — Diagrams showing the Development of the Atjriculo-ventricular
 
Valves.
 
b, Muscular trabecule; cht, chordae tendinae; mk and vtk 1 , valve; pm, musculus papillaris;
 
tc, trabeculse carneae; v, ventricle. — (From Hertwig, after Gegenbaur.)
 
 
 
may retain a transverse direction, passing across the ventricular
cavity and forming the so-called moderator bands; while others, again,
retaining their attachment to the valves, condense only at their lower
ends to form the musculi papillares, their upper portions undergoing conversion into strong though slender fibrous cords, the
chorda tendinece (Fig. 144).
 
The endocardial lining of the ventricles is at first a simple sac
separated by a distinct interval from the myocardium, but when the
condensation of the muscle trabecular occurs the endocardium applies
itself closely to the irregular surface so formed, dipping into all the
crevices between the trabeculse carneae and wrapping itself around
 
 
 
 
THE FORMATION OF THE HEART 239
 
the musculi papillares and chordae tendineae so as to form a complete
 
lining of the inner surface of the myocardium.
 
The aortic and pulmonary semilunar valves make their appearance,
 
before the aortic bulb undergoes its longitudinal splitting, as four
 
tubercle-like thickenings of connective tissue situated on the inner
 
wall of the bulb just where it arises from the ventricle. When the
 
division of the bulb occurs, two of the thickenings, situated on
 
opposite sides, are divided, so that both the
 
pulmonary and systemic aorta? receive three
 
thickenings (Fig. 145). Later the thickenings
 
become hollowed out on the surfaces directed
 
away from the ventricles and are so converted
 
into the pouch-like valves of the adult.
 
Changes in the Heart after Birth. — The T FlG - 145-— Diagrams
 
/ . Illustrating the For
heart when first formed lies far forward in the mation of the Semi
neck region of the embryo, between the head £toarValves.-(G^«»and the anterior surface of the yolk-sac, and
from this position it gradually recedes until it reaches its final
position in the thorax. And not only does it thus change its relative position, but the direction of its axes also changes. For at an
early stage the ventricles lie directly in front of (i. e., ventrad to)
the atria and not below them as in the adult heart, and this primitive condition is retained until the diaphragm has reached its final
position (see p. 322).
 
In addition to these changes in position, which are antenatal,
important changes also occur in the atrial septum after birth.
Throughout the entire period of fetal life the foramen ovale persists,
permitting the blood returning from the placenta and entering the
right atrium to pass directly across to the left atrium, thence to the
left ventricle, and so out to the body through the systemic aorta
(see p. 267). At birth the lungs begin to function and the placental
circulation is cut off, so that the right atrium receives only venous
blood and the left only arterial; a persistence of the foramen ovale
beyond this period would be injurious, since it would permit of a
mixture of the arterial and venous bloods, and, consequently, it
 
 
 
240 DEVELOPMENT OF THE ARTERIAL SYSTEM
 
closes completely soon after birth. The closure is made possible
by the fact that during the growth of the heart in size the portion of
the atrial septum which is between the edge of the foramen ovale
and the dorsal wall of the atrium increases in width, so that the foramen is carried further and further away from the dorsal wall of the
atrium and comes to be almost completely overlapped by the annulus
of Vieussens (Fig. 141). This process continuing, the dorsal portion
of the atrial septum finally overlaps the free edge of the annulus,
and after birth the fusion of the.overlapping surfaces takes place and
the foramen is completely closed.
 
In a large percentage (25 to 30 per cent.) of individuals the fusion of
the surfaces of the septum and annulus is not complete, so that a slit-like
opening persists between the two atria. This, however, does not allow of
any mingling of the blood in the two cavities, since when the atria contract
the pressure of the blood on both sides will force the overlapping folds
together and so practically close the opening. Occasionally the growth
of the dorsal portion of the septum is imperfect or is inhibited, in which
case closure of the foramen ovale is impossible.
 
The Development of the Arterial System.- — It has been seen
(p. 221) that the formation of the blood-vessels begins in the extraembryonic splanchnic mesoderm surrounding the yolk-sac and extends thence toward the embryo. Furthermore, it has been seen
that the vessels appear as capillary networks from which definite
stems are later elaborated. This seems also to be the method of
formation of the vessels developed within the body of the embryo,
the arterial and venous stems being first represented by a number
of anastomosing capillaries, from which, by the enlargement of some
and the disappearance of the others, the definite stems are formed.
 
The earliest known embryo that shows a blood circulation is
that described by Eternod (Fig. 43). From the plexus of vessels
on the yolk-sack two veins arise which unite with two other veins
returniDg from the chorion by the belly-stalk and passing forward to
the heart as the two umbilical veins (Fig. 146, Vu). There is as yet
no vitelline vein, the chorionic circulation in the human embryo
apparently taking precedence over the vitelline. From the heart
a short arterial stem arises, which soon divides so as to form three
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
241
 
 
 
branches* passing dorsally on either side of the pharynx. The
branches of each side then unite to form a paired dorsal aorta (dAr,
dAs) which extends caudally and is continued into the belly-stalk
and so to the chorion as the umbilical arteries (Au). There is as
yet no sign of vitelline arteries passing to the yolk-sack, again
an indication of the subservience of the vitelline to the chorionic
circulation in the human embryo.
 
 
 
 
Fig. 146. — Diagram showing the Arrangement of the Blood-vessels in an
 
Embryo 1.3 mm. in Length.
 
Au, Umbilical artery; All, allantois; Ch, chorionic villus; dAr and dAs, right and left
 
dorsal aortae; Vu, umbilical veins; Ys, yolk-sack. — (From Kollmann after Eternod.)
 
In later stages when the branchial arches have appeared the
dorsally directed arteries are seen to lie in these, forming what are
termed the branchial arch vessels, and later also the two dorsal
 
 
 
* Evans (Keibel-Mall, Human Embryology, Vol. 11, 1912) considers two of these
branches to be probably plexus formations rather than definite stems, since there is
evidence to indicate that only one such stem exists at such an early stage of development.
16
 
 
 
242
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
aortae fuse as far forward as the region of the eighth cervical segment
to form a single trunk from which segmental branches arise.
 
It will be convenient to consider first the history of the vessels
which pass dorsally in the branchial arches. Altogether, six of these
vessels are developed, the fifth being rudimentary and transitory, and
when fully formed they have an arrangement which may be understood from the diagram (Fig.
147). This arrangement represents a condition which is permanent in the lower vertebrates.
In the fishes the respiration is
performed by means of gills
developed upon the branchial
arches, and the heart is an organ
which receives venous blood from
the body and pumps it to the
gills, in which it becomes arterialized and is then collected into
the dorsal aortae, which distribute it to the body. But in terrestrial animals, with the loss of the
gills and the development of the
lungs as respiratory organs, the
capillaries of the gills disappear
and the afferent and efferent
branchial vessels become continuous, the condition represented in the diagram resulting.
But this condition is merely temporary in the mammalia and
numerous changes occur in the arrangement of the vessels before
the adult plan is realized. The first change is a disappearance of
the vessel of the first arch, the ventral stem from which it arose being
continued forward to form the temporal arteries, giving off near the
point where the branchial vessel originally arose a branch which
represents the internal maxillary artery in part, and possibly also a
 
 
 
 
Fig. 147. — Diagram Illustrating the
Primary Arrangement of the Branchial Arch Vessels.
 
a, aorta; db, aortic bulb; ec, external
carotid; ic, internal carotid; sc, subclavian;
I-VI, branchial arch vessels.
 
 
 
' DEVELOPMENT OF THE ARTERIAL SYSTEM 243
 
second branch which represents the external maxillary (His).
A little later the second branchial vessel also degenerates (Fig. 148),
a branch arising from the ventral trunk near its former origin,
possibly representing the future lingual artery (His), and then the
portion of the dorsal trunk which intervenes between the third and
fourth branchial vessels vanishes, so that the dorsal trunk anterior
to the third branchial arch is cut off from its connection with the
dorsal aorta and forms, together with the vessel of the third arch, the
internal carotid, while the ventral trunk, anterior to the point of
 
 
 
Fig. 148. — Arteriat, System of an Embryo of 10 mm.
 
Ic, Internal carotid; P, pulmonary artery; Ve, vertebral artery; III to VI, persistent
 
branchial vessels. — (His.)
 
origin of the third vessel, becomes the external carotid, and the portion which intervenes between the third and fourth vessels becomes
the common carotid (Fig. 149).
 
The rudimentary fifth vessel, like the first and second, disappears,
but the fourth persists to form the aortic arch, there being at this
stage of development two complete aortic arches. From the
sixth vessel a branch arises which passes backward to the lungs,
forming the pulmonary artery, and the portion of the vessel of the
right side which intervenes between this and the aortic arch disappears, while the corresponding portion of the left side persists
 
 
 
244
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
until after birth, forming the ductus arteriosus {ductus Botalli) (Fig.
149). When the longitudinal division of the aortic bulb occurs
(p. 236), the septum is so arranged as to place the sixth arch in
communication with the right ventricle and the remaining vessels
in connection with the left ventricle, the only direct communication
 
between the systemic and
ec pulmonary vessels being by
 
way of the ductus arteriosus,
whose significance will be explained later (p. 267).
 
One other change is still
necessary before the vessels
acquire the arrangement
which they possess during
fetal life, and this consists in
the disappearance of the
lower portion of the right
aortic arch (Fig. 149), so that
the left arch alone forms the
connection between the heart
and the dorsal aorta. The
upper part of the right aortic
arch persists to form the proximal part of the right subclavian artery, the portion of
the ventral trunk which unites
the arch with the aortic bulb
becoming the innominate
artery.
 
From the entire length of the thoracic aorta, and in the embryo
from the aortic arches, lateral branches arise corresponding to each
segment and accompanying the segmental nerves. The first of
these branches arises just below the point of union of the vessel
of the sixth arch with the dorsal trunk and accompanies the hypoglossal nerve (Fig. 150, h), and that which accompanies the seventh
 
 
 
 
Fig. 149. — Diagram Illustrating the
changes in the branchial arch vessels.
 
a, Aorta; da, ductus arteriosus; ec, external
carotid; ic, internal carotid; pa, pulmonary artery; sc, subclavian; I- VI, aortic arch vessels.
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
245
 
 
 
cervical nerve arises just above the point of union of the two aortic
arches (Fig. 150, s), and extends out into the limb bud, forming the
subclavian artery.*
 
Further down twelve pairs of lateral branches, arising from the
thoracic portion of the aorta, represent the intercostal arteries,
and still lower four pairs of lumbar arteries are formed, the fifth
lumbars being represented by
two large branches, the common
iliacs, which seem from their size
to be the continuations of the
aorta rather than branches of it.
The true continuation of the
aorta is, however, the middle sacral artery, which represents in
a degenerated form the caudal
prolongation of the aorta of
other mammals, and, like this,
gives off lateral branches corresponding to the sacral segments.
 
In addition to the segmental FlG . I50 .— diagram showing the Re
lateral branches arising from nations op the Lateral Branches to
 
the Aortic Arches.
 
the aorta, Visceral branches, EC> External carotid; h, lateral branch
 
Which have their origin rather cacompanying the hypoglossal nerve; IC,
 
° internal carotid; ICo, intercostal; IM, m
from the Ventral surface, also ternal mammary; s, subclavian; v, verte
^„„,,~ TV, ~™u„mr, ~t - mm bral; I to VIII, lateral cervical branches;
 
OCCUr. In embryos of 5 mm. I; 2) lateral thoracic branches.
 
these branches are arranged in
 
a segmental manner in threes, a median unpaired vessel passing
to the digestive tract and a pair of more lateral branches
passing to the mesonephros (see p. 339) corresponding to each of
the paired branches passing to the body wall (Fig. 151). As
 
* It must be remembered that the right subclavian of the adult is more than equivalent to the left, since it represents the fourth branchial vessel + a portion of the dorsal
longitudinal trunk + the lateral segmental branch (see Fig. 142).
 
 
 
 
246
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
development proceeds the great majority of these visceral
branches disappear, certain of the lateral ones persisting, however,
to form the renal, internal spermatic, and hypogastric arteries of
the adult, while the unpaired branches are represented only by the
c celiac artery and the superior and inferior mesenteries. The
superior mesenteric artery is the adult representative of the vitelline
artery of the embryo and arises from the aorta by two, three or more
roots, which correspond to the fifth, fourth and higher thoracic
 
 
 
 
Fig. 151. — Diagram showing the Arrangement of the Segmental Branches
 
arising from the aorta.
A, Aorta; B, lateral somatic branch; c, lateral visceral branch; D, median visceral
 
branch; E, peritoneum.
 
segments. Later, all but the lowest of the roots disappear and the
persisting one undergoes a downward migration in accordance with
the recession of the diaphragm and viscera (see p. 322), until in
embryos of 17 mm. it lies opposite the first lumbar segment. Similarly the cceliac and inferior mesenteric arteries, which when first
recognizable in embryos of 9 mm. correspond with the fourth and
twelfth thoracic segments respectively, also undergo a secondary
downward migration, the cceliac artery in embryos of 17 mm. arising
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
247
 
 
 
opposite the twelfth thoracic and the inferior mesenteric opposite
the third lumbar segment.
 
The umbilical arteries of the embryo seem at first to be the direct
continuations of the dorsal aortas (Fig. 146), but as development
proceeds they come to arise from the aorta opposite the third
lumbar segment, where they are in line with the lateral visceral
segmental branches. They pass ventral to the Wolffian duct (see
p. 339) and are continued out
along with the allantois to the
chorionic villi. Later this
original stem is joined, not far
from its origin, by what appears to be the lateral somatic
branch of the fifth lumbar segment, whereupon the proximal
part of the original umbilical
vessel degenerates and the umbilical comes to arise from the
somatic branch, which is the
common iliac artery of adult
anatomy (Fig. 152). Hence
it is that this vessel in the adult
gives origin both to branches
such as the external iliac, the
gluteal, the sciatic and the internal pudendal, which are
distributed to the body walls
 
or their derivatives, and to others, such as the vesical, inferior haemorrhoidal and uterine, which are distributed to the pelvic viscera. At
birth the portions of the umbilical arteries beyond the umbilicus are
severed when the umbilical cord is cut, and their intra-embryonic
portions, which have been called the hypogastric arteries, quickly
undergo a reduction in size. Their proximal portions remain
functional as the superior vesical arteries, carrying blood to the
urinary bladder, but the portions which intervene between the
 
 
 
 
Fig. 152. — Diagram Illustrating the
Development of the Umbilical Arteries.
 
A, Aorta; CIl, common iliac; Ell, external iliac; G, gluteal; III, internal iliac; IP,
internal pudic; IV, inferior vesical; Sc, sciatic; U, umbilical; U', primary proximal portion of the umbilical; wd, Wolffian duct.
 
 
 
248 DEVELOPMENT OF THE ARTERIAL SYSTEM
 
bladder and the umbilicus become reduced to solid cords, forming
the obliterated hypogastric arteries of adult anatomy.
f~ In its general plan, accordingly, the arterial system may be
regarded as consisting of a pair of longitudinal vessels which fuse
together throughout the greater portion of their length to form
the dorsal aorta, from which there arise segmentary arranged
lateral somatic branches and ventral and lateral visceral branches.
With the exception of the aortic trunks (together with their anterior
continuations, the internal carotids) and the external carotids, no
longitudinal arteries exist primarily. In the adult, however, several
longitudinal vessels, such as the vertebrals, internal mammary,
and epigastric arteries, exist. The formation of these secondary
longitudinal trunks is the result of a development between adjacent
vessels of anastomoses, which become larger and more important
blood-channels than the original vessels.
 
At an early stage each of the lateral branches of the dorsal aorta
gives off a twig which passes forward to anastomose with a backwardly directed twig from the next anterior lateral branch, so as to
form a longitudinal chain of anastomoses along each side of the
neck. In the earliest stage at present known the chain starts from
the lateral branch corresponding to the first cervical (suboccipital)
segment and extends forward into the skull through the foramen
magnum, terminating by anastomosing with the internal carotid.
To this original chain other links are added from each of the
succeeding cervical lateral branches as far back as the seventh
(Figs. 150 and 153). But in the meantime the recession of the
heart toward the thorax has begun, with the result that the common
carotid stems are elongated and the aortic arches are apparently
shortened so that the subclavian arises on the left side almost
opposite the point where the aorta was joined by the sixth branchial
vessel. As this apparent shortening proceeds, the various lateral
branches which give rise to the chain of anastomoses, with the
exception of the seventh, disappear in their proximal portions and
the chain becomes an independent stem, the vertebral artery, arising
from the seventh lateral branch, which is the subclavian.
 
 
 
DEVELOPMENT OF THE ARTERIAL SYSTEM
 
 
 
249
 
 
 
The recession of the heart is continued until it lies below the
level of the upper intercostal arteries, and the upper two of these,
together with the last cervical branch on each side, lose their connection with the dorsal aorta, and, sending off anteriorly and posteriorly
 
 
 
-A.VCK
 
 
 
 
Fig. 153. — The Development of the Vertebral Artery in a Rabbit Embryo
 
of Twelve Days.
 
IIIA.B to VIA.B, Branchial arch vessels; Ap, pulmonary artery. A.v.c.b and
A.v.cv, cephalic and cervical portions of the vertebral artery; A.s, subclavian; C.d
and C.v internal and external carotid ; ISp.G, spinal ganglion. — (Hochstetter.)
 
 
 
anastomosing twigs, develop a short longitudinal stem, the superior
intercostal, which opens into the subclavian.
 
The intercostals and their abdominal representatives, the
 
 
 
250
 
 
 
DEVELOPMENT OF ARTERIES OF LIMBS
 
 
 
lumbars and iliacs, also give rise to longitudinal anastomosing
twigs near their ventral ends (Fig. 154), and these increasing in
size give rise to the internal mammary and inferior epigastric arteries,
which together form continuous stems extending from the subclavian to the external iliacs in the ventral abdominal walls. The
superficial epigastrics and other secondary longitudinal vessels are
formed in a similar manner.
 
The Development of the Arteries of the Limbs. — The earliest
stages in the development of the limb arteries are unknown in man,
 
 
 
 
Fig. 154,
 
 
 
-Embryo of 13 mm. showing the Mode of Development of the Internal
Mammary and Deep Epigastric Arteries. — (Mall.)
 
 
 
but it has been found that in the mouse the primary supply of the
anterior limb bud is from five branches arising from the sides of the
aorta. These anastomose to form a plexus from which later a single
stem, the subclavian artery, is elaborated, occupying the position
of the seventh cervical segmental vessel, the remaining branches of
the plexus having disappeared. The common iliac artery similarly
 
 
 
DEVELOPMENT OF ARTERIES OF LIMBS 25 1
 
represents the fifth lumbar segmental artery, but whether or not it
also is elaborated from a plexus is as yet unknown.
 
The later history of the limb arteries is also but imperfectly
known and one must rely largely upon the facts of comparative
anatomy and on the anomalies that occur in the adult for indications
of what the development is likely to be. The comparative evidence
indicates the existence of several stages in the development of the
limb vessels, and so far as embryological observations go they
confirm the conclusions drawn from this source, although the various
stages show apparently a great amount of overlapping owing to a
concentration of the developmental stages. In the simplest arrangement the subclavian is continued as a single trunk along the axis
of the limb as far as the carpus, where it divides into digital branches
for the fingers. In its course through the forearm it lies in the
interval between the radius and ulna, resting on the interosseous
membrane, and in this part of its course it may be termed the arteria
interossea. In the second stage a new artery accompanying the
median nerve appears, arising from the main stem or brachial
artery a little below the elbow-joint. This may be termed the
arteria mediana, and as it develops the arteria interossea gradually
diminishes in size, becoming finally the small volar interosseous
artery of the adult (Fig. 155), and the median, uniting with its
lower end, takes from it the digital branches and becomes the principal stem of the forearm.
 
A third stage is then ushered in by the appearance of a branch
from the brachial which forms the arteria ulnaris, and this, passing
down the ulnar side of the forearm, unites at the wrist with the
median to form a superficial palmar arch from which the digital
branches arise. A fourth stage is marked by the diminution of the
median artery until it finally appears to be ,a small branch of the
interosseous, and at the same time there develops from the brachial,
at about the middle of the upper arm, what is known as the arteria
radialis superficial (Fig. 155, rs). This extends down the radial
side of the forearm, following the course of the radial nerve, and at
the wrist passes upon the dorsal surface of the hand to form the
 
 
 
252
 
 
 
DEVELOPMENT OF ARTERIES OP LIMBS
 
 
 
dorsal digital arteries of the thumb and index finger. At first this
artery takes no part in the formation of the palmar arches, but later
it gives rise to the superficial volar branch, which usually unites
with the superficial arch, while from its dorsal portion a perforating
branch develops which passes between the first and second meta
 
 
 
r
 
 
 
Fig. 155. — Diagrams showing an Early and a Late Stage in the Development
 
of the Arteries of the Arm.
 
b, Brachial; i, interosseous; m, median; r, radial; rs, superficial radial; u, ulnar.
 
 
 
carpal bones and unites with a deep branch of the ulnar to form the
deep arch. The fifth or adult stage is reached by the development
from the brachial below the elbow of a branch (Fig. 155, r) which
passes downward and outward to unite with the superficial radial,
whereupon the upper portion of that artery degenerates until it is
 
 
 
DEVELOPMENT OF ARTERIES OF LIMBS 253
 
represented only by a branch to the biceps muscle (Schwalbe), while
the lower portion persists as the adult radial.
 
The various anomalies seen in the arteries of the forearm are, as a
rule, due to the more or less complete persistence of one or other of the
stages described above, what is described, for instance, as the high branching of the brachial being the persistence of the superficial radial.
 
In the leg there is a noticeable difference in the arrangement of
the arteries from what occurs in the arm, in that the principal artery
of the thigh, the femoral, does not accompany the principal nerve,
the sciatic. This difference is apparently secondary, but, as in the
case of the upper limb, it is necessary to rely largely on the facts of
comparative anatomy and on anomalies which occur in the human
body for an idea of the probable development of the arteries of the
lower limb. It has already been seen that the common iliac artery
is to be regarded as a lateral branch of the dorsal aorta, and in the
simplest condition of the limb arteries its continuation, the anterior
division of the hypogastric, passes down the leg as a well-developed
sciatic artery as far as the ankle (Fig. 156,5). At the knee it occupies
the position of the popliteal of adult anatomy, and below the knee
gives off a branch corresponding to the anterior tibial (at) which,
passing forward to the extensor surface of the leg, quickly loses itself
in the extensor muscles. The main artery continues downward on
the interosseous membrane, and some distance above the ankle
divides into a strong anterior and a weaker posterior branch; the
former perforates the membrane and is continued down the extensor
surface of the leg to form the lower part of the anterior tibial and
the dorsalis pedis arteries, while the latter, passing upon the plantar
surface of the foot, is lost in the plantar muscles. At this stage the
external iliac is a secondary branch of the common iliac, being but
poorly developed and not extending as far as the knee.
 
In the second stage the external iliac artery increases in size until it
equals the sciatic, and it now penetrates the adductor magnus
muscle and unites with the popliteal portion of the sciatic. Before
doing this, however, it gives off a strong branch (sa) which accompanies the long saphenous nerve down the inner side of the leg, and,
 
 
 
254
 
 
 
DEVELOPMENT OF ARTERIES OE LIMBS
 
 
 
passing behind the internal malleolus, extends upon the plantar
surface of the foot, where it gives rise to the digital branches. From
this arrangement the adult condition may be derived by the continued increase in size of the external iliac and its continuation, the
femoral (/), accompanied by a reduction of the upper portion of the
sciatic and its separation from its popliteal portion (p) to form the
inferior gluteal artery of the adult. The continuation of the popli
 
 
n
 
 
 
i ,°
 
 
 
 
p
 
 
 
pe
 
 
 
 
f
 
 
 
at
 
 
 
\s
 
 
 
P
 
 
 
pe
 
 
 
t\
 
 
 
Pt
 
 
 
C
 
 
 
Fig. 156. — Diagrams Illustrating Stages in the Development of the Arteries
 
of the Leg.
 
at, Anterior tibial; dp, dorsalis pedis;/, femoral; p, popliteal; pe, peroneal pt, posterior
 
tibial; s, sciatic (inferior gluteal); sa, saphenous.
 
teal down the leg is the peroneal artery (pe) and the upper perforating
branch of this unites with the lower one to form a continuous anterior tibial, the lower connection of which with the peroneal persists
in part as the anterior peroneal artery. A new branch arises from
the upper part of the peroneal and passes down the back of the leg
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM 255
 
to unite with the lower part of the arteria saphena, forming the
posterior tibial artery (pt), and the upper part of the saphenous
becomes much reduced, persisting as the superficial branch of the
art. genu suprema and a rudimentary chain of anastomoses which
accompany the long saphenous nerve.
 
The Development of the Venous System. — The earliest veins
to develop are those which accompany the first-formed arteries, the
umbilicals, but it will be more convenient to consider first the veins
which carry the blood from the body of the embryo back to the
heart. These make their appearance, while the heart is still in the
pharyngeal region, as two pairs of longitudinal trunks, the anterior
and posterior cardinal veins, into which lateral branches, arranged
more or less segmentally, open. The anterior cardinals appear
somewhat earlier than the posterior and form the internal jugular
veins of adult anatomy. Each vein extends forward from the heart
at the side of the notochord and is continued on the under surface
of the brain, lying medial to the roots of the cranial nerves. Later
sprouts arising from the vein form loops around the nerve roots and
the portion of the loops formed by the original vein then disappear,
so that the vessel now lies lateral to the nerve roots, except in the case
of the trigeminus, where the original vessel persists to form the
cavernous sinus. From the vena capitis lateralis so formed three
veins, an anterior, a middle and a posterior cerebral, pass to the
brain, the anterior cerebral together with the ophthalmic vein opening
into the anterior end of the cavernous sinus, the middle cerebral into
the posterior extremity of the same sinus and the posterior cerebral
into the vena capitis lateralis behind the ear vesicle (Fig. 157). The
branches of the anterior cerebral vein extending over the cerebral hemisphere unite with their fellows of the opposite side to form a longitudinal trunk, the superior sagittal sinus, lying between the two cerebral hemispheres. At first this sinus drains by way of the anterior
cerebral vein (Fig. 158, A), but as the cerebral hemispheres increase
in size it is gradually carried backward and makes connections first
with the middle cerebral and later with the posterior cerebral vein
(Fig. 158, B and C), each of these becoming in turn the principal
 
 
 
256
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM
 
 
 
drainage of the sinus. The connections which join the veins to the
sinus become the proximal portion of the transverse sinus, the posterior cerebral vein itself becoming the distal portion, the middle
cerebral vein becomes the superior petrosal sinus, while the anterior
cerebral vein persists as the middle cerebral vein of adult anatomy
 
 
 
m vc i vcv
 
 
 
 
Fig. 157. — Reconstruction of the Head of a Human Embryo of 9 mm. showing
 
the Cerebral Veins.
acv, Anterior cerebral vein; au, auditory vesicle; cs, cavernous sinus; fa, facial
nerve; mcv, middle cerebral vein; pcv, posterior cerebral vein; tr, trigeminal nerve;
vcl, lateral cerebral vein. — {Mall.)
 
 
 
(Fig. 158, C). Additional sprouts from the terminal portion of the
superior sagittal sinus give rise to the straight and inferior sagittal
sinuses, and, after the disappearacne of the vena capitis lateralis, a
new stem develops between the cavernous and transverse sinuses,
passing medial to the ear vesicle, and forms the inferior petrosal
sinus (Fig. 158, C). This joins the transverse sinus at the jugular
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM
 
 
 
257
 
 
 
foramen and from this junction onward the anterior cardinal vein
may now be termed the internal jugular vein.
 
Passing backward from the jugular foramen the internal jugular
veins unite with the posterior cardinals to form on each side a common
trunk, the ductus Cuvieri, and then passing transversely toward the
median line open into the sides of the sinus venosus. So long as the
heart retains its original position in the pharyngeal region the jugular
 
 
 
 
Fig. 158. — Diagrams showing the Arrangement of the Cerebral Veins in
Embryos of (A) the Fifth Week, (B) the Beginning of the Third Month and
in (C) an Older Fetus.
 
acv, Anterior cerebral vein; cs, cavernous sinus; Us, inferior sagittal sinus; Inf.
Pet., inferior petrosal sinus; Is, transverse sinus; ov, ophthalmic vein; sis, superior
sagittal sinus; sps, spheno-parietal sinus; sr, straight sinus; 55, middle cerebral vein
(Sylvian); sup. pet, superior petrosal sinus; th, torcular Herophili; v, trigeminal nerve;
vca, anterior cerebral vein; vol. lateral cerebral vein; vcm, middle cerebral vein; vcp,
posterior cerebral vein; vg, vein of Galen; vj, internal jugular. — (Mall.)
 
is a short trunk receiving lateral veins only from the uppermost segments of the neck and from the occipital segments, the remaining
segmental veins opening into the inferior cardinals. As the heart
recedes, however, the jugulars become more and more elongated
 
17
 
 
 
2 5 8
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM
 
 
 
and the cervical lateral veins shift their communication from the
cardinals to the jugulars, until, when the subclavians have thus
shifted, the jugulars become much larger than the cardinals. When
the sinus venosus is absorbed into the wall of the right auricle, the
course of the left Cuvierian duct becomes a little longer than that
of the right, and from the left jugular, at the point where it is joined
by the left subclavian, a branch arises which extends obliquely across
to join the right jugular, forming the left innominate vein. When
this is established, the connection between the left jugular and
Cuvierian duct is dissolved, the blood from the left side of the head
and neck and from the left subclavian vein passing over to empty
 
 
 
 
Fig. 159. — Diagrams showing the Development of the Superior Vena Cava.
a, Azygos vein; cs, coronary sinus; ej, external jugular; h, hepatic vein; ij, internal
jugular; inr and inl, right and left innominate veins; s, subclavian; vci and vcs, inferior
and superior venae cava?.
 
into the right jugular, whose lower end, togethei with the right
Cuvierian duct, thus becomes the superior vena cava. The left
Cuvierian duct persists, forming with the left horn of the sinus
venosus the coronary sinus (Fig. 159).
 
The external jugular vein develops somewhat later than the
internal. The facial vein, which primarily forms the principal
affluent of this stem, passes at first into the skull along with the fifth
nerve and communicates with the internal jugular system, but later
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM 259
 
this original communication is broken and the facial vein, uniting
with other superficial veins, passes over the jaw and extends down
the neck as the external jugular. Later still the facial anastomoses
with the ophthalmic at the inner angle of the eye and also makes
connections with the internal jugular just after it has crossed the jaw,
and so the adult condition is acquired.
 
It is interesting to note that in many of the lower mammals the external
jugular becomes of much greater importance than the internal, the latter
in some forms, indeed, eventually disappearing and the blood from the
interior of the skull emptying by means of anastomoses which have
developed into the external jugular system. In man the primitive condition is retained, but indications of a transference of the intracranial
blood to the external jugular are seen in the emissary veins.
 
The posterior cardinal veins, or, as they may more simply be
termed, the cardinals, extend backward from their union with the
jugulars along the sides of the vertebral column, receiving veins
from the mesentery and also from the various lateral segmental
veins of the neck and trunk regions, with the exception of that of
the first cervical segment which opens into the jugular. Later,
however, as already described (p. 258), the cervical veins shift to
the jugulars, as do also the first and second thoracic (intercostal)
veins, but the remaining intercostals, together with the lumbars
and sacrals, continue to open into the cardinals. In addition, the
cardinals receive in early stages the veins from the primitive kidneys
(meson ephros), which are exceptionally large in the human embryo,
but when they are replaced later on by the permanent kidneys
(metanephros) their afferent veins undergo a reduction in number
and size, and this, together with the shifting of the upper lateral veins,
produces a marked diminution in the size of the cardinals. The
changes by which they acquire their final arrangement are, however,
so intimately associated with the development of the inferior vena
cava that their description may be conveniently postponed until the
history of the vitelline and umbilical veins has been presented.
 
The vitelline veins are two in number, a right and a left, and pass
in along the yolk-stalk until they reach the embryonic intestine,
along the sides of which they pass forward to unite with the corre
 
 
20O
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM
 
 
 
sponding umbilical veins. These are represented in the bellystalk by a single venous trunk which, when it reaches the body of
the embryo, divides into two stems which pass forward, one on each
side of the umbilicus, and thence on each side of the median line of
the ventral abdominal wall, to form with the corresponding vitelline
veins common trunks which open into the ductus Cuvieri. As the
liver develops it comes into intimate relation with the vitelline veins,
which receive numerous branches from its substance and, indeed,
seem to break up into a network (Fig. 160, A) traversing the liver
 
 
 
DC,
 
 
 
DC,
 
 
 
 
DC
 
 
 
Vus
 
 
 
Vom.s
 
 
 
 
Vud.
 
 
 
DC
 
 
 
D.K4
 
 
 
^drus
 
 
 
Vorn.s.
 
 
 
 
Kl/J.
 
 
 
Vamd. Vb.7ns.
 
 
 
-Diagrams Illustrating the Transformations of the Vitelline and
Umbilical Veins.
 
D.C, Ductus Cuvieri; D.V.A, ductus venosus; V.o.m.d and V.o.m.s, right and left
vitelline veins; V.u.d and V.u.s, right and left umbilical veins. — {Hochstetter.)
 
 
 
substance and uniting again to form two stems which represent the
original continuations of the vitellines. From the point where the
common trunk formed by the right vitelline and umbilical veins
opens into the Cuvierian duct a new vein develops, passing downward and to the left to unite with the left vitelline; this is the ductus
venosus (Fig. 160, B, D.V.A). In the meantime three cross-connections have developed between the two vitelline veins, two of which
pass ventral and the other dorsal to the intestine, so that the latter is
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM
 
 
 
26l
 
 
 
surrounded by two venous loops (Fig. 161, A), and a connection is
developed between each umbilical vein and the corresponding
vitelline (Fig. 160, B), that of the left side being the larger and uniting
with the vitelline just where it is joined by the ductus venosus so as
to seem to be the continuation of this vessel (Fig. 160, C). When
these connections are complete, the upper portions of the umbilical
veins degenerate (Fig. 161), and now the right side of the lower of the
two vitelline loops which surround the intestine disappears, as does
also that portion of the left side of the upper loop which intervenes
 
 
 
 
 
Fig. 161. — A, The Venous Trunks of an Embryo of 5 mm. seen from the
Ventral Surface; B, Diagram Illustrating the Transformation to the Adult
Condition.
 
Vcd and Vcs, Right and left superior venae cavae; Vj, jugular vein; V.om, vitelline
vein; Vp, vena porta; Vu, umbilical vein (lower part); Vu', umbilical vein (upper
part); Vud and Vus, right and left umbilical veins (lower parts). — (His.)
 
between the middle cross-connection and the ductus venosus, and
so there is formed from the vitelline veins the vena porta.
 
While these changes have been progressing the right umbilical
vein, originally the larger of the two (Fig. 160, A and B, V.u.d),
has become very much reduced in size and, losing its connection
with the left vein at the umbilicus, forms a vein of the ventral abdominal wall in which the blood now flows from above downward. The
 
 
 
262
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM
 
 
 
left umbilical now forms the only route for the return of blood from
the placenta, and appears to be the direct continuation of the ductus
venosus (Fig. 161, C), into which open the hepatic veins, returning
the blood distributed by the portal vein to the substance of the liver.
Returning now to the posterior cardinal veins, it has been found
that in the rabbit the branches which come to them from the mesentery anastomose longitudinally to form a vessel lying parallel and
slightly ventral to each cardinal. These may be termed the sub
 
 
 
A £
 
Fig. 162. — Diagrams Illustrating the Development or the Inferior Vena Cava.
The cardinal veins and ductus venosus are black, the subcardinal system blue,
and the supracardinal yellow, cs, coronary sinus; dv, ductus venosus; il, iliac vein;
r, renal; s, internal spermatic; scl, subclavian; sr, suprarenal; va, azygos; vha, hemiazygos; vi, innominate; vj, internal jugular.
 
 
 
cardinal veins (Lewis), and in their earliest condition they open at
either end into the corresponding cardinal, with which they are also
united by numerous cross-branches. Later, in rabbits of 8.8 mm.,
these cross-branches begin to disappear and give place to a large
cross-branch situated immediately below the origin of the superior
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM 263
 
mesenteric artery, and at the same point a cross-branch between the
two subcardinals also develops. The portion of the right subcardial which is anterior to the cross-connection now rapidly enlarges
and unites with the ductus venosus about where the hepatic veins
open into that vessel (Fig. 162, A), and the portion of each posterior
cardinal immediately above the entrance of the renal veins degenerates, so that all the blood received by the posterior portions of the
cardinals is returned to the heart by way of the right subcardinal,
its cross-connections, and the upper part of the ductus venosus.
 
When this is accomplished the lower portions of the subcardinals
disappear, while the portions above the large cross-connection persist, greatly diminished in size, as the suprarenal veins (Fig. 162, B).
 
In the early stages the veins which drain the posterior abdominal
walls empty into the posterior cardinals, and later they form, in the
region of the kidney on each side, a longitudinal anastomosis which
opens at either extremity into the posterior cardinal. The ureter
thus becomes surrounded by a venous ring, the dorsal limb of which
is formed by the new longitudinal anastomosis, which has been
termed the supracardinal vein (McClure and Huntington), while the
ventral limb is formed by a portion of the posterior cardinal (Fig.
162, B). Still later the ventral limb of the loop disappears and the
dorsal supracardinal limb replaces a portion of the more primitive
posterior cardinal. An anastomosis now develops between the
right and left cardinals at the point where the iliac veins open into
them (Fig. 162, B), and the portion of the left cardinal which intervenes between this anastomosis and the entrance of the internal
spermatic vein disappears, the remainder of it, as far forward as the
renal vein, persisting as the upper part of the left internal spermatic
vein, which thus comes to open into the renal vein instead of into
the vena cava as does the corresponding vein of the right side of the
body (Fig. 162, C, s). The renal veins originally open into the
cardinals at the point where these are joined by the large crossconnection, and when the lower part of the left cardinal disappears,
this cross-connection forms the proximal part of the left renal vein,
which consequently receives the left suprarenal (Fig. 162, C).
 
 
 
264 DEVELOPMENT OF THE VENOUS SYSTEM
 
The observations upon which the above description is based
have been made upon the rabbit, but it seems probable from the
partial observations that have been made that similar changes
occur also in the human embryo. It will be noted from what has
been said that the inferior vena cava is a composite vessel, consisting
of at least four elements: (1) the proximal part of the ductus venosus;
(2) the anterior part of the right subcardinal; (3) the right supracardinal; and (4) the posterior part of the right cardinal.
 
The complicated development of the inferior vena cava naturally
gives rise to numerous anomalies of the vein due to inhibitions of its
development. These anomalies affect especially the post-renal portion, a
persistence of both cardinals (interpreting the conditions in the terms of
what occurs in the rabbit) giving rise to a double post-renal cava, or a
persistence of the left cardinal and the disappearance of the right to a
vena cava situated on the left side of the vertebral column and crossing
to the right by way of the left renal vein. So, too, the occurrence of
accessory renal veins passing dorsal to the ureter is explicable on the
supposition that they represent portions of the supracardinal system of
veins.
 
It has already been noted that the portions of the posterior
cardinals immediately anterior to the entrance of the renal veins
disappear. The upper part of the right vein persists, however, and
becomes the vena azygos of the adult, while the upper portion of the
left vein sends a cross-branch over to unite with the azygos and then
separates from the coronary sinus to form the vena hemiazygos. At
least this is what is described as occurring in the rabbit. In the cat,
however, only the very uppermost portion of the right posterior
cardinal persists and the greater portion of the azygos and perhaps
the entire hemiazygos vein is formed from the prerenal portions of
the supracardinal veins, the right one joining on to the small persisting upper portion of the right posterior cardinal, while the crossconnection between the hemiazygos and azygos represents one of the
originally numerous cross-connections between the supracardinals.
 
The ascending lumbar veins, frequently described as the commencements of the azygos veins, are in reality secondary formations developed
by the anastomoses of anteriorly and posteriorly directed branches of the
lumbar veins,
 
 
 
DEVELOPMENT OF THE VENOUS SYSTEM 26
 
 
 
The Development of the veins of the Limbs. — The development of
the limb veins of the human embryo requires further investigation,
but from a comparison of what is known with what has been observed
in rabbit embryos it may be presumed that the changes which take
place are somewhat as follows : In the anterior extremity the blood
brought to the limb is collected by a vein which passes distally along
the radial border of the limb bud, around its distal border, and proximally along its ulnar border to open into the anterior cardinal vein;
this is the primary ulnar vein. Later a second vein grows out from
the external jugular along the radial border of the limb, representing
the cephalic vein of the adult, and on its appearance the digital veins,
which were formed from the primary ulnar vein, become connected
with it, and the distal portion of the primary ulnar vein disappears.
Its proximal portion persists, however, to form the basilic vein, from
which the brachial vein and its continuation, the ulnar vein, are
developed, while the radial vein develops as an outgrowth from the
cephalic, which at an early stage secures an opening into the axillary
vein, its original communication with the external jugular forming
the jugulo-cephalic vein.
 
In the lower limb a primary fibular vein, exactly comparable to
the primary ulnar of the arm, surrounds the distal border of the limbbud and passes up its fibular border to open with the posterior
cardinal vein. The further development in the lower limb differs
considerably, however, from that of the upper limb. From the primary fibular vein an anterior tibial vein grows out, which receives
the digital branches from the toes, and from the posterior cardinal,
anterior to the point where the primary fibular opens into it, a vein
grows down the tibial side of the leg, forming the long saphenous vein.
From this the femoralvein is formed and from it the posterior tibial
vein is continued down the leg. An anastomosis is formed between
the femoral and the primary fibular veins at the level of the knee and
the proximal portion of the latter vein then becomes greatly reduced,
while its distal portion possibly persists as the small saphenous vein
(Hochstetter).
 
The Pulmonary Veins. — The development of the pulmonary veins
 
 
 
266
 
 
 
THE FETAL CIRCULATION
 
 
 
has already been described in connection with the development of
the heart (see p. 234).
 
The Fetal Circulation. — During fetal life while the placenta is
the sole organ in which occur the changes in the blood on which the
 
 
 
 
Fig. 163. — The Fetal Circulation.
ao, Aorta; a.pu., pulmonary artery; au, umbilical artery; da, ductus arteriosus;
dv, ductus venosus; int, intestine; vci and vcs, inferior and superior vena cava; vh,
hepatic vein; vp, vena portas; v.pu, pulmonary vein; vu, umbilical vein. — {From
Kollmann.)
 
 
 
nutrition of the embryo depends, the course of the blood is necessarily somewhat different from what obtains in the child after birth.
Taking the placenta as the starting-point, the blood passes along the
 
 
 
THE FETAL CIRCULATION 267
 
umbilical vein to enter the body of the fetus at the umbilicus, whence
it passes forward in the free edge of the ventral mesentery (see p. 321)
until it reaches the liver. Here, owing to the anastomoses between
the umbilical and vitelline veins, a portion of the blood traverses the
substance of the liver to open by the hepatic veins into the inferior
vena cava, while the remainder passes on through the ductus venosus
to the cava, the united streams opening into the right atrium. This
blood, whose purity is only slightly reduced by mixture with the
blood returning from the inferior vena cava, is prevented from passing into the right ventricle by the Eustachian valve, which directs it
to the foramen ovale, and through this it passes into the left atrium,
thence to the left ventricle, and so out by the systemic aorta.
 
The blood which has been sent to the head, neck, and upper
extremities is returned by the superior vena cava also into the right
atrium, but this descending stream opens into the atrium to the right
of the annulus of Vieussens (see Fig. 141) and passes directly to the
right ventricle without mingling to any great extent with the blood
returning by way of the inferior cava. From the right ventricle
this blood passes out by the pulmonary artery; but the lungs at this
period are collapsed and in no condition to receive any great amount
of blood, and so the stream passes by way of the ductus arteriosus into
the systemic aorta, meeting there the placental blood just below the
point where the left subclavian artery is given off. From this point
onward the aorta contains only mixed blood, and this is distributed
to the walls of the thorax and abdomen and to the lungs and abdominal viscera, the greater part of it, however, passing off in the hypogastric arteries and so out again to the placenta.
 
This is the generally accepted account of the fetal circulation and it
is based upon the idea that the foramen ovale is practically a connection
between the inferior vena cava and the left atrium. If it be correct the
right ventricle receives only the blood returning to the heart by the vena
cava superior, while the left receives all that returns by the inferior vena
cava together with what returns by the pulmonary veins. One would,
therefore, expect that the capacity and pressure of the right ventricle
would in the fetus be less than those of the left. Pohlman, who has
recently investigated the question in embryo pigs, finds, on the contrary,
that the capacities and pressures of the two ventricles are equal and
 
 
 
268 DEVELOPMENT OF THE LYMPHATIC SYSTEM
 
maintains that the foramen ovale is actually a connection between the
two atria. That is to say, he holds that there is an actual mingling of the
blood from the two venae cava? in the right atrium, whence the mixed
blood passes to the right ventricle, a certain amount of it, however,
passing through the foramen ovale and so to the left ventricle to equalize
the deficiency that would otherwise exist in that chamber owing to the
small amount of blood returning by the pulmonary veins. According
to this view there would be no difference in the quality of the blood distributed to different portions of the body, such as is provided for by the
current theory; all the blood leaving the heart would be mixed blood and
in favor of this view is the fact that starch granules injected into either
the superior or the inferior vena cava in living pig embryos were in all
cases recovered from both sides of the heart.
 
At birth the lungs at once assume their functions, and on the
cutting of the umbilical cord all communication with the placenta
ceases. Shortly after birth the foramen ovale closes more or less
perfectly, and the ductus arteriosus diminishes in size as the pulmonary arteries increase and becomes eventually converted into a
fibrous cord. The hypogastric arteries diminish greatly, and after
they have passed the bladder are also reduced to fibrous cords, a fate
likewise shared by the umbilical vein, which becomes converted
into the round ligament of the liver.
 
The Development of the Lymphatic System. — Concerning
the development of the lymphatic system two discordant views
exist, one (Sabin, Lewis) regarding it in its entirety as a direct
development from the venous system, while the other (Huntington,
McClure) recognizes for it a dual origin, a portion being derived
directly from the venous system and the rest from a series of mesenchymal spaces developing in relation to veins but quite unconnected
with them.
 
The portion of the system concerning which harmony prevails
is that which forms the connection with the venous system in the
adult and constitutes what in the embryo is termed the jugular
lymph sac. In the early stages of development a capillary network
extends along the line of the jugular veins, communicating with
them at various points. In embryos of 10 mm. a portion of this
network, on either side of the body, becomes completely separated
 
 
 
bEVELOPMENT OF THE LYMPHATIC SYSTEM
 
 
 
269
 
 
 
from the jugular and gives rise to a number of closed cavities lined
with endothelium and situated in the neighborhood of the junction
of the primary ulnar and cephalic veins with the jugular. In
 
 
 
 
 
Fig. 164. — Diagrams showing the Arrangement of the Lymphatic Vessels in
Pig Embryos of (4) 20 mm. and (B) 40 mm.
ACV, Jugular vein; ADR, suprarenal body; ALU, jugular lymph sac; Ao, aorta
Arm D, deep lymphatics to the arm; D, diaphragm; Du, branches to duodenum
FV, femoral vein; H, branches to heart; K, kidney; LegD, deep lymphatics to leg
Lu, branches to lung; MP, branches to mesenteric plexus; CE, branch to oesophagus
PCV, cardinal vein: PLH, posterior lymph sac; RC, cisterna chyli; RLD, right
lymphatic duct; ScV, subclavian vein; SV, sciatic vein; St, branches to stomach; TD,
thoracic duct; WB. Wolffian body. — (Sdbin.)
 
later stages these cavities enlarge and unite to form a large sac, the
jugular lymph sac (Fig. 164, ALU), and this, still later, makes a
 
 
 
270 DEVELOPMENT OF THE LYMPHATIC SYSTEM
 
new connection with the jugular, the opening being guarded by a
valve. This communication becomes the adult communication of
the thoracic duct or right lymphatic duct with the venous system,
but the sac itself, as development proceeds, becomes divided into
smaller portions and gives rise to a number of lymph nodes.
 
A similar pair of lymph sacs also develop in relation to the
sciatic vein, but their exact mode of origin is uncertain. In embryos
of 20 mm. venous plexuses, similar to the jugular plexuses of
younger stages, are found accompanying the sciatic veins, and a
little later there are found in the same region a pair of posterior or
sciatic lymph sacs (Fig. 164, PLH), which, like the jugular sacs,
later give rise to a series of lymph nodes. At about the same stage
of development & retroperitoneal sac (Fig. 165, Lsr) is also formed in
the root of the mesentery cranial to the origin of the superior mesenteric artery, and this, too, later gives rise to a plexus of lymphatic
vessels in connection with which the mesenteric lymphatic nodes
develop. This last sac is much more pronounced in the pig embryo
than in man, and in that form it has been found to have its origin
from a capillary network that separates from the renal veins
(Baetjer).
 
There are thus formed five sacs, all of which are associated with
the formation of groups of lymphatic nodes, and in the case of one
pair at least it is agreed that they are directly developed from venous
capillaries. It is in connection with the remaining sac and especially with the formation of the thoracic duct and the peripheral
lymphatics that the want of harmony referred to above occurs.
The first portion of the thoracic duct to appear is the cisterna chyli,
which is found in embryos of 23 mm. in the region of the third and
fourth lumbar segments, in close proximity to the vena cava (Fig.
165, Cc). After its appearance the rest of the thoracic duct develops
quickly, it being completely formed in embryos of 30 mm., and it is
interesting to note that at this stage the duct is paired in its caudal
portion, two trunks passing forward from the cisterna chyli, the
right one passing behind the aorta and uniting with the left after it
has entered the thorax.
 
 
 
DEVELOPMENT OF THE LYMPHATIC SYSTEM
 
 
 
271
 
 
 
The mode of origin of the duct has not yet been made out in
human embryos. In the pig and rabbit isolated spaces lined with
endothelium occur along the course of the duct, but without communicating with it, and the fact that some of these showed connection with the neighboring azygos veins gave basis for the view that
they were the remains of a venous capillary plexus from which the
duct had developed. It is possible, however, that the duct is formed
 
 
 
 
T Fig. 165. — Diagram of the Posterior Portion of the Body of a Human
Embryo of 23 mm., showing the Relations of the Retroperitoneal Lymph
Sac and the Cisterna Chyli to the Veins.
 
Am, Superior mesenteric artery; Ao, aorta; Cc, cisterna chyli; Isr, retroperitoneal
lymph sac; S, suprarenal body; Va, vena azygos; Vci, vena cava inferior; vl u first
lumbar vertebra; vs u first sacral vertebra. — (After Sabin.)
 
by the union of outgrowths from the cisterna chyli and jugular sac,
in which case it would also be a derivative of the venous system,
provided that the cisterna chyli is formed in the same way as the
jugular sac. Huntington and McClure, however, maintain that it
 
 
 
272
 
 
 
DEVELOPMENT OF THE LYMPHATIC SYSTEM
 
 
 
is formed by the fusion of spaces appearing in the mesenchyme
immediately external to the intima of degenerating veins; hence the
spaces are termed extraintimal spaces. These at first have no
endothelial lining and they are never in connection with the lumina
of the veins. They are perfectly independent structures and any
connections they may«nake with the venous system are entirely
secondary. This mode of origin from extraintimal spaces is not
 
confined to the thoracic duct, according
to the authors mentioned, but is the
method of development of all parts of
the lymphatic system, with the exception
of the jugular sacs. According to the
supporters of the direct venous origin
the peripheral lymphatic stems develop,
like blood-vessels, as outgrowths from
the stems already present.
 
Lymph nodes nave not been observed
in human embryos until toward the end
of the third month of development, but
' ! .<l'-V''\LY. they appear in pig embryos of 3 cm.
X^Hi^. Their unit of structure is a blood-vessel,
breaking up at its termination into a
leash of capillaries, around which a condensation of lymphocytes occurs in the
mesenchyme. A structure of this kind
forms what is termed a lymphoid follicle
and may exist, even in this simple condition, in the adult. More
frequently, however, there are associated with the follicle lymphatic
vessels, or rather the follicle develops in a network of lymphatic
vessels, which, become an investment of the follicle and form with it a
simple lymph node (Fig. 166). This condition is, however, in many
cases but transitory, the artery branching and collections of lymphoid tissue forming around each of the branches, so that a series
of follicles are formed, which, together with the surrounding lymphatic vessels, become enclosed by a connective-tissue capsule to
 
 
 
 
Fig. 166.— Diagram of a
Primary Lymph Node of an
Embryo Pig of 8 cm.
a, Artery; aid, afferent lymph
duct; eld, efferent lymph duct;
/, follicle. — (Sabin.)
 
 
 
DEVELOPMENT OF THE LYMPHATIC SYSTEM
 
 
 
273
 
 
 
form a compound lymph node. Later trabecular of connective tissue
extend from the capsule toward the center of the node, between the
follicles, the lymphatic network gives rise to peripheral and central
lymph sinuses, and the follicles, each with its arterial branch, constitute the peripheral nodules and the medullary cords, the portions
of these immediately surrounding the leash of capillaries into which
 
 
 
 
 
 
 
tt- be
 
 
 
Fig. 167. — Developing H^emolymph Node.
 
be, central blood-vessel; bh, blood-vessel at hilus; ps, peripheral blood sinus. — (Sabin
 
from Morris' Human Anatomy.)
 
 
 
the artery dissolves, constituting the so-called germ centers in which
multiplication of the lymphocytes occurs.
 
In various portions of the body, but especially along the root of
 
the mesentery, what are termed hcemolymph nodes occur. In these
 
the lymph sinus is replaced by a blood sinus, but with this exception
 
their structure resembles that of an ordinary lymph node, a simple
 
18
 
 
 
274 DEVELOPMENT OF THE SPLEEN
 
one consisting of a follicle, composed of adenoid tissue with a central
blood-vessel, and a peripheral blood sinus (Fig. 167).
 
The Development of the Spleen. — Recent studies (Mall) have
shown that the spleen may well be regarded as possessing a structure
comparable to that of the lymph nodes, the pulp being more or less
distinctly divided by trabecular into areas termed pulp cords, the axis
of each of which is occupied by a twig of the splenic artery. The
spleen, therefore, seems to fall into the same category of organs as
the lymph and hsemolymph nodes, differing from these chiefly in
the absence of sinuses. It has generally been regarded as a development of the mesenchyme situated between the two layers of the
mesogastrium. To this view, however, recent observers have
taken exception, holding that the ultimate origin of the organ is in
part or entirely from the ccelomic epithelium of the left layer of the
mesogastrium. The first indication of the spleen has been observed
in embryos of the fifth week as a slight elevation on the left (dorsal)
surface of the mesogastrium, due to a local thickening and vascularization of the mesenchyme, accompanied by a thickening of the
ccelomic epithelium which covers the elevation. The mesenchyme
thickening presents no differences from the neighboring mesenchyme,
but the epithelium is not distinctly separated from it over its entire
surface, as it is elsewhere in the mesentery. In later stages, which
have been observed in detail in pig and other amniote embryos,
cells separate from the deeper layers of the epithelium (Fig. 168) and
pass into the mesenchyme thickening, whose tissue soon assumes a
different appearance from the surrounding mesenchyme by its cells
being much crowded. This migration soon' Ceases, however, and
in embryos of forty-two days the ccelomic epithelium covering the
thickening is reduced to a simple layer of cells.
 
The later stages of development consist of an enlargement of
the thickening and its gradual constriction from the surface of the
mesogastrium, until it is finally united to it only by a narrow band
through which the large splenic vessels gain access to the organ
The cells differentiate themselves into trabecular and pulp cords
 
 
 
DEVELOPMENT OF THE SPLEEN
 
 
 
7d
 
 
 
special collections of lymphoid cells around the branches of the
splenic artery forming the Malpighian corpuscles.
 
It has already been pointed out (p. 225) that during embryonic life
the spleen is an important haematopoietic organ, both red and white
corpuscles undergoing active formation within its substance. The
Malpighian corpuscles are collections of lymphocytes in which multiplication takes place, and while nothing is as yet known as to the fate of the
cells which are contributed to the spleen from the ccelomic epithelium,
since they quickly come to resemble the mesenchyme cells with which
they are associated, yet the growing number of observations indicating
an epithelial origin for lymphocytes suggests the possibility that the cells
in question may be responsible for the first leukocytes of the spleen.
 
 
 
" ' . .
 
 
 
ms
 
 
 
Fig. 168. — Section through the Left Layer of the Mesogastrium of a Chick
 
Embryo of Ninety-three Hours, Showing the Origin of the Spleen.
 
ep, Ccelomic epithelium; ms, mesenchyme. — {Tonkoff.)
 
The Coccygeal or Luschka's Ganglion. — In embryos of about 15
cm. there is to be found on the ventral surface of the apex of the
coccyx a small oval group of polygonal cells, clearly separated from
the surrounding tissue by a mesenchymal capsule. Later, connective-tissue trabecular make their way into the mass, which thus
becomes divided into lobules, and, at the same time, a rich vascular
supply, derived principally from branches of the middle sacral artery,
penetrates the body, which thus assumes the adult condition in
which it presents a general resemblance to a group of lymph follicles.
 
It has generally been supposed that the coccygeal ganglion was in
part derived from the sympathetic nervous system and belonged to
the same group of organs as the suprarenal bodies. The most recent
 
 
 
276 LITERATURE
 
work on its development (Stoerk) tends, however, to disprove this
view, and the ganglion seems accordingly to find its place among
the lymphoid organs.
 
LITERATURE.
 
W. A. Baetjer: "On the Origin of the Mesenteric Sac and the Thoracic Duct in the
 
Embryo Pig," Amer. Journ. Anat., vin, 1908.
E. van Beneden and C. Julln: "Recherches sur la formation des annexes fcetales
 
chez les mammiferes," Archives de Biolog., v, 1884.
A. C. Bernays: " Entwickehingsgeschichte der Atrioventricularklappen," Morphol.
 
Jahrbuch, 11, 1876.
G. Born: "Beitrage zur Entwicklungsgeschichte des Saugethierherzens," Archiv
 
fiir mikrosk. Anat., xxxiii, 1889.
J. L. Bremer: " On the Origin of the Pulmonary Arteries in Mammals," Anat. Record,
 
in, 1909.
I. Broman: "Ueber die Entwicklung, Wanderung und Variation der Bauchaorten
zweige bei den Wirbeltiere," Ergeb. Anat. und Entwick., xvi, 1906.
I. Broman: " Ueber die Entwicklung und "Wanderung" der Zweige der aorta abdom
inalis beim Menschen," Anat. Hefte, XXXVI, 1908.
E. E. Butterfield: "Ueber die ungranulierte Vorstufen der Myelocyten und ihre
 
Bildung in Milz, Leber und Lymphdriisen," Deutsch. Arch. f. klin. Med., xcn,
 
1908.
 
E. R. Clark: " Observations on Living Growing Lymphatics in the Tail of the Frog
 
Larva," Anat. Record, in, 1909.
 
C. B. Coulter: "The Early Development of the Aortic Arches of the Cat, with
 
Especial Reference to the Presence of a Fifth Arch." Anat. Record, III, 1909.
 
D . M. Davis: " Studies on the Chief Veins in Early Pig Embryos and the Origin of the
 
Vena Cava Inferior," Amer. Journ. Anat., x, 1910.
J. Disse: "Die Entstehung des B lutes und der ersten Gefasse im Huhnerei," Archiv
 
fiir mikrosk. Anat., xvi, 1879.
A. C. F. Eternod: "Premiers stades de la circulation sanguine dans l'ceuf et Pembryon
 
humain," Anat. Anzeiger, xv, 1899.
H. M. Evans: "On the Development of the Aortae, Cardinal and Umbilical Veins,
 
and the other Blood-vessels of Vertebrate Embryos from Capillaries," Anat.
 
Record, in, 1909.
V. Federow: "Ueber die Entwicklung der Lungenvene," Anat. Hefte, xl, 1910.
W. Felix: " Zur Entwicklungsgeschichte der Rumpfarterien des menschlichen Embryo,"
 
Morphol. Jahrb., xli, 1910.
G. J. Heuer: "The Development of the Lymphatics in the Small Intestine of the
 
Pig," Amer. Journ. Anat., ix, 1909.
W. His: "Anatomie menschlicher Embryonen," Leipzig, 1880-1882.
 
F. Hochstetter: "Ueber die ursprungliche Hauptschlagader der hinteren Gliedmasse
 
des Menschen und der Saugethiere, nebst Bemerkungen iiber die Entwicklung der
Endaste der Aorta abdominalis," Morphol. Jahrbuch, xvi, 1890.
 
 
 
LITERATURE 277
 
F. Hochstetter: "Ueber die Entwicklung der A. vertebralis beim Kaninchen, nebst
Bemerkungen uber die Entstehung der Ansa Vieusseni," Morphol. Jahrbuch, XVI,
1890.
 
F. Hochstetter: "Beitrage zur Entwicklungsgeschichte des Venensystems der
 
Amnioten." Morphol. Jahrbuch, xx, 1893.
W. H. Howell: "The Life-history of the Formed Elements of the Blood, Especially
 
the Red Blood-corpuscles," Journ. of Morphol., iv, 1890.
W. H. Howell: "Observations on the Occurrence, Structure, and Function of the
 
Giant-cells of the Marrow," Journ. of M or ph., rv, 1890.
 
G. S. Huntington: "The Genetic Principles of the Development of the Systemic
 
Lymphatic Vessels in the Mammalian Embryo," Anal. Record, iv, 1910.
G. S. Huntington: "The Anatomy and Development of the Systemic Lymphatic
 
Vessels of the Domestic Cat," Memoirs of Wistar Institute, 1, 1912.
G. S. Huntington and C. F. W. McClure: "Development of Post-cava and Tributaries in the Domestic Cat," Amer. Journ. Anat., vi, 1907.
G. S. Huntington and C. F. W. McClure: "The Development of the Main Lymph
 
Channels of the Cat in their Relations to the Venous System," Amer. Journ
 
Anat., vi, 1907.
G. S. Huntington and C. F. W. MtjClure: "The Anatomy and Development of
 
the Jugular Lymph Sacs in the Domestic Cat," Amer. Journ. Anat., x, 1910.
H. E. Jordan: "A Microscopical Study of the Umbilical Vesical of a 13 mm. Human
 
Embryo, with Special Reference to the Entodermal Tubules and the Blood
 
Islands," Anat. Anzeiger, xxxvn, 1910.
C. A. Kling: "Studien uber die Entwicklung der Lymphdriisen beim Menschen,"
 
Archiv.fiir mikrosk. Anal., lxiii, 1904.
H. Lehmann: " On the Embryonic History of the Aortic Arches in Mammals," Anat.
 
Anzeiger, xxvi, 1905.
F. T. Lewis: "The Development of the Vena Cava Inferior," Amer. Journ. of Anat.,
 
1, 1902.
F. T. Lewis: "The Development of the Veins in the Limbs of Rabbit Embryos."
 
Amer. Journ. Anat., v, 1906.
F. T. Lewis: "The Development of the Lymphatic System in Rabbits," Amer. Journ.
 
Anat., v, 1906.
F. T. Lewis: "On the Cervical Veins and Lymphatics in Four Human Embryos,"
 
Amer. Journ. Anat., ix, 1909.
F. T. Lewis: "The First Lymph Glands in Rabbit and Human Embryos," Anat.
 
Record, in, 1909.
W. A. Locy: "The Fifth and Sixth Aortic Arches in Chick Embryos, with Comments
 
on the Condition of the same Vessels in other Vertebrates," Anat. Anzeiger
 
xxix, 1906.
F. P. Mall: "Development of the Internal Mammary and Deep Epigastric Arteries
 
in Man," Johns Hopkins Hospital Bulletin, 1898.
F. P. Mall: "On the Developmennt of the Blood-vessels of the Brain in the Human
 
Embryo," Amer. Journ. Anat., iv, 1905.
A. Maximow: " Untersuchungen liber Blut und Bindegewebe," Arch, fur mikr. Anat.,
 
Lxxni, 1909; lxxiv, 1909; lxxvi, 1910.
 
 
 
278 LITERATURE
 
C. F. W. McClure: "The Development of the Thoracic and Right Lymphatic Ducts
 
in the Domestic Cat (Felis Domestica)," Anat. Anzeiger, xxxii, 1908.
C. F. W. McClure: " The Extra-intimal Theory of the Development of the Mesenteric
 
Lymphatics in the Domestic Cat," Verhandl. Anat. Gesellsch., xxiv, 1910.
C. S. Minot: "On a Hitherto Unrecognized Form of Blood Circulation without
 
Capillaries in the Organs of Vertebrata," Proc. Boston Soc. Nat. Hist., xxix, 1900.
S. Molleer: "Die Blutbildung in der Embryonalen Leber des Menschen und der
 
Saugetiere," Arch.filr mikrosk. Anat., Lxxrv, 1909.
A. G. Pohlman: "The Course of the Blood through the Fetal Mammalian Heart,"
 
Anat. Record, n, 1908.
F. Reagan: "The Fifth Aortic Arch of Mammalian Embryos." Amer. Journ. Anat...
 
xii, 1912.
 
E. Retterer: "Sur la part que prend 1' epithelium a la formation de la bourse de
 
Fabricius, des amygdales et des plaques de Peyer," Journ. de I' Anat. et de la
 
Physiol., xxix, 1893.
R. Retzer: "Some Results of Recent Investigations on the Mammalian Heart,"
 
Anat. Record, 11, 1908.
C. Rose: "Zur Entwicklungsgeschichte des Saugethierherzens," Morphol. Jahrbuch,
 
xv, 1889.
Florence R. Sabln: "On the Origin of the Lymphatic System from the Veins and
 
the Development of the Lymph Hearts and Thoracic Duct in the Pig," Amer.
 
Journ. of Anat., I, 1902.
Florence R. Sabin: "The Development of the Lymphatic Nodes in the Pig and
 
their Relation to the Lymph Hearts," Amer. Journ. Anat., rv, 1905.
Florence R. Sabin: "Further Evidence on the Origin of the Lymphatic Endothelium
 
from the Endothelium of the Blood Vascular System," Anat. Record, 11, 1908.
Florence R. Sabin: On the Development of the Lymphatic System in Human
> Embryos with a Consideration of the Morphology of the System as a Whole,"
 
Amer. Journ. Anat., ix, 1909.
Florence R. Sabin: "A Critical Study of the Evidence Presented in Several Recent
 
Articles on the Development of the Lymphatic System," Anat. Record, v, 1911.
 
F. Saxer: "Ueber die Entwicklung und der Bau normaler Lymphdrusen und die
 
Entsehung der roten und weissen Blutkorperchen," Anat. Hefte, vi, 1896.
H. Schridde: "Die Entstehung der ersten embryonalen Blutzellen des Menschen,"
 
Folia hcematol, rv, 1907.
P. Stohr: "Ueber die Entwicklung der Darmlymphknotchen und iiber die Riick
bildung von Darmdrusen," Archiv fur mikrosk. Anat., LI, 1898.
O. van der Stricht: " Nouvelles recherches sur la genese des globules rouges et des
 
globules blancs du sang," Archives de Biolog., xn, 1892.
O. van der Stricht: "De la premiere origine du sang et des capillaires sanguins dans
 
l'aire vasculaire du Lapin," Comptes Rendus de la Soc. de Biolog. Paris, -Ser. 10,
 
11, 1895.
O. Stoerk: "Ueber die Chromreaktion der Glandula coccygea und die Beziehung,
 
dieser Druse zum Nervus sympathicus," Arch, fur mikroskop. Anat., lxix, 1906.
J. Tandler: "Zur Entwicklungsgeschichte der Kopfarterien bei den Mammalia."
 
Morphol. Jahrbuch, xxx, 1902.
 
 
 
LITERATURE 279
 
J. Tandler: "Zur Entwickelungsgeschichte der menschlichen Darmarterien," Anat.
 
Hefte, xxiii, 1903.
J. Tandler: " Ueber die Varietaten der arteria coeliaca und deren Entwicklung," Anat.
 
Hefte, xxv, 1904.
J. Tandler: " Ueber die Entwicklung des fiinften Aortenbogens und der fiinften
 
Schlundtasche beim Menschen," Anat. Hefte, xxxvin, 1909.
W. Tonkoff: " Die Entwickelung der Milz bei den Amnioten," Arch, fiir mikrosk.
 
Anat., lvi, 1900.
Bertha de Vriese: "Recherches sur revolution des vaissaux sanguins des membres
 
chez l'homme," Archives de Biolog., xvili, 1902.
F. Weidenreich: "Die roten Blutkorperchen," Ergeb. Anat. und Entwick., xiii, 1903
 
xiv, 1904.
F. Weidenreich: "Die Leucocyten und verwandte zellformen," Ergeb. Anat. und;
 
Entwick., xvi, 191 1.
J. H. Wright: "The Histogenesis of the Blood Platelets," Journ. of Morph., xxr, 1910.
 
 
 
CHAPTER X.
 
THE DEVELOPMENT OF THE DIGESTIVE
TRACT AND GLANDS.
 
The greatest portion of the digestive tract is formed by the constriction off of the dorsal portion of the yolk-sac, as shown in Fig. 52,
the result being the formation of a cylinder, closed at either end,
and composed of a layer of splanchnic mesoderm lined on its inner
surface by endoderm. This cylinder is termed archenteron and has
connected with it the yolk-stalk and the allantois, the latter communicating with its somewhat dilated terminal portion, which also
receives the ducts of the primitive kidneys and is known as the
cloaca (Fig. 170).
 
At a very early stage of development the anterior end of the
embryo begins to project slightly in front of the yolk-sac, so that a
shallow depression is formed between the two structures. As the
constriction of the embryo from the sac proceeds, the anterior portion
of the brain becomes bent ventrally and the heart makes its appearance immediately in front of the anterior surface of the yolk-sac,
and so the depression mentioned above becomes deepened (Fig. 169)
to form the oral sinus. The floor of this, lined by ectoderm, is
immediately opposite the anterior end of the archenteron, and, since
mesoderm does not develop in this region, the ectoderm of the sinus
and the endoderm of the archenteron are directly in contact, forming
a thin pharyngeal membrane separating the two cavities (Fig. 169, pm)
In embryos of 2.15 mm. this membrane is still existent, but soon after
it becomes perforated and finally disappears, so that the archenteron
and oral sinus become continuous.
 
Toward its posterior end trr; archenteron comes into somewhat
similar relations with the ectoderm, though a marked difference is
noticeable in that the area over which the cloacal endoderm is in
 
280
 
 
 
DEVELOPMENT OF THE DIGESTIVE TRACT
 
 
 
281
 
 
 
O-A
 
 
 
contact with the ectoderm to form the cloacal membrane (Fig. 170, cm)
lies a little in front of the actual end of the archenteric cylinder, the
portion of the latter which lies posterior to the membrane forming
what has been termed the postanal gut {p. an). This diminishes in
size during development and early disappears altogether, and the
pouch-like fold seen in Fig. 170 between the intestinal portion of the
archenteron and the allantoic stalk (al) deepening until its floor
comes into contact with the cloacal membrane, the cloaca becomes divided into a ventral portion, with which the allantois
and the primitive excretory ducts
(w) are connected, and a dorsal
portion which becomes the lower
end of the rectum. This latter
abuts upon the dorsal portion
of the cloacal membrane, and
this eventually ruptures, so that
the posterior communication of
the archenteron with the exterior
becomes established. This rupture, however, does not occur until a comparatively late period of
development, until after the embryo has reached the fetal stage;
nor does the position of the membrane correspond with the adult
anus, since later there is a considerable development of mesoderm
around the mouth of the cloaca, bulging out, as it were, the surrounding ectoderm, more especially anteriorly where it forms the
large genital tubercle (see Chapter XIII), and posteriorly where it produces the anal tubercle. This appears as a rounded elevation on
each side of the median line, immediately behind the cloacal membrane and separated from the root of the caudal projection by a depression, the precaudal recess. Later the two elevations unite across
the median line to form a transverse ridge, the ends of which curve
 
 
 
 
Fig. 169. — Reconstruction of the
Anterior Portion of an Embryo of 2.15
 
MM.
 
ab. Aortic bulb; h, heart; 0, auditory capsule; op, optic evagination;/>?w, pharyngeal
membrane. — {His.)
 
 
 
282
 
 
 
DIGESTIVE TRACT AND GLANDS
 
 
 
forward and eventually meet in front of the original anal orifice.
From the mesoderm of the circular elevation thus produced the external sphincter ani muscle is formed, and it would seem that so
much of the lower end of the rectum as corresponds to this muscle
is formed by the inner surface of the elevation and is therefore
ectodermal. The definitive anus being at the end of this terminal
portion of the gut is therefore some distance away from the position of the original cloacal membrane.
 
 
 
 
nc
 
 
 
Fig. 170. — Reconstruction of the Hind End of an Embryo 6.5 mm. Long
 
al, Allantois; b, belly-stalk; cl, cloaca; cm, cloacal membrane; i, intestine; n, spinal
cord; nc, notochord; p.an, postanal gut; ur, outgrowth to form ureter and metanephros;
w, Wolffian duct. — (Keibel.)
 
 
 
It will be noticed that the digestive tract thus formed consists of
three distinct portions, an anterior, short, ectodermal portion, an
endodermal portion representing the original archenteron, and a
posterior short portion which is also ectodermal. The differentiation of the tract into its various regions and the formation of the
various organs found in relation with these may now be considered.
 
 
 
DEVELOPMENT OF THE MOUTH REGIONS 283
 
The Development of the Mouth Region. — The deepening
of the oral sinus by the development of the first branchial arch and
its separation into the oral and nasal cavities by the development
of the palate have already been described (p. 99), but, for the sake
of continuity in description, the latter process may be briefly recalled.
At first the nasal pits communicate with the oral sinus by grooves
lying one on each side of the fronto-nasal process, but by the union
of the latter, through its processus globularis, with the maxillary
processes these communications are interrupted and the floors of
the nasal pits are separated from the oral cavity by thin bucco-nasal
membranes, formed of the nasal epithelium in contact with that
of the oral cavity. In embryos of about 15 mm. these membranes
break through and disappear, and the nasal and oral cavities are
again in communication, but the communications are now behind
the maxillary processes and constitute what are termed the primitive
choance. The oral cavity at this stage does not, however, correspond
with the adult mouth cavity, since there is as yet no palate, the roof
of the oral cavity being the base of the skull. From the maxillopalatine portions of the upper jaw, shelf-like ridges begin to grow,
being at first directed downward so that their surfaces are parallel
with the sides of the tongue, which projects up between them.
Later, however, they become bent upward to a horizontal position
(Fig. 171) and eventually meet in the median line to form the palate,
separating the nasal cavities from the mouth cavity. All that portion of the original oral cavity which lies behind the posterior edge
of the palatal shelf is now known as the pharynx, the boundary
between this and the mouth cavity being emphasized by the prolongation backward and downward of the posterior angles of the
palatal shelf as ridges, which form the pharyn go -palatine arches
{posterior pillars of the fauces) . The nasal cavities now communicate
with the upper part of the pharynx (naso-pharynx) by the posterior
choanae. The palatal processes are entirely derived from the
maxillary processes, the premaxillary portion of the upper jaw,
which is a derivative of the fronto-nasal processes, not taking part
in their formation/ Consequently a gap exists between the palatal
 
 
 
284
 
 
 
DEVELOPMENT OE THE MOUTH REGIONS
 
 
 
shelves and the premaxillae for a time, by which the nasal and
mouth cavities communicate; it places the organ of Jacobson (see
p. 429) in communication with the mouth cavity and may persist
until after birth. Later it becomes closed over by mucous membrane, but may be recognized in the dried skull as the foramen
incisivum (anterior palatine canal).
 
Occasionally there is a failure of the union of the palatal plates, the
condition known as cleft palate resulting. The inhibition of development
which brings about this condition may take place at different stages, but
frequently it occurs while the plates still have an almost vertical direction.
Typically cleft palate is a deficiency in the median line of the roof of the
 
 
 
 
Fig. 171. — View of the Roof of the Oral Fossa of Embryo showing the Lipgroove and the Formation of the Palate. — (His.)
 
mouth, not affecting the upper jaw, but very frequently it is combined
with the defect which produces hare-lip (see p. 100), in which case the
cleft may be continued through the upper jaw between its maxillary and
premaxillary portions on either or both sides, according to the extent of
the defect.
 
At about the fifth week of development a downgrowth of epithelium into the substance of both the maxillary and fronto-nasal
processes above and the mandibular process below takes place, and
the surface of the downgrowth becomes marked by a deepening
groove (Fig. 171), which separates an anterior fold, the Up, from
the jaw proper (Fig. 172). Mention should also be made of the
 
 
 
DEVELOPMENT OF THE TEETH 2S5
 
fact that at an early stage of development a pouch is formed in the
median line of the roof of the oral sinus, just in front of the pharyngeal membrane, by an outgrowth of the epithelium. This pouch,
known as Rathke's pouch, comes in contact above with a downgrowth
from the floor of the brain and forms with it the pituitary body
(seep. 399).
 
The Development of the Teeth. — When the epithelial downgrowth
which gives rise to the lip groove is formed, a horizontal outgrowth
develops from it which extends backward into the substance of the
jaw, forming what is termed the dental shelf (Fig. 172, A). This
at first is situated on the anterior surface of the jaw, but with the
continued development of the lip fold it is gradually shifted until it
comes to lie upon the free surface (Fig. 172, B), where its superficial
edge is marked by a distinct groove, the dental groove (Fig. 171).
At first the dental shelf of each jaw is a continuous plate of cells,
uniform in thickness throughout its entire width, but later ten thickenings develop upon its deep edge, and beneath each of these the
mesoderm condenses to form a dental papilla, over the surface of
which the thickening moulds itself to form a cap, termed the enamel
organ (Fig. 172, B). These ten papillae in each jaw, with their
enamel caps, represent the teeth of the first dentition.
 
The papillae do not, however, project into the very edge of the
dental shelf, but obliquely into what, in the lower jaw, was originally
its under surface (Fig. 172, B), so that the edge of the shelf is free
to grow still deeper into the surface of the jaw. This it does, and
upon the extension so formed there is developed in each jaw a second
set of thickenings, beneath each of which a dental papilla again
appears. These tooth-germs represent the incisors, canines, and
premolars of the permanent dentition. The lateral edges of the
dental shelf being continued outward toward the articulations of
the jaws as prolongations which are not connected with the surface
epithelium, opportunity is afforded for the development of three
additional thickenings on each side in each jaw, and, papillae developing beneath these, twelve additional tooth-germs are formed.
These represent the permanent molars; their formation is much
 
 
 
286
 
 
 
DEVELOPMENT OF THE TEETH
 
 
 
later than that of the other teeth, the germ of the second molar not
appearing until about the sixth week after birth, while that of the
third is delayed until about the fifth year.
 
As the tooth-germs increase in size, they approach nearer and
nearer to the surface of the jaw, and at the same time the enamel
organs separate from the dental shelf until their connection with it
is a mere neck of epithelial cells. In the meantime the dental shelf
itself has been undergoing degeneration and is reduced to a reticulum
 
 
 
 
 
 
W'- : ^^^^^0^ :i '
 
 
Mill
 
 
 
3
 
 
 
Fig. 172. — Transverse Sections through the Lower Jaw showing the
Formation of the Dental Shelf in Embryos of (A) 17 mm. and (B) 40 mm. —
(Rose.)
 
which eventually completely disappears, though fragments of it may
occasionally persist and give rise to various malformations. With
the disappearance of the last remains of the shelf, the various toothgerms naturally lose all connection with one another.
 
It will be seen, from what has been said, that each tooth-germ
consists of two portions, one of which, the enamel organ, is derived
from the ectoderm, while the other, the dental papilla, is mesen
 
 
DEVELOPMENT OF THE TEETH 287
 
chymatous. Each of these gives rise to a definite portion of the
fully formed tooth, the enamel organ, as its name indicates, producing the enamel, while from the dental papilla the dentine and pulp
are formed.
 
The cells of the enamel organ which are in contact with the surface of the papilla, at an early stage assume a cylindrical form and
become arranged in a definite layer, the enamel membrane (Fig.
173, SEi), while the remaining cells (SEa) apparently degenerate
eventually, though they persist for a time to form what has been
termed the enamel pulp. The formation of the enamel seems to be
due to the direct transformation of the enamel cells, the process beginning at the basal portion of each cell, and as a result, the enamel
consists of a series of prisms, each of which represents one of the
cells of the enamel membrane. The transformation proceeds
until the cells have become completely converted into enamel
prisms, except at their very tips, which form a thin membrane, the
enamel cuticle, which is shed soon after the eruption of the teeth.
 
The dental papillae are at first composed of a closely packed mass
of mesenchyme cells, which later become differentiated into connective tissue into which blood-vessels and nerves penetrate. The
superficial cells form a more or less definite layer (Fig. 173, od),
and are termed odontoblasts, having the function of manufacturing
the dentine. This they accomplish in the same manner as that in
which the periosteal osteoblasts produce bone, depositing the dentine between their surfaces and the adjacent surface of the enamel.
The outer surface of each odontoblast is drawn out into a number
of exceedingly fine processes which extend into the dentine to occupy
the minute dentinal tubules, just as processes of the osteoblasts
occupy the canaliculi of bone.
 
At an early stage the enamel membrane forms an almost complete investment for the dental papilla (Fig. 173), but as the ossification of the tooth proceeds, it recedes from the lower part, until
finally it is confined entirely to the crown. The dentine forming the
roots of the tooth then becomes enclosed in a layer of cement, which
is true bone and serves to unite the tooth firmly to the walls of its
 
 
 
288
 
 
 
DEVELOPMENT OF THE TEETH
 
 
 
socket. As the tooth increases in size, its extremity is brought
nearer to the surface of the gum and eventually breaks through, the
eruption of the first teeth usually taking place during the last half
of the first year after birth. The growth of the permanent teeth
 
 
 
-£p.
 
 
 
 
-Od.
 
 
 
Fig. 173. — Section through the First Molar Tooth of a Rat, Twelve Days Old.
Ap, Periosteum; E, dentine; Ep, epidermis; Od, odontoblasts; S, enamel; SEa
and SEi, outer and inner layers of the enamel organ; SE, portion of the enamel organ
which does not produce enamel. — (von Brunn.)
 
 
 
proceeds slowly at first, but later it becomes more rapid and produces pressure upon the roots of the primary teeth. These roots
then undergo partial absorption, and the teeth are thus loosened
 
 
 
DEVELOPMENT OF THE TONGUE 289
 
in their sockets and are readily- pushed out by the further growth of
the permanent teeth.
 
The dates and order of the eruption of the teeth are subject to considerable variation, but the usual sequence is somewhat as follows:
 
Primary Dentition.
 
Median incisors 6th to 8th month.
 
Lateral incisors 8th to 12 month.
 
First molars Beginning of 2d year.
 
Canines i£ years.
 
Second molars 3 to 3^ years.
 
The teeth of the lower jaw generally precede those of the upper.
 
Permanent Dentition.
 
First molars 7th year.
 
Middle incisors 8th year.
 
Lateral incisors 9th year.
 
First premolars 10th year.
 
Second premolars nth year.
 
Canines
 
 
 
13th to 14th years.
 
Second molars J
 
Third molars 17th to 40th years.
 
In a considerable percentage of individuals the third molars (wisdom
teeth) never break through the gums, and frequently when they do so
they fail to reach the level of the other teeth, and so are only partly functional. These and other peculiarities of a structural nature shown
by these teeth indicate that they are undergoing a retrogressive evolution.
 
The Development of the Tongue. — Strictly speaking, the
tongue is largely a development of the pharyngeal region of the
digestive tract and only secondarily grows forward into the floor of
the mouth. In embryos of about 3 mm. there may be seen in the
median line of the floor of the mouth, between the ventral ends of
the first and second branchial arches, a small rounded elevation
which has been termed the tuberculum impar (Fig. 174, Ti). It was
at one time believed that this gave rise to the anterior portion of the
tongue, but recent observations seem to show that it reaches its
greatest development in embryos of about 8 mm., after which it
becomes less prominent and finally unrecognizable. But before
19
 
 
 
290
 
 
 
DEVELOPMENT OF THE TONGUE
 
 
 
this occurs a swelling appears in the anterior part of the mouth on
each side of the median line (Fig. 174, t), and these gradually increase
 
 
 
n
 
 
 
X-'
 
 
 
 
x
 
 
 
 
 
 
-Cap
 
 
 
/
 
 
 
/
 
 
 
/<â– 
 
 
 
Fig. 174. — Floor of the Mouth and Pharynx of an Embryo of 7.5 mm., from
 
a Reconstruction.
 
Cop, Copula; /, furcula; t, swelling that gives rise to the body of the tongue; Ti,
 
tuberculum impar; I-III, branchial arches.
 
in size and eventually unite in the median line to form the main
mass of the body of the tongue. They are separated from the
 
neighboring portions of the first
branchial arch by a deep groove,
the alveolo-lingual groove, and posteriorly are separated from the
second arch by a groove which later becomes distinctly V-shaped
(Fig. 175), a deep depression, which
gives rise to the thyreoid body lying
at the apex of the V. Behind the
thyreoid pouch the ventral ends of
the second and third branchial arches
unite to form an elevation, the
copula (Fig. 174, cop), and from this
and the adjacent portions of the
second and third arches the posterior portion of the tongue develops.
The tongue then consists of two distinct portions, which even
 
 
 
Fig. 175. — The Floor of the
Pharynx of an Embryo of about
20 MM.
 
ep, Epiglottis; fc, foramen caecum;
t 1 and t 2 median and lateral portions
of the tongue. — (His.)
 
 
 
THE SALIVARY GLANDS 2 9 1
 
tually fuse together, but the groove which originally separated them
remains more or less clearly distinguishable (Fig. 175), the vallate
papillae (see p. 430) developing immediately anterior to it.
 
The tongue is essentially a muscular organ, being formed of a central
mass of muscular tissue, enclosed at the sides and dorsally by mucous
membrane derived from the floor of the mouth and pharynx. The
muscular tissue consists partly of fibers limited to the substance of the
tongue and forming the m. lingualis, and also of a number of extrinsic
muscles, the hyoglossi, genioglossi, styloglossi, glos so palatini, and chondroglossi. The last two muscles are innervated by the vagus nerve, and
the remaining extrinsic muscles receive fibers from the hypoglossal, while
the lingualis is supplied partly by the hypoglossal and partly, apparently,
by the facial through the chorda tympani. That the facial should take
part in the supply is what might be expected from the mode of development of the tongue, but the hypoglossal has been seen to correspond to
certain primarily postcranial metameres (p. 169), and its relation to
structures taking part in the formation of an organ belonging to the anterior
part of the pharynx seems somewhat anomalous. It may be supposed
that in the evolution of the tongue the extrinsic muscles, together with a
certain amount of the lingualis, have grown into the tongue thickenings
from regions situated much further back, for the most part from behind
the last branchial arch.
 
Such an invasion of the tongue by muscles from posterior segments
would explain the distribution of its sensory nerves (Fig. 176). The
anterior portion, from its position, would naturally be supplied by branches
from the fifth and seventh nerves, while the posterior portion might be
expected to be supplied by the seventh. There seems, however, to have
been a dislocation forward, if it may be so expressed, of the mucous membrane, the sensory distribution of the ninth nerve extending forward upon
the posterior part of the anterior portion of the tongue, while a considerable amount of the posterior portion is supplied by the tenth nerve.
The distribution of the sensory fibers of the facial is probably confined
entirely to the anterior portion, though further information is needed to
determine the exact distribution of both the motor and sensory fibers of
this nerve in the tongue.
 
The Development of the Salivary Glands. — In embryos of
about 8 mm. a slight furrow may be observed in the floor of the
groove which connects the lip grooves of the upper and lower jaws
at the angle of the mouth and may be known as the cheek groove.
In later stages this furrow deepens and eventually becomes closed
in to form a hollow tubular structure, which in embryos of 17 mm.
 
 
 
292
 
 
 
THE SALIVARY GLANDS
 
 
 
has separated from the epithelium of the floor of the cheek groove
except at its anterior end and has become embedded in the connective tissue of the cheek. This tube is readily recognizable as the
parotid gland and duct, and from the latter as it passes across the
masseter muscle a pouch-like outgrowth is early formed which probably represents the soda parotidis.
 
 
 
 
Fig. 176. — Diagram of the Distribution of the Sensory Nerves of the Tongue.
The area supplied by the fifth (and seventh) nerve is indicated by the transverse
lines; that of the ninth by the oblique lines; and that of the tenth by the small circles.
— {Zander.)
 
The submaxillary gland and duct appear in embryos of about
13 mm. as a longitudinal ridge-like thickening of the epithelium
of the floor of the alveolo-lingual groove (see p. 290). This ridge
 
 
 
THE SALIVARY GLANDS
 
 
 
2 93
 
 
 
gradually separates from behind forward from the floor of the
groove and sinks into the subjacent connective tissue, retaining,
however, its connection with the epithelium at its anterior end,
which indicates the position of the opening of the duct. In the
vicinity of this there appear in embryos of 24.4 mm. five small
bud-like downgrowths of the epithelium (Fig. 177, SL), which later
increase considerably in number as well as in size, and constitute a
group of glands which are generally spoken of as the sublingual
gland.
 
As these representatives of the various glands increase in length,
 
 
 
ZAl\
 
 
 
 
Man.
 
 
 
Fig. 177. — Transverse Section of the Lower Jaw and Tongue of an Embryo
 
of about 20 mm.
D, Digastric muscle; GGl., genioglossus, GH.\ geniohyoid; T.Al, inferior alveolar
nerve; Man, mandible; MK, Meckel's cartilage; My, mylohyoid; SL, sublingual gland;
S.Mx, submaxillary duct; T, tongue.
 
they become lobed at their deeper ends, and the lobes later give
rise to secondary outgrowths which branch repeatedly, the terminal
branches becoming the alveoli of the glands. A lumen early appears in the duct portions of the structures, the alveoli remaining
solid for a longer time, although they eventually also become hollow.
 
It is to be noted that each parotid and submaxillary consists of a single
primary outgrowth, and is therefore a single structure and not a union of
a number of originally separate parts. The sublingual glands of adult
 
 
 
294 THE PHARYNX
 
anatomy are usually described as opening upon the floor of the mouth by
a number of separate ducts. This arises from the fact that the majority
of the glands which form in the vicinity of the opening of Wharton's
duct remain quite small, only one of them on each side giving rise to the
sublingual gland proper. The small glands have been termed the
alveolo-lingual glands, and each one of them is equivalent to a parotid or
submaxillary gland. In other words, there are in reality not three pairs
of salivary glands, but from fourteen to sixteen pairs, there being usually
from eleven to thirteen alveolo-lingual glands on each side.
 
The Development of the Pharynx. — The pharynx represents
the most anterior part of the archenteron, that portion in which the
branchial arches develop, and in the embryo it is relatively much
longer than in the adult, the diminution being brought about by
the folding in of the posterior arches and the formation of the sinus
prsecervicalis already described (p. 97). Between the various
branchial arches, grooves occur, representing the endodermal
portions of the grooves which separate the arches. During development the first of these becomes converted into the tympanic cavity
of the ear and the Eustachian tube (see Chapter XV) ; the second
disappears in its upper part, the lower persisting as the fossa in
which the tonsil is situated; while the lower parts of the remaining
two are represented by the sinus piriformis of the larynx (His), and
also leave traces of their existence in detached portions of their epithelium which form what are termed the branchial epithelial bodies ,
and take part in the formation of the thyreoid and thymus glands.
 
In the floor of the pharynx behind the thickenings which produce the tongue there is to be found in early stages a pair of thickenings passing horizontally backward and uniting in front so that
they resemble an inverted U (Fig. 178, /). These ridges, which
form what is termed the furcula (His), are concerned in the formation of parts of the larynx (see p. 334). In the part of the roof of
the pharynx which comes to lie between the openings of the Eustachian tubes, a collection cf lymphatic tissue takes place beneath
the mucous membrane, forming the pharyngeal tonsil, and immediately behind this there is formed in the median line an upwardly
projecting pouch, the pharyngeal bursa, first certainly noticeable
in embryos 6.5 mm. in length.
 
 
 
THE BRANCHIAL EPITHELIAL BODIES
 
 
 
295
 
 
 
This bursa has very generally been regarded as the persistent remains
of Rathke's pouch (p. 285), especially since it is much more pronounced
in fetal than in adult life. It has been shown, however, that it is formed
quite independently of and posterior to the true Rathke's pouch (Killian),
though what its significance may be is still uncertain.
 
The tonsils are formed from the epithelium of the second branchial groove. At about the fourth month solid buds begin to grow
from the epithelium into the subjacent mesenchyme, and depressions
appear on the surface of this region. Later the buds become hollow
by a cornification of their central cells, and open upon the floor of
the depressions which represent the
crypts of the tonsil. In the meantime
lymphocytes, concerning whose origin
there is a difference of opinion, collect in
the subjacent mesenchyme and eventually aggregate to form lymphatic follicles
in close relation with the buds. Whether
the lymphocytes wander out from the
blood into the mesenchyme or are derived
directly from the epithelium or the mesenchyme cells is the question at issue.
 
The tonsil may grow to a size sufficient
to fill up completely the groove in which
it forms, but not infrequently a marked
depression, the fossa supratonsillaris, exists above it and represents
a portion of the original second branchial furrow.
 
The groove of Rosenmuller, which was at one time thought to be
also a remnant of the second furrow, is a secondary depression
which appears in embryos of 11.5 cm. behind the opening of the
Eustachian tube, in about the region of the third branchial furrow.
 
The Development of the Branchial Epithelial Bodies. — These are
structures which arise either as thickenings or as outpouchings of
the epithelium lining the lower portions of the inner branchial furrows. Five pairs of these structures are developed and, in addition,
there is a single unpaired median body. This last makes its appearance in embryos of about 3 mm., and gives rise to the major por
 
 
 
Fig. 178. — The Floor of
the Pharynx of an Embryo
of 2.15 MM.
 
/. Furcula; t, tuberculum impair. — (His.)
 
 
 
296
 
 
 
THE BRANCHIAL EPITHELIAL BODIES
 
 
 
tion of the thyreoid body. It is situated immediately behind the
anterior portion of the tongue, at the apex of the groove between
this and the posterior portion, and is first a slight pouch -like depression. As it deepens, its extremity becomes bilobed, and after the
embryo has reached a length of 6 mm. it becomes completely separated from the floor of the pharynx. The point of its original
origin is, however, permanently marked by a circular depression,
the foramen cacum (Fig. 175, fc). Later the bilobed body migrates
down the neck and becomes a solid transversely elongated mass
(Fig. 179, th), into the substance of which trabecule of connective
tissue extend, dividing it into a network of anastomosing cords which
 
 
 
 
Fig. 179. — Reconstructions of the Branchial Epithelial Bodies of Embryos.
 
of (a) 14 mm. and (b) 26 mm.
 
ao, Aorta; Ith, lateral thyreoid; ph, pharynx; pth 1 and pth 2 , parathyreoids; th, thyreoid;
 
thy, thymus; vc, vena cava superior. — (Tourneux and Verdun.)
 
later divide transversely to form follicles. When the embryo has
reached a length of 2.6 cm., a cylindrical outgrowth arises from the
anterior surface of the mass, usually a little to the left of the median
line, and extends up the neck a varying distance, forming, when it
persists until adult life, the so-called pyramid of the thyreoid body.
 
This account of the pyramid follows the statements made by recent
workers on the question (Tourneux and Verdun) ; His has claimed that
it is the remains of the stalk connecting the thyreoid with the floor of the
pharynx, and which he terms the thyreo- glossal duct.
 
Two other pairs of bodies enter into intimate relations with the
 
 
 
THE BRANCHIAL EPITHELIAL BODIES
 
 
 
297
 
 
 
thm IV
 
 
 
thyreoid, forming what have been termed the parathyreoid bodies
(Fig. 179, pth 1 and pth 2 ). One of these pairs arises as a thickening
of the dorsal portion of the fourth branchial groove and the other
comes from the corresponding
portion of the third groove.
The members of the former
pair, after separating from their pthm IV
points of origin, come to lie on
 
the dorsal surface of the lateral sd ^v^B — pthm ill
 
portions of the thyreoid body
(Fig. 180, pthm IV) in close
proximity to the lateral thyreoids, while those of the other
pair, passing further backward,
come to rest behind the lower
border of the thyreoid (Fig. 180,
pthm III). The cells of these
bodies do not become divided
into cords by the ingrowth of
connective tissue to the same
extent as those of the thyreoids,
nor do they become separated
into follicles, so that the bodies
are readily distinguishable by
their structure from the thyreoid.
 
From the ventral portion of
the third branchial groove a
pair of evaginations develop,
similar to those which produce
the lateral thyreoids. These elongate greatly, and growing downward ventrally to the thyreoid and separating from their points of origin, come to lie below the thyreoids, forming the thymus gland (Fig.
179, thy). As development proceeds they pass further backward
and come eventually to rest upon the anterior surface of the peri
 
 
 
thm HI
 
 
 
Fig. 180. — Thyreoid, Tyhmtjs and
Epithelial Bodies of a New-born
Child.
 
pthm 111 and pthm IV, Para thyreoids;
sd, thyreoid; thm III, thymus; thm 7T",
lateral thyreoid. — (Groschuff.)
 
 
 
298
 
 
 
THE BRANCHIAL EPITHELIAL BODIES
 
 
 
cardium. The cavity which they at first contain is early obliterated
and the glands assume a lobed appearance and become traversed by
trabecular of connective tissue. Lymphocytes, derived, according
to some recent observations, directly from the epithelium of the
glands, make their appearance and gradually increase in number
until the original epithelial cells are represented only by a number
of peculiar spherical structures, consisting of cells arranged in concentric layers and known as Hassall's corpuscles.
 
The glands increase in size until about the fifteenth year, after
 
 
 
 
Fig. 181. — Diagram showing the Origin of the Various Branchial Epithelial
 
Bodies.
 
Ith, Lateral thyreoids; pp, ultimobranchial bodies; pht 1 and phi 2 , parathyreoids; th,
median thyreoid; thy, thymus; I to IV, branchial grooves. — (Kohn.)
 
which they gradually undergo degeneration into a mass of fibrous
and adipose tissue.
 
A pair of evaginations very similar to those that give rise to the
thymus are also formed from the ventral portion of the fourth
branchial groove (Figs. 179, A and 181, lih). As a rule they completely disappear in later stages of development, but occasionally
 
 
 
THE (ESOPHAGUS 299
 
they undergo differentiation into small masses of thymus-like tissue,
which remain associated with the parathyreoids from the same arch
(Fig. 180, thm IV). They have been termed lateral thyreoids, but
the term is a misnomer, since they take no essential part in the formation of the thyreoid body.
 
Finally, a pair of outgrowths arise from the floor of the pharynx
just behind the fifth branchial arch, in the region where the fifth
groove, if developed, would occur. These ultimo-branchial bodies,
as they have been called, usually undergo degeneration at an early
stage and disappear completely, though occasionally they persist
as cystic structures embedded in the substance of the thyreoid.
 
The relation of these various structures to the branchial grooves is
shown by the annexed diagram (Fig. 181), and from it, it will be seen
that the bodies derived from the third and fourth grooves are serially
equivalent. Comparative embryology makes this fact still more evident,
since, in the lower vertebrates, each branchial groove contributes to the
formation of the thymus gland. The terminology used above for the
various bodies is that generally applied to the mammalian organs, but it
would be better, for the sake of comparison with other vertebrates, to
adopt the nomenclature proposed by Groschuff, who terms each lateral
thyreoid a thymus IV, while each thymus lobe is a thymus III. Similarly
the parathyreoids are termed parathymus III and IV, the term thyreoid
being limited to the median thyreoid.
 
The Musculature of the Pharynx. — The pharynx differs from
other portions of the archenteron in the fact that its walls are furnished with voluntary muscles, the principal of which are the constrictors and the stylo-pharyngeus. This peculiarity arises from
the relations of the pharynx to the branchial arches. It has been
seen that in the higher mammalia the dorsal ends of the third,
fourth, and fifth branchial cartilages disappear; the muscles originally associated with these structures persist, however, and give rise
to the muscles of the pharynx, which consequently are innervated
by the ninth and tenth nerves.
 
The Development of the (Esophagus. — From the ventral
side of the lower portion of the pharynx an evagination develops
at an early stage which is destined to give rise to the organs of
 
 
 
3°°
 
 
 
THE STOMACH
 
 
 
respiration; the development of this may, however, be convenientlypostponed to a later chapter (Chapter XII) .
 
The oesophagus is at first a very short portion of the archenteron
(Fig. 182, A), but as the heart and diaphragm recede into the
thorax, it elongates (Fig. 182, B) until it eventually forms a considerable portion of the digestive tract. Its endodermal lining, like that
of the rest of the digestive tract except the pharynx, is surrounded
 
 
 
 
Fig. 182. — Reconstructions of the Digestive Tract of Embryos of (^4) 4.2 mm.
 
and (2?) 5 MM.
 
all, Allantois; cl, cloaca; I, lung; li, liver; Rp, Rathke's pouch; 5, stomach; t, tongue; th>
 
thyreoid body; Wd, Wolffian duct; y, yolk-stalk. — (His.)
 
by splanchnic mesoderm whose cells become converted into nonstriated muscular tissue, which, by the fourth month, has separated
into an inner circular and an outer longitudinal layer.
 
The Development of the Stomach and Intestines. — By the
time the embryo has reached a length of about 5 mm. its constriction
 
 
 
THE INTESTINE 30I
 
from the yolk-sac has proceeded so far that a portion of the digestive
tract anterior to the yolk-sac can be recognized as the stomach and
a portion posterior as the intestine. As first the stomach is a simple,
spindle-shaped enlargement (Fig. 182) and the intestine a tube
without any coils or bends, but since in later stages the intestine
grows much more rapidly in length than the abdominal cavity, a
coiling of the intestine becomes necessary.
 
The elongation of the stomach early produces changes in its
position, its lower end bending over toward the right, while its upper
end, owing to the development of the liver, is forced somewhat
toward the left. At the same time the entire organ undergoes a
rotation about its longitudinal axis through nearly ninety degrees,
so that, as the result of the combination of these two changes, what
was originally its ventral border becomes its lesser curvature and
what was originally its left surface becomes its ventral surface.
 
Hence it is that the left vagus nerve passes over the ventral and
the right over the dorsal surface of the stomach in the adult.
 
In the meantime the elongation of the oesophagus has carried
the stomach further away from the lower end of the pharynx, and
from being spindle-shaped it has become more pyriform, as in the
adult. The fundus, it may be noted, is not due to a general enlargement of the organ but to a local outpouching of the upper
dorsal portion of its wall.
 
The growth of the intestine results in its being thrown into a loop
opposite the point where the yolk-stalk is still connected with it,
the loop projecting ventrally into the portion of the ccelomic cavity
which is contained within the umbilical cord, and being placed so
that its upper limb lies to the right of the lower one. Upon the latter
a slight pouch-like lateral outgrowth appears which is the beginning
of the cacum and marks the line of union of the future small and large
intestine. The small intestine, continuing to lengthen more rapidly
than the large, assumes a sinuous course (Fig. 183), in which it is
possible to recognize six primary coils which continue to be recognizable until advanced stages of development and even in the adult
(Mall). The first of these is at first indistinguishable from the
 
 
 
302
 
 
 
THE INTESTINE
 
 
 
pyloric portion of the stomach and can be recognized as the duodenum only by the fact that it has connected with it the ducts of the
liver and pancreas; as development proceeds, however, its caliber
diminishes and it assumes the appearance of a portion of the
intestine.
 
The remaining coils elongate rapidly and are thrown into
numerous secondary coils, all of which are still contained within the
 
 
 
 
Fig. 183.— Reconstruction of Embryo of 20 mm.
C, Caecum; K, kidney; L, liver; S, stomach; SC, suprarenal bodies; W, mesonephros. —
 
{Mall.)
 
ccelom of theumbilical cord (Fig. 184). When the embryo has
reached a length of about 40 mm. the coils rather suddenly return
to the abdominal cavity, and now the caecum is thrown over toward
the right, so that it comes to lie immediately beneath the liver on the
right side of the abdominal cavity, a position which it retains until
about the fourth month after birth (Treves). The portion of the
large intestine which formerly projected into the umbilical ccelom now
 
 
 
THE INTESTINE 303
 
lies transversely across the upper part of the abdomen, crossing in
front of the duodenum and having the remaining portion of the small
intestine below it. The elongation continuing, the secondary coils
of the small intestine become more numerous and the lower portion
of the large intestine is thrown into a loop which extends transversely across the lower part of the abdominal cavity and represents
the sigmoid flexure of the colon. At the time of birth this portion
of the large intestine is relatively much longer than in the adult,
amounting to nearly half the entire length of the colon (Treves),
but after the fourth month after birth a readjustment of the relative
 
 
 
 
 
Fig. 184. — Reconstruction of the Intestine of an Embryo of 19 mm. The
Figures on the Intestine Indicate the Primary Coils. — {Mall.)
 
lengths of the parts of the colon occurs, the sigmoid flexure becoming
shorter and the rest of the colon proportionally longer, whereby the
caecum is pushed downward until it lies in the right iliac fossa, the
ascending colon being thus established.
 
When this condition has been reached, the duodenum, after
passing downward for a short distance so as to pass dorsally to the
transverse colon, bends toward the left and the secondary coils
derived from the second and third primary coils come to occupy
the left upper portion of the abdominal cavity. Those from the
fourth primary coil pass across the middle line and occupy the right
 
 
 
3°4
 
 
 
THE INTESTINE
 
 
 
upper part of the abdomen, those from the fifth cross back again to
the left lumbar and iliac regions, and those of the sixth take possession of the false pelvis and the right iliac region (Fig. 185).
 
Slight variations from this arrangement are not infrequent, but it
occurs with sufficient frequency to be regarded as the normal. A failure
 
 
 
 
Fig. 185. — Representation of the Coilings of the Intestine in the Adult
Condition. The Numbers indicate the Primary Coils. — (Mall.)
 
in the readjustment of the relative lengths of the different parts of the
colon may also occasionally occur, in which case the caecum will retain its
embryonic position beneath the liver.
 
The yolk-stalk is continuous with the intestine at the extremity
of the loop which extends out into the umbilical coelom, and when the
 
 
 
THE INTESTINE 305
 
primary coils become apparent its point of attachment lies in the
region of the sixth coil. As a rule, the caliber of the stalk does not
increase proportionally with that of the intestine, and eventually
its embryomic portion disappears completely. Occasionally, however, this portion of it does partake of the increase in size which
occurs in the intestine, and it forms a blind pouch of varying length,
known as Meckel's diverticulum (see p. 113).
 
The ccecum has been seen to arise as a lateral outgrowth at a
time when the intestine is first drawn out into the umbilicus. During
subsequent development it continues to increase in size until it forms a conical pouch
arising from the colon just where it is joined
by the small intestine (Fig. 186). The enlargement of its terminal portion does not keep
pace, however, with that of the portion nearest the intestine, but it becomes gradually
more and more marked off from it by its lesser
caliber and gives rise to the vermiform appendix. At birth the original conical form Fig. 186.— caecum of
of the entire outgrowth is still quite evident, B „ . *°' 3
 
^ c, Colon; 1, ileum.
 
though it is more properly described as funnelshaped, but later the proximal part, continuing to increase in diameter at the same rate as the colon, becomes sharply separated from
the appendix, forming the caecum of adult anatomy.
 
Up to the time when the embryo has reached a length of 14 mm.,
the inner surface of the intestine is quite smooth, but when a length
of 19 mm. has been reached, the mucous membrane of the upper
portion becomes thrown into longitudinal folds, and later these make
their appearance throughout its entire length (Fig. 187). Later, in
embryos of 60 mm., these folds break up into numbers of conical
processes, the villi, which increase in number with the development
of the intestine, the new villi appearing in the intervals between those
already present. Villi are formed as well in the large as in the small
intestine, but in the former they decrease in size as development
proceeds and practically disappear toward the end of fetal life.
 
 
 
 
3° 6
 
 
 
THE LIVER
 
 
 
In the early stages the endodermal lining of the digestive tract assumes
a considerable thickness, the lumen of the oesophagus and upper part of
the small intestine being reduced to a very small caliber. In later stages
a rapid increase in the size of the lumen occurs, apparently associated
with the formation of cavities or vacuoles in the endodermal epithelium.
These increase in size, the neighboring cells arrange themselves in an
epithelial layer around their walls and they eventually break through into
the general lumen. They are sometimes sufficiently large to give the
appearance of diverticula of the gut, but later they flatten out, their
cavities becoming portions of the general lumen.
 
In the case of the duodenum the thickening of the endodermal
lining proceeds to such an, extent that in embryos of from 12.5 mm. to
14.5 mm. the lumen is completely obliterated immediately below the
opening of the hepatic and pancreatic ducts. This condition is interesting
in connection with the occasional occurrence in new-born children of an
atresia of the duodenum. Under normal conditions, however, the lumen
is restored by the process of vacuolization described above.
 
 
 
 
Fig. 187. — Reconstruction of a Portion of the Intestine of an Embryo of 28
 
mm. showing the longitudinal folds from which the villi are formed.
 
{Berry.)
 
The Development of the Liver. — The liver makes its appearance in embryos of about 3 mm. as a longitudinal groove upon the
ventral surface of the archenteron just below the stomach and
between it and the umbilicus. The endodermal cells lining the
anterior portion of the groove early undergo a rapid proliferation,
and form a solid mass which projects ventrally into the substance
 
 
 
THE LIVER
 
 
 
3°7
 
 
 
of a horizontal shelf, the septum transversum (see p. 318), attached
to the ventral wall of the body. This solid mass (Fig. 188, L)
forms the beginning of the liver proper, while the lower portion of
the groove, which remains hollow, represents the future gall-bladder
(Fig. 188, B). Constrictions appearing between the intestine and
both the hepatic and cystic portions of the organ gradually separate
these from the intestine, until they are united to it only by a stalk
which represents the ductus choledochus (Fig. 188).
 
The further development of the liver, so far as its external
 
 
 
. SS 2
 
 
 
 
£---'*
 
 
 
 
' r
 
 
 
Fig. 188. — Reconstruction of the Liver Outgrowths of Rabbit Embryos of
 
(a) 5 mm. and (b) of 8 mm.
 
B, Gall-bladder; d, duodenum; DV, ductus venosus;L, liver; p, dorsal pancreas; pm,
 
ventral pancreas; rL, right lobe of the liver; S, stomach. — (Hammar.)
 
 
 
form is concerned, consists in the rapid enlargement of the hepatic
portion until it occupies the greater part of the upper half of the
abdominal cavity, its ventral edge extending as far down as the
umbilicus. In the rabbit its substance becomes divided into four
lobes corresponding to the four veins, umbilical and vitelline, which
traverse it, and the same condition occurs in the human embryo,
although the lobes are not so clearly indicated upon' the surface as in
the rabbit. The two vitelline lobes are in close apposition and may
 
 
 
3 o8
 
 
 
THE LIVER
 
 
 
almost be regarded as one, a median ventral lobe which embraces
the ductus venosus (Fig. 188, B, DV), while the umbilical lobes are
more lateral and dorsal and represent the right (rL) and left lobes
of the adult liver. The remaining definite lobes, the caudate
(Spigelian) and quadrate, are of later formation, standing in relation
to the vessels which cross the lower surface of the liver.
 
The ductus choledochus is at first wide and short, and near its
proximal end gives rise to a small outgrowth on each side, one of
which becomes the ventral pancreas (Fig. 188, B, pm). Later the
duct elongates and becomes more slender, and the gall-bladder is
 
 
 
 
Fig. 189. — Transverse Section through the Liver oe an Embryo of Four
 
Months.
in, Intestine; I, liver; W, Wolffian body. — {Toldt and Zuckerkandl.)
 
constricted off from it, the connecting stalk becoming the cystic
duct. The hepatic ducts are apparently developed from the liver
substance and are relatively late in appearing.
 
Shortly after the hepatic portion has been differentiated its substance becomes permeated by numerous blood-vessels (sinusoids)
and so divided into anastomosing trabeculae (Fig. 189). These are
at first irregular in size and shape, but later they become more slender
and more regularly cylindrical, forming what have been termed the
 
 
 
THE LIVER
 
 
 
3°9
 
 
 
hepatic cylinders. In the center of each cylinder, where the cells
which form it meet together, a fine canal appears, the beginning of
a bile capillary, the cylinders thus becoming converted into tubes
with fine lumina. This occurs at about the fourth week of development and at this time a cross-section of a cylinder shows it to be
composed of about three or four hepatic cells (Fig. 190, A), among
which are to be seen groups of smaller cells (e) which are erythrocytes, the liver having assumed by this time its haematopoietic function (see p. 225). This condition of affairs persists until birth, but
 
 
 
 
Fig. 190.- — Transverse Sections of Portions of the Liver of (.4) a Fetus of Six
 
Months and (B) a Child of Four Years.
 
be, Bile capillary; e, erythrocyte; he, hepatic cylinder. — (Toldt and Zuckerkandl.)
 
 
 
later the cylinders undergo an elongation, the cells of which they are
composed slipping over one another apparently, so that the cylinders become thinner as well as longer and show for the most part
only two cells in a transverse section (Fig. 190, B); and in still later
periods the two cells, instead of lying opposite one another, may
alternate, so that the cylinders become even more slender.
 
The bile capillaries seem to make their appearance first in cylinders which lie in close relation to branches of the portal vein (Fig. 191) ,
 
 
 
3io
 
 
 
THE LIVER
 
 
 
and thence extend throughout the neighboring cylinders, anastomosing with capillaries developing in relation to neighboring portal
branches. As the extension so proceeds the older capillaries continue to enlarge and later become transformed into bile-ducts (Fig.
191, C), the cells of the cylinders in which these capillaries were
situated becoming converted into the epithelial lining of the ducts.
 
The lobules, which form so characteristic a feature of the adult
liver, are late in appearing, not being fully developed until some
time after birth. They depend upon the relative arrangement of
the branches of the portal and hepatic veins; these at first occupy
distinct territories of the liver substance, being separated from one
another by practically the entire thickness of the liver, although of
 
 
 
 
 
 
Fig. 191. — Injected Bile Capillaries of Pig Embryos of (A) 8 cm., (B) 16 cm., and
(C) of Adult Pig. — (Hendrickson.)
 
course connected by the sinusoidal capillaries which lie between the
hepatic cylinders. During development the two sets of branches
extend more deeply into the liver substance, each invading the
territory of the other, but they can readily be distinguished from one
another by the fact that the portal branches are enclosed within a
sheath of connective tissue (Glisson's capsule) which is lacking to
the hepatic vessels. At about the time of birth the branches of the
hepatic veins give off at intervals bunches of terminal vessels, around
which branches of the portal vein arrange themselves, the liver tissue
becoming divided up into a number of areas which may be termed
 
 
 
THE PANCREAS 3II
 
hepatic islands, each of which is surrounded by a number of portal
branches and contains numerous dichotomously branching hepatic
terminals. Later the portal branches sink into the substance of the
islands, which thus become lobed, and finally the sinking in extends
so far that the original island becomes separated into a number of
smaller areas or lobules, each containing, as a rule, a single hepatic
terminal (the intralobular vein) and being surrounded by a number
of portal terminals {interlobular veins) , the two systems being united
by the capillaries which separate the cylinders contained within the
area. The lobules are at first very small, but later they increase in
size by the extension of the hepatic cylinders.
 
Frequently in the human liver lobules are to be found containing two
intralobular veins, a condition with results from an imperfect subdivision
of a lobe of the original hepatic island.
 
The liver early assumes a relatively large size, its weight at one
time being equal to that of the rest of the body, and though in later
embryonic stages its relative size diminishes, yet at birth it is still a
voluminous organ, occupying the greater portion of the upper half of
the abdominal cavity and extending far over into the left hypochondrium. Just after birth there is, however, a cessation of
growth, and the subsequent increase proceeds at a much slower rate
than that of the rest of the body, so that its relative size bcomes
still more diminished (see Chap. XVII). The cessation of growth
affects principally the left lobe and is accompanied by an actual
degeneration of portions of the liver tissue, the cells disappearing
completely, while the ducts and blood-vessels originally present
persist, the former constituting the vasa aberrantia of adult anatomy.
These are usually especially noticeable at the left edge of the liver,
between the folds of the left lateral ligament, but they may also be
found along the line of the vena cava, around the gall-bladder, and
in the region of the left longitudinal fissure.
 
The Development of the Pancreas. — The pancreas arises a
little later than the liver, as two or three separate outgrowths, one
from the dorsal surface of the duodenum (Fig. 192, DP) usually a
little above the liver outgrowth, and one or two from the lower part
 
 
 
312
 
 
 
THE PANCREAS
 
 
 
of the common bile-duct. Of the latter outgrowths, that upon the
left side (Vps) may be wanting and, if formed, early disappears,
while that of the right side (Vpd) continues its development to form
what has been termed the ventral pancreas. Both this and the
dorsal pancreas continue to elongate, the latter lying to the left of
^^ the portal vein, while the former,
 
at first situated to the right of
the vein, later grows across its
ventral surface so as to come into
contact with the dorsal gland,
with which it fuses so intimately
that no separation line can be
distinguished. The body and
tail of the adult pancreas represent the original dorsal outgrowth, while the right ventral
pancreas becomes the head.
 
Both the dorsal and ventral
outgrowths early become lobed,
and the lobes becoming secondarily lobed and this lobation repeating itself several times, the
compound tubular structure of
the adult gland is acquired, the
very numerous terminal lobules
becoming the secreting acini,
while the remaining portions
become the ducts. Of the principal ducts, there are at first two;
that of the dorsal pancreas, the
duct of Santorini, opens into the duodenum on its dorsal surface,
while that of the ventral outgrowth, the duct of Wirsung, opens
into the ductus choledochus. When the fusion of the two portions
of the gland occurs, an anastomosis of branches of the two ducts
develops and the proximal portion of the duct of Santorini may
 
 
 
 
Fig. iq2. — Reconstruction of the
Pancreatic Outgrowths of an Embryo
of 7.5 MM.
 
D, Duodenum; Dc, ductus communis
choledochus; DP, dorsal pancreas; Vpd,
and Vps, right and left ventral pancreas.
—{Helly.)
 
 
 
LITERATURE 313
 
degenerate, so that the secretion of the entire gland empties into
the common bile-duct through the duct of Wirsung.
 
In the connective tissue which separates the lobules of the gland,
groups of cells occur, which have no connection with the ducts of
the gland, and form what are termed the areas ofLangerhans. They
arise by a differentiation of the cells which form the original pancreatic outgrowths, and have been distinguished in the dorsal pancreas
of the guinea-pig while it is still a solid outgrowth. They gradually
separate from the remaining cells of the outgrowth and come to lie
in the mesenchyme of the gland in groups into which, finally, bloodvessels penetrate.
 
LITERATURE.
 
E. T. Bell: "The Development of the Thymus," Amer. Journ. of Anat., v, 1906.
J. M. Berry: "On the Development of the Villi of the Human Intestine," Anat.
 
Anzeiger, xvi, 1900.
L. Bolk: "Die Entwicklungsgeschichte der menschlichen Lippen," Anat. Hefte.
 
xliv, 1908.
L. Bolk: "Ueber die Gaumenentwicklung und die Bedeutung der oberen Zahn
leiste beim Menschen," Zeit.fiir Morphol. und Anthropol., xiv, 191 1.
J. Bracket: "Recherches sur le developpement du pancreas et du foie," Journ. de
 
I' Anat. et de la Physiol., xxxii, 1896.
O. C. Bradley: "A Contribution to the Morphology and Development of the Mammalian Liver," Journ. Anat. and Physiol., xliii, 1908.
H. M. de Burlet: "Die ausseren Formverhaltnisse der Leber beim menschlichen
 
Embryo," Morphol. Jahrb., XLII, 1910.
R. V. Chamberldst: "On the Mode of Disappearance of the Villi from the Colon of
 
Mammals," Anat. Record, in, 1909.
J. H. Chievitz: "Beitrage zur Entwicklungsgeschichte der Speicheldrusen," Archiv
 
fiir Anat. und Physiol., Anat. Abth., 1885.
H. Fox: "The Pharyngeal Pouches and Their Derivatives in the Mammalia," Amer.
 
Journ. Anat., vin, 1908.
K. Groschtjff: " Ueber das Vorkommen eines Thymussegmentes der vierten Kiemen
tasche beim Menschen," Anat. Anzeiger, xvn, 1900.
O. Grosser: "Zur Kenntnis des ultimobranchialen Korpers beim Menschen," Anat.
 
Anzeiger, xxxvn, 19 10.
L. Grunwald: "Ein Beitrag zur Entstehung und Bedeutung der Gaumenmandeln,"
 
Anat. Anzeiger, xxxvn, 1910.
J. A. Hammar: "Einige Plattenmodelle zur Beleuchtung der friiheren embryonal
 
Leberentwicklung," Arch.f. Anat. undPhys., Anat. Abth., 1893.
 
 
 
314 LITERATURE
 
J. A. Hammar: "Notiz iiber die Entwicklung der Zunge und der Mundspeicheldrtisen
 
beim Menschen," Anat. Anzeiger, xix, 1901.
J. A. Hammar: "Studien iiber die Entwicklung des Vorderdarms und einiger angren
zender Organe," Arch./, mikrosk. Anat., lix and lx, 1902.
K. Helly: "Zur Entwickelungsgeschichte der Pancreasanlagen und Duodenalpapillen
 
des Menschen," Archivfiir mikrosk. Anat., lvi, 1900.
K. Helly: "Studien iiber Langerhanssche Inseln," Arch, filr mikrosk. Anat.,fXMU,
 
1907.
W. F. Hendrickson: "The Development of the Bile-capillaries as revealed by Golgi's
 
Method," Johns Hopkins Hospital Bulletin, 1898.
W. His: "Anatomie menschlicher Embryonen," Leipzig, 1882-1886.
F. Hochstetter: "Ueber die Bildung der primitiven Choanen beim Menschen,"
 
Anat. Anzeiger, vii, 1892.
N. W. Ingalls : " A Contribution to the Embryology of the Liver and Vascular System
 
in Man," Anat. Record, II, 1908.
C. M. Jackson: " On the Development and Topography of the Thoracic and Abdominal
 
Viscera," Anat. Record, in, 1909.
F. P. Johnson: "The Development of the Mucous Membrane of the (Esophagus,
 
Stomach and Small Intestine in the Human Embryo," Amer. J own. Anat., x,
 
1910.
 
E. Kallius: "Beitrage zur Entwicklung der Zunge, 3teTh. Saugetiere. I. Sus scrofa,"
 
Anat. Hefte, xli, 1910.
 
F. Keibel: "Zur Entwickelungsgeschichte des menschlichen Urogenital-apparatus,"
 
Archivfiir Anat. und Physiol., Anat. Abth., 1896.
 
G. Killian: "Ueber die Bursa und Tonsilla pharyngea," Morphol. Jahrbuch, xiv,
 
1888.
A. Kohn: "Die Epithelkorperchen," Ergebnisse der Anat. und Entwicklungsgesch., ix,
 
1899.
H. Kuster: "Zur Entwicklungsgeschichte der Langerhans'schen Inseln im Pancreas
 
beim menschlichen Embryo," Arch, filr mikrosk., Anat., lxiv, 1904.
F. T. Lewis and F. W. Thyng: "The Regular Occurrence of Intestinal Diverticula
 
in Embryos of the Pig, Rabbit and Man," Amer. Journ. Anat., vii, 1908.
F. P. Mall: "Ueber die Entwickelung des menschlichen Darmes und seiner Lage
 
beim Erwachsenen," Archivfiir Anat. und Physiol., Anat. Abth., Supplement, 1897.
F. P. Mall: "A Study of the Structural Unit of the Liver," Amer. Journ. of Anat., V,
 
1906.
R. Mayer: " Ueber die Bildung des Recessus pharyngeus medius s. Bursa pharyngis in
 
zusammenhang mit der Chorda bei menschlichen Embryonen," Anat. Anzeiger,
 
xxxvii, 1910.
J. F. Meckel: " Bildungsgeschichte des Darmkanals der Saugethiere und namentlich
 
des Menschen," Archivfiir Anat. und Physiol., in, 1817.
T. Mironescu: " Ueber die Entwicklung der Langerhans' schen Inseln bei menschlichen Embryonen," Arch, fur mikrosk. Anat., lxxvi, 1911.
W. J. Otis: " Die Morphogenese und Histogenese des Analhockers nebst Bemerkungen
 
iiber die Entwicklung der Sphincter ani externus beim Menschen," Anat. Hefte,
 
xxx, 1906.
 
 
 
LITERATURE 315
 
R. M. Pearce: "The Development of the Islands of Langerhans in the Human
 
Embryo," Amer. Journ. of Anal., 11, 1902.
C. Rose: "Ueber die Entwicklung der Zahne des Menschen," Archiv fur mikrosk.
 
Anat., xxxviii, 1891.
G. Schorr: " Zur Entwickelungsgeschichte des secundaren Gaumens," Anat. Hefte,
 
xxxvi, 1908.
G. Schorr: "Ueber Wolfsrachen von Standpunkt der Embryologie und pathologischen
 
Anatomie," Arch, fur palholog. Anal., cxcvn, 1909.
A, Swaen: "Recherches sur le developement du foie, du tube digestif, de l'arriere
cavite du peritoine et du mesentere," Journ. de I' Anal, et de la Physiol., xxxii,
 
1896, and xxxiii, 1897.
J. Tandler: "Zur Entwickelungsgeschichte des menschlichen Duodenum in frtihen
 
Embryonalstadien," Morphol. Jahrbuch, xxix, 1900.
P. Thompson: "A Note on the Development of the Septum Transversum and the
 
Liver," Journ. Anat. and Phys., xlii, 1908.
F. W. Thyng: "Models of the Pancreas in Embryos of the Pig, Rabbit, Cat and
 
Man," Amer, Journ. Anat., vn, 1908.
C. Toldt and E. Zuckerkandl: "Ueber die Form und Texturveranderungen der
 
menschlichen Leber wahrend des Wachsthums," Sitzungsber. der kais. Akad.
 
Wissensch. Wien., M ath.-N aturwiss . Classe, lxxii, 1875.
F. Tourneux and P. Verdun: "Sur les premiers developpements de la Thyroide, du
 
Thymus et des glandes parathyroidiennes chez l'homme," Journ. de I' Anat. et
 
de la Physiol., xxxiii, 1897.
F. Treves: "Lectures on the Anatomy of the Intestinal Canal and Peritoneum in
 
Man," British Medical Journal, 1, 1885.
 
 
 
CHAPTER XI
 
THE DEVELOPMENT OF THE PERICARDIUM, THE
PLEURO-PERITONEUM AND THE DIAPHRAGM.
 
It has been seen (p. 229) that the heart makes its appearance at
a stage when the greater portion of the ventral surface of the intestine is still open to the yolk-sac. "The ventral mesoderm splits to
form the somatic and splanchnic layers and the heart develops as a
fold in the latter on each side of the median line, projecting into the
ccelomic cavity enclosed by the two layers (Fig. 136, A). As the
constriction of the anterior part of the embryo proceeds the two
heart folds are brought nearer together and later meet, so that the
heart becomes a cylindrical structure lying in the median line of the
body and is suspended in the ccelom by a ventral band, the ventral
tnesocardium, composed of two layers of splanchnic mesoderm
which extend to it from the ventral wall of the body, and by a
similar band, the dorsal tnesocardium, which unites it with the
splanchnic mesoderm surrounding the digestive tract. The ventral mesocardium soon disappears (Fig. 136 C) and the dorsal one
also vanishes somewhat later, so that the heart comes to lie freely
in the ccelomic cavity, except for the connections which it makes
with the body-walls by the vessels which enter and arise from it.
 
The ccelomic cavity of the embryo does not at first communicate
with the extra-embryonic ccelom, which is formed at a very early
period (see p. 67), but later when the splitting of the embryonic
mesoderm takes place the two cavities become continuous behind
the heart, but not anteriorly, since the ventral wall of the body is
formed in the heart region before the union can take place. It is
possible, therefore, to recognize two portions in the embryonic
ccelom, an anterior one, the parietal cavity (His), which is never
connected laterally with the extra-embryonic cavity, and a posterior
one, the trunk cavity, which is so connected.^The heart is situated
 
316
 
 
 
THE PERICARDIUM AND PLEURO-PERITONEUM
 
 
 
3*7
 
 
 
in the parietal cavity, a considerable portion of which is destined to
become the pericardial cavity.
 
Since the parietal cavity lies immediately anterior to the still
wide yolk-stalk, as may be seen from the position of the heart in the
embryo shown in Fig. 53, it is bounded
posteriorly by the yolkstalk. This
boundary is complete, however, only
in the median line, the cavity being
continuous on either side of the yolkstalk with the trunk-cavity by passages which have been termed the
recessus parietales (Fig. 193, Bp and
Rca). Passing forward toward the
heart in the splanchnic mesoderm
which surrounds the yolkstalk are the
large vitelline veins, one on either side,
and these shortly become so large as
to bring the splanchnic mesoderm in
which they lie in contact with the somatic mesoderm which forms the lateral wall of each recess. Fusion of
the two layers of mesoderm along the
course of the veins now takes place,
and each recess thus becomes divided
into two parallel passages, which have
been termed the dorsal (Fig. 194, rpd)
and ventral irpv) parietal recesses.
Later the two veins fuse in the upper
portion of their course to form the beginning of the sinus venosus, with the result that the ventral recesses become closed below and their continuity with the trunkcavity is interrupted, so that they form two blind pouches extending
downward a short distance from the ventral portion of the floor of
the parietal cavity. The dorsal recesses, however, retain their
continuity with the trunk-cavity until a much later period.
 
 
 
 
Om
 
 
 
Rca
 
 
 
Fig. 1 93 . — Reconstruction
of a Rabbit Embryo of Eight
Days, with the Pericardial
Cavity Laid Open.
 
A, Auricle; Aob, aortic bulb;
A. V., atrio- ventricular communication; Bp, ventral parietal recess; Om, vitelline vein; Pc, pericardial cavity; Rca, dorsal parietal recess; Sv, sinus venosus; V,
ventricle. — (His.)
 
 
 
3^
 
 
 
THE PERICARDIUM AND PLEURO-PERITONEUM
 
 
 
By the fusion of the vitelline veins mentioned above, there is
formed a thick semilunar fold which projects horizontally into the
ccelom from the ventral wall of the body and forms the floor of the
ventral part of the parietal recess. This is known as the septum
transversum, and besides containing the anterior portions of the
vitelline veins, it also furnishes a passage by which the ductus
Cuvieri, formed by the union of the jugular and cardinal veins,
reach the heart. Its dorsal edge is continuous in the median line
with the mesoderm surrounding the digestive tract just opposite
the region where the liver outgrowth will form, but laterally this
edge is free and forms the ventral walls of the dorsal parietal recess.
An idea of the relations of the septum at this stage may be obtained
 
 
 
 
 
V0771
 
 
 
rpv
 
 
 
Fig. 194. — Transverse Sections of a Rabbit Embryo showing the Division of
 
the Parietal Recesses by the Vitelline Veins.
 
am, Amnion; rp, parietal recess; rpd and rpv, dorsal and ventral divisions of the parietal
 
recess; vom, vitelline vein. — (Ravn.)
 
from Fig 195, which represents the anterior surface of the septum,
together with the related parts, in a rabbit embryo of nine days.
 
The Separation of the Pericardial Cavity. — The septum transversum is at first almost horizontal, but later it becomes decidedly
oblique in position, a change associated with the backward movement of the heart. As the closure of the ventral wall of the body
extends posteriorly the ventral edge of the septum gradually slips
downward upon it, while the dorsal edge is held in its former position by its attachment to the wall of the digestive tract and the
ductus Cuvieri. The anterior surface of the septum thus comes to
 
 
 
THE PERICARDIUM AND PLEURO-PERITONEUM
 
 
 
3 J 9
 
 
 
look ventrally as well as forward, and the parietal cavity, having
taken up into itself the blind pouches which represented the ventral
recesses, comes to lie to a large extent ventral to the posterior recesses.
As may be seen from Fig. 195, the ductus Cuvieri, as they bend
from the lateral walls of the body into the free edges of the septum,
form a marked projection which diminishes considerably the opening of the dorsal recesses into the parietal cavity. In later stages
 
 
 
 
am
 
 
 
Fig. 195. — Reconstruction from a Rabbit Embryo of Nine Days showing the
 
Septum Transversum from Above.
 
am, Amnion; at, atrium; dc, ductus Cuvieri; rpd, dorsal parietal recess. — (Ravn.)
 
this projection increases and from its dorsal edge a fold, which
may be regarded as a continuation of the free edge of the septum,
projects into the upper portions of the recesses and eventually fuses
with the median portion of the septum attached to the wall of the gut.
In this way the parietal cavity becomes a completely closed sac, and
is henceforward known as the pericardial cavity, the original ccelom
 
 
 
3 2 °
 
 
 
THE DIAPHRAGM
 
 
 
being now divided into two portions, (i) the pericardial, and (2) the
pleuro -peritoneal cavities, the latter consisting of the abdominal
ccelom together with the two dorsal parietal recesses which have
been separated from the pericardial (parietal) cavity and are destined to be converted into the pleural cavities.
 
The Formation of the Diaphragm. — It is to be remembered that
the attachment of the transverse septum to the ventral wall of the
digestive tract is opposite the point where the liver outgrowth
develops. When, therefore, the outgrowth appears, it pushes its
 
 
 
 
 
 
Fig. 196, — Diagrams of (A) a Sagittal Section of an Embryo showing the
Liver Enclosed within the Septum Transversum; (B) a Frontal Section of the
Same; (C) a Frontal Section of a Later Stage when the Liver has Separated
from the Diaphragm.
 
All, Allantois; CI, cloaca; D, diaphragm ;Li, liver;Ls, falciform ligament of the liver;
M, mesentery; Mg, mesogastrium; Pc, pericardium; S, stomach;5T, septum transversum;
U, umbilicus.
 
way into the substance of the septum, which thus acquires a very
considerable thickness, especially toward its dorsal edge, and it
furthermore becomes differentiated into two layers, an upper one,
which forms the floor of the ventral portion of the pericardial cavity
and encloses the Cuvierian ducts, and a lower one which contains the
liver. The upper layer is comparatively thin, while the lower forms
the greater part of the thickness of the septum, its posterior surface
meeting the ventral wall of the abdomen at the level of the anterior
margin of the umbilicus (Fig. 196, A).
 
 
 
THE DIAPHRAGM 32 1
 
In later stages of development the layer containing the liver
becomes separated from the upper layer by two grooves which,
appearing at the sides and ventrally immediately over the liver
(Fig. 196, B), gradually deepen toward the median line and dorsally.
These grooves do not, however, quite reach the median line, a portion of the lower layer of the septum being left in this region as a
fold, situated in the sagittal plane of the body and attached above
to the posterior surface of the upper layer and below to the anterior
surface of the liver, beyond which it is continued down the ventral
wall of the abdomen to the umbilicus (Fig. 196, C,Ls). This is the
falciform ligament of the liver of adult anatomy, and in the free
edge of its prolongation down the ventral wall of the abdomen the
umbilical vein passes to the under surface of the liver, while the free
edge of that portion which lies between the liver and the digestive
tract contains the vitelline (portal) vein, the common bile-duct, and
the hepatic artery. The diagram given in Fig. 196 will, it is hoped,
make clear the mode of formation and the relation of this fold,
which, in its entirety, constitutes what is sometimes termed the
ventral mesentery.
 
And not only do the grooves fail to unite in the median line, but
they also fail to completely separate the liver from the upper layer
of the septum dorsally, the portion of the lower layer which persists
in this region forming the coronary ligament of the liver. The
portion of the lower layer which forms the roof of the grooves becomes the layer of peritoneum covering the posterior surface of the
upper layer (which represents the diaphragm), while the portion
which remains connected with the liver constitutes its peritoneal
investment.
 
I In the meantime changes have been taking place in the upper
layer of the septum. As the rotation of the heart occurs, so that its
atrial portion comes to lie anterior to the ventricle, the Cuvierian
ducts are drawn away from the septum and penetrate the posterior
wall of the pericardium, the separation being assisted by the continued descent of the attachment of the edge of the septum to the
ventral wall of the body. During the descent, when the upper
 
 
 
3 22
 
 
 
THE PLEURAE
 
 
 
layer of the septum has reached the level of the fourth cervical segment, portions of the myotomes of that segment become prolonged
into it and the layer assumes the characteristics of the diaphragm,
the supply of whose musculature from the fourth cervical nerves is
thus explained.
 
The Pleurce. — The diaphragm is as yet, however, incomplete
dors ally, where the dorsal parietal recesses are still in continuity with
the trunk-cavity. With the increase in thickness of the septum
transversum, these recesses have acquired a considerable length
antero-posteriorly, and into their upper portions the outgrowths
from the lower part of the pharynx which form the lungs (see page
331) begin to project. The recesses thus become transformed
into the pleural cavities, and as the diaphragm continues to descend,
slipping down the ventral wall of the body and drawing with it the
pericardial cavity, the latter comes to lie entirely ventral to the pleural
cavities. The free borders of the diaphragm, which now form the
ventral boundaries of the openings by which the pleural and peritoneal cavities communicate, begin to approach the dorsal wall of
the body, with which they finally unite and so complete the separation of the cavities. The pleural cavities continue to enlarge after
their separation and, extending laterally, pass between the pericardium and the lateral walls of the body until they finally almost
completely surround the pericardium. The intervals between the
two pleurae form what are termed the mediastina.
 
The downward movement of the septum transversum extends
through a very considerable interval, which may be appreciated
from the diagram shown in Fig. 197. From this it may be seen
that in early embryos the septum is situated just in front of the first
cervical segment and that it lies very obliquely, its free edge being
decidedly posterior to its ventral attachment. When the downward
displacement occurs, the ventral edge at first moves more rapidly
than the dorsal, and soon comes to lie at a much lower level. The
backward movement continues throughout the entire length of the
cervical and thoracic regions, and when the level of the tenth thoracic segment is reached the separation of the pleural and peritoneal
 
 
 
THE PERITONEUM
 
 
 
3 2 3
 
 
 
'atxJidbJb
 
 
 
1 Cuw%ea£
 
 
 
1 SaUai
 
 
 
cavities is completed, and then the dorsal edge begins to descend
more rapidly than the ventral, so that the diaphragm again becomes
oblique in the same sense as in the beginning, a position which it
retains in the adult.
 
The Development of the Peritoneum. — The peritoneal cavity is
developed from the trunk-cavity of early stages and is at first in free
communication on all sides of theyolk-stalk with the extra-embryonic
ccelom. As the ventral wall of the
body develops the two cavities become
more and more separated, and with
the formation of the umbilical cord
the separation is complete. Along
the middorsal line of the body the
archenteron forms a projection into
the cavity and later moves further out
from the body-wall into the cavity,
pushing in front of it the peritoneum,
which thus comes to surround the intestine, forming its serous coat, and
from it is continued back to the dorsal
body- wall forming the mesentery.
 
It has already been seen that on
the separation of the liver from the
septum transversum, the tissue of the
latter gives rise to the peritoneal
covering of the liver and of the posterior surface of the diaphragm, and also to the ventral mesentery.
When the separation is taking place, the rotation of the stomach already described (p. 301) occurs, with the result that the portion of the
ventral mesentery which stretches between the lesser curvature of the
stomach and the liver shares in the rotation and comes to lie in a plane
practically at right angles with that of the suspensory ligament, its surfaces looking dorsally and ventrally and its free edge being directed
toward the right. This portion of the ventral mesentery forms
 
 
 
 
Fig. 197.— Diagram showing
the Position of the Diaphragm
in Embryos of Different Ages.
—{M all.)
 
 
 
324 THE PERITONEUM
 
what is termed the lesser omentum, and between it and the dorsal
surface of the stomach as the ventral boundaries, and the dorsal
wall of the abdominal cavity dorsally, there is a cavity, whose floor
is formed by the dorsal mesentery of the stomach, the mesogastrium,
the roof by the under surface of the left half of the liver, while to the
right it communicates with the general peritoneal cavity dorsal to
the free edge of the lesser omentum. This cavity is known as the
bursa omentalis (lesser sac of the peritoneum), and the opening into
it from the general cavity or greater sac is termed the epiploic foramen
(foramen of Winslow). Later, the floor of the lesser sac is drawn
downward to form a broad sheet of peritoneum lying ventral to the
coils of the small intestine and consisting of four layers; this represents the great omentum of adult anatomy (Fig. 201).
 
Although the form assumed by the bursa omentalis is associated
with the rotation of the stomach, it seems probable that its real
origin is independent of that process (Broman). The subserous
tissue of the transverse septum is at first thick and includes not only
the liver, but also the pancreas and the portion of the digestive tract
which becomes the stomach and the upper part of the duodenum
(Fig. 196, A). The shrinkage of this tissue by which these organs
become separated from the septum cannot take place evenly on
account of the relations which the organs bear to one another, so
that on the right side certain peritoneal recesses are formed, one
between the right lung and the stomach, a second between the liver
and the stomach, and a third between the pancreas and the same
structure. In man these three recesses communicate with one
another to form the primary bursa omentalis, and open by a common epiploic foramen into the general peritoneal cavity. The rotation of the stomach, which takes place later, merely serves to modify
the original bursa.
 
In the human embryo a small recess also forms upon the left side
between the left lung and the stomach. Later it separates from the rest
of the bursa omentalis and passes up along the side of the oesophagus,
coming to lie on its right side between it and the diaphragm. It gives rise
to a small serous sac that lies beneath the infracardial lobe of the right
 
 
 
THE PERITONEUM
 
 
 
3 2 5
 
 
 
lung, when this is present, and hence has been termed the infracardial
bursa.
 
Below the level of the upper part of the duodenum the ventral
mensentery is wanting; only the dorsal mesentery occurs. So long
as the intestine is a straight tube the length of the intestinal edge of
this mesentery is practically equal to that of its dorsal attached edge.
The intestine, however, increasing in length much more rapidly
than the abdominal walls, the intestinal edge of the mesentery soon
becomes very much longer than the attached edge, and when the intestine grows
out into the umbilical ccelom the mesentery
accompanies it (Fig. 198). As the coils of
the intestine develop, the intestinal edge of
the mesentery is thrown, into corresponding
folds, and on the return of the intestine to
the abdominal cavity the mesentery is
thrown into a somewhat funnel-like form
by the twisting of the intestine to form its
primary loop (Fig. 199). All that portion
of the mesentery which is attached to the
part of the intestine which will later become
the jejunum, ileum, ascending and transverse colon, is attached to the body-wall
at the apex of the funnel, at a point which bryo of Six Weeks.
lies to the left of the duodenum. S p%^-VoMn° m ^''
 
Up to this stage or to about the middle
of the fourth month the mesentery has retained its attachment to the
median line of the dorsal wall of the abdomen throughout its entire
length, but later fusions of certain portions occur, whereby the original condition is greatly modified. One of the earliest of these fusions
takes place at the apex of the funnel, where the portion of the mesentery which passes to the tranverse colon and arches over the duodenum fuses with the ventral surface of the latter portion of the
intestine and also with the peritoneum covering the dorsal wall of the
abdomen both to the right and to the left of the duodenum. In this
 
 
 
 
Fig. 198. — Diagram
showing the arrangement
of the Mesentery and Visceral Branches of the Abdominal Aorta in an Em
 
 
326
 
 
 
THE PERITONEUM
 
 
 
way the attachment of the transverse mesocolon takes the form of a
transverse line instead of a point, and this portion of the mesenterydivides the abdominal cavity into two portions, the upper (anterior)
of which contains the liver and stomach, while the lower contains
the remainder of the digestive tract with the exception of the duodenum. By passing across the ventral surface of the duodenum
and fusing with it, the transverse mesocolon forces that portion of
the intestine against the dorsal wall of the abdomen and fixes it in
that position, and its mesentery thereupon degenerates, becoming
 
 
 
 
 
md'
 
 
 
Fig. 199.
 
 
-Diagrams Illustrating the Development of the Great Omentum
 
and the Transverse Mesocolon.
 
bid, Caecum; dd, small intestine; dg, yolk-stalk; di, colon; du, duodenum; gc, greater
 
curvature of stomach; gg, bile duct; gn, mesogastrium; k, point where the loops of the
 
intestine cross; mc, mesocolon; md, rectum; mes, mesentery; wf, vermiform appendix.
 
— (Hertwig.)
 
subserous areolar tissue, the duodenum assuming the retroperitoneal position which characterizes it in the adult.
 
The descending colon, which on account of the width of its mesentery is at first freely movable, lies well over to the left side of the
abdominal cavity, and in consequence the left layer of its mesentery
lies in contact with the parietal layer of the peritoneum. A fusion
of these two layers, beginning near the middle line and thence extending outward, takes place, the fused layers becoming converted into
 
 
 
THE PERITONEUM
 
 
 
3 2 7
 
 
 
connective tissue, and this portion of the colon thus loses its mesentery and becomes fixed to the abdominal wall. The process by
which the fixation is accomplished may be understood from the
diagrams which constitute Fig. 200. When the ascending colon is
formed, its mesentery undergoes a similar fusion, and it also becomes
fixed to the abdominal wall.
 
The fusion of the mesentery of the ascending and descending colon
remains incomplete in a considerable number of cases (one-fourth to onethird of all cases examined), and in these the colons are not perfectly
fixed to the abdominal wall. It may also be pointed out that the caecum
and appendix, being primarily a lateral outpouching of the intestine, do
 
 
 
 
 
Fig. 200— Diagrams Illustrating the Manner in Which the Fixation of the
Descending Colon (C) takes Place.
 
not possess any true mesentery, but are completely enclosed by peritoneum.
Usually a falciform fold of peritoneum may be found extending along one
surface of the appendix to become continuous with the left layer of the
mesentery of the ileum. This, however, is not a true mesentery, and is
better spoken of as a mesenteriole.
 
One other fusion is still necessary before the adult condition of
the mesentery is acquired. The great omentum consists of two
folds of peritoneum which start from the greater curvature of the
stomach and pass downward to be reflected up again to the dorsal
wall of the abdomen, which they reach just anterior to (above) the
line of attachment of the transverse mesocolon (Fig. 201, A). At
 
 
 
328
 
 
 
THE PERITONEUM
 
 
 
first the attachment of the omentum is vertical, since it represents
the mesogastrium, but later, by fusion with the parietal peritoneum,
it assumes a transverse direction, while at the same time the pancreas,
which originally lay between the two folds of the mesogastrium, is
carried dorsally and comes to have a retroperitoneal position in the
line of attachment of the omentum. By this change the lower layer
of the omentum is brought in contact with the upper layer of the
 
 
 
 
 
Fig. 201. — Diagrams showing the Development of the Great Omentum and its
Fusion with the Transverse Mesocolon.
 
B, Bladder; c, transverse colon; d, duodenum; Li, liver; p, pancreas; R, rectum; S,
stomach; U, uterus. — {After Allen Thomson.)
 
transverse mesocolon and a fusion and degeneration of the two results (Fig. 201 B), a condition which brings it about that the omentum seems to be attached to the transverse colon and that the pancreas seems to lie in the line of attachment of the transverse mesocolon. This mesentery, as is occurs in the adult, really consists
partly of a portion of the original transverse mesocolon and partly
of a layer of the great omentum.
 
 
 
LITERATURE 329
 
By these various changes the line of attachment of the mesentery to the dorsal wall of the body has become somewhat complicated and has departed to a very considerable extent from its original simple vertical arrangement. If all the viscera be removed
from the body of an adult and the mesentery be cut close to the line
of its attachment, the course of the latter will be seen to be as follows: Descending from the under surface of the diaphragm are
the lines of attachment of the suspensory ligament, which on
reaching the liver spread out to become the coronary and lateral
ligaments of that organ. At about the mid-dorsal line these lines
become continuous with those of the mesogastriumr which curve
downward toward the left and are continued into the transverse lines
of the transverse mesocolon. Between these last, in a slight prolongation, there may be seen to the right the cut end of the first portion
of the duodenum as it passes back to the dorsal wall of the abdomen,
and at about the mid-dorsal line the cut ends of its last part become
visible as it passes ventrally again to become the jejunum. From the
transverse mesocolon three lines of attachment pass downward; the
two lateral broad ones represent the lines of fixation of the ascending
and descending colons, while the narrower median one, which
curves to the right, represents the attachment of the mesentery of
the small intestine other than the duodenum. Finally, from the
lower end of the fixation line of the descending colon the mesentery
of the sigmoid is continued downward.
 
The special developments of the peritoneum in connection with
the genito-urinary apparatuus will be considered in Chapter XIII.
 
LITERATURE.
 
I. Broman: "Ueber die Entwicklung und Bedeutung der Mesenterial und der
 
Korperhohlen bei den Wirbeltieren," Ergebn. der Anat. u. Entw., XV, 1906.
A. Bracket: "Die Entwickelung der grossen Korperhohlen und ihre Trennung von
 
Einander," Ergebnisse der Anat. und Eniwickelungsgesch., vn, 1898.
W. His: " Mittheilungen zur Embryologie der Saugethiere und des Menschen,"
 
Archiv fur Anat. und Physiol., Anat. Abth., 1881.
F. P. Mall: "Development of the Human Ccelom," Journal of Morphol., xii, 1897.
F. P. Mall: "On the Development of the Human Diaphragm," Johns Hopkins
 
Hospital Bull., xii, 1901.
 
 
 
330 LITERATURE
 
E. Ravn: "Ueber die Bildung der Scheidewand zwischen Brust- und Bauchhohle in
 
Saugethierembryonen," Archiv fur Anat. und Physiol., Anat, Abth., 1889.
A. Swaen: "Recherches sur le developpement du foie, du tube digestif, de l'arriere
cavite du peritoine et du mesentere," Journ. de I' Anat. et de la Physiol., xxxii,
 
1896; xxxni, 1897.
C. Toldt: "Bau und Wachstumsveranderungen der Gekrose des menschlichen
 
Darmkanals," Denkschr. der kais. Akad. Wissensch. Wien, Math.-Naturwiss.
 
Classe, xli, 1879.
C. Toldt: "Die Darmgekrose und Netze im gesetzmassigen und gesetzwidrigen
 
Zustand," Denkschr. der kais. Akad. Wissensch. Wien. Math.-Naturwiss. Classe,
 
lvt, 1889.
 
F. Treves: "Lectures on the Anatomy of the Intestinal Canal and Peritoneum,"
 
British Medical Journal, I, 1885.
 
 
 
CHAPTER XII.
 
 
 
THE DEVELOPMENT OF THE ORGANS OF RESPIRATION.
 
 
 
The Development of the Lungs. — The first indication of the
lungs and trachea is found in embryos of about 3.2 mm. in the
form of a groove on the ventral surface of the oesophagus, at first extending almost the entire length of that portion of the digestive
tract. As the oesophagus lengthens the lung groove remains connected with its upper portion (Fig. 182, A), and furrows which appear along the line of junction of the groove and the oesophagus
gradually deepen and separate
the two structures (Fig. 182, B).
The separation takes place earliest
at the lower end of the groove
and thence extends upward, so
that the groove is transformed
into a cylindrical pouch lying ventral to the oesophagus and dorsal
to the heart and opening with the
oesophagus into the terminal portion of the pharynx.
 
Soon after the separation of
the groove from the oesophagus
its lower end becomes enlarged
and bilobed, and since this lower
end lies, with the oesophagus, in
 
the median attached portion of the dorsal edge of the septum transversum, the lobes, as they enlarge, project into the dorsal parietal
recesses (Fig. 202), and so become enclosed within the peritoneal
lining of the recesses which later become the pleural cavities.
 
The lobes, which represent the lungs, do not long remain simple,
 
33i
 
 
 
 
RP
 
 
 
Fig. 202. — Portion of a Section
 
THROUGH AN EMBRYO OF THE FOURTH
 
Week.
 
A, Aorta; DC, ductus Cuvieri; L,
lung; O, oesophagus; RP, parietal recess; VOm, vitelline vein. — (Toldt.)
 
 
 
33 2
 
 
 
THE LUNGS
 
 
 
but bud-like processes arise from their cavities, three appearing in
the right lobe and two in the left (Fig. 203, A), and as these increase
in size and give rise to additional outgrowths, the structure of the
lobes rapidly becomes complicated (Fig. 203, B and C).
 
The lower primary process on each side may be regarded as a
prolongation of the bronchus, while the remaining process or processes represent lateral outgrowths from it. Considerable difference
of opinion has existed as to the nature of the further branching of the
bronchi, some authors regarding it as a succession of dichotomies,
one branch of each of these placing itself so as to be in the line of the
 
 
 
 
/\
 
 
 
 
Vp
 
\
 
 
 
V
 
 
 
c
 
 
 
Fig. 203. — Reconstruction of the Lung Outgrowths of Embryos of (/I) 4.3,
 
(5) 8.5, and (C) 10.5 MM.
 
Ap, Pulmonary artery; Ep, eparterial bronchus; Vp, pulmonary vein; 7, second lateral
 
bronchus; II, main bronchi. — (His.)
 
original main bronchus, while the other comes to resemble a lateral
outgrowth, and other observers have held that the main bronchus
has an uninterrupted growth, all other branches being lateral outgrowths from it, and the branching therefore a monopodial process.
The recent thorough study by Flint of the development of the lung of
the pig shows that, in that form at least, the branching is a monopodial one, and that from the main bronchus as it elongates four sets
of secondary outgrowths develop, namely, a strong lateral, a dorsal,
a ventral, and a weak and variable medial set.
 
 
 
THE LUNGS
 
 
 
333
 
 
 
There is a general tendency for the individual branches of the
various sets to be arranged in regular succession and for their development to be symmetrical in the two lungs. But on account of the
necessity under which the lungs are placed of adapting themselves
to the neighboring structures and at the same time affording a
respiratory surface as large as possible, an amount of asymmetry
supervenes. Thus, it has already been noted that in the earliest
branching a single lateral bronchus is formed in the left lung and two
in the right. The uppermost of these
latter, the first lateral bronchus, is unrepresented in the left lung, and is peculiar in that it lies behind the right pulmonary artery (Fig. 203, C), or in the
adult, after the recession of the heart,
above it, whence it is termed the eparterial bronchus. Its absence on the left
side is perhaps due to its suppression to
permit the normal recession of the aortic
arch (Flint).
 
So, too, the inclination of the heart
causes a suppression of the second ventral bronchus in the left lung, but at
the same time it affords opportunity for
an excessive development of the corresponding bronchus of the right lung,
which pushes its way between the heart
and the diaphragm and is known as the
infra-cardiac bronchus.
 
As soon as the unpaired first lateral bronchus and the paired second lateral bronchi are formed mesenchyme begins to collect around
each of them and also around the main bronchi, the lobes of the
adult lung, three in the right lung and two in the left, being thus
outlined. A development of mesenchyme also takes place around
the excessively developed right second ventral bronchus, and sometimes produces a well-marked infra-cardiac lobe in the right lung.
 
 
 
 
Fig. 204. — Diagram of the
Final Branches of the Mammalian Bronchi.
 
A, Atrium; B, bronchus; S,
air-sac. — (Miller.)
 
 
 
334
 
 
 
THE LARYNX
 
 
 
In later stages the various bronchi of each lobe give rise to
additional branches and these again to others, and the mesenchyme
of each lobe grows in between the various branches. At first the
amount of mesenchyme separating the branches is comparatively
great, but as the branches continue, the growth of the mesenchyme
fails to keep pace with it, so that in later stages the terminal enlargements are separated from one another by only very thin partitions
of mesenchyme, in which the pulmonary vessels form a dense network. The final branching of each ultimate bronchus or bronchiole
results in the formation at its extremity of from three to five enlargements, the atria (Fig. 204, A), from which arise a number of air-sacs
(S) whose walls are pouched out into slight diverticula, the air-cells
or alveoli. Such a combination of atria, air-sacs, and air-cells
 
constitutes a lobule, and each lung
is composed of a large number of
such units.
 
The greater part of the original pulmonary groove becomes
converted into the trachea, and in
the mesenchyme surrounding it
the incomplete cartilaginous rings
develop at about the eighth or
ninth week. The cells of the epithelial lining of the trachea and
bronchi remain columnar or cubical in form and become ciliated
at about the fourth month, but
those of the epithelium of the airsacs become greatly flattened and
constitute an exceedingly thin
layer of pavement epithelium.
The Development of the Larynx. — The opening of the upper
end of the pulmonary groove into the pharynx is situated at first
just behind the fourth branchial furrow and is surrounded anteriorly
and laterally by the PI -shaped ridge already described (p. 294) as
 
 
 
 
Fig. 205. — Reconstruction of the
Opening into the Larynx in an Embryo of Twenty-eight Days, Seen
from Behind and Above, the Dorsal
Wall of the Pharynx being Cut
Away.
 
co, Cornicular, and cu, cuneiform tubercle; Ep, epiglottis; T, unpaired portion of the tongue. — (Kallius.)
 
 
 
THE LARYNX 335
 
the furcula, this separating it from the posterior portion of the
tongue (Fig. 178). The anterior portion of this ridge, which is
apparently derived from the ventral portions of the third branchial
arch, gradually increases in height and forms the epiglottis, while
the lateral portions, which pass posteriorly into the margins of the
pulmonary groove, form the ary epiglottic folds. When the pulmonary groove separates from the oesophagus, the opening of the trachea
into the pharynx is somewhat slit-like and is bounded laterally by
the aryepiglottic folds, whose margins present two elevations which
may be termed the comicular and cuneiform tubercles (Fig. 205, co
and cu, and Fig. 175). The opening is, however, for a time almost
obliterated by a thickening of the epithelium covering the ridges,
 
 
 
Fig. 206.— Reconstruction of the Mesenchyme Condensations which Represent
 
the Hyoid and Thyreoid Carthages in an Embryo of Forty Days.
 
The darkly shaded areas represent centers of chondrification. c.ma, Greater cornu of
 
hyoid; c.mi, lesser cornu; Th, thyreoid cartilage. — (Kallius.)
 
and it is not until the tenth or eleventh week of development that
it is re-established. Later than this, at the middle of the fourth
month, a linear depression makes its appearance on the mesial
surface of each ary-epiglottic fold, forming the beginning of the
ventricle, and although at first the depression lies horizontally, its
lateral edge later bends anteriorly, so that its surfaces look outward
and inward. The lips which bound the opening of the ventricle
into the laryngeal cavity give rise to the ventricular and vocal folds.
The cartilages of the larynx can be distinguished during the
seventh week as condensations of mesenchyme which are but
indistinctly separated from one another. The thyreoid cartilage is
represented at this stage by two lateral plates of mesenchyme,
 
 
 
336 THE LARYNX
 
separated from one another both ventrally and dorsally, and each
of these plates undergoes chondrification from two separate centers
(Fig. 206) . These, as they increase in size, unite together and send
prolongations ventrally which meet in the mid-ventral line with the
corresponding prolongations of the plates of the opposite side, so
as to enclose an area of mesenchyme into which the chondrification
only extends at a later period, and occasionally fails to so extend,
producing what is termed a foramen thyreoideum.
 
The mesenchymal condensations which represent the cricoid
and arytenoid cartilages are continuous, but each arytenoid has a
distinct center of chondrification, while the cartilage of the cricoid
appears as a single ring which is at first open dorsally and only later
becomes complete. The epiglottis cartilage resembles the thyreoid
in being formed by the fusion of two originally distinct cartilages,
from each of which a portion separates to form the cuneiform
cartilages {cartilages of Wrisberg) which produce the tubercles of
the same name on the ary-epiglottic fold, while the corniculate
cartilages (cartilages of Santorini) are formed by the separation of a
small portion of cartilage from each arytenoid.
 
The formation of the thyreoid cartilage by the fusion of two pairs
of lateral elements finds an explanation from the study of the
comparative anatomy of the larynx. In the lowest group of the
mammalia, the Monotremata, the four cartilages do not fuse
together and are very evidently serially homologous with the cartilages which form the cornua of the hyoid. In other words, the
thyreoid results from the fusion of the fourth and fifth branchial
cartilages. The cricoid, in its development, presents such striking
similarities to the cartilaginous rings of the trachea that it is probably
to be regarded as the uppermost cartilage of that series, but the
epiglottis seems to be a secondary chondrification in the glossolaryngeal fold (Schaffer). The arytenoids possibly represent an
additional pair of branchial cartilages, such as occur in the lower
vertebrates (Gegenbaur).
 
These last arches have undergone almost complete reduction in
the mammalia, the cartilages being their only representatives, but,
 
 
 
LITERATURE 337
 
in addition to the cartilages, the fourth and fifth arches have also
preserved a portion of their musculature, part of which becomes
transformed into the muscles of the larynx. Since the nerve which
corresponds to these arches is the vagus, the supply of the larynx is
derived from that nerve, the superior laryngeal nerve probably
corresponding to the fourth arch, while the inferior (recurrent)
answers to the fifth.
 
The course of the recurrent nerve finds its explanation in the relation
of the nerve to the fourth branchial artery. When the heart occupies
its primary position ventral to the floor of the pharynx, the inferior
laryngeal nerve passes transversely inward to the larynx beneath the
fourth branchial artery. As the heart recedes the nerve is caught by the
vessel and is carried back with it, the portion of the vagus between it and
the superior laryngeal nerve elongating until the origins of the two
laryngeal nerves are separated by the entire length of the neck. Hence it
is that the right recurrent nerve bends upward behind the right subclavian artery, while the left curves beneath the arch of the aorta (see
Fig. 149).
 
LITERATURE.
 
J. M. Flint: "The Development of the Lungs," Amer. Journ. Anal., vi, 1906.
 
J. E. Frazer: "The Development of the Larynx," Journ. Anat. and Phys., xliv, 1910.
 
E. Goppert: "Ueber die Herkunft der Wrisbergschen Knorpels," Morphol. Jahrbuch,
 
xxi, 1894.
W. His: "Zur Bildungsgeschichte des Lungen beim menschlichen Embryo," Archiv
 
fiir Anat. und Physiol., Anat. Abth., 1887.
E. Kallius: "Beitrage zur Entwickelungsgeschichte des Kehlkopfes," Anat. Hefle,
 
ix, 1897.
E. Kallius: "Die Entwickelung des menschlichen Kehlkopfes," Verhandl. der Anat.
 
Gesellsch., xii, 1898.
A. Lisser: "Studies on the Development of the Human Larynx," Amer. Journ.
 
Anat., xii, 191 1.
A. Narath: "Der Bronchialbaum der Saugethiere und des Menschen," Bibliotheca
 
Medica, Abth. A, Heft 3, 1901.
J. Schaffer: "Zur Histologie Histogenese und phylogenetischen Bedeutung der
 
Epiglottis," Anat. Hefte, xxxin, 1907.
A. Sotjlie! and E. Bardier: "Recherches sur le developpement du larynx chez
 
l'homme," Journ. de V Anat. et de la Physiol., xxiii, 1907.
 
 
 
CHAPTER XIII.
 
THE DEVELOPMENT OF THE URINOGENITAL SYSTEM.
 
The excretory and reproductive systems of organs are so closely
related in^their development that they must be considered together.
They both owe their origin to the mesoderm which constitutes, the
intermediate cell-mass (p. 77), this, at an early period of development, becoming thickened so as to form a ridge projecting into the
dorsal portion of the ccelom and forming what is known as the
Wolffian ridge (Fig. 207, wr). The greater portion of the substance
 
 
 
,nc
 
 
 
 
 
 
 
 
 
6 y L m y 7
 
 
 
otr wr
 
 
 
Fig. 207. — Transverse Section through the Abdominal Region of a Rabbit
 
Embryo of 12 mm.
 
a, Aorta; gl., glomerulus; gr, genital ridge; m, mesentery; nc, notochord; t, tubule of
 
mesonephros; wd, Wolffian duct; wr, Wolffian ridge. — (Mihalkovicz.)
 
of this ridge is concerned in the development of the primary and
secondary excretory organs, but on its mesial surface a second ridge
appears which is destined to give rise to the ovary or testis, and
hence is termed the genital ridge (gr).
 
The development of the excretory organs is remarkable in that
three sets of organs appear in succession. The first of these, the
pronephros, exists only in a rudimentary condition in the human
 
338
 
 
 
THE PRONEPHROS
 
 
 
339
 
 
 
embryo, although its duct, the pronephric or Wolffian duct, undergoes
complete development and plays an important part in the development of the succeeding organs of excretion and also in that of the
reproductive organs. The second set, the mesonephros or Wolffian
body, reaches a considerable development during embryonic life,
but later, on the development of the final set, the definite kidney or
metanephros, undergoes degeneration, portions only persisting as
rudimentary structures associated for the most part with the reproductive organs.
 
The Development of the Pronephros and the Pronephric
 
Duct. — The first portions of thppjrrejl? r y system to make their
appearance are the pronephric orWolffian ducts, which develop as
 
 
 
 
1/71
 
 
 
nc
 
 
 
en
 
 
 
Fig.^2o8. — Transverse Section through Chick Embryo of about Thirty-six
 
Hours.
 
en, Endoderm; im, intermediate cell mass; ms, mesodermic somite; nc, notochord; so,
 
somatic, and sp, splanchnic mesoderm; wd, Wolffian duct. — (Waldeyer.)
 
outgrowths of the dorsal wallsof_t he intermedi atecejljnasses ; At first
ThT outgrowths are solid cords of cells (Fig. 208, wd), but" later a
lumen appears in the center of each and the canal so formed from
each intermediate cell mass, bending backward at its free end, comes
into contact and fuses with the canal from the next succeeding
segment. Two longitudinal canals, the pronephric or Wolffian ducts,
are thus formed, with which the cavities of the intermediate cell masses
communicate. The formation of the ducts begins in the anterior
segments before the segmentation of the posterior portions of the
mesoderm has taken place, and the further backward extension of
the ducts takes place independently of the formation of excretory
tubules, apparently by a process of terminal growth. The free end
 
 
 
340 THE PRONEPHROS
 
of each duct comes into intimate relation with the ectoderm above it,
so much so that its posterior portion has been held^by some observers
to be formed from that layer, but it seems more probable that the
relation to the ectoderm is a secondary process and that the ducts
are entirely of mesodermal origin. They reach the cloaca in embryos of a little over 4 mm., and later they unite with that organ, so
that their lumina open into its cavity.
 
The pronephric tubules make their appearance in embryos of
about 1.7 mm., while as yet there are only nine or ten mesodermic
somites, and they are formed from the intermediate cell masses of the
seventh to the fourteenth segment, and perhaps from those situated
 
 
 
 
En Ao
 
 
 
Fig. 209. — Diagram showing the Structure of a Fully Developed
Pronephric tubule.
Ao, Aorta; Coe, ccelom; ec, Ectoderm; eg, external glomerulus; en, endoderm; Ms,
mesodermic somite; iV, nervous system; n, nephrostome; nc, notochord; pc, pronephric
chamber; Wd, Wolffian duct. — (Modified from Felix.)
 
still more anteriorly. The entire series, however, is never in existence at any one time, for before the more posterior tubules are
formed, those of the anterior segments have undergone degeneration.
Each pronephric tubule, when fully formed, consists of a portion
which unites it to the Wolffian duct, and opens at its other end into
an enlargement, the pronephric chamber, (Fig. 209, pc), which, on
its part opens mto the ccelomic cavity by means of a nephrostome
canal. In the neighborhood of the ccelomic opening, or nephrostome,
an outgrowth of the ccelomic epithelium is formed, and a branch
from the aorta penetrates into this to form a stalked external glomerulus lying free in the coelomic cavity (Fig. 209, e.g.). Internal
 
 
 
THE MESONEPHROS
 
 
 
341
 
 
 
glomeruli, such as occur in connection with the mesonephric tubules
do not occur in the pronephros of the human embryo, and this fact,
together with the presence of external glomeruli and the participation of the tubules in the formation of the Wolffian duct, serve to
distinguish the pronephros from the mesonephros.
 
The pronephric tubules, are, as has been stated, transitory
structures and by the time the embryo has reached a length of about
5 mm. they have all disappeared. Before their disappearance is
complete, however, a second series of tubules has commenced to
develop, forming what is termed the mesonephros or Wolffian body.
 
The Development of the Mesonephros. — The pronephric
duct does not disappear with the degeneration of the pronephric
tubules, but persists to serve as
the duct for the mesonephros and
to play an important part in the
development of the metanephros
also. In the Wolffian ridge there
appear in embryos of between 3
and 4 mm. a number of coiled
tubules, which arise by some of
the cells of the ridge aggregating
to form solid cords, at first entirely unconnected with either the
coelomic epithelium or the
Wolffian duct. Later the cords
become connected with the ccelomic epithelium and acquire a
lumen, and near the coelomic end
 
of the tubule, at a region corresponding to the chamber of a pronephric tubule, a condensation of the mesenchyme of the Wolffian
ridge occurs to form a glomerulus into which a branch extends from
the neighboring aorta. The tubules finally acquire connection with
the Wolffian duct and at the same time lose their connections with
the coelomic epithelium, their nephrostomes being accordingly but
transitory structures. The tubules rapidly increase in length and
 
 
 
 
Fig. 210. — Transverse Section of
the Wolffian Ridge of a Chick Embryo of Three Days.
 
ao, Aorta; gl, glomerulus; gr, genital
ridge; mes, mesentery; ml, mesonephric
tubule; vc, cardinal vein; Wd, Wolffian
duct. — (Mihalkovicz.)
 
 
 
342
 
 
 
THE MESONEPHROS
 
 
 
become coiled, and the glomeruli project into their cavities, pushing
in front of them the wall of the tubule so that it has the appearance
represented in Fig. 210.
 
In its anterior portion the Wolffian ridge is formed by distinct
intermediate cell masses, but posterior to the tenth segment it
becomes distinguishable from the rest of the mesoderm before this
has become segmented, and, failing to undergo transverse division
into segments, it forms a continuous column of cells, known as the
nephrogenic cord. The anterior tubules of the mesonephros make
their appearance in the intermediate cell masses belonging to the
sixth cervical segment, its tubules thus overlapping those of the
pronephros, and from this level they appear in all succeeding segments and in the nephrogenic cord as far back as the region of the
third or fourth lumbar segment, where the cord is partially interrupted. This interruption marks the dividing line between the mesonephric and metanephric portions of the cord, the portions posterior
to it being destined to give rise to the metanephros. But, as is the
case with the pronephros, the entire series of mesonephric tubules is
never in existence at any one time, a degeneration of the anterior
ones supervening even before the posterior ones have differentiated,
and the degeneration proceeds to such an extent that in an embryo
of about 21 mm. all the tubules of the cervical and thoracic segments
have disappeared, only those of the lumbar segments persisting.
 
This does not mean, however, that the number of persisting
tubules corresponds with that of the segments in which they occur, for
the tubules are not segmental in their arrangement, but are much
more numerous than such an arrangement would allow. Two,
three, or even as many as nine may correspond with the extent of
a mesodermic somite and when the reduction is complete in an embryo
of 21 mm., where only the tubules corresponding with four or five
segments remain, they may number twenty-six in each mesonephros
(Felix). This arrangement of the tubules together with the size
which they assume when fully developed brings it about that the
Wolffian ridges become somewhat voluminous structures in their
mesonephric portions, projecting markedly into the ccelomic cavity
 
 
 
THE METANEPHROS
 
 
 
343
 
 
 
(Fig. 211). Each is attached to the dorsal wall of the body by a distinct mesentery and has in its lateral portion, embedded in its
substance, the Wolffian duct, while on its mesial surface anteriorly
is the but slightly developed genital ridge (/). This condition is
reached in the human embryo at about the sixth or seventh week of
development, and after that period the mesonephros again begins to
undergo rapid degeneration, so that at about the sixteenth week
 
 
 
 
Fig. 211. — Urinogenital Apparatus of a Male Pig Embryo of 6 cm.
 
ao, Aorta; b, bladder; gh, gubernaculum testis; k, kidney; md, Mullerian duct; sr,
 
suprarenal body; t, testis; w, Wolffian body; wd, Wolffian duct. — (Mihalkovicz.)
 
 
 
nothing remains of it except the duct and a few small rudiments
whose history will be given later.
 
The Development of the Metanephros. — The first indication
of the metanephros or permanent kidney is a tubular outgrowth
from the dorsal surface of the Wolffian duct shortly before its
entrance into the cloaca (Fig. 170). When first formed this outgrowth lies lateral to the posterior portion of the Wolffian ridge,
 
 
 
344
 
 
 
THE METANEPHROS
 
 
 
 
which, as has already been noted (p. 342), is separated from the
portion that gives rise to the mesonephros. This terminal portion of
the ridge forms what is termed the metanephric blastema and in
embryos of 7 mm. it has come into relation with the outgrowth from
the Wolffian duct and covers its free extremity as a cap. Since
both the blastema and the outgrowth from the Wolffian duct take
 
part in the formation of the
uriniferous tubules, these have
a double origin.
 
The outgrowth from the
Wolffian duct as it continues to
elongate comes to lie dorsal to
the mesonephros, carrying the
cap of blastema with it, and
it soon assumes a somewhat
club-shaped form, its terminal
enlargement or ampulla forming what may be termed the
primary renal pelvis, while the
remainder represents theureter.
The primary renal pelvis then gives rise to from three to six, usually
four, tubular outgrowths, which may be termed primary collecting
tubules, and with their formation the original cap of metanephric
blastema undergoes a division into as many portions as there are
tubules, so that each of the latter has its own cap of blastema.
As soon as each tubule has reached a certain length it begins to
enlarge at its free extremity to form an ampulla, just as did the
primary renal pelvis, and from this ampulla there grow out from
two to four secondary collecting tubules, a further corresponding
division of the metanephric blastema taking place. In their turn
these secondary tubules similarly enlarge at their extremities to
form ampullae (Fig. 212, A) from which tertiary collecting tubules
are budded out, accompanied by a third fragmentation of the blastema
and so the process goes on until about the fifth fetal month, the
number of generations of collecting tubules formed being between
 
 
 
Fig. 212. — Diagrams of Early Stages in
the Development of the Metanephric
Tubules.
 
t, Urinary tubule; Ur, ureter; v, renal ampulla. — (Haycrqft.)
 
 
 
THE METANEPHROS
 
 
 
34
 
 
eleven and thirteen, each tubule of the final generation having its
cap of blastema.
 
In this way there is formed a complicated branching system of
tubules all of which ultimately communicate with the primary
renal pelvis, and all of which have, in the last analysis, had their
origin from the Wolffian duct. They represent, however, only the
collecting portions of the uriniferous tubules, their excreting por
 
 
 
Fig. 213. — Four Stages in the Development of a Uriniferous Tubule of a Cat.
A, Arched collecting tubule, C, distal convoluted tubule; C, proximal convoluted
tubule; H, loop of Henle; M, glomerulus; T, renal vesicle; V, ampulla (drawn from
reconstructions prepared by G. C. Huber).
 
tions having yet to form, and these take their origin from the metanephric blastema.
 
When the terminal collecting tubules have been formed the
blastemic cap in connection with each one condenses to form a renal
vesicle (Fig. 213, A, T), which is at first solid, but later becomes
hollow and proceeds to elongate to an S-shaped tubule, one end of
which becomes continuous with the neighboring ampulla (Figs.
212, B, and 213, B), and in the space enclosed by_what may be
termed the lower loop of the S a collection of mesenchyme cells
 
 
 
346 THE METANEPHROS
 
appears, into which branches penetrate at an early stage from the
renal artery to form a glomerulus, the neighboring walls of the
tubule becoming exceedingly thin and being transformed into a
capsule of Bowman. The upper loop of the S now begins to elongate (Fig. 213, C), growing toward the hilus of the kidney, parallel
to the branch of the outgrowth from the Wolffian duct to which it is
attached and between this and the glomerulus, and forms a loop of
Henle. From the portion of the horizontal limb of the S which lies
between the glomerulus and the descending limb of the loop of
Henle the proximal convoluted tubule (C) arises, while the distal
convoluted and the arched collecting tubules (C and A) are formed
from the uppermost portion of the upper loop (Fig. 213, D). The
entire length of each uriniferous tubule from Bowman's capsule to
the arched collecting tubule inclusive is thus derived from a renal
vesicle, that is to say, from the metanephric blastema.
 
Since the tubules of the kidney are formed by the union of two originally
distinct structures it is conceivable that in the cases of certain tubules
there may be a failure of the union. The blastemic portion of the tubules
would, nevertheless, continue their development and become functional
and, since there would be not means of escape for the secretion, the result
would be a cystic kidney. Occasionally the two blastemata of opposite
sides fuse across the middle line, the result being the formation of a
single transverse or horse-shoe shaped kidney, or, what is much rarer, the
blastema of one side may cross the middle line to fuse with that of the
other, the result being an apparently single kidney with two ureters which
open normally into the bladder.
 
The primary renal pelvis is the first formed ampulla and does not
exactly represent the definitive pelvis. This is produced partly by
the enlargement of the primary pelvis and partly by the enlargement
of the collecting tubules of the first four generations, those of the third
and fourth generations later being taken up or absorbed into those
of the second generation, so that the tubules of the fifth generation
appear to open directly into those of the second, which form the
calices minores, while those of the first constitute the calices majores.
In some kidneys the process of reduction of the earlier formed
collecting tubules proceeds a step further, those of the first generation
 
 
 
THE MULLERIAN DUCT 347
 
being taken up into the primary renal pelvis, the secondaries then
forming a series of short calices arising from a single pelvic cavity.
 
At about the tenth week of development the surface of the human
kidney becomes marked by shallow depressions into lobes, of which
there are about eighteen, one corresponding to each of the groups
of tubules which arise from the same renal vesicle. This lobation
persists until after birth and then disappears completely, the surface
of the kidney becoming smooth.
 
The Development of the Mullerian Duct and of the Genital
Ridge. — At the time when the Wolffian body has almost reached
its greatest development the Wolffian ridge is distinctly divided into
three portions (Fig. 214), a median or mesonephric portion attached
to the body wall, a lateral or tubal portion containing the Wolffian
duct and attached to the mesonephric portion, and a genital portion,
formed by the genital ridge and also attached to the mesonephric
portion, but to its medial surface. In the tubal portion a second
longitudinal duct, known as the Mullerian duct (Fig. 214, Md),
makes its appearance. Near the anterior end of each Wolffian
ridge there is formed on the free edge of the tubal portion an invagination of the peritoneal covering, and by the proliferation of the
cells at its tip this invagination gradually extends backward in the
substance of the tubal portion and reaches the cloaca in embryos of
about 22 mm. The primary peritoneal invagination becomes the
abdominal ostium of the Mullerian duct, the backward prolongation
forming the duct itself.
 
In Fig. 214 it will be seen that the tubal portion of the left
Wolffian ridge is somewhat bent inward toward the median line
and in the lower parts of their extent this becomes more pronounced
in both tubal portions until finally their free edges come in contact
and fuse in the median line, while at the same time their lower edges
fuse with the floor of the ccelomic cavity. In this way a transverse
partition is formed across what will eventually be the pelvis of the
adult, this cavity being thus divided into two compartments, a
posterior one containing the lower portion of the intestine and an anterior one containing the bladder. With the formation of this trans
 
 
348
 
 
 
THE GENITAL RIDGE
 
 
 
N
 
 
 
M
 
 
 
â– sg
 
 
 
M
 
 
 
M
 
 
 
S
 
 
 
V s>
 
 
 
Ao
 
 
 
 
V
 
 
 
-r
 
 
 
M
 
 
 
Ur
 
 
 
Wd
 
 
 
Md
M
 
 
 
(B
 
 
 
I
 
!
 
UA
 
 
 
RA
 
 
 
Fig. 214. — Transverse Section through the Abdominal Region oe an Embryo
 
of 25 MM.
_ Ao, Aorta; B, bladder; I, intestine; L, liver; M, muscle; Md, Miillerian duct; N,
spinal cord; Ov, ovary; RA, rectus abdominis; Sg, spinal ganglion; UA, umbilical
artery; Ur, ureter; V, vertebra; W, Wolffian body; Wd, Wolffian duct. — (Keibel.)
 
 
 
THE GENITAL RIDGE 349
 
verse fold, which is represented by the broad ligament in the female,
the Miillerian ducts of opposite sides are brought into contact and
finally fuse in the lower portions of their course to form an unpaired
utero-vaginal canal.
 
Upon the lateral surface of the mesonephric portion of the
Wolffian ridge a longitudinal elevation is formed at about this time.
It is the inguinal fold and on the union of the transverse fold with
the floor of the ccelomic cavity it comes into contact and fuses with the
lower part of the anterior abdominal wall, just lateral to the lateral
border of the rectus abdominis muscle. In the substance of the
fold the mesenchyme condenses to form a ligament-like cord, the
inguinal ligament, whose further history will be considered later on.
 
The genital ridge makes its apearance as a band-like thickening
of the epithelium covering the mesial surface of the Wolffian ridge
(Fig. 207, gr). Later columns of cells grow down from the thickening into the substance of the Wolffian ridge, displacing the mesonephric tubules to a greater or less extent. These columns are composed of two kinds of cells: (i) smaller epithelial cells with a relatively small amount of cytoplasm and (2) large, spherical cells with
more abundant and clear cytoplasm known as sex-cells. The
growth of the cell-columns down into the substance of the Wolffian
body does not take place, however, to an equal extent in all portions of the length of the genital ridge. Indeed, three regions
may be recognized in the ridge; an anterior one in which a relatively
small number of cell-columns, extending deeply into the stroma, is
formed; a middle one in which numerous columns are formed; and
a posterior one in which practically none are formed. The first
region has been termed the rete region and its cell-columns the retecords, the second region the sex-gland region and its columns the
sex-cords, and the posterior region is the mesenteric region and plays
no part in the actual formation of the ovary or testis.
 
In the human embryo all the sex-cells seem to have their origin from
the epithelium of the genital ridge, but in the lower vertebrates and also
in mammals (Allen, Rubaschkin) they have been found to make their
appearance in the endoderm of the digestive tract. Thence they wander
 
 
 
35°
 
 
 
THE TESTIS
 
 
 
into the mesentery and some of them eventually into the peritoneum
covering the mesial surface of the Wolffian ridge, where they give rise
to the sex-cells found in the epithelium of the genital ridge. This origin
of the sex-cells has not yet been observed in the human embryo.
 
The various steps in the differentiation of the reproductive
organs so far described occur in all embryos, no matter what their
future sex may be. The later stages, however, differ according to
sex, and consequently it will be necessary to follow the further
development first of the testis and then of the ovary, the changes
 
 
 
 
Fig. 215. — Section through the Testis and the Broad Ligament of the Testis
of an Embryo of 5.5 mm.
 
ep, Epithelium; md, Miillerian duct; mo, mesorchium; re, rete-cords; sc, sex-cords; wd,
Wolffian duct. — (Mihalkovicz.)
 
that take place in the ducts and other accessory structures being
reserved for a special section.
 
The Development of the Testis. — At about the fourth or fifth week
there appears in the sex-gland region of the genital ridge a structure
which serves to characterize the region as a testis. This is a layer
of somewhat dense connective tissue which grows in between the
epithelial and stroma layers of the sex-gland region and gradually
extends around almost the entire sex-gland to form the tunica albuginea. By its development the sex-cords are separated from the
 
 
 
THE TESTIS
 
 
 
351
 
 
 
epithelium, which later becomes much flattened and eventually
almost disappears. Shortly after the appearance of the albuginea
the sex-cords unite to from a complicated network and the rete-cords
grow backward along the line of attachment of the testis to the
mesonephric portion of the Wolffian ridge, coming to lie in the hilus
 
 
 
 
Mc —
 
 
 
ep
 
 
 
â– - R
 
 
 
— Mn
 
 
 
Fig. 216. — Longitudinal Section of the Ovary of an Embryo Cat of 9.4 cm.
cor, Cortical layer; ep, epoophoron; Mc, medullary cords; Mn, mesonephros; pf,
peritoneal fold containing Fallopian tube; R, rete; T, Fallopian tube. — {Coert, from
Bilhler.)
 
of the testis (Fig. 215). They then develop a lumen and send off
branches which connect with the sex-cord reticulum and they also
make connection with the glomerular portions of the tubules belonging to the anterior part of the mesonephros. Since like the sexcords, they have by this time separated from the epithelium that
 
 
 
352 THE OVARY
 
gave rise to them, they now extend between the sex-cord reticulum
and the anterior mesonephric tubules. Certain portions of the
sex-cords now begin to break down leaving other portions to form
convoluted stems which eventually become the seminiferous tubules,
while from the rete-cords are formed the tubuli recti and rete testis,
by which the spermatozoa are transmitted to the mesonephric
tubules and so to the Wolffian duct (see p. 355).
 
The development of the seminiferous tubules is not, however,
completed until puberty. The stems derived from the sex-cords
form cylindrical cords, between which lie stroma cells and interstitial cells derived from the stroma; but until puberty these cords
remain solid, a lumen developing only at that period. The cords
contain the same forms of cells as were described as occurring in the
epithelium of the germinal ridge, and while in the early stages
transitional forms seem to occur, in later periods the two varieties of
cells are quite distinct, the sex-cells becoming spermatogonia
(see p. 14) and being the mother cells of the spermatozoa, while the
remaining epithelial cells perhaps become transformed into the connective-tissue walls of the tubules.
 
The Development of the Ovary. — In the case of the ovary, after
the formation of the sex-cords, connective tissue grows in between
these and the epithelium, forming a layer equivalent to the tunica
albuginea of the testis. It is, however, a much looser tissue than
its homologue in the male, and, indeed, does not completely isolate
the sex-cords from the epithelium, although the majority of the cords
are separated and sink into the deeper portions of the ovary where
they form what have been termed the medullary cords. In the meantime the germinal epithelium has continued to bud off cords which
unite to form a cortical layer of cells lying below the epithelium and
separated from the medullary cords by the tunica albuginea
(Fig. 216).
 
Later the cortical layer becomes broken up by the ingrowth of
stroma tissue into spherical or cord-like masses, consisting of sexcells and epithelial cells (Fig. 217). The invasion of the stroma
continuing, these spheres or cords (Pfluger's cords) become divided
 
 
 
THE OVARY
 
 
 
353
 
 
 
into smaller masses, the primary ovarian follicles, each of which
consists as a rule of a single sex-cell surrounded by a number of
epithelial cells, the whole being enclosed by a- zone of condensed
stroma tissue, which eventually becomes richly vascularized and
forms a theca folliculi (Fig. 10). The epithelial cells in each follicle
are at first comparatively few in number and closely surround the
sex-cell (Fig. 217,/), which is destined to become an ovum, but in
certain of the follicles they undergo an increase by mitosis, becoming
extremely numerous, and later
secrete a fluid, the liquor folliculi, which collects at one side of
the follicle and eventually forms
a considerable portion of its contents. The follicular cells are
differentiated by its appearance
into the stratum granulosum,
which surrounds the wall of the
follicle, and the discus froligerus,
in which the ovum is embedded
(Fig. 10, dp), and the cells which
immediately surround the ovum,
becoming cylindrical in shape,
give rise to the corona radiata
(Fig. 11, cr).
 
A somewhat similar fate is
shared by the medullary cords, these also breaking up into a number of follicles, but sooner or later these follicles undergo degeneration so that shortly after birth practically no traces of the cords remain. It must be noted that degeneration of the follicles formed
from the cortical layer also takes place even during fetal life and
continues to occur throughout the entire periods of growth and functional activity, numerous atretic follicles being found in the ovary
at all times. Indeed it would seem that degeneration is the fate of
the great majority of the follicles and sex-cells of the ovary, but few
ova coming to maturity during the life-time of any individual.
23
 
 
 
 
Fig. 217. — Section of the Ovary of
a New-born Child.
 
a, Ovarial epithelium; b, proximal part
of a Pfl tiger's cord; c, sex-cell in epithelium; d and e, spherical masses; /, primary follicle; g, blood-vessel. — (From
Gegenbaur, after Waldeyer.)
 
 
 
354 THE GENITAL DUCTS
 
Rete-cords developed from the rete portion of the germinal
ridge occur in connection with the ovary as well as with the. testis
and form a rete ovarii (Fig. 216, R). They do not, however, extend
so deeply into the ovary, remaining in the neighborhood of the
mesovarium, and they do not become tubular, but resemble closely
the medullary cords with which they are serially homologous.
They separate from the epithelium and make connections with the
glomeruli of the anterior portion of the mesonephros, on the
one hand, and on the other with medullary cords, and in later
stages show a tendency to break up into primary follicles, which
early degenerate and disappear like those of the medullary cords.
 
The Transformation of the Mesonephros and the Ducts. —
At one period of development there are present, as representatives
of the urinogenital apparatus, the Wolffian body (mesonephros)
and duct, the Miillerian duct, and the developing ovary or testis.
Such a condition forms an indifferent stage from which the development proceeds in one of two directions according as the genital
ridge becomes a testis or an ovary, the Wolffian body in part undergoing degeneration and in part persisting to form organs which for
the most part are rudimentary, while in the female the Wolffian
duct also degenerates except for certain rudiments and in the male
the Miillerian duct behaves similarly.
 
In the Male. — It has been seen that the Wolffian body, through
the rete cords, enters into very intimate relations with the testis,
and it may be regarded as divided into two portions, an upper
genital and a lower excretory. In the male the genital portion
persists in its entirety, serving as the efferent ducts of the testis,
which, beginning in the spaces of the rete testis, already shown to be
connected with the capsules of Bowman, open into the upper part of
the Wolffian duct and form the globus major of the epididymis.
The excretory portion undergoes extensive degeneration, a portion
of it persisting as a mass of coiled tubules ending blindly at both
ends, situated near the head of the epididymis and known as the
paradidymis or organ of Giraldes, while a single elongated tubule,
arising from the portion of the Wolffian duct which forms the
 
 
 
THE GENITAL DUCTS 355
 
globus minor of the epididymis, represents another portion of it and
is known as the vas aberrans.
 
The Wolffian duct is retained complete, the portion of it nearest
the testis becoming greatly elongated and thrown into numerous
coils, forming the body and globus minor of the epididymis, while the
remainder of it is converted into the vas deferens and the ductus
ejaculatorius. A lateral outpouching of the wall of the duct to
form a longitudinal fold appears at about the third month and
gives rise to the vesicula seminalis, the lateral position of the outgrowth explaining the adult position of the vesiculse lateral to the
vasa deferentia.
 
With the Mullerian ducts the case is very different, since they
disappear completely throughout the greater part of their course,
only their upper and lower ends persisting, the former giving rise to a
small sac-like body, the sessile hydatid of Morgagni, attached to the
upper end of each testis near the epididymis. It has been seen (p. 349)
that the lower ends of the Mullerian ducts, in the male as well as the
female, fuse to form the utero-vaginal canal, and the lower portion
of this also persists to form what is termed the uterus masculinus,
although it corresponds to the vagina of the female rather than to the
uterus. It is a short cylindrical pouch of varying length, that opens
into the urethra at the bottom of a depression known as the utriculus
prostaticus {sinus pocularis).
 
The transverse pelvic partition, produced by the union of the two
tubal portions of the Wolffian body, is formed in the male embryo,
but at an early stage its anterior surface fuses with the posterior
surface of the bladder and consequently there is in the male no pelvic
compartment equivalent to the vesico-uterine pouch of the female.
The male recto-vesical pouch is, however, the homologue of the rectouterine pouch of the female.
 
The formation of the inguinal ligament on the surface of the
mesonephros has been described on p. 349. On the degeneration of
the mesonephros the layer of peritoneum that covered it persists to
form a mesorchium extending from the body wall to the hilus of the
testis and the inguinal ligament now comes to have its origin from
 
 
 
356 THE GENITAL DUCTS
 
the lower pole of that organ, whence it extends to the anterior abdominal wall. Owing to the rudimentary nature of the uterus
masculinus and the slight development of its walls the inguinal
ligament does not become involved with it, but remains independent
and forms the gubemaculum testis of the adult, whose adult position is brought about by the descent of the testis into the scrotum
(see p. 366).
 
In the Female. — In the female the transverse partition of the
pelvis does not fuse w'th the bladder but remains distinct as the
broad ligament. Consequently there is in the female both a vesicouterine and a recto-uterine pouch. Since the genital ridges form
upon the mesial surfaces of the Wolffian ridges and the tubal
portions are their lateral portions, when these latter unite to form
the broad ligament the ovary will come to lie upon the posterior
surface of that structure, projecting into the recto-vesical pouch.
On the degeneration of the mesonephros the peritoneum that
covered it becomes a part of the broad ligament, forming that part
of it which contains the Fallopian tubes and hence is known as the
mesosalpinx, while the lower part of the ligament, on account of its
relation to the uterus,. is termed the mesometrium.
 
The genital portion of the mesonephros, though never functional
as ducts in the female, persists as a group of ten to fifteen tubules,
situated between the two layers of the broad ligament and in close
proximity to the ovary; these constitute what is known as the
epoophoron (parovarium or organ of Rosenmuller) . The tubules terminate blindly at the ends nearest the ovary, but at the other extremity, where they are somewhat coiled, they open into a collecting
duct which represents the upper end of the Wolffian duct. Near this
rudimentary body is another, also composed of tubules, representing
the remains of the excretory portion of the mesonephros and termed
the paroophoron which, however, degenerates during the early years
of extra-uterine life. So far as the mesonephros is concerned, therefore, the persisting rudiments in the female are comparable to those
occurring in the male.
 
As regards the ducts, however, the case is different, for in the
 
 
 
THE GENITAL DUCTS 357
 
female it is the Mtillerian ducts which persist, while the Wolmans
undergo degeneration, a small portion of their upper ends persisting
in connection with the epoophora, while their lower ends persist as
straight tubules lying at the sides of the vagina and forming what
are known as the canals of Gartner. The Mtillerian ducts, on the
other hand, become converted into the Fallopian tubes {tubas uterince),
and in their lower portions into the uterus and vagina. From the
margins of the openings by which the Mullerian ducts communicate
with the ccelom projections develop at an early period and give rise
to the fimbria, with the exception of the one connected with the
ovary, the fimbria ovarica, which is the persisting upper portion of
the original genital ridge. From the utero-vaginal canal the two
structures which give it its name are formed, the entire canal being
transformed into the mucous membrane of the uterus and vagina.
Indeed, the lower ends of the Fallopian tubes are also taken up
into the uterus, for the condensation of mesenchyme that takes
place around the mucosa to form the muscular wall of the uterus
is so voluminous that it includes not only the utero-vaginal canal
but also the adjacent portions of the Mullerian ducts. The histological differentiation of the uterus from the vagina begins to
manifest itself at about the third month, and during the fourth
month the vaginal portion of the duct becomes flattened and the
epithelium lining its lumen fuses so as to completely occlude it
and, a little later, there appears at its lower opening a distinct semicircular fold. This is the hymen, a structure which seems to be
represented in the male by the colliculus seminalis. The obliteration
of the lumen of the vagina persists until about the sixth month,
when the cavity is re-established by the breaking down of the central
epithelial cells.
 
The extent of the mesenchymal condensation to form the
muscularis uteri also produces a modification of the relations of the
inguinal ligament in the female. For the ligament becomes for a
short portion of its length included in the condensation and thus
attached to the upper portion of the uterus. It is consequently
divided into two portions, one extending from the lower pole of
 
 
 
358
 
 
 
THE GENITAL DUCTS
 
 
 
the ovary to the uterus and forming the ligamentum ovarii proprium
and the other extending from the uterus to the anterior abdominal
wall and forming what is known in the adult as the round ligament
of the uterus.
 
The diagram, Fig. 218, illustrates the transformation from the
indifferent condition which occurs in the two sexes, and that the
 
 
 
 
UM
 
 
 
female indifferent male
 
Fig. 218. — Diagrams Illustrating the Transformation of the Mullerian and
 
Wolffian Ducts.
B, Bladder; C, clitoris; CG, canal of Gaertner; CI, cloaca; Eo, epoophoron; Ep, epididymis; F, Fallopian tube; G, genital gland; HE, hydatid of epididymis; HM, hydatid
of Morgagni; K, kidney; MD, Mullerian duct; O, ovary; P, penis; Po, paroophron; Pr,
prostate gland; R, rectum; T, testis; U, urethra; UM, uterus masculinus; Ur, ureter;
US, urogenital sinus; Ut, uterus; V, vagina; Va, vas aberrans; VD, vas deferens; VS,
vesicula seminalis; WB, Wolffian body; WD, Wolffian duct. — (Modified from Huxley.)
 
 
 
homologies of the various parts may be clearly understood they
may also be stated in tabular form as on the next page.
 
 
 
THE BLADDER
 
 
 
359
 
 
 
Indifferent Stage.
 
 
Male. Female.
 
 
Genital ridge J
 
 
Testis.
 
 
Fimbria ovarica.
Ovary.
 
Ovarian ligament.
Round ligament.
 
 
Wolffian body <
 
 
Globus major of epididymis.
 
Paradidymis.
 
Vasa aberrantia.
 
 
Epoophoron.
Paroophoron.
 
 
Wolffian ducts. . . . •
 
 
Body and globus minor of
 
epididymis.
Vasa deferentia.
Seminal vesicles.'
Ejaculatory ducts.
 
 
Collecting tubules of epoophoron.
 
Canal of Gartner.
 
 
Miillerian ducts . . . <
 
 
Sessile hyatid.
Uterus masculinus.
 
 
Fallopian tubes.
 
Uterus.
 
Vagina.
 
 
 
In addition to the sessile hydatid, a stalked hydatid also occurs in
connection with the testis, and a similar structure is attached to the
fimbriated opening of each Fallopian tube. The significance of these
structures is uncertain, though it has been suggested that they are persisting rudiments of the pronephros.
 
A failure of the development of the various parts just described to be
completed in the normal manner leads to various abnormalities in connection with the reproductive organs. Thus there may occur a failure
in the fusion of the lower portions of the Miillerian ducts, a bihorned or
bipartite uterus resulting, or the two ducts may come into contact and
their adjacent walls fail to disappear, the result being a median partition
separating the vagina or both the vagina and uterus into two compartments. The excessive development of the fold which gives rise to the
hymen may lead to a complete closure of the lower opening of the
vagina, while, on the other hand, a failure of the Miillerian ducts to
fuse may produce a biperforate hymen.
 
The Development of the Urinary Bladder and the Urogenital Sinus. — So far the relations of the lower ends of the urinogenital ducts have not been considered in detail, although it has been
 
 
 
s6o
 
 
 
THE BLADDER
 
 
 
seen that in the early stages of development the Wolffian and
Miillerian ducts open into the sides of the ventral portion of the
cloaca; that the ureters communicate with the lower portions of the
Wolffian ducts; that from the ventral anterior portion of the cloaca
the allantoic duct extends outward into the belly-stalk; and, finally
(p. 281), that the cloaca becomes divided into a dorsal portion, which
forms the lower part of the rectum, and a ventral portion, which is
continuous with the allantois and receives the urinogenital ducts
 
 
 
 
Fig. 219. — Reconstruction of the Cloacal Region op an Embryo of 14 mm.
 
al, Allantois; b, bladder; gt, genital tubercle; i, intestine; n, spinal cord; nc, notochord ;
 
r, rectum; sg, urogenital sinus; ur, ureter; w, Wolffian duct.— (Keibel.)
 
(Fig. 219). It is the history of this ventral portion of the cloaca
which is now to be considered.
 
It may be regarded as consisting of two portions, an anterior and
a posterior, the line of insertion of the urinogenital ducts marking the
junction of the two. The anterior or upper portion is destined to
give rise to the urinary bladder (Fig. 219, b), while the lower one
forms what is known for a time as the urogenital ^inus (sg). The
bladder, when first differentiated, is a tubular structure, whose
lumen is continuous with that of the allantois, but after the second
 
 
 
THE BLADDER
 
 
 
361
 
 
 
month it enlarges to become more sac-like, while the intra-embryonic
portion of the allantois degenerates to a solid cord extending from
the apex of the bladder to the umbilicus and is known as the urachus.
During the enlargement of the bladder the terminal portions of the
urinogenital ducts are taken up into its walls, a process which
continues until finally the ureters and Wolffian ducts open into it
separately, the ureters opening to the sides of and a little anterior
to the ducts. This condition is reached in embryos of about 14 mm.
 
 
 
 
Fig. 220. — Reconstruction of the Cloacal Structures of an Embryo of 25 mm.
 
bl, Bladder; m, Mullerian duct; r, rectum; sg, urogenital sinus; sy, symphysis pubis; u,
 
ureter; ur, urethra; w. Wolffian duct. — (Adapted from Keibel.)
 
(Fig. 219), and in later stages the interval between the two pairs of
ducts is increased (Fig. 220), resulting in the formation of a short
canal connecting the lower end of the bladder which receives the
ureters with the upper end of the urogenital sinus, into which the
Wolffian and Mullerian ducts open. This connecting canal represents the urethra (Fig. 220, ur), or rather the entire urethra of the
female and the proximal part of that of the male, since a considerable
portion of the latter canal is still undeveloped (see p. 364). From
 
 
 
362 THE UROGENITAL SINUS
 
this urethra there is developed, at about the third month, a series of
solid longitudinal folds which project upon the outer surface and
separate from the urethra from above downward. These represent
the tubules of the prostate gland and are developed in both sexes,
although they remain in a somewhat rudimentary condition in the
female. The muscular tissue, so characteristic of the gland in the
adult male, is developed from the surrounding mesenchyme at a
later stage.
 
The bladder is, accordingly, essentially a derivative of the cloaca
and its mucous membrane is therefore largely of endodermal origin.
Portions of the Wolffian ducts which are of mesodermal origin are,
however, taken up into the wall of the bladder and form a portion
of it. The extent of the portion so formed is indicated by the
position of the orifices of the ureters above and of the ejaculatory
ducts below, and it corresponds therefore with what is termed the
trigonum vesica together with the floor of the urethra as far as the
openings of the ejaculatory ducts. Throughout this region the
mucous membrane is of mesodermal origin.
 
The urogenital sinus is in the early stages also tubular in its
upper part, though it expands considerably below, where it is
closed by the cloacal membrane. This, by the separation of the
cloaca into rectum and sinus, has become divided into two portions,
the more ventral of which closes the sinus and the dorsal the rectum,
the interval between them having become considerably thickened
to form the perineal body. In embryos of about 17 mm. the urogenital portion of the membrane has broken through, and in later
stages the tubular portion of the sinus is gradually taken up into
the more expanded lower portion, until finally the entire sinus forms
a shallow depression, termed the vestibule, into the upper part of
which the urethra opens, while below are the openings of the
Wolffian (ejaculatory) ducts in the male or the orifice of the vagina
in the female. From the sides of the lower part of the sinus a pair
of evaginations arise toward the end of the fourth month and give
rise to the bulbo-vestibular glands (Bartholin's) of the female or the
corresponding bulbo-urethral glands (Cowper's) in the male.
 
 
 
THE EXTERNAL GENITALIA 363
 
The Development of the External Genitalia. — At about the
fifth week, before the urogenital sinus has opened to the exterior,
the mesenchyme on its ventral wall begins to thicken, producing a
slight projection to the exterior. This eminence, which is known
as the genital tubercle (Fig. 219, gt), rapidly increases in size, its
extremity becomes somewhat bulbously enlarged (Fig. 221, gl) and
a groove, extending to the base of the terminal enlargement, appears
upon its vestibular surface, the lips of the groove forming two wellmarked genital folds (Fig. 221, gf). At about the tenth week there
appears on either side of the tubercle an enlargement termed the
genital swelling (Fig. 221, gs), which is due to a thickening of the
mesenchyme of the lower part of the ventral^abdominal wall in the
 
 
 
V^
 
 
 
 
Fig. 221. — The External Genitalia of an Embryo of 25 mm.
a, Anus; gf, genital fold; gl, glans; gs, genital swelling; p, perineal body. — (Keibel.)
 
region where the inguinal ligament is attached, and with the appearance of these structures the indifferent stage of the external genitals
is completed.
 
In the female the growth of the genital tubercle proceeds rather
slowly and it becomes transformed into the clitoris, the genital folds
becoming prolonged to form the labia minora. The genital swellings
increase in size, their mesenchyme becomes transformed into a mass
of adipose and fibrous tissue and they become converted into the
labia majora, the interval between them constituting the vulva.
 
In the male the early stages of development are closely similar to
 
 
 
3 6 4
 
 
 
THE EXTERNAL GENITALIA
 
 
 
those of the female; indeed, it has been well said that the external
genitals of the adult female resemble those of the fetal male. In
early stages the genital tubercle elongates to form the penis and the
integument which covers the proximal part of it grows forward as a
fold which encloses the bulbous enlargement or glans and forms the
prepuce, whose epithelium fuses with that covering the glans and
only separates from it later by a cornification of the cells along the
plane of fusion. The genital folds meet together and fuse, converting
the vestibule and the groove upon the vestibular surface of the penis
into the terminal portion of the male urethra and bringing it about
that the vasa deferentia and the uterus masculinus open upon the
floor of that passage. The two genital swellings are at the same
time brought closer together, so as to lie between the base of the
penis and the perineal body and, eventually, they form the scrotum.
The mesenchyme of which they were primarily composed differentiates into the same layers as are found in the wall of the abdomen and
a peritoneal pouch is prolonged into them from the abdomen, so that
they form sacs into which the testes descend toward the close of fetal
life (p. 366).
 
The homologies of the portions of the reproductive apparatus
derived from the cloaca and of the external genitalia in the two sexes
may be perceived from the following table.
 
 
 
 
 
Male
 
 
 
 
Female
 
 
 
 
Urinary bladder.
 
 
 
 
Urinary bladder.
 
 
 
 
Proximal portion of urethra.
 
 
 
 
Urethra.
 
 
 
 
Bulbo-urethral glands.
 
 
 
 
Bulbo-vestibular glands.
 
 
Urogenital sinus ....
 
 
The rest of the urethra.
 
 
 
 
Vestibule.
 
 
Genital tubercle. . . .
 
 
Penis.
 
 
 
 
Clitoris.
 
 
Genital folds
 
 
Prepuce and integument of
 
 
penis.
 
 
Labia minora
 
 
Genital swellings... .
 
 
Scrotum.
 
 
 
 
Labia majora.
 
 
 
It is stated above that the layers which compose the walls of the scrotum are identical with those of the abdominal wall. This may be seen in
detail from the following scheme:
 
 
 
THE DESCENT OF THE OVARIES 365
 
Abdominal Walls. Scrotum.
 
Integument. Integument.
 
Superficial fascia. Dartos.
 
External oblique muscle. Intercolumnar fascia.
 
Internal oblique muscle. Cremasteric fascia.
 
Transverse muscle. Infundibuliform fascia.
 
Peritoneum. Tunica vaginalis.
 
Numerous anomalies, depending upon an inhibition or excess of the
development of the parts, may occur in connection with the external
genitalia. Should, for instance, the lips of the groove on the vestibular
surface of the penis fail to fuse, the penial portion of the urethra remains
incomplete, constituting a condition known as hypospadias, a condition
whic,h offers a serious bar to the fulfilment of the sexual act. If the
hypospadias is complete and there be at the same time an imperfect
development of the penis, as frequently occurs in such cases, the male
genitalia closely resemble those of the female and a condition is produced
which is usually known as hermaphroditism. It is noteworthy that in
such cases there is frequently a somewhat excessive development of the
uterus masculinus, and a similar condition may be produced in the
female by an excessive development of the clitoris. Such cases, however,
which concern only the accessory organs of reproduction, are instances of
what is more properly termed spurious hermaphroditism, true hermaphroditism being a term which should be reserved for possible cases in
which the genital ridges give rise in the same individual to both ova and
spermatozoa. Such cases are of exceeding rarity in the human species,
although occasionally observed in the lower vertebrates, and the great
majority of the examples of hermaphroditism hitherto observed are cases
of the spurious variety.
 
The Descent of the Ovaries and Testes. — The positions
finally occupied by the ovaries and testes are very different from
those which they possess in the earlier stages of development, and
this is especially true in the case of the testes. The change of position
is partly due to the rate of growth of the inguinal ligaments being
less than that of the abdominal walls, the reproductive organs being
thereby drawn downward toward the inguinal regions where the
ligaments are attached. The point of attachment is beneath the
bottom of a slight pouch of peritoneum which projects a short distance into the substance of the genital swellings and is known as the
canal of Nuck in the female, and in the male as the vaginal process.
 
In the female a second factor combines with that just mentioned.
 
 
 
3 66
 
 
 
THE DESCENT OF THE TESTES
 
 
 
The relative shortening of the inguinal ligaments acting alone
would draw the ovaries toward the inguinal regions, but since they
are united to the uterus by the ovarian ligaments movement in that
direction is prevented and the ovaries come to lie in the recto-uterine
compartment of the pelvic cavity.
 
With the testes the case is more complicated, since in addition to
the relative shortening of the inguinal ligaments there is an elongation of the vaginal processes into the substance of the genital swellings, and it must be remembered that the testes, like the ovaries, are
primarily connected with the peritoneum. Three stages may be
recognized in the descent of the testes. The first of these depends
 
 
 
 
Fig. 222. — Diagrams Illustrating the Descent of the Testis.
il, Inguinal ligament; m, muscular layer; s, skin and dartos of the scrotum; t, testis;
tv, tunica vaginalis ; vd, vas deferens ; vp, vaginal process of peritoneum. — (After Hertwig.)
 
on the slow rate of elongation of the inguinal ligaments or gubernacula. It lasts until about the fifth month of development, when
the testes lie in the inguinal region of the abdomen, but during this
month the elongation of the gubernaculum becomes more rapid and
brings about the second stage, during which there is a slight ascent
of the testes, so that they come to lie a little higher in the abdomen.
This stage is, however, of short duration, and is succeeded by the
stage of the final descent, which is characterized by the elongation
of the vaginal processes of the peritoneum into the substance of the
scrotum (Fig. 222, A). Since the gubernaculum is attached to the
 
 
 
THE DESCENT OF TBE TESTES 367
 
abdominal wall beneath this process, and since its growth has again
diminished, the testes gradually assume again their inguinal position,
and are finally drawn down into the scrotum with the vaginal
processes.
 
The condition which is thus acquired persists for some time after
birth, the testicles being readily pushed upward into the abdominal
cavity along the cavity by which they descended. Later, however,
the size of the openings of the vaginal processes into the general
peritoneal cavity becomes greatly reduced, so that each process
becomes converted into an upper narrow neck and a lower sac-like
cavity (Fig. 222, B), and, still later, the walls of the neck portion fuse
and become converted into a solid cord, while the lower portion,
wrapping itself around the testis, becomes the tunica vaginalis (tv).
By these changes the testes become permanently located in the scrotum. During the descent of the testes the remains of each Wolffian
body, the epididymis, and the upper part of each vas deferens
together with the spermatic vessels and nerves, are drawn down into
the scrotum, and the mesenterial fold in which they were originally
contained also practically disappears, becoming converted into a
sheath of connective tissue which encloses the vas deferens and the
vessels and nerves, binding them together into what is termed the
spermatic cord. The mesorchium, which united the testis to the
peritoneum enclosing the Wolffian body, does not share in the degeneration of the latter, but persists as a fold extending between the
epididymis and the testis and forming the sinus epididymis.
 
In the text-books of anatomy the spermatic cord is usually described
as lying in an inguinal canal which traverses the abdominal walls obliquely
immediately above Poupart's ligament. So long as the lumen of the neck
portion of the vaginal process of peritoneum remains patent there is such
a canal, placing the cavity of the tunica vaginalis in communication with
the general peritoneal cavity, but the cord does not traverse this canal,
but lies outside it in the retroperitoneal connective tissue. When,
however, the neck of the vaginal process disappears, a canal no longer
exists, although the connective tissue which surrounds the spermatic
cord and unites it with the tissues of the abdominal walls is less dense than
the neighboring tissues, so that the cord may readily be separated from
these and thus appear to He in a canal.
 
 
 
368 LITERATURE
 
LITERATURE.
 
B. M. Allen: "The Embryonic Development of the Ovary and Testes in Mammals,"
 
Amer. Journ. of AnaL, in, 1904.
J. L. Bremer: "Morphology of the Tubules of the Human Testis and Epididymis,"
 
Amer. Journ. Anat., xi, 1911.
E. J. Evatt: "A Contribution to the Development of the Prostate in Man," Journ.
 
Anat. and Phys., xliii, 1909.
E. J. Evatt: " A Contribution to the Development of the Prostate Gland in the Human
 
Female," Journ. Anat. and Phys., xlv, 1911.
W. Felix: " Entwickelungsgeschichte des Exkretions-sy stems," Ergebn. der Anat. und
 
Entwicklungsgesch., xni, 1903.
W. Felix: "Die Entwicklung der Ham- und Geschlechtsorgane," in Keibel-Mall
 
Human Embryology, II, 1912.
A. Fleischmann: " Morphologische Studien liber Kloake und Phallus der Amnioten,
 
Morphol. Jarhbuch, xxx, xxxii und xxxvi, 1902, 1904, 1907.
O. Frankl: "Beitrage zur Lehre vom Descensus testiculorum," Sitzungsber. der kais.
 
Akad. Wissensch. Wien, Math.-Naturwiss. Classe, cix, 1900.
S. P. Gage: "A Three Weeks Human Embryo, with especial reference to the Brain
 
and the Nephric System," Amer. Journ. of Anat., rv, 1905.
D. B. Hart: " The Nature and Cause of the Physiological Descent of the Testes,"
 
Journ. Anat. and Phys., xliv, 1909.
 
D. B. Hart: " The Physiological Descent of the Ovaries in the HumanFoetus," Journ.
 
Anat. and Phys., xliv, 1909.
 
E. Hauch: "Ueber die Anatomie und Entwicklung der Nieren," Anat. Hefte, xxii,
 
1903.
G. C. Huber: "On the Development and Shape of the Uriniferous Tubules of Certain
 
of the Higher Mammals," Amer. Journ. of Anat., rv, Suppl. 1905.
J. Janosik: "Histologisch-embryologische Untersuchungen uber das Urogenitalsystem,"
 
Sitzungsber. der kais. Akad. Wissensch. Wien, Math.-Naturwiss. Classe, xci, 1887
J. Janosik: "Ueber die Entwicklung der Nachniere bei den Amnioten," Arch, fur
 
Anat. u. Phys., Anat. Abth., 1907.
J. Janosik: "Entwicklung des Nierenbeckens beim Menschen," Arch, fitr mikrosk.
 
Anat., lxxviii, 191 1.
 
F. Keibel: "Zur Entwickelungsgeschichte des menschlichen Urogenital-apparatus,"
 
Archiv fiir Anat. und Physiol., Anat. Abth., 1896.
J. B. Macallum: "Notes on the Wolffian Body of Higher Mammals," Amer. Journ.
 
Anat., 1, 1902.
E. Martin: "Ueber die Anlage der Urniere beim Kaninchen," Archiv fiir Anat. und
 
Physiol., Anat. Abth., 1888.
H. Meyer: "Die Entwickelung der Urnieren beim Menschen," Archiv fiir mikrosk.
 
Anat., xxxvi, 1890.
R. Meyer: "Zur Kenntnis des Gartner'schen Ganges besonders in der Vagina und
 
dem Hymen des Menschen," Arch, fur mikrosk. Anat., lxxiii, 1909.
R. Meyer: "Zur Entwicklungsgeschichte und Anatomie des utriculus prostaticus beim
 
Menschen," Arch, fiir mikrosk. Anat., lxxtv, 1909
 
 
 
LITERATURE 369
 
G. VON Mihalkovicz : " Untersuchungen iiber die Entwickelung des Ham- und
 
Geschlechtsapparates der Amnioten," Internat. Monatsschrift fiir Anat. und
 
Physiol., 11, 1885.
W. Nagel: "Ueber die Entwickelung des Urogenitalsystems des Menschen," Archiv
 
fiir mikros. Anat., xxxiv, 1889.
W. Nagel: "Ueber die Entwickelung des Uterus und der Vagina beim Menschen,"
 
Archiv fiir mikros k. Anat., xxxvn, 1891.
W. Nagel: "Ueber die Entwickelung der innere und aussere Genitalien biem mensch
lichen Weibes," Archiv fiir Gynakol., xlv, 1894.
K. Peter: "Untersuchungen iiber Bau und Entwicklung der Niere. I. Die Nieren
kanalchen des Menschen und einiger Saugetiere, Jena, 1909.
A. G. Pohlman: "The Development of the Cloaca in Human Embryos." Amer. Journ.
 
of Anat., xii, 191 1.
W. Rubaschkin: " Ueber die Urgeschlechtszellen bei Saugetiere,'Mwa<. Hefte, xxxix,
 
1909.
K. E. Schrelner: "Ueber die Entwicklung der Amniotenniere," Zeit. fiir wissensch.
 
Zool., lxxi, 1902.
O. Stoerk: "Beitrag zur Kenntnis des Aufbaues der menschlichen Niere," Anat.
 
Hefte, xxill, 1904.
J. Tandler: "Ueber Vornieren-Rudimente beim menschliche Embryo," Anat. Hefte,
 
xxvni, 1905.
F. J. Taussig: "The Development of the Hymen," Amer. Journ. Anat., viii, 1908.
F. Tourneux: " Sur le developpement et revolution du tubercule genital chez le foetus
 
humain dans les deux sexes," Journ. de I' Anat. et de la Physiol., xxv, 1889.
S. Weber: " Zur Entwickelungsgeschichte des uropoetischen Apparates bei Saugern,
 
mit besonderer Beriicksichtigung der Urniere zur Zeit des Auftretens der blei
benden Niere," Morphol. Arbeiten, vil, 1897.
 
 
 
24
 
 
 
CHAPTER XIV.
THE SUPRARENAL SYSTEM OF ORGANS.
 
To the suprarenal system a number of bodies of peculiar structure, probably concerned with internal secretion, may be assigned.
In the fishes they fall into two distinct groups, the one containing
organs derived from the ccelomic epithelium and known as intervened
organs, and the other consisting of organs derived from the sympathetic nervous system and which, on account of the characteristic
affinity they possess for chromium salts, have been termed the
chroma ffine organs. But in the amphibia and amniote vertebrates,
while both the groups are represented by independent organs, yet
they also become intimately associated to form the suprarenal bodies,
so that, notwithstanding their distinctly different origins, it is
convenient to consider them together.
 
The Development of the Suprarenal Bodies. — The suprarenal bodies make their appearance at an early stage, while the
Wolffian bodies are still in a well-developed condition, and they are
situated at first to the medial side of the upper ends of these structures (Fig. 211, sr). Their final relation to the metanephros is a
secondary event, and is merely a topographic relation, there being
no developmental relation between the two structures.
 
In the human embryo they make their appearance at about the
beginning of the fourth week of development as a number of proliferations of the ccelomic epithelium, which project into the subjacent mesenchyme, and are situated on either side of the median
line between the root of the mesentery and the upper portion of the
Wolffian body. The various proliferations soon separate from the
epithelium and unite to form two masses situated in the mesenchyme,
one on either side of the upper portion of the abdominal aorta. In
certain forms, such as the rabbit, the primary proliferations arise
 
37°
 
 
 
DEVELOPMENT OF THE SUPRARENAL BODIES
 
 
 
371
 
 
 
from the bottom of depressions of the ccelomic epithelium (Fig. 223),
but in the human embryo these depressions do not form.
 
Up to this stage the structure is a pure interrenal organ, but
during the fifth week of development masses of cells, derived from
the abdominal portion of the sympathetic nervous system, begin to
penetrate into each of the interrenal masses (Fig. 224), and form
strands traversing them. At about the ninth or tenth week fatty
granules begin to appear in the interrenal cells and somewhat later,
about the fourth month, the sympathetic constituents begin to show
their chromaffine characteristics. The two tissues, however, remain
intermingled for a considerable time, and it is not until a much later
 
 
 
 
 
 
Ao
 
 
 
& Sr ns
 
tern J „-.-—
 
 
 
WC
 
 
 
-fvd
 
 
 
Fig. 223. — Section through a Portion of the Wolffian Ridge of a Rabbit
 
Embryo of 6.5 mm.
 
Ao, Aorta; ns, nephrostome; Sr, suprarenal body; vc, cardinal vein; wc, tubule of
 
Wolffian body; wd, Wolffian duct. — (Aichel.)
 
period that they become definitely separated, the sympathetic
elements gradually concentrating in the center of the compound
organ to become its medullary substance, while the interrenal tissue
forms the cortical substance. Indeed, it is not until after birth that
the separation of the two tissues and their histological differentiation
is complete, occasional masses of interrenal tissue remaining
imbedded in the medullary substance and an immigration of
sympathetic cells continuing until at least the tenth year (Wiesel).
 
A great deal of difference of opinion has existed in the past concerning
the origin of the suprarenal glands. By several authors they have been
regarded as derivatives in whole or in part of the excretory apparatus,
some tracing their origin to the mesonephros and others even to the pronephros. The fact that in some mammals the cortical (interrenal) cells are
 
 
 
372 DEVELOPMENT OF THE SUPRARENAL BODIES
 
formed from the bottom of depressions of the coelomic epithelium seemed
to lend support to this view, but it is now pretty firmly established that
the appearances thus presented do not warrant the interpretation placed
upon them and that the interrenal tissue is derived from the ccelomic
epithelium quite independently of the nephric tubules. That the chromaffine tissue is a derivative of the sympathetic nervous system has long
been recognized.
 
During the development of the suprarenal glands portions of
their tissue may be separated as the result of unequal growth and
form what are commonly spoken of as accessory suprarenal glands,
although, since they are usually composed solely of cortical sub
 
 
 
... â–  /
 
'•'§'''•. ' . . .■/.
 
 
 
$M
 
 
 
S.B.
 
Fig. 224. — Section through the Suprarenal Body of an Embryo of 17 mm.
 
A, Aorta; R, interrenal portion; S, sympathetic nervous system; SB, sympathetic cells
 
penetrating the interrenal portion. — (Wiesel.)
 
stance, the term accessory interrenal bodies would be more appropriate.
They may be formed at different periods of development and occur
in various situations, as for instance, in the vicinity of the kidneys
or even actually imbedded in their substance, on the walls of neighboring blood-vessels, in the retroperitoneal tissue below the level of
the kidneys, and in connection with the organs of reproduction, in
the spermatic cord, epididymis or rete testis of the male and in the
broad ligament of the female.
 
It seems probable that the bodies associated with the reproductive
 
 
 
DEVELOPMENT OF THE SUPRARENAL BODIES
 
 
 
373
 
 
 
apparatus are separated from the main mass of interrenal tissue
before the immigration of the sympathetic tissue and before the
descent of the ovaries or testes, while those which occur at higher
levels are of later origin, and in some cases may contain some medullary substance, being then true accessory suprarenals. Such
bodies are, however, comparatively rare, the great majority of the
accessory bodies being composed of interrenal tissue alone.
 
Independent chromamne organs also occur, among them the
 
 
 
 
Fig. 225. — Section of a Cell Ball from the Intercarotid Ganglion of Man
 
be, Blood capillaries; ev, efferent vein; S, connective-tissue septum; I, trabecular —
 
(From Bohm and Davidoff, after Schaper.)
 
 
 
intercarotid ganglia and the organs of Zuckerkandl being especially
deserving of note. It may also be pointed out, however, that the
chromamne cells have the same origin as the cells of the sympathetic
ganglia and may sometimes fail to separate from the latter, so that
the sympathetic ganglia and plexuses frequently contain chromamne
cells.
 
The Intercarotid Ganglia. — These structures, which are fre
 
 
374 T TTF. INTERCAROTID GANGLIA
 
quently though incorrectly termed carotid glands, are small bodies
about 5 mm. in length, which lie usually to the mesial side of the
upper ends of the common carotid arteries. They possess a very
rich arterial supply and stand in intimate relation with the branches
of an intercarotid sympathetic plexus, and, furthermore, they are
characterized by possessing as their specific constituents markedly
chromamne cells, among which are scattered stellate cells resembling
the cells of the sympathetic ganglia.
 
They have been found to arise in pig embryos of 44 mm. by the
separation of cells from the ganglionic masses scattered throughout
the carotid sympathetic plexuses. These cells, which become the
chromamne cells, arrange themselves in round masses termed cell
balls, many of which unite to form each ganglion, and in man each
cell ball becomes broken up into trabecule by the blood-vessels
(Fig. 225) which penetrate its substance, and the individual balls are
separated from one another by considerable quantities of connective
tissue.
 
Some confusion has existed in the past as to the origin of this structure.
The mesial wall of the proximal part of the internal carotid artery becomes
considerably thickened during the early stages of development and the
thickening is traversed by numerous blood lacunae which communicate
with the lumen of the vessel. This condition is perhaps a relic of the
branchial capillaries which in the lower gill-breathing vertebrates represent the proximal portion of the internal carotid, and has nothing to do
with the formation of the intercarotid ganglion, although it has been
believed by some authors (Schaper) that the ganglion was derived from
the thickening of the wall of the vessel. The fact that in some animals,
such as the rat and the dog, the ganglion stands in relation with the
external carotid and receives its blood- supply from that vessel is of importance in this connection.
 
The thickening of the internal carotid disappears in the higher
vertebrates almost entirely, but in the Amphibia it persists throughout
life, the lumen of the proximal part of the vessel being converted into a
fine meshwork by the numerous trabecular which traverse it. This
carotid labyrinth has been termed the carotid gland, a circumstance
which has probably assisted in producing confusion as to the real significance of the intercarotid ganglion.
 
The Organs of Zuckerkandl. — In embryos of 14.5 mm. there
have been found, in front of the abdominal aorta, closely packed
 
 
 
THE ORGANS OF ZUCKERKANDL
 
 
 
375
 
 
 
groups of cells which resemble in appearance the cells composing
the ganglionated cord, two of these groups, which extend downward
along the side of the aorta to below the point of origin of the inferior
mesenteric artery, being especially distinct. These cell groups give
rise to the ganglia of the prevertebral sympathetic plexuses and also
 
 
 
 
Fig. 226. — Organs of Zuckerkandl from a New-born Child.
a, Aorta; ci, inferior vena cava; i.c, common iliac artery; mi, inferior mesenteric
artery; n.l and n.r, left and right accessory organs; pl.a, aortic plexus; u, ureter; v.r.s,
left renal vein. — (Zuckerkandl.)
 
to peculiar bodies which, from their discoverer, may be termed the
organs of Zuckerkandl. Each body stands in intimate relation with
the fibers of the sympathetic plexuses and has a rich blood-supply,
resembling in these respects the intercarotid ganglia, and the resem
 
 
376 LITERATURE
 
blance is further increased by the fact that the specific cells of the
organ are markedly chromamne.
 
i At birth the bodies situated in the upper portion of the abdominal
cavity have broken up into small masses, but the two lower ones,
mentioned above, are still well defined (Fig. 226). Even these, how- #
ever, seem to disappear later on and no traces of them have as yet
been found in the adult.
 
LITERATURE.
 
A. Kohn: "Ueber den Bau und die Entwickelung der sog. Carotisdruse," Archiv.
 
fur mikrosk. Anat., lvi, 1900.
A. Kohn: "Das chromaffine Gewebe," Ergebn. der Anat. und Entwickelungsgesch.,
 
xii, 1902.
H. Poll: "Die vergleichende Entwicklungsgeschichte der Nebennierensysteme der
 
Wirbeltiere," Hertwig's Handb. der vergl. und exper. Entwicklungslehre der Wirbel
tiere, in, 1906.
A. Sotjlie: "Recherches sur le developpement des capsules surrenales chez les
 
Vertebres," Journ. de V Anat. et de la Physiol., xxxix, 1903.
J. Wiesel: "Beitrage zur Anatomie und Entwickelung der menschlichen Nebenniere,"
 
Anat. Heft., xix, 1902.
E. Zuckerkandl: "Ueber Nebenorgane des Sympathicus im Retroperitonealraum
 
des Menschen," Verhandl. Anat. Gesellsch., xv, 1901.
 
 
 
CHAPTER XV.
 
THE DEVELOPMENT OF THE NERVOUS
SYSTEM.
 
The Histogenesis of the Nervous System. — The entire central
nervous system is derived from the cells lining the medullary groove,
whose formation and conversion into the medullary canal has already
been described (p. 72). When the groove is first formed, the cells
lining it are somewhat more columnar in shape than those on either
side of it, though like them they are arranged in a single layer;
later they increase by mitotic division and arrange themselves in
several layers, so that the ectoderm of the groove becomes very much
thicker than that of the general surface of the body. At the same
time the cell boundaries, which were originally quite distinct,
gradually disappear, the tissue becoming a syncytium. While its
tissue is in this condition the lips of the medullary groove unite,
and the subsequent differentiation of the canal so formed differs
somewhat in different regions, although a fundamental plan may be
recognized. This plan is most readily perceived in the region which
becomes the spinal cord, and may be described as seen in that region.
 
Throughout the earlier stages, the cells lining the inner wall of
the medullary tube are found in active proliferation, some of the
cells so produced arranging themselves with their long axes at right
angles to the central canal (Fig. 227), while others, whose destiny
is for the most part not yet determinable, and which therefore may
be termed indifferent cells are scattered throughout the syncytium.
At this stage a transverse section of the medullary tube shows it to
be composed of two well-defined zones, an inner one immediately
surrounding the central canal and composed of the indifferent cells
and the bodies of the inner or ependymal cells, and an outer one consisting of branched prolongations of the syncytial cytoplasm. This
 
377
 
 
 
378 THE HISTOGENESIS OF THE NERVOUS SYSTEM
 
outer layer is termed the marginal velum (Randschleier) (Fig. 227,
m). The indifferent cells now begin to wander outward to form
a definite layer, termed the mantle layer, lying between the marginal
velum and the bodies of the ependymal cells (Fig. 228), and when
this layer has become well established the cells composing it begin
to divide and to differentiate into (1) cells termed neuroblasts,
destined to become nerve-cells, and (2) others which appear to be
supportive in character and are termed neuroglia cells (Fig. 228, B).
 
 
 
 
6r ° %
 
 
 
 
 
 
,'. I"' «9
 
 
 
W:
 
 
 
cs
 
 
 
Fig. 227. — Transverse Section through the Spinal Cord of a Pig Embryo
of 30 mm., the Upper Part showing the Appearance produced by the Silver
Method of Demonstrating the Neuroglia Fibers.
 
a, Ependyma of floor plate; b, boundary between mantle layer and marginal
zone; cs, mesenchymal connective- tissue syncytium; ep, ependymal cells; i, ingrowth
of connective tissue; m, marginal velum; mn, mantle layer; mv, mantle layer of floor
plate; p, pia mater; r, neuroglia fibers. — (Hardesty.)
 
The latter are for the" most part small and are scattered among the
neuroblasts, these, on the other hand, being larger and each early
developing a single strong process which grows out into the marginal
velum and is known as an axis-cylinder. At a later period the
 
 
 
THE HISTOGENESIS OF THE NERVOUS SYSTEM
 
 
 
379
 
 
 
neuroblasts also give rise to other processes, termed dendrites, more
slender and shorter than the axis-cylinders, branching repeatedly,
and, as a rule, not extending beyond the limits of the mantle layer.
In connection with the neuroglia cells peculiar neuroglia fibrils
develop very much in the same way as the fibers are formed in mesenchymal connective tissue. That is to say, they are formed from the
peripheral portions of the cytoplasm of the neuroglial and ependymal cells. But since these cells are connected i together to form a
syncytium the fibrils are not confined to the territories of the indi
 
 
o ^i^r
 
Ote%0«»
 
OqQ q ®»,
 
■ooq.©^
 
u rtOO^*>^ a
 
 
 
D ooo§
 
 
 
 
 
 
o o°b
 
u o u o °
 
 
 
 
 
 
Fig. 228. — Diagrams showing the Development of the Mantle Layer in the
 
Spinal Cord.
The circles, indifferent cells; circles with dots, neuroglia cells; shaded cells, germinal
cells; circles with cross, germinal cells in mitosis; black cells, nerve-cells. — {Schaper.)
 
vidual cells, but may extend far beyond these, passing in the syncytium from the territory of one neuroglial cell to another, many of
those, indeed, arising in connection with the ependymal cells extending throughout the entire thickness of the medullary wall (Fig. 227).
The fibrils branch abundantly and form a supportive network
extending through all portions of the central nervous system.
The axis-cylinder processes of the majority of the neuroblasts on
reaching the marginal velum bend upward or downward and, after
 
 
 
3 8o
 
 
 
THE HISTOGENESIS OF THE NERVOUS SYSTEM
 
 
 
traversing a greater or less length of the cord, re-enter the mantle
layer and terminate by dividing into numerous short branches which
come into relation with the dendrites of adjacent neuroblasts.
The processes of certain cells situated in the ventral region of the
mantle zone pass, however, directly through the marginal velum
out into the surrounding tissues and constitute the ventral nerveroots (Fig. 231).
 
The dorsal nerve-roots have a very different origin. In embryos
 
of about 2.5 mm., in which the
medullary canal is only partly
closed (Fig. 53), the cells which
lie along the line of transition
between the lips of the groove
and the general ectoderm form
a distinct ridge readily recognized in sections and termed the
neural crest (Fig. 229, A). When
the lips of the groove fuse together the cells of the crest unite
to form a wedge-shaped mass,
completing the closure of the
canal (Fig. 229, B), and later
proliferate so as to extend outward over the surface of the
canal (Fig. 229, C). Since this
proliferation is most active in the
regions of the crest which correspond to the mesodermic somites
there is formed a series of cell masses, arranged segmentally
and situated in the mesenchyme at the sides of the medullary
canal (Fig. 214). These cell masses represent the dorsal root
ganglia, and certain of their constituent cells, which may also be
termed neuroblasts, early assume a fusiform shape and send out a
process from each extremity. One of these processes, the axiscylinder, grows inward toward the medullary canal and penetrates its
 
 
 
 
Fig. 229. — Three Sections through
the Medullary Canal of an Embryo
of 2.5 mm. — (vonLenhossek.)
 
 
 
THE HISTOGENESIS OF THE NERVOUS SYSTEM 381
 
marginal velum, and, after a longer or shorter course in this zone,
enters the mantle layer and comes into contact with the dendrites of
some of the central neuroblasts. The other process extends peripherally and is to be regarded as an extremely elongated dendrite.
The processes from the cells of each ganglion aggregate to form a
nerve, that formed by the axis-cylinders being the posterior root of
a spinal nerve, while that formed by the dendrites soon unites with
the ventral nerve-root of the corresponding segment to form the
main stem of a spinal nerve.
 
There is thus a very important difference in the mode of development of the two nerve-roots, the axis-cylinders of the ventral roots
 
 
 
 
Fig. 230. — Cells from the Gasserian Ganglion of a Guinea-pig Embryo.
a, Bipolar cell; b and c, transitional stages to d, T-shaped cells. — (von Gehuchten.)
 
arising from cells situated in the wall of the medullary canal and growing outward (centrif ugally) , while those of the dorsal root spring
from cells situated peripherally and grow inward (centripetally)
toward the medullary canal. In the majority of the dorsal root
ganglia the points of origin of the two processes of each bi-polar
cell gradually approach one another (Fig. 230, b) and eventually
come to rise from a common stem, a process of the cell-body, which
thus assumes a characteristic T form (Fig. 230, d).
 
From what has been said it will be seen that each axis-cylinder is
an outgrowth from a single neuroblast and is part of its cell-body, as are
also the dendrites. Another view has, however, been advanced to the
 
 
 
382 THE HISTOGENESIS OF THE NERVOUS SYSTEM
 
effect that the nerve fibers first appear as chains of cells and that the axiscylinders, being differentiated from the cytoplasm of the chains, are really
multicellular products. Many difficulties stand in the way of the acceptance of this view and recent observations, both histogenetic (Cajal)
and experimental (Harrison), tend to confirm the unicellular origin of
the axis-cylinders. The embryological evidence therefore goes to support
the neurone theory, which regards the entire nervous system as composed of definite units, each of which corresponds to a single cell and is
termed a neurone.
 
By the development of the axis-cylinders which occupy the meshes
of the marginal velum, that zone increases in thickness and comes
to consist principally of nerve-fibers, while the cell-bodies of the
neurones of the cord are situated in the mantle zone. No such definite distinction of color in the two zones as exists in the adult is,
however, noticeable until a late period of development, the medullary
sheaths, which give to the nerve-fibers their white appearance not
beginning to appear until the fifth month and continuing to form
from that time onward until after birth. The origin of the myelin
which composes the medullary sheaths is as yet uncertain, although
the more recent observations tend to show that it is picked out from
the blood and deposited around the axis-cylinders in some manner
not yet understood. Its appearance is of importance as being
associated with the beginning of the functional activity of the
nerve-fibers.
 
In addition to the medullary sheaths the majority of the fibers
of the peripheral nervous system are provided with primitive sheaths,
which are lacking, however, to the fibers of the central system.
They are formed by cells which wander out from the dorsal
root-ganglia and are therefore of ectodermal origin. Frog larvae
deprived of their neural crests at an early stage of development
produce ventral nerve-fibers altogether destitute of primitive
sheaths (Harrison).
 
Various theories have been advanced to account for the formation of
the medullary sheaths. It has been held that the myelin is formed at the
expense of the outermost portions of the axis-cylinders themselves (von
Kolliker), and on the other hand, it has been regarded as an excretion
of the cells which compose the primitive sheaths surrounding the fibers
 
 
 
THE SPINAL CORD 383
 
(Ranvier) , a theory which is, however, invalidated by the fact that myelin is
formed around the fibers of the central nervous system which possess no
primitive sheaths. As stated above, the more recent observations
(Wlassak) indicate its exogenous origin.
 
It has been seen that the central canal is closed in the mid-dorsal
line by a mass of cells derived from the neural crest. These cells
do not take part in the formation of the mantle layer, but become
completely converted into ependymal tissue, and the same is true of
the cells situated in the mid-ventral line of the canal. In these two
regions, known as the roof -plate and floor -plate respectively, the
wall of the canal has a characteristic structure and does not share
to any great extent in the increase of thickness which distinguishes
the other regions (Fig. 231). In the lateral walls of the canal there
is also noticeable a differentiation into two regions, a dorsal one
standing in relation to the ingrowing fibers from the dorsal root
ganglia and known as the dorsal zone, and a ventral one, the ventral
zone, similarly related to the ventral nerve-roots. In different
regions of the medullary tube these zones, as well as the roof- and
floor-plates, undergo different degrees of development, producing
peculiarities which may now be considered.
 
Trie Development of the Spinal Cord. — Even before the lips
of the medullary groove have met a marked enlargement of the
anterior portion of the canal is noticeable, the region which will
become the brain being thus distinguished from the more posterior
portion which will be converted into the spinal cord. When the
formation of the mesodermic somites is completed, the spinal cord
terminates at the level of the last somite, and in this region still
retains its connection with the ectoderm of the dorsal surface of
the body; but in that portion of the cord which is posterior to the
first coccygeal segment the histological differentiation does not
proceed beyond the stage when the walls consist of several layers of
similar cells, the formation of neuroblasts and nerve-roots ceasing
with the segment named. After the fourth month the more differentiated portion elongates at a much slower rate than the surrounding tissues and so appears to recede up the spinal canal, until its
 
 
 
384 THE SPINAL CORD
 
termination is opposite the second lumber vertebra. The less
differentiated portion, which retains its connection with the ectoderm
until about the fifth month, is, on the other hand, drawn out into a
slender filament whose cells degenerate during the sixth month,
except in its uppermost part, so that it comes to be represented
throughout the greater part of its extent by a thin cord composed
of pia mater. This cord is the structure known in the adult as the
filum terminate, and lies in the center of a leash of nerves occupying
the lower part of the spinal canal and termed the cauda equina.
The existence of the cauda is due to the recession of the cord which
necessitates for the lower lumbar, sacral and coccygeal nerves, a
descent through the spinal canal for a greater or less distance,
before they can reach the intervertebral foramina through which
they make their exit.
 
In the early stages of development the central canal of the cord
is quite large and of an elongated oval form, but later it becomes
somewhat rhomboidal in shape (Fig. 231, A), the lateral angles
marking the boundaries between the dorsal and ventral zones.
As development proceeds the sides of the canal in the dorsal region
gradually approach one another and eventually fuse, so that this
portion of the canal becomes obliterated (Fig. 231, B) and is indicated by the dorsal longitudinal fissure in the adult cord, the central
canal of which corresponds to the ventral portion only of the embryonic cavity. While this process has been going on both the roofand the floor-plate have become depressed below the level of the
general surface of the cord, and by a continuance of the depression
of the floor-plate — a process really due to the enlargement and
consequent bulging of the ventral zone — the anterior median fissure
is produced, the difference between its shape and that of the dorsal
fissure being due to the difference in its development.
 
The development of the mantle layer proceeds at first more
rapidly in the ventral zone than in the dorsal, so that at an early
stage (Fig. 231, A) the anterior column of gray matter is much more
pronounced, but on the development of the dorsal nerve-roots the
formation of neuroblasts in the dorsal zone proceeds apace, resulting
 
 
 
THE SPINAL CORD
 
 
 
385
 
 
 
in the formation of a dorsal column. A small portion of the zone,
situated between the point of entrance of the dorsal nerve-roots and
the roof-plate, fails, however, to give rise to neuroblasts and is
entirely converted into ependyma. This represents the future
funiculus gracilis (fasciculus of Goll) (Fig. 231, A, cG), and at the
point of entrance of the dorsal roots into the cord a well-marked
oval bundle of fibers is formed (Fig. 231, A, ob) which, as develop
 
 
 
Fig. 231. — Transverse Sections through the Spinal Cords of Embryos .of (A)
about Four and a Half Weeks and (B) about Three Months'.
cB, Fasciculus of Burdach; cG, fasciculus of Goll; dh, dorsal column; dz, dorsal
zone; fp, floor-plate; ob, oval bundle; rp, roof-plate; vh, ventral column; vz, ventral zone.
— {His.)
 
ment proceeds, creeps dorsally over the surface of the dorsal horn
until it meets the lateral surface of the fasciculus of Goll, and, its
further progress toward the median line being thus impeded, it
insinuates itself between that fasciculus and the posterior horn to
form the funiculus cuneatus {fasciculus of Burdach) (Fig. 231, B, cB).
 
Little definite is as yet known concerning the development of the
other fasciculi which are recognizable in the adult cord, but it seems
 
25
 
 
 
3 86
 
 
 
THE BRAIN
 
 
 
A "
 
;
 
 
t
 
 
ffe
 
 
V/tt
 
 
1 — H
 
 
 
 
/»//
 
 
\-mt
 
 
 
certain that the lateral and anterior cerebro-spinal (pyramidal) fasciculi
are composed of fibers which grow downward in the meshes of the
marginal velum from neuroblasts situated in the cerebral cortex, while
the cerebellospinal (direct cerebellar) fasciculi and the fibers of the
ground-bundles have their origin from cells of the mantle layer of the
cord.
 
The myelination of the fibers of the spinal cord begins between the
fifth and sixth months and appears first in the funiculi cuneati, and about
 
a month later in the funiculi graciles.
The myelination of the great motor paths,
the lateral and anterior cerebro-spinal fasciculi, is the last to develop, appearing toward the end of the ninth month of fetal
life.
 
The Development of the Brain.
 
— The enlargement of the anterior
portion of the medullary canal does
not take place quite uniformly, but is
less along two transverse lines than else
where, so that the brain region early
becomes divided into three primary
vesicles which undergo further differentiation as follows. Upon each side
of the anterior vesicle an evagination
appears and becomes converted into a
club-shaped structure attached to the
ventral portion of the vesicle by a
pedicle. These evaginations (Fig.
232, op) are known as the optic evaginations, and being concerned in the
formation of the eye will be considered
in the succeeding chapter. After their
formation the antero-lateral portions
of the vesicle become bulged out into two protuberances (h) which
rapidly increase in size and give rise, eventually to the two cerebral
hemispheres, which form, together with the portion of the vesicle
which lies between them, what is termed the telencephalon or forebrain, the remainder of the vesicle giving rise to what is known as
 
 
 
 
Fig. 232. — Reconstruction of
the Brain of an Embryo of 2.15
 
MM.
 
h, Hemisphere; i, isthmus; m,
mesencephalon; mf, mid-brain flexure; mt, metencephalon ; myl, myelencephalon; nf, nape flexure; ot, otic
capsule; op, optic evagination; t,
diencephalon. — (His.)
 
 
 
THE BRAIN
 
 
 
387
 
 
 
the diencephalon or Hween-brain (Fig. 232, /). The middle vesicle is
bodily converted into the mesencephalon or mid-brain (m), but the
posterior vesicle differentiates so that three parts may be recognized :
(1) a rather narrow portion which immediately succeeds the midbrain and is termed the isthmus (i); (2) a portion whose roof and
floor give rise to the cerebellum and pons respectively, and which is
termed the metencephalon or hind-brain (mi) ; and (3) a terminal portion which is known as the medulla oblongata, or, to retain a consistent nomenclature, the myelencephalon or after-brain {my). From
each of these six divisions definite structures arise whose relations
to the secondary divisions and to the primary vesicles may be understood from the following table and from the annexed figure (Fig.
233), which represents a median longitudinal section of the brain
of a fetus of three months.
 
 
 
3rd Vesicle
 
 
 
Myelencephalon
 
 
 
Metencephalon
 
 
 
Isthmus
 
 
 
Medulla oblongata (I) .
 
/ Pons (II 1).
 
^ Cerebellum (II 2).
 
SBrachia conjunctiva (III).
Cerebral peduncles (posterior
portion) .
 
 
 
2nd Vesicle Mesencephalon
 
 
 
Cerebral peduncles (anterior portion) (IV 1).
Corpora quadrigemina (IV 2).
 
 
 
1st Vesicle <
 
 
 
Diencephalon
 
 
 
Telencephalon
 
 
 
Pars mammillaris (V 1).
Thalamus (V 2).
Epiphysis (V 3).
 
Infundibulum (VI 1).
Corpus striatum (VI 2).
Olfactory bulb (VI 3).
Hemispheres (VI 4).
 
 
 
But while the walls of the primary vesicles undergo this complex
differentiation, their cavities retain much more perfectly their
original relations, only that of the first vesicle sharing to any great
extent the modifications of the walls.
 
 
 
388 THE BRAIN
 
The cavity of the third vesicle persists in the adult as the fourth
ventricle, traversing all the subdivisions of the vesicle; that of the
second, increasing but little in height and breadth, constitutes the
aquaductus cerebri; while that of the first vesicle is continued into
the cerebral hemispheres to form the lateral ventricles, the remainder
of it constituting the third ventricle, which includes the cavity of
the median portion of the telencephalon as well as the entire cavity
of the diencephalon.
 
During the differentiation of the various divisions of the brain
certain flexures appear in the roof and floor, and to a certain extent
 
 
 
'V'i L-/
 
 
 
 
IV Z
 
 
 
iVi
 
 
 
02
 
 
 
Fig. 233. — Median Longitudinal Section of the Brain of an Embryo of the
Third Month. — (His.)
 
correspond with those already described as occurring in the embryo.
The first of these flexures to appear occurs in the region of the midbrain, the first vesicle being bent ventrally until it comes to lie at
practically a right angle with the axis of the mid-brain. This may
be termed the mid-brain flexure (Fig. 232, mf) and corresponds with
the head-bend of the embryo. The second flexure occurs in the
region of the medulla oblongata and is known as the nape flexure
(Fig. 232, nf); it corresponds with the similarly named bend of the
embryo and is produced by a bending ventrally of the entire head, so
 
 
 
THE MYELENCEPHALON 389
 
that the axis of the mid-brain comes to lie almost at right angles
with that of the medulla and that of the first vesicle parallel with it.
Finally, a third flexure occurs in the region of the metencephalon
and is entirely peculiar to the nervous system; it consists of a bending
ventrally of the floor of the hind-brain, the roof of this portion of the
brain not being affected by it, and it may consequently be known as
the pons flexure (Fig. 233).
 
In the later development the pons flexure practically disappears,
owing to the development in this region of the transverse fibers and
nuclei of the pons, but the mid-brain and nape flexures persist,
though greatly reduced in acuteness, the axis of the anterior portion
of the adult brain being inclined to that of the medulla at an angle of
about 134 degrees.
 
The Development of the Myelencephalon. — In its posterior portion
the myelencephalon closely resembles the spinal cord and has a very
similar development. More anteriorly, however, the roof-plate
(Fig. 234, rp) widens to form an exceedingly thin membrane, the
posterior velum; with the broadening of the roof-plate there is associated a broadening of the dorsal portion of the brain cavity, the
dorsal and ventral zones bending outward, until, in the anterior
portion of the after-brain, the margins of the dorsal zone have a
lateral position, and are, indeed, bent ventrally to form a reflected
lip (Fig. 234, I). The portion of the fourth ventricle contained in
this division of the brain becomes thus converted into a broad shallow
cavity, whose floor is formed by the ventral zones separated in the
median line by a deep groove, the floor of which is the somewhat
thickened floor-plate. About the fourth month there appears in the
roof-plate a transverse groove into which the surrounding mesenchyme dips, and, as the groove deepens in later stages, the mesenchyme contained within it becomes converted into blood-vessels,
forming the chorioid plexus of the fourth ventricle, a structure which,
as may be seen from its development, does not lie within the cavity
of the ventricle, but is separated from it by the portion of the roofplate which forms the floor of the groove.
 
In embryos of about 9 mm. the differentiation of the dorsal
 
 
 
39o
 
 
 
THE MYELENCEPHALON
 
 
 
and ventral zones into ependymal and mantle layers is clearly visible
(Fig. 234), and in the ventral zone the marginal velum is also well
developed. Where the fibers from the sensory ganglion of the vagus
nerve enter the dorsal zone an oval area (Fig. 234, fs) is to be seen
which is evidently comparable to the oval bundle of the cord and
consequently with the fasciculus of Burdach. It gives rise to the
solitary fasciculus of adult anatomy, and in embryos of 11 to 13 mm.
it becomes covered in by the fusion of the reflected lip of the dorsal
zone with the sides of the myelencephalon, this fusion, at the same
time, drawing the margins of the roof-plate ventrally to form a
 
 
 
 
Fig. 234. — Transverse Section through the Medulla Oblongata of
 
an Embryo of 9.1 mm.
 
dz, Dorsal zone; fp, floor-plate; /s, fasciculus solitarius; I, lip; rp, roof-plate; vz, ventral
 
zone; X and XII, tenth and twelfth nerves. — (His.)
 
secondary lip (Fig. 235). Soon after this a remarkable migration
ventrally of neuroblasts of the dorsal zone begins. Increasing
rapidly in number the migrating cells pass on either side of the solitary fasciculus toward the territory of the ventral zone, and, passing
ventrally to the ventral portion of the mantle layer, into which
fibers have penetrated and which becomes the formatio reticularis
(Fig. 235, fr), they differentiate to form the olivary body (ol).
 
The thickening of the floor-plate gives opportunity for fibers to
pass across the median line from one side to the other, and this
opportunity is taken advantage of at an early stage by the axis-cylin
 
 
THE MYELENCEPHALON
 
 
 
39 1
 
 
 
ders of the neuroblasts of the ventral zone, and later, on the establishment of the olivary bodies, other fibers, descending from the cerebellum, decussate in this region to pass to the olivary body of the
opposite side. In the lower part of the medulla fibers from the
neuroblasts of the nuclei gracilis and cuneatus, which seem to be
 
 
 
 
ol z*
 
Fig. 235. — Transverse Section through the Medulla Oblongata of an Embryo
 
of about Eight Weeks.
 
av, Ascending root of the trigeminus ;fr, reticular formation; ol, olivary body; sf, solitary
 
fasciculus; tr, restiform body; XII, hypoglossal nerve. — (His.)
 
developments from the mantle layer of the dorsal zone, also decussate
in the substance of the floor-plate; these fibers, known as the arcuate
fibers, pass in part to the cerebellum, associating themselves with
fibers ascending from the spinal cord and with the olivary fibers to
form a round bundle situated in the dorsal portion of the marginal
velum and known as the restiform body (Fig. 235, tr).
 
The principal differentiations of the zones of the myelencephalon
may be stated in tabular form as follows:
 
Roof-plate Posterior velum.
 
(Nuclei of termination of sensory roots of cranial nerves.
Nuclei gracilis and cuneatus.
The olivary bodies.
 
. ( Nuclei of origin of the motor roots of cranial nerves.
 
Ventral zones < _,, ...
 
I I he reticular formation.
 
Foor-plate The median raphe.
 
 
 
39 2
 
 
 
THE CEREBELLUM
 
 
 
The Development of the Metencephalon and Isthmus. — Our knowledge of the development of the metencephalon, isthmus, and mesencephalon is by no means as complete as is that of the myelencephalon.
The pons develops as a thickening of the portion of the brain floor
which forms the anterior wall of the pons flexure, and its transverse
fibers are well developed by the fourth month (Mihalkovicz), but all
details regarding the origin of the pons nuclei are as yet wanting.
If one may argue from what occurs in the myelencephalon, it seems
probable that the reticular formation of the metencephalon is derived
from the ventral zone, and that the median raphe represents the
floor-plate. Furthermore, the relations of the pons nuclei to the
reticular formation on the one hand, and its connection by means of
 
 
 
 
 
Fig. 236. — A, Dorsal View of the Brain or a Rabbit Embryo of 16 mm.; B, Median
 
Longitudinal Section of a Calf Embryo of 3 cm.
 
c, Cerebellum; m, mid-brain. — {Mihalkovicz?)
 
the transverse pons fibers with the cerebellum on the other, suggest
the possibility that they may be the metencephalic representatives
of the olivary bodies and are formed by a migration ventrally of
neuroblasts from the dorsal zones, such a migration having been
observed to occur (Essick).
 
The cerebellum is formed from the dorsal zones and roof-plate
of the metencephalon and is a thickening of the tissue immediately
anterior to the front edge of the posterior velum. This latter structure has in early stages a rhomboidal shape (Fig. 236, A) which
causes the cerebellar thickening to appear at first as if composed
of two lateral portions inclined obliquely toward one another. In
reality, however, the thickening extends entirely across the roof of
 
 
 
THE CEREBELLUM
 
 
 
393
 
 
 
the brain (Fig. 236, B), the roof-plate probably being invaded by
cells from the dorsal zones and so giving rise to the vermis, while the
lobes are formed directly from the dorsal zones. During the second
month a groove appears on the ventral surface of each lobe, marking
out an area which becomes the flocculus, and later, during the third
month, transverse furrows appear upon the vermis dividing it into
five lobes, and later still extend out upon the lobes and increase in
number to produce the lamellate structure characteristic of
the cerebellum.
 
The histogenetic development of the cerebellum at first
proceeds along the lines which
have already been described
as typical, but after the development of the mantle layer the
cells lining the greater portion
of the cavity of the ventricle
 
rease to rrmltinlv onlv those FlG - 237-— Diagram Representing the
cease to multiply, oniy tnose DifferenT iation of the Cerebellar Cells.
 
which are situated in the roof- The circles, indifferent cells; circles with
 
plate of the metencephalon d °f ' n< r ur . g lia c f s > shaded c ? lls : g™. al
 
1 r cells; circles with cross, germinal cells in
 
and along the line of junction mitosis; black cells, nerve-cells. L, Lateral
 
. , , ,, ,i • i • recess; M, median furrow, and R, floor of IV,
 
of the cerebellar thickening fourth ven tricle.— (Schaper.)
with the roof-plate continuing
 
to divide. The indifferent cells formed in these regions migrate
outward from the median line and forward in the marginal velum to form a superficial layer, known as the epithelioid layer,
and cover the entire surface of the cerebellum (Fig. 237). The
cells of this layer, like those of the mantle, differentiate into neuroglia
cells and neuroblasts, the latter for the most part migrating centrally
at a later stage to mingle with the cells of the mantle layer and to
become transformed into the granular cells of the cerebellar cortex.
The neuroglia cells remain at the surface, however, forming the
principal constituent of the outer or, as it is now termed, the molecular
layer of the cortex, and into this the dendrites of the Purkinje cells,
 
 
 
 
394 THE isthmus
 
probably derived from the mantle layer, project. The migration
of the neuroblasts of the epithelial layer is probably completed
before birth, at which time but few remain in the molecular layer
to form the stellate cells of the adult. The origin of the dentate and
other nuclei of the cerebellum is at present unknown, but it seems
probable that they arise from cells of the mantle layer.
 
The nerve-fibers which form the medullary substance of the
cerebellum do not make their appearance until about the sixth
month, when they are to be found in the ependymal tissue on the
inner surface of the layer of granular cells. Those which are not
commissural or associative in function converge to the line of junction
of the cerebellum with the pons, and there pass into the marginal
velum of the pons, myelencephalon, or isthmus as the case may be.
 
The dorsal surface of the isthmus is at first barely distinguishable
from the cerebellum, but as development proceeds its roof-plate
undergoes changes similar to those occurring in the medulla oblongata and becomes converted into the anterior velum. In the
dorsal portion of its marginal velum fibers passing to and from the
cerebellum appear and form the brachia conjunctiva, while ventrally
fibers, descending from the more anterior portions of the brain, form
the cerebral peduncles. Nothing is at present known as to the history
of the gray matter of this division of the brain, although it may be
presumed that its ventral zones take part in the formation of the
tegmentum, while from its dorsal zones the nuclei of the brachia conjunctiva are possibly derived.
 
The following table gives the origin of the principal structures of
the metencephalon and isthmus:
 
Metencephalon. Isthmus.
 
/ Posterior velum. Anterior velum.
 
^ Vermis of cerebellum.
 
 
 
Dorsal zones.
 
 
 
Lobes of cerebellum. Brachia conjunctiva.
 
Flocculi.
 
Nuclei of termination of sensory roots of cranial nerves.
Pons nuclei.
 
 
 
THE MESENCEPHALON 395
 
Metencephalon. Isthmus.
 
f Nuclei of origin of motor Posterior part of cerebral
 
Ventral zones -j roots of cranial nerves. peduncles.
 
[ Reticular formation. Posterior part of tegmentum.
 
Floor-plate Median raphe. Median raphe.
 
The Development of the Mesencephalon. — Our knowledge of the
development of this portion of the brain is again very imperfect.
During the stages when the flexures of the brain are well marked
(Figs. 232 and 233) it forms a very prominent structure and possesses for a time a capacious cavity. Later, however, it increases in
size less rapidly than adjacent parts and its walls thicken, the roofand floor-plates as well as the zones, and, as a result, the cavity
becomes the relatively smaller canal-like cerebral aquaeduct. In the
marginal velum of its ventral zone fibers appear at about the third
month, forming the anterior portion of the cerebral peduncles, and,
at the same time, a median longitudinal furrow appears upon the
dorsal surface, dividing it into two lateral elevations which, in the
fifth month, are divided transversely by a second furrow and are
thus converted from corpora bigemina (in which form they are
found in the lower vertebrates) into corpora quadrigemina.
 
Nothing is known as to the differentiation of the gray matter of the
dorsal and ventral zones of the mid-brain. From the relation of the parts
in the adult it seems probable that in addition to the nuclei of origin of
the oculomotor and trochlear nerves, the ventral zones give origin to the
gray matter of the tegmentum, which is the forward continuation of the
reticular formation. Similarly it may be supposed that the corpora
quadrigemina are developments of the dorsal zones, as may also be the
red nuclei, whose relations to the brachia conjunctiva suggest a comparison with the olivary bodies and the nuclei of the pons.
 
A tentative scheme representing the origin of the mid-brain structures
may be stated thus:
 
Roof -plate (?)
 
J Corpora quadrigemina.
.LJorsal zones. ...... \ .
 
^ Red nuclei.
 
[ Nuclei of origin of the third and fourth nerves.
Ventral zones \ Anterior part of tegmentum.
 
[ Anterior part of cerebral peduncles.
Floor-plate Median raphe.
 
 
 
396
 
 
 
THE DIENCEPHALON
 
 
 
The Development of the Diencephalon. — A transverse section
through the diencephalon of an embryo of about five weeks (Fig.
238) shows clearly the differentiation of this portion of the brain into
the typical zones, the roof-plate {rp) being represented by a thinwalled, somewhat folded area, the floor-plate (fp) by the tissue
forming the floor of a well-marked ventral groove, while each lateral
wall is divided into a dorsal and ventral zone by a groove known as
the sulcus Monroi (Sm), which extends forward and ventrally
 
toward the point of origin of the optic
evagination (Fig. 240). At the posterior end of the ridge-like elevation
which represents the roof-plate is a
rounded elevation (Fig. 239, p) which,
in later stages, elongates until it almost reaches the dermis, forming a
hollow evagination of the brain roof
known as the pineal process. The distal extremity of this process enlarges to
a sac-like structure which later beFig. 238.— Transverse Section comes lobed, and, by an active pro
of the Diencephalon of an Em- Hferation of the cells lining the cavibryo of Five Weeks. ^
 
dz, Dorsal zone; fp, floor-plate; tieS ° f the various lobes, finally be
 
 
 
rp, roof-plate; Sm, sulcus Monroi
vz, ventral zone. — (His.)
 
 
 
comes a solid structure, the pineal body.
 
The more proximal portion of the
evagination, remaining hollow, forms the pineal stalk, and the entire structure, body and stalk, constitutes what is known as the
epiphysis.
 
The significance of this organ in the Mammalia is doubtful. In the
Reptilia and other lower forms the outgrowth is double, a secondary
outgrowth arising from the base or from the anterior wall of the primary
one. This anterior evagination elongates until it reaches the dorsal
epidermis of the head, and, there expanding, develops into an unpaired
eye, the epidermis which overlies it becoming converted into a transparent cornea. In the Mammalia this anterior process does not develop
and the epiphysis in these forms is comparable only to the posterior
process of the Reptilia.
 
In addition to the epiphysial evaginations, another evagination arises
 
 
 
THE DIENCEPHALON
 
 
 
397
 
 
 
 
from the roof-plate of the first brain vesicle, further forward, in the region
which becomes the median portion of the telencephalon. This paraphysis
as it has been called, has been observed in the lower vertebrates and in the
Marsupials (Selenka), but up to the
present has not been found in other
groups of the Mammalia. It seems to
be comparable to a chorioid plexus
which is evaginated from the brain
surface instead of being invaginated
as is usually the case. There is no evidence that a paraphysis is developed
in the human brain.
 
The portion of the roof-plate
which lies in front of the epiphysis
represents the velum interpositum
of the adult brain, and it forms at
first a distinct ridge (Fig. 239, rp).
At an early stage, however, it becomes reduced to a thin membrane
upon the surface of which bloodvessels, developing in the surrounding mesenchyme, arrange themselves at about the third month in
two longitudinal plexuses, which,
with the subjacent portions of the
 
velum, become invaginated into the „ „,
 
riG. 239. — Dorsal View of the
 
cavity of the third ventricle to form Brain, the Roof of the Lateral
 
its chorioid Mexu* Ventricles being Removed, of an
 
us cnomoia plexus. Embryo of 13.6 mm.
 
The dorsal zones thicken in b, Superior brachiuui; eg, lateral
 
their more dorsal and anterior S eniculate . bod y; C P> chorioid plexus;
tneir more aorsai ana anterior cqa> anterior corpu3 quadrigeminum;
 
portions to form massive Structures, h > hippocampus; hf, hippocampal fis, 7 7 . , sure; ot, thalamus; p, pineal body; rp,
 
the thatami [rigs. 233, V2, and roof-plate.— (Aw.)
 
239, ot), which, encroaching upon
 
the cavity of the ventricle, transform it into a narrow slit-like
 
space, so narrow, indeed, that at about the fifth month the inner
 
surfaces of the two thalami come in contact in the median line,
 
forming what is known as the intermediate mass. More ventrally
 
 
 
/, .,
 
 
 
-\P
 
 
 
cqn
 
 
 
398 THE TELENCEPHALON
 
and posteriorly another thickening of the dorsal zone occurs, giving
rise on each side to the pulvinar of the thalamus and to a lateral
geniculate body, and two ridges extending backward and dorsally
from the latter structures to the thickenings in the roof of the midbrain which represent the anterior corpora quadrigemina, give a
path along which the nerve-fibers which constitute the superior
quadrigeminal brachia pass.
 
From the ventral zones what is known as the hypothalamic region
develops, a mass of fibers and cells whose relations and development
are not yet clearly understood, but which may be regarded as the
forward continuation of the tegmentum and reticular formation.
In the median line of the floor of the ventricle an unpaired thickening
appears, representing the corpora mamillaria, which during the
third month becomes divided by a median furrow into two rounded
eminences; but whether these structures and the posterior portion
of the tuber cinereum, which also develops from this region of the
brain, are. derivatives of the ventral zones or of the floor-plate is as
yet uncertain.
 
Assuming that the mamillaria and the tuber cinereum are derived
from the ventral zones, the origins of the structures formed from the
walls of the diencephalon may be tabulated as follows:
 
^ . , f Velum interpositum.
 
Roof-plate < ^ . . . ^
 
(_ Epiphysis.
 
{Thalami.
Pulvinares.
Lateral geniculate bodies.
{Hypothalamic region.
Corpora mamillaria.
Tuber cinereum (in part) .
Floor-plate Tissue of mid-ventral line.
 
The Development of the Telencephalon.- — For convenience of
description the telencephalon may be regarded as consisting of a
median portion, which contains the anterior part of the third ventricle, and two lateral outgrowths which constitute the cerebral
hemispheres. The roof of the median portion undergoes the same
transformation as does the greater portion of that of the diencephalon
 
 
 
THE TELENCEPHALON 399
 
and is converted into the anterior part of the velum interpositum
(Fig. 240, vi). Anteriorly this passes into the anterior wall of the
third ventricle, the lamina terminalis {It), a structure which is to be
regarded as formed by the union of the dorsal zones of opposite
sides, since it lies entirely dorsal to the anterior end of the sulcus
Monroi. From the ventral part of the dorsal zones the optic
evaginations are formed, a depression, the optic recess (or), marking
their point of origin.
 
The ventral zones are but feebly developed, and form the anterior
part of the hypothalamic region, while at the anterior extremity
of the floor-plate an evagination occurs, the infundibular recess (ir),
which elongates to form a funnel-shaped structure known as the
hypophysis. At its extremity the hypophysis comes in contact
during the fifth week with the enlarged extremity of Rathke's pouch
formed by an invagination of the roof of the oral sinus (see p. 285),
and applies itself closely to the posterior surface of this (Fig. 233)
to form with it the pituitary body. The anterior lobe at an early
stage separates from the mucous membrane of the oral sinus, the
stalk by which it was attached completely disappearing, and toward
the end of the second month it begins to send out processes from
its walls into the surrounding mesenchyme and so becomes converted into a mass of solid epithelial cords embedded in a mesenchyme rich in blood and lymphatic vessels. The cords later on
divide transversely to a greater or less extent to form alveoli, the
entire structure coming to resemble somewhat the parathyreoid
bodies (see p. 297), and, like these, having the function of producing
an internal secretion. The posterior lobe, derived from the brain,
retains its connection with that structure, its stalk being the injundibidum, but its terminal portion does not undergo such extensive
modifications as does the anterior lobe, although it is claimed that
it gives rise to a glandular epithelium which may become arranged
so as to form alveoli.
 
The cerebral hemispheres are formed from the lateral portions
of the dorsal zones, each possessing also a prolongation of the roofplate. From the more ventral portion of each dorsal zone there is
 
 
 
400 THE TELENCEPHALON
 
formed a thickening, the corpus striatum (Figs. 240, cs, and 233, VI 2),
a structure which is for the telencephalon what the optic thalamus
is for the diencephalon, while from the more dorsal portion there is
formed the remaining or mantle {pallial) portions of the hemispheres
(Figs. 240, h, and 233, VI 4). When first formed, the hemispheres
are slight evaginations from the median portion of the telencephalon,
the openings by which their cavities communicate with the third
ventricle, the interventricular foramina, being relatively very large
(Fig. 240), but, in later stages (Fig. 233), the hemispheres increase
more markedly and eventually surpass all the other portions of the
 
 
 
<y
 
 
 
 
— •■/
 
 
 
Fig. 240. — Median Longitudinal Section of the Brain of an Embryo of 16.3 mm.
br, Anterior brachium; eg, corpus geniculatum laterale; cs, corpus striatum; h,
cerebral hemisphere; ir, infundibular recess; It, lamina terminalis; or, optic recess; ot,
thalamus; p, pineal process; sm, sulcus Monroi; st, hypothalamic region; vi, velum
interpositum. — (His.)
 
brain in magnitude, overlapping and completely concealing the
roof and sides of the diencephalon and mesencephalon and also the
anterior surface of the cerebellum. In this enlargement, however,
the interventricular foramina share only to a slight extent, and
consequently become relatively smaller (Fig. 233), forming in the
adult merely slit-like openings lying between the lamina terminalis
and the thalami and having for their roof the anterior portion of the
velum interpositum.
 
The velum Interpositum — that is to say, the roof-plate — where
 
 
 
THE TELENCEPHALON
 
 
 
401
 
 
 
it forms the roof of the interventricular foramen, is prolonged out
upon the dorsal surface of each hemisphere, and, becoming invaginated, forms upon it a groove.' As the hemispheres, increasing in
height, develop a mesial wall, the groove, which is the so-called
chorioidal fissure, comes to lie along the ventral edge of this wall,
and as the growth of the hemispheres continues it becomes more and
more elongated, being carried at first backward (Fig. 241), then
ventrally, and finally forward to end at the tip of the temporal lobe.
After the establishment of the grooves the mesenchyme in their
vicinity dips into them, and, developing blood-vessels, becomes the
chorioid plexuses of the lateral ventricles, and at first these plexuses
grow much more rapidly than the ventricles, and so fill them almost
completely. Later, however, the walls
of the hemispheres gain the ascendancy
in rapidity of growth and the plexuses
become relatively much smaller. Since
the portions of the roof-plate which form
the chorioidal fissures are continuous
with the velum interpositum in the roofs
of the interventricular foramina, the
chorioid plexuses of the lateral and third
ventricles become continuous also at that
point.
 
 
 
 
Fig. 241.
 
 
 
-Median Longitudinal Section of the Brain
of an Embryo Calf of 5 cm.
 
cb, Cerebellum; cp, chorioid
plexus; cs, corpus striatum; JM,
interventricular foramen; in,
The mode of growth of the chorioid hypophysis; m, mid-brain; oc,
, optic commissure; t, posterior
 
fissures seems to indicate the mode of par t of the diencephalon —
growth of the hemispheres. At first the Wihalkovicz.)
growth is more or less equal in all directions, but later it becomes more
extensive posteriorly, there being more room for expansion in that
direction, and when further extension backward becomes difficult
the posterior extremities of the hemispheres bend ventrally toward
the base of the cranium, and reaching this, turn forward to form the
temporal lobes. As a result the cavities of the hemispheres, the
lateral ventricles, in addition to being carried forward to form an
anterior horn, are also carried backward and ventrally to form the
lateral or descending horn, and the corpus striatum likewise extends
26
 
 
 
402 THE TELENCEPHALON
 
backward to the tip of each temporal lobe as a slender process known
as" the tail of the caudate nucleus. In addition to the anterior and
lateral horns, the ventricles of the human brain also possess posterior
horns extending backward into the occipital portions of the hemispheres, these portions, on account of the greater persistence of the
mid-brain flexure (see p. 388), being enabled to develop to a greater
extent than in the lower mammals.
 
The scheme of the origin of parts in the telencephalon may be
stated as follows:
 
Median Part. Hemispheres.
 
„ , , f Anterior part of velum inter- f _ , n ..,,,.
 
Roof-plate < . < Moor of chonoidai nssure.
 
(^ positum. [
 
r , . ... Pallium.
 
-r. , Lamina terminahs. _
 
Dorsal zones â– (_... < Corpus striatum.
 
Optic evaginations. _,, , , . ..
 
> . , . Olfactory lobes (see p. 406)
 
Anterior part of hypothalamic [
 
Ventral zones < region.
 
[ Anterior part of tuber cinereum.
 
The Convolutions of the Hemispheres. — The growth of the
hemispheres to form the voluminous structures found in the adult
depends mainly upon an increase of size of the pallium. The
corpus striatum, although it takes part in the elongation of each
hemisphere, nevertheless does not increase in other directions as
rapidly and extensively as the pallium, and hence, even in very early
stages, a depression appears upon the surface of the hemispheres
where the corpus is situated (Fig. 242). This depression is the
lateral cerebral fossa, and for a considerable period it is the only sign
of inequality of growth on the outer surfaces of the hemispheres.
Upon the mesial surfaces, however, at about the time that the
choroid fissure appears, another linear depression is formed dorsal
to the chorioid, and when fully formed extends from in front of the
interventricular foramen to the tip of the temporal lobe (Fig. 244, h).
It affects the entire thickness of the pallial wall and consequently
produces an elevation upon the inner surface, a projection into the
cavity of the ventricle which is known as the hippocampus, whence
 
 
 
THE CEREBRAL CONVOLUTIONS
 
 
 
403
 
 
 
the fissure may be termed the hippocampal fissure. The portion of
the pallium which intervenes between this fissure and the chorioidal
forms what is known as the dentate gyrus.
 
Toward the end of the third or the beginning of the fourth month
two prolongations arise from the fissure just where it turns to be
continued into the temporal lobe, and these, extending posteriorly,
give rise to the parieto-occipital and calcarine fissures. Like the
hippocampal, these fissures produce elevations upon the inner
surface of the pallium, that formed by the parieto-occipital early
disappearing, while that produced by the calcarine persists
to form the calcar {hippocampus minor) of adult anatomy.
 
The three fissures just
described, together with the
chorioidal and the lateral
cerebral fossa, are all formed
by the beginning of the fourth
month and all the fissures
affect the entire thickness of
the wall of the hemisphere,
and hence have been termed
the primary or total fissures.
Until the beginning of the fifth
 
month they are the only fissures present, but at that time secondary
fissures, which, with one exception, are merely furrows of the surface of the pallium, make their appearance and continue to form
until birth and possibly later. Before considering these, however,
certain changes which occur in the neighborhood of the lateral
cerebral fossa may be described.
 
The fossa is at first a triangular depression situated above the
temporal lobe on the surface of the hemisphere. During the fourth
month it deepens considerably, so that its upper and lower margins
become more pronounced and form projecting folds, and, during
the fifth month, these two folds approach one another and eventually
 
 
 
 
Fig. 242. — Brain of an Embryo of the
 
Fourth Month.
c, Cerebellum; p, pons; s, lateral cerebral
fossa.
 
 
 
404
 
 
 
TEE CEREBRAL CONVOLUTIONS
 
 
 
cover in the floor of the fossa completely, the groove which marks
the line of their contact forming the lateral cerebral fissure, while the
floor of the fossa becomes known as the insula.
 
The first of the secondary fissures to appear is the sulcus cinguli,
which is formed about the middle of the fifth month on the mesial
surface of the hemispheres, lying parallel to the anterior portion of
the hippocampal fissure and dividing the mesial surface into the
gyri marginalis and fornicatus. A little later, at the beginning of
the sixth month, several other fissures make their appearance upon
 
 
 
ptc
 
 
 
 
Fig. 243. — Cerebral Hemisphere oe an Embryo of about the Seventh Month.
fs, Superior frontal sulcus; ip, interparietal; IR, insula; pet, inferior pre-central; pes,
superior pre-central; ptc, post-central; R, central; S, lateral; t 1 , first temporal. —
{Cunningham )
 
 
 
the outer surface of the pallium, the chief of these being the central
sulcus, the inter-parietal, the pre- and post-central, and the temporal
sulci, the most ventral of these last running parallel with the lower
portion of the hippocampal fissure and differing from the others in
forming a ridge on the wall of the ventricle termed the collateral
eminence, whence the fissure is known as the collateral. The position
of most of these fissures may be seen from Fig. 243, and for a more
 
 
 
THE CORPUS CALLOSUM
 
 
 
405
 
 
 
complete description of them reference may be had to text-books of
descriptive anatomy.
 
In later stages numerous tertiary fissures make their appearance
and mask more or less extensively the secondaries, than which they
are, as a rule, much more inconstant in position and shallower.
The Corpus Callosum and Fornix. — While these fissures have been
forming, important structures have developed in connection with
the lamina terminalis. Up to about the fourth month the lamina
is thin and of nearly uniform thickness throughout, but at this time
it begins to thicken near its dorsal edge and fibers appear in the
thickening. These fibers belong to three sets. In the first place,
certain of them arise in connection with the olfactory tracts (see p.
407) and from the region of the hippocampal gyrus, which is also
associated with the olfactory sense, and, passing through tbe substance of the lamina terminalis, they extend across the median line
to the corresponding regions of the opposite cerebral hemisphere.
They are therefore commissural fibers and form what is termed the
anterior commissure (Figs. 244, ca and 245, ac). Secondly, fibers,
which have their origin from the cells of the hippocampus, develop
along the chorioidal edge of that structure, forming what is termed
the fimbria. They follow along the edge of the chorioidal fissure
and, when this reaches the interventricular foramen, they enter as
the pillars of the fornix (Figs. 244, cf; Fig. 245,/) the substance of the
lamina terminalis and, passing ventrally in it, eventually reach the
hypothalamic region, where they terminate in the corpora
mammillaria.
 
Thirdly, as the mantle develops fibers radiate from all parts of
it toward the dorsal portion of the lamina terminalis and traversing
it are distributed to the corresponding portions of the mantle of the
opposite side. There fibers are also commissural in character and
form the corpus callosum (Figs. 244 and 245, cc). With the development of these three sets of fibers and especially those forming the
corpus callosum, the dorsal portion of the lamina terminalis becomes enlarged so as to form a triangular area extending between
the two cerebral hemispheres (Fig. 245), the corpus callosum form
 
 
4<o6
 
 
 
THE CORPUS CALLOSUM
 
 
 
ing its dorsal portion and base, which is directed anteriorly, the
pillars of the fornix its ventral portion, while the anterior commissure
occupies its ventral anterior angle.
 
The portion of the triangle included between the callosum and
the fornix remains thin and forms the septum pellucidum, and a split
occurring in the center of this gives rise to the so-called^///* ventricle,
 
which, from its mode of formation, is a completely closed cavity and is not lined with ependymal tissue of the same nature
as that found in the other ventricles.
 
Owing to the very considerable size reached by the triangular area whose history has just
been described, important
changes are wrought in the adjoining portions of the mesial
surface of the hemispheres. Before the development of the area
the gyrus dentatus and the hippocampus extend forward into
the anterior portion of the hemispheres (Fig. 244), but on account of their position they become encroached upon by the
enlargement of the corpus callosum, with the result that the hippocampus becomes practically
obliterated in that portion of its course which lies in the region
occupied by the corpus callosum, its fissure in this region becoming
known as the callosal fissure, while the corresponding portions
of the dentate gyrus become reduced to narrow and insignificant
bands of nerve tissue which rest upon the upper surface of the corpus
callosum and are known as the lateral longitudinal stria.
 
The Olfactory Lobes. — At the time when the cerebral hemispheres
 
 
 
 
Fig. 244. — Median Longitudinal Section or the Brain of an Embryo of
Four Months.
 
c, Calcarine fissure; ca, anterior commissure; cc, corpus callosum; cf. chorioidal
fissure; dg, dentate gyrus; fm, interventricular foramen; h, hippocampal fissure;
po, parieto-o c c i p i t a 1 fissure. — (Mihalkovicz.)
 
 
 
THE OLFACTORY LOBES
 
 
 
407
 
 
 
 
begin to enlarge — that it to say, at about the fourth week — a slight
furrow, which appears on the ventral surface of each anteriorly,
marks off an area which, continuing to enlarge with the hemispheres,
gradually becomes constricted off from them to form a distinct lobelike structure, the olfactory lobe (Fig. 233, VI 3). In most of the
lower mammalia these lobes
reach a very considerable size,
and consequently have been
regarded as constituting an
additional division of the
brain, known as the rhinencephalon, but in man they
remain smaller, and although
they are at first hollow, containing prolongations from the
lateral ventricles, the cavities
later on disappear and the
lobes become solid. Each
lobe becomes differentiated
into two portions, its terminal
portion becoming converted
into the club-shaped structure, the olfactory bulb and stalk, while its proximal portion gives
rise to the olfactory tracts, the trigone, and the anterior perforated
substance.
 
Histogenesis of the Cerebral Cortex. — A satisfactory study of the
histogenesis of the cortex has not yet been made. In embryos of
three months a marginal velum is present and probably gives rise
to the stratum zonale of the adult brain; beneath this is a cellular
layer, perhaps representing the mantle layer; beneath this, again, a
layer of nerve-fibers is beginning to appear, representing the white
substance of the pallium; and, finally, lining the ventricle is an
ependymal layer. In embryos of the fifth month, toward the innermost part of the second layer, cells are beginning to differentiate
into the large pyramidal cells, but almost nothing is known as to the
 
 
 
Fig. 245. — Median Longitudinal Section
of the Brain oe an Embryo of the Fifth
Month.
 
ac, Anterior commissure; cc, corpus callosum; dg, dentate gyrus;/, fornix; i, infundibulum; mc, intermediate mass; si, septum
pellucidum; vi, velum interpositum. — (Mihal
kovicz.)
 
 
 
408 THE SPINAL NERVES
 
origin of the other layers recognizable in the adult cortex, nor is it
known whether any migration, similar to what occurs in the cerebellar cortex, takes place. The fibers of the white substance do not
begin to acquire their myelin sheaths until toward the end of the
ninth month, and the process is not completed until some time after
birth (Flechsig), while the fibers of the cortex continue to undergo
myelination until comparatively late in life (Kaes).
 
The Development of the Spinal Nerves. — It has already been
seen that there is a fundamental difference in the mode of development of the two roots of which the typical spinal nerves are composed,
the ventral root being formed by axis-cylinders which arise from
neuroblasts situated within the substance of the spinal cord, while
the dorsal roots arise from the cells of the neural crests, their axiscylinders growing into the substance of the cord while their dendrites
become prolonged peripherally to form the sensory fibers of the
nerves. Throughout the thoracic, lumbar and sacral regions of the
cord the fibers which grow out from the anterior horn cells converge
to form a single nerve-root in each segment, but in the cervical region
fibers which arise from the more laterally situated neuroblasts make
their exit from the cord independently of the more ventral neuroblasts and form the roots of the spinal accessory nerve (see p. 416).
In the cervical region there are accordingly three sets of nerve-roots,
the dorsal, lateral, and ventral sets.
 
In a typical spinal nerve, such as one of the thoracic series, the
dorsal roots as they grow peripherally pass ventrally as well as outward, so that they quickly come into contact with the ventral roots
with whose fibers they mingle, and the mixed nerve so formed soon
after divides into two trunks, a dorsal one, which is distributed to the
dorsal musculature and integument, and a larger ventral one. The
ventral division as it continues its outward growth soon reaches the
dorsal angle of the pleuro-peritoneal cavity, where it divides, one
branch passing into the tissue of the body- wall while the other passes
into the splanchnic mesoderm. The former branch, continuing its
onward course in the body- wall, again divides, one branch becoming
the lateral cutaneous nerve, while the other continues inward to
 
 
 
THE CRANIAL NERVES 409
 
terminate in the median ventral portion of the body as the anterior
cutaneous nerve. The splanchnic branch forms a ramus communicans to the sympathetic system and will be considered more fully
later on.
 
The conditions just described are those which obtain throughout
the greater part of the thoracic region. Elsewhere the fibers of the
ventral divisions of the nerves as they grow outward tend to separate
from one another and to become associated with the fibers of adjacent nerves, giving rise to plexuses. In the regions where the limbs
occur the formation of the plexuses is also associated with a shifting
of the parts to which the nerves are supplied, a factor in plexus formation which is, however, much more evident from comparative
anatomical than from embryological studies.
 
The Development of the Cranial Nerves.— During the last
thirty years the cranial nerves have received a great deal of attention
in connection with the idea that an accurate knowledge of their
development would afford a clue to a most vexed problem of vertebrate morphology, the metamerism of the head. That the metamerism which was so pronounced in the trunk should extend into the
head was a natural supposition, strengthened by the discovery of
head-cavities in the lower vertebrates and by the indications of
metamerism seen in the branchial arches, and the problem which
presented itself was the correlation of the various structures belonging
to each metamere and the determination of the modifications which
they had undergone during the evolution of the head.
 
In the trunk region a nerve forms a conspicuous element of each
metamere and is composed, according to what is known as Bell's
law, of a ventral or efferent and a dorsal or afferent root. Until
comparatively recently the study of the cranial nerves has been
dominated by the idea that it was possible to extend the application
of Bell's law to them and to recognize in the cranial region a number
of nerve pairs serially homologous with the spinal nerves, some of
them, however, having lost their afferent roots, while in others a dislocation, as it were, of the two roots had occurred.
 
The results obtained from investigation along this line have not,
 
 
 
4IO . THE CRANIAL NERVES
 
however, proved entirely satisfactory, and facts have been elucidated
which seem to show that it is not possible to extend Bell's law, in its
usual form at least, to the cranial nerves. It has been found that
it is not sufficient to recognize simply afferent and efferent roots,
but these must be analyzed into further components, and when this
is done it is found that in the series of cranial nerves certain components occur which are not represented in the nerves of the spinal
series.
 
Before proceeding to a description of these components it will be
well to call attention to a matter already alluded to in a previous
chapter (p, 84) in connection with the segmentation of the mesoderm of the head. It has been pointed out that while there exist
"head-cavities" which are serially homologous with the mesodermal
somites of the trunk, there has been imposed upon this primary
cranial metamerism a secondary metamerism represented by
the branchiomeres associated with the branchial arches, and,
it may be added, this secondary metamerism has become the more
prominent of the two, the primary one, as it developed, gradually
slipping into the background until, in the higher vertebrates, it has
become to a very considerable extent rudimentary. In accordance
with this double metamerism it is necessary to recognize two sets of
cranial muscles, one derived from the cranial myotomes and represented by the muscles of the eyeball, and one derived from the
branchiomeric mesoderm, and it is necessary also to recognize
for these two sets of muscles two sets of motor nerves, so
that, with the dorsal or sensory nerve-roots, there are altogether
three sets of nerve-roots in the cranial region instead of only two, as
in the spinal region.
 
These three sets of roots are readily recognizable both in the embryonic and in the adult brain, especially if attention be directed to
the cell groups or nuclei with which they are associated (Fig. 246).
Thus there can be recognized: (1) a series of nuclei from which
nerve-fibers arise, situated in the floor of the fourth ventricle and
aquaeduct close to the median line and termed the ventral motor
nuclei; (2) a second series of nuclei of origin, situated more laterally
 
 
 
THE CRANIAL NERVES
 
 
 
411
 
 
 
and in the substance of the formatio reticularis, and known as the
lateral motor nuclei; and (3) a series of nuclei in which afferent nervefibers terminate, situated still more laterally in the floor of the ventricle and forming the dorsal or sensory nuclei. None of the twelve
cranial nerves usually recognized in the text-books contains fibers
associated with all three of these nuclei; the fibers from the lateral
motor nuclei almost invariably unite with sensory fibers to form a
 
 
 
 
Fig. 246. — Transverse Section through the Medulla Oblongata of an
Embryo of 10 mm., showing the Nuclei of Origin of the Vagus (X) and Hypoglossal (XII) Nerves. — (His.)
 
mixed nerve, but those from all the ventral motor nuclei form independent roots, while the olfactory and auditory nerves alone, of all
the sensory roots (omitting for the present the optic nerve), do not
contain fibers from either of the series of motor nuclei. The relations
of the various cranial nerves to the nuclei may be seen from the
following table, in which the + sign indicates the presence and the
— sign the absence of fibers from the nuclear series under which it
stands':
 
 
 
412
 
 
 
THE CRANIAL NERVES
 
 
 
Number
 
 
Name
 
 
Ventral Motor
 
 
Lateral Motor
 
 
Sensory
 
 
I.
 
 
Olfactory.
 
 
_
 
 
 
 
 
+
 
 
III.
 
 
Oculomotor.
 
 
+
 
 
 
 
 
 
TV.
 
 
Trochlear.
 
 
+
 
 
 
 
 
 
V.
 
 
Trigeminus.
 
 
 
 
+
 
 
+
 
 
VI
 
 
Abducens.
 
 
+
 
 
 
 
 
 
VII.
 
 
Facial.
 
 
 
 
+
 
 
+
 
 
VIII.
 
 
Auditory.
 
 
 
 
 
 
+
 
 
IX.
 
 
Glossopharyngeal.
 
 
 
 
+
 
 
+
 
 
X.
 
XI.
 
 
Vagus. 1
Spinal Accessory. J
 
 
 
 
+
 
 
+
 
 
 
Two nerves — namely, the second and twelfth — have been omitted
from the above table. Of these, the second or optic nerve undoubtedly belongs to ah entirely different category from the other peripheral nerves, and will be considered in the following chapter in
connection with the sense-organ with which it is associated (see
especially p. 460). The twelfth or hypoglossal nerve, on the other
hand, really belongs to the spinal series and has only secondarily
been taken up into the cranial region in the higher vertebrates. It
has already been seen (p. 170) that the bodies of four vertebrae are
included in the basioccipital bone, and that three of the nerves
corresponding to these vertebrae are represented in the adult by the
hypoglossal and the fourth by the first cervical or suboccipital nerve.
The dorsal roots of the hypoglossal nerves seem to have almost
disappeared, although a ganglion has been observed in embryos of
7 and 10 mm. in the posterior part of the hypoglossal region (His),
and probably represents the dorsal root of the most posterior portion
of the hypoglossal nerve. This ganglion disappears, as a rule, in
later stages, and it is interesting to note that the ganglion of the
suboccipital nerve is also occasionally wanting in the adult condition.
The hypoglossal roots are to be regarded, then, as equivalent to the
ventral roots of the cervical spinal nerves, and the nuclei from
which they arise lie in series with the cranial ventral motor roots, a
 
 
 
THE CRANIAL NERVES 413
 
fact which indicates the equivalency of these latter with the fibers
which arise from the neuroblasts of the anterior horns of the spinal
cord.
 
The equivalents of the lateral motor roots may more conveniently
be considered later on, but it may be pointed out here that these are
the fibers which are distributed to the muscles of the branchiomeres.
In the case of the sensory nerves a further analysis is necessary
before their equivalents in the spinal series can be determined.
For this the studies which have been made in recent years of the
components entering into the cranial nerves of the amphibia (Strong)
and fishes (Herrick) must supply a basis, since as yet a direct analysis
of the mammalian nerves has not been made. In the forms named
it has been found that three different components enter into the
formation of the dorsal roots of the cranial nerves: (i) fibers belonging to a general cutaneous or somatic sensory system, distributed to
the skin without being connected with any special sense-organs; (2)
fibers belonging to what is termed the communis or viscerosensory
system, distributed to the walls of the mouth and pharyngeal region
and to special organs found in the skin of the same character as
those occurring in the mouth; and (3) fibers belonging to a special
set of cutaneous sense-organs largely developed in the fishes and
known as the organs of the lateral line.
 
The fibers of the somatic sensory system converge to a group of
cells, situated in the lateral part of the floor of the fourth ventricle
and forming what is termed the trigeminal lobe, and also extend
posteriorly in the substance of the medulla (Fig. 247), forming what
has been termed the spinal root of the trigeminus and terminating
in a column of cells which represents the forward continuation of the
posterior horn of the cord. In the fishes and amphibia fibers
belonging to this system are to be found in the fifth, seventh, and
tenth nerves, but in the mammalia their distribution has apparently
become more limited, being confined almost exclusively to the
trigeminus, of whose sensory divisions they form a very considerable
part. Since the cells around which the fibers of the spinal root of the
trigeminus terminate are the forward continuations of the posterior
 
 
 
414
 
 
 
THE CRANIAL NERVES
 
 
 
horn of the cord, it seems probable that the fibers of this system
are the cranial representatives of the posterior roots of the spinal
nerves, which, it may be noted, are also somatic in their distribution.
The fibers of the viscero-sensory system are found in the lower
forms principally in the ninth and tenth nerves (see Fig. 247),
although groups of them are also incorporated in the seventh and
fifth. They converge to a mass of cells, known as the lobus vagi,
and like the first set are also continued down the medulla to form
 
 
 
 
rix
 
 
 
Fig. 247.— Diagrams showing the Sensory Components of the Cranial Nerves
 
of a Fish (Menidia) .
The somatic sensory system is unshaded, the viscero-sensory is cross-hatched, and
the lateral line system is black, asc.v, Spinal root of trigeminus; brx, branchial branches
of vagus; Ix, lobus vagi; ol, olfactory bulb; op, optic nerve; rc.x, cutaneous branch of the
vagus; rix, intestinal branch of vagus; rl, lateral line nerve; rl.acc, accessory lateral
line nerve; ros, superficial ophthalmic; rp, ramus palatinus of the facial; thy, hyomandibular branch of the facial; t.inf, infraorbital nerve. — {Herrick.)
 
a tract known as the fasciculus solitarius or: fasciculus communis. In
the mammalia the system is represented by the sensory fibers of the
glosso-pharyngeo-vagus set of nerves, of which it represents practically the entire mass; by the sensory fibers of the facial arising from
the geniculate ganglion and included in the chorda tympani and
probably also the great superficial petrosal; and also, probably, by
 
 
 
THE CRANIAL NERVES 415
 
the lingual branch of the trigeminus. Furthermore, since the
mucous membrane of the palate is supplied by branches from the
trigeminus which pass by way of the spheno-palatine (Meckel's)
ganglion, and the same region is supplied in lower forms by a palatine branch from the facial, it seems probable that the palatine nerves
of the mammalia are also to be assigned to this system.* If this
be the case, a very evident clue is afforded to the homologies of the
system in the spinal nerves, for since the spheno-palatine ganglion
is to be regarded as part of the sympathetic system, the sensory
fibers which pass from the viscera to the spinal cord by way of the
sympathetic system (p. 420) present relations practically identical
with those of the palatine nerves.
 
Finally, with regard to the system of the lateral line, there seems
but little doubt that it has no representation whatsoever in the spinal
nerves. It is associated with a peculiar system of cutaneous senseorgans found only in aquatic or marine animals, and also with the
auditory and possibly the olfactory organs, the former of which are
certainly and the latter possibly primarily parts of the lateral line
system of organs. The organs are principally confined to the head,
although they also extend upon the trunk, where they are followed
by a branch from the vagus nerve, the entire system being accordingly
supplied by cranial nerves. In the fishes, in which the development
of the organs is at a maximum, fibers belonging to the system are
found in all the branchiomeric nerves and all converge to a portion
of the medulla known as the tuberculum acusticum. In the Mammalia, with the disappearance of the lateral line organs there has
been a disappearance of the associated nerves, and the only certain
representative of the system which persists is the auditory nerve.
 
The table given on page 412 may now be expanded as follows,
though it must be recognized that such an analysis of the mammalian
nerves is merely a deduction from what has been observed in lower
 
* The fact that the palatine branches are associated with the trigeminus in the
Mammalia and with the facial in the Amphibia is readily explained by the fact that
in the latter the Gasserian and geniculate ganglia are not always separated, so that
it is possible for fibers originating from the compound ganglion to pass into either
 
 
 
416
 
 
 
THE CRANIAL NERVES
 
 
 
forms, and may require some modifications when the components
have been subjected to actual observation:
 
 
 
Nerve
 
 
Ventral
 
 
Lateral
 
 
Somatic
 
 
Visceral
 
 
Lateral
 
 
Motor
 
 
Motor
 
 
Sensory
 
 
Sensory
 
 
Line
 
 
I.
 
 
 
 
_
 
 
 
 
 
 
 
 
+
 
 
III.
 
 
+
 
 
 
 
 
 
 
 
 
 
IV.
 
 
+
 
 
 
 
 
 
 
 
 
 
V.
 
 
 
 
+
 
 
+
 
 
+
 
 
 
 
VI.
 
 
+
 
 
 
 
 
 
 
 
 
 
vii.
 
 
 
 
+
 
 
 
 
+
 
 
 
 
VIII.
 
 
 
 
 
 
 
 
 
 
+
 
 
IX.]
 
 
 
 
 
 
 
 
 
 
 
 
X.
 
 
 
 
+
 
 
+
 
 
+
 
 
 
 
XL J
 
 
 
 
 
 
 
 
 
 
 
 
XII.
 
 
+
 
 
 
 
 
 
 
 
 
 
Spinal.
 
 
+
 
 
(?)
 
 
+
 
 
+
 
 
 
 
 
An additional word is necessary concerning the spinal accessory
nerve, for it presents certain interesting relations which possibly
furnish a clue to the spinal equivalents of the lateral motor roots.
In the first place, the neuroblasts which give rise to those fibers of
the nerve which come from the spinal cord are situated in the dorsal
part of the ventral zones. As the nuclei of origin are traced anteriorly they will be found to change their position somewhat as the
medulla is reached and eventually come to lie in the reticular formation, the most anterior of them being practically continuous with
the motor nucleus of the vagus. Indeed, it seems that the spinal
accessory nerve is properly to be regarded as an extension of the
vagus downward into the cervical region (Furbringer, Streeter),
a process which reaches its greatest development in the mammalia
and seems to-stand in relation to the development of those portions
of the trapezius and sterno-mastoid muscles which are supplied by
the spinal accessory nerve.
 
It is believed that the white rami communicantes which pass
from the spinal cord to the thoracic and upper lumbar sympathetic
 
 
 
THE CRANIAL NERVES 417
 
ganglia arise from cells situated in the dorso-lateral portions of the
ventral horns, and it is noteworthy that white rami are wanting in
the region in which the spinal accessory nerve occurs. Since this
nerve represents a cranial lateral motor root the temptation is great
to regard the cranial lateral motor roots as equivalent to the white
rami of the cord, and this temptation is intensified when it is recalled
that there are both embryological and topographical reasons for
regarding the branchiomeric muscles, to which the cranial lateral
motor nerves are supplied, as equivalent to the visceral muscles of
the trunk. But in view of the fact that a sympathetic neurone is
always interposed between a white ramus fiber and the visceral
musculature (see Fig. 249), while the lateral motor fibers connect
directly with the branchiomeric musculature, it seems advisable to
await further studies before yielding to the temptation.
 
As regards the actual development of the cranial nerves, they
follow the general law which obtains for the spinal nerves, the
motor fibers being outgrowths from neuroblasts situated in the
walls of the neural tube, while the sensory nerves are outgrowths
from the cells of ganglia situated without the tube. In the lower
vertebrates a series of thickenings, known as the suprabranchial placodes, are developed from the ectoderm along a line corresponding
with the level of the auditory invagination, while on a line corresponding with the upper extremities of the branchial clefts another
series occurs which has been termed that of the epibranchial placodes,
and with both of these sets of placodes the cranial nerves are in
connection. In the human embryo epibranchial placodes have
been found in connection with the fifth, seventh, ninth and tenth
nerves, to whose ganglia they contribute cells. The suprabranchial
placodes, which in the lower vertebrates are associated with the
lateral line nerves, are unrepresented in man, unless, as has been
maintained, the sense-organs of the internal ear are their
representatives.
 
From what has been said above it is clear that the usual arrangement
of the cranial nerves in twelve pairs does not represent their true relationships with one another. The various pairs are serially homologous neither
 
27
 
 
 
418 THE SYMPATHETIC SYSTEM
 
with one another nor with the typical spinal nerves, nor can they be
regarded as representing twelve cranial segments. Indeed, it would seem
that comparatively little information with regard to the number of
myotomic segments which have fused together to form the head is to be
derived from the cranial nerves, for while there are only four of these
nerves which are associated with structures equivalent to the mesodermic
somites of the trunk, a much greater number of head cavities or mesodermic somites has been observed in the cranial region of the embryos
of the lower vertebrates, Dohrn, for instance, having found nineteen and
Killian eighteen in the cranial region of Torpedo. Furthermore, it is not
possible to say at present whether the branchiomeres and their associated
nerves correspond with one or several of the cranial mesodermic somites,
or whether, indeed, any correspondence whatever exists.
 
In early stages of development a series of constrictions have been
observed in the cranial portion of the neural tube and have been regarded
as indicating a primitive segmentation of that structure. The neuromeres,
as the intervals between successive constrictions have been termed, seem
to correspond with the cranial nerves as usually recognized and hence
cannot be regarded as primitive segmental structures. They are more
probably secondary and due to the arrangement of the neuroblasts corresponding to the various nerves.
 
The Development of the Sympathetic Nervous System. —
 
From the embryological standpoint the distinction which has been
generally recognized between the sympathetic and central nervous
systems does not exist, the former having been found to be an
outgrowth from the latter. This mode of origin has been observed
with especial clearness in the embryos of some of the lower vertebrates, in which masses of cells have been seen to separate from the
posterior root ganglia to form the ganglia of the ganglionated cord
(Fig. 248). In the mammalia, including man, the relations of the
two sets of ganglia to one another is by no means so apparent, since
the sympathetic cells, instead of being separated from the posterior
root ganglion en masse, migrate from it singly or in groups, and are
therefore less readily distinguishable from the surrounding mesodermal tissues.
 
To understand the development of the sympathetic system it
must be remembered that it consists typically of three sets of ganglia. One of these is constituted by the ganglia of the ganglionated
cord (Fig. 249, GC), the second is represented by the ganglia of the
 
 
 
THE SYMPATHETIC SYSTEM
 
 
 
419
 
 
 
'"v
 
 
 
 
 
 
 
 
 
 
^
 
 
 
 
 
 
 
â– 
 
 
 
Fig. 248. — Transverse Section through an Embryo Shark (Scyllium) of ii mm.,
 
SHOWING THE ORIGIN OF A SYMPATHETIC GANGLION.
 
Ch, Notochord; E, ectoderm; G, posterior root ganglion; Gs, sympathetic ganglion; .1/,
 
spinal cord. — (Onodi.)
 
 
 
420
 
 
 
THE SYMPATHETIC SYSTEM
 
 
 
prevertebral plexuses (PVG), such as the cardiac, solar, hypogastric, and pelvic, while the third or peripheral set {PG) is formed by
the cells which occur throughout the tissues of probably most of the
visceral organs, either in small groups or scattered through plexuses
such as the Auerbach and Meissner plexuses of the intestine. Each
cell in these various ganglia stands in direct contact with the axiscylinder of a cell situated in the central nervous system, probably in
the lateral horn of the spinal cord or the corresponding region of the
brain, so that each cell forms the terminal link of a chain whose first
link is a neurone belonging to the central system (Huber) . Through
 
 
 
Fig. 249. — Diagram showing the Arrangement of the Neurones of the Sympathetic System.
The fibers from the posterior root ganglia are represented by the broken black lines;
those from the anterior horn cells by the solid black; the white rami by red; and the
sympathetic neurones by blue. DR, Dorsal ramus of spinal nerve; GC, ganglionated
cord; GR, gray ramus communicans; PG, peripheral ganglion; PVG, prsevertebral
ganglion; VR, ventral ramus of spinal nerve; WR, white ramus communicans. —
{Adapted from Huber.)
 
out the thoracic and upper lumbar regions of the body the central
system neurones form distinct cords known as the white rami communicantes (Fig. 249, WR), which pass from the spinal nerves to the
adjacent ganglia of the ganglionated cord, some of them terminating around the cells of these ganglia, others passing on to the cells of
the prsevertebral ganglia, and others to those of the peripheral
plexuses. In the cervical, lower lumbar and sacral regions white
rami are wanting, the central neurones in the first-named region
probably making their way to the sympathetic cells by way of the upper
 
 
 
THE SYMPATHETIC SYSTEM
 
 
 
421
 
 
 
thoracic nerves, while in the lower regions they may pass down the
ganglionated cord from higher regions or may join the prevertebral
and peripheral ganglia directly without passing through the proximal ganglia. In addition to these white rami, what are known as
gray rami also extend between the proximal ganglia and the spinal
nerves; these are composed of fibers, arising from sympathetic cells,
 
 
 
 
Fig. 250. — Transverse Section through the Spinal Cord of an Embryo of 7 mm.
 
c, Notochord; g, posterior root ganglion; m, spinal cord; s, sympathetic cell migrating
 
from the posterior root ganglion; wr, white ramus.- — (His.)
 
which join the spinal nerves in order to pass with them to their
ultimate distribution.
 
The brief description here given applies especially to the sympathetic system of the neck and trunk. Representatives of the
system are also found in the head, in the form of a series of ganglia
connected with the trigeminal and facial nerves and known as the
ciliary, spheno-palatine, otic, and submaxillary ganglia; and, as will
 
 
 
422 THE SYMPATHETIC SYSTEM
 
be seen later, there are probably some sympathetic cells which owe
their origin to the root ganglia of the vagus and glossopharyngeal
nerves. There is nothing, however, in the head region corresponding
to the longitudinal bundles of fibers which unite the various proximal
ganglia of the trunk to form the ganglionated cord.
 
The first distinct indications of the sympathetic system are to be
seen in a human embryo of about 7 mm. As the spinal nerves
reach the level of the dorsal edge of the body-cavity, they branch,
one of the branches continuing ventrally in the body-wall, while the
other (Fig 250, wr) passes mesially toward the aorta, some of its
fibers reaching that structure, while others bend so as to assume a
longitudinal direction. These mesial branches represent the white
rami communicantes, but as yet no ganglion cells can be seen in
their course. The cells of the posterior root ganglia have already,
for the most part, assumed their bipolar form, but among them there
may still be found a number of cells in the neuroblast condition, and
these (Fig. 250, s), wandering out from the ganglia, give rise to a
column of cells standing in relation to the white rami. At first there
is no indication of a segmental arrangement of the cells of the column
(Fig. 251), but at about the seventh week such an arrangement
makes its appearance in the cervical region, and later, extends
posteriorly, until the column assumes the form of the ganglionated
cord.
 
This origin of the ganglionated cord from cells migrating out
from the posterior root ganglia has been described by various
authors, but recently the origin of the cells has been carried a step
further back, to the mantle layer of the central nervous system
(Kuntz). Indifferent cells and neuroblasts are said to wander out
from the walls of the medullary canal by way of both the posterior
and anterior nerve roots and it is claimed that these are the cells that
give rise to the ganglionated cord in the manner just described.
 
Before, however, the segmentation of the ganglionated cord becomes marked, thickenings appear at certain regions of the cell
column, and from these, bundles of fibers may be seen extending
ventrally toward the viscera. The thickenings represent certain of
 
 
 
THE SYMPATHETIC SYSTEM
 
 
 
423
 
 
 
the prevertebral ganglia, and later cells wander out from them and
take a position in front of the aorta. In an embryo of 10.2 mm. two
ganglionic masses (Fig. 251, pc) occur in the vicinity of the origin
 
 
 
 
Fig. 251. — Reconstruction of the Sympathetic System of an Embryo of 10.2 mm.
am, Vitelline artery; ao, aorta; au, umbilical artery; bg, ganglionic mass representing
the pelvic plexus; d, intestine; oe, oesophagus; pc, ganglia of the cceliac plexus; ph,
pharynx; rv, right vagus nerve; sp, splanchnic nerves; sy, ganglionated cord; t, trachea;
*, peripheral sympathetic ganglia in the walls of the stomach. — (His, Jr.)
 
 
 
of the vitelline artery (am), one lying above and the other below
that vessel; these masses represent the ganglia of the cceliac
 
 
 
424 LITERATURE
 
plexus and have separated somewhat from the ganglionated cord,
the fiber bundles which unite the upper mass with the cord representing the greater and lesser splanchnic nerves (sp), while that connected
with the lower mass represents the connection of the cord with the
superior mesenteric ganglion. Lower down, in the neighborhood
of the umbilical arteries, is another enlargement of the cord (bg),
which probably represents the inferior mesenteric and hypogastric
ganglia which have not yet separated from the cell column.
 
With the peripheral ganglia the conditions are slightly different,
in that they are formed very largely, if not exclusively, from cells
that migrate from the walls of the hind-brain by way of the vagus
nerves (Fig. 251). In this way the ganglia of the myenteric, pulmonary and cardiac plexuses are formed, though in the case of the
last named it is probable that contributions are also received from
the ganglionated cord.
 
The elongated courses of the cardiac sympathetic and splanchnic
nerves in the adult receive an explanation from the recession of the heart
arid diaphragm (see pp. 239 and 322), the latter process forcing downward the coeliac plexus, which originally occupied a position opposite
the region of the ganglionated cord from which the splanchnic nerves
arise.
 
As regards the cephalic sympathetic ganglia, the observations
of Remak on the chick and Kolliker on the rabbit show that the
ciliary, sphenopalatine, and otic ganglia arise by the separation of
cells from the semilunar (Gasserian) ganglion, and from their adult
relations it may be supposed that the cells of the submaxillary and
sublingual ganglia have similarly arisen from the geniculate ganglion
of the facial nerve. Evidence has also been obtained from human
embryos that sympathetic cells are derived from the ganglia of the
vagus and glossopharyngeal nerves, but, instead of forming distinct
ganglia in the adult, these, in all probability, associate themselves
with the first cervical ganglia of the ganglionated cord.
 
LITERATURE.
 
C. R. Bardeen: "The Growth and Histogenesis of the Cerebrospinal Nerves in
Mammals," Amer. Journ. AnaL, 11, 1903.
 
 
 
LITERATURE 425
 
S. R. Cajal: "Nouvelles Observations sur revolution des neuroblasts avec quelques
 
remarques sur l'hypothese neurogenetique de Hensen-Held," Anal. Anzeiger,
 
xxxii, 1908.
A. F. Dixon: "On the Development of the Branches of the Fifth Cranial Nerve in
 
Man," Sclent. Trans. Roy. Dublin Soc, Ser. 1, VI, 1896.
C. R. Essick: "The Development of the Nuclei pontis and the Nucleus Arcuatus in
 
Man," Amer. Journ. Anat., xiii, 1912.
E. Giglio-Tos: "Sugli organi branchiali e laterali di senso nell' uomo nei primordi
 
del suo sviluppo," Monit. Zool. Ital., xill, 1902.
E. Giglio-Tos: "SulP origine embrionale del nervo trigemino nell' uomo," Anat.
 
Anzeiger, xxi, 1902.
E. Giglio-Tos: "Sui primordi dello sviluppo del nervo acustico-faciale nell' uomo,"
 
Anat. Anzeiger, xxi, 1902.
K. Goldstein: "Die erste Entwicklung der grossen Hirncommissuren und die
 
'Verwachsung' von Thalamus und Striatum" Archiv jiir Anat. und Physiol.,
 
Anat. Abth., 1903.
G. Groenberg: "Die Ontogenese einer niederen Saugergehirns nach Untersuchungen
 
an Erinaceus europaeus," Zoolog. Jahrb. Abth. f. Anat. und Ontogen., xv, 1901.
I. Hardesty: "On the Development and Nature of the Neuroglia," Amer. Journ
 
Anat., in, 1904.
R. G. Harrison: "Further Experiments on the Development of Peripheral Nerves,'
 
Amer. Journ. of Anat., v, 1906.
W. His: "Zur Geschichte des menschlichen Ruckenmarkes und der Nervenwurzeln,'
 
Abhandl. der konigl. Sachsischen Gesellsch., Math.-Physik. Classe, xiii, 1886.
W. His: "Zur Geschichte des Gehirns sowie der centralen und peripherischen Nerven
bahnen beim menschlichen Embryo," Abhandl. der konigl. Sachsischen Gesellsch.,
 
Math.-Physik. Classe, xiv, 1888.
W. His: "Die Formentwickelung des menschlichen Vorderhirns vom Ende des ersten
 
bis zum Beginn des dritten Monats," Abhandl. der konigl. Sachsischen Gesellsch.,
 
Math.-Physik. Classe, xv. 1889.
W. His: "Histogenese und Zusammenhang der Nervenelemente," Archiv fur Anat.
 
und Physiol., Anat. Abth., Supplement, 1890.
W. His: "Die Entwickelung des menschlichen Gehirns wahrend der ersten Monate,"
 
Leipzig, 1904.
W. His, Jr.: "Die Entwickelung des Herznervensystem bei Wirbelthieren," Abhandl.
 
der konigl. Sachsischen Gesellsch., Math.-Physik. Classe, xvni, 1893.
W. His, Jr.: "Ueber die Entwickelung des Bauchsympathicus beim Huhnchen und
 
Menschen," Archiv filr Anat. und Physiol., Anat. Abth., Supplement, 1S97.
C. J. Herrick: " The Cranial and First Spinal Nerves of Menidia: A Contribution upon
 
the Nerve Components of the Bony Fishes," Journ. of Comp. Neurol., ix, 1899.
C. J. Herrick: "The Cranial Nerves and Cutaneous Sense-organs of the North
 
American Siluroid Fishes," Journ. of Comp. Neurol., xi, 1901.
G. C. Huber: "Four Lectures on the Sympathetic Nervous System," Journ. of Comp.
 
Neurol., vn, 1897.
A. Kuntz: "A Contribution to the Histogenesis of the Sympathetic System," Anat.
 
Record, in, 1909.
 
 
 
426 LITERATURE
 
A. Kuntz: "The role of the Vagi in the Development of the Sympathetic Nervous
 
System," Anat. Anzeiger, xxxv, 1909.
A. Kuntz: "The Development of the Sympathetic Nervous System in Mammals,'
 
Journ. Compar. Neurol., xx, 1910.
M. von Lenhossek: "Die Entwickelung der Ganglienanlagen bei dem menschlichen
 
Embryo," Archiv filr Anat. und Physiol., Anat. Abth., 1891.
 
F. Marchand: "Ueber die Entwickelung des Balkens im menschlichen Gehirn,"
 
Archiv filr mikrosk. Anat., xxxvn, 1891.
V. VON Mihalkovicz: " Entwickelungsgeschichte des Gehirns," Leipzig, 1877.
A. D. Onodi: "Ueber die Entwickelung des sympathischen Nervensystems," Archiv
 
filr mikrosk. Anat., xxvn, 1886.
 
G. Retzius: "Das Menschenhirn," Stockholm, 1896.
 
A. Schaper: "Die friihesten Differenzirungsvorgange im Central-nerven-system,'
 
Archiv filr Entwicklungsmechanik, v, 1897.
G. L. Streeter: " The Development of the Cranial and Spinal Nerves in the Occipita
 
Region of the Human Embryo," Amer. Journ. Anat., iv, 1904.
O. S. Strong: "The Cranial Nerves of Amphibia," Journal of Morphol., x, 1895.
R. Wlassak: "Die Herkunft des Myelins," Archiv filr Entwicklungsmechanik, VI
 
1898.
E. Zuckerkandl: "Zur Entwicklung des Balkens," Arbeiten aus neurol. Inst. Wien.
 
xvii, 1909.
 
 
 
CHAPTER XVI.
 
THE DEVELOPMENT OF THE ORGANS OF
SPECIAL SENSE.
 
Like the cells of the central nervous system, the sensory cells
are all of ectodermal origin, and in lower animals, such as the earthworm, for instance, they retain their original position in the ectodermal epithelium throughout life. In the vertebrates, however,
the majority of the sensory cells relinquish their superficial position
and sink more or less deeply into the subjacent tissues, being represented by the posterior root ganglion cells and by the sensory cells
of the special sense-organs, and it is only in the olfactory organ that
the original condition is retained. Those cells which have withdrawn from the surface receive stimuli only through overlying cells,
and in certain cases these transmitting cells are not specially differentiated, the terminal branches of the sensory dendrites e ding
among ordinary epithelial cells or in such structures as the Pacinian
bodies or the end-bulbs of Krause situated beneath undifferentiated
epithelium. In other cases, however, certain specially modified
superficial cells serve to transmit the stimuli to the peripheral sensory
neurones, forming such structures as the hair-cells of the auditory
epithelium or the gustatory cells of the taste-buds.
 
Thus three degrees of differentiation of the special sensory cells
may be recognized and a classification of the sense-organs may be
made upon this basis. One organ, however, the eye, cannot be
brought into such a classification, since its sensory cells present
certain developmental peculiarities which distinguish them from
those of all other sense-organs. Embryologically the retina is a
portion of the central nervous system and not a peripheral organ,
and hence it will be convenient to arrange the other sense-organs
 
427
 
 
 
428 THE OLFACTORY ORGANS
 
according to the classification indicated and to discuss the" history
of the eye at the close of the chapter.
 
The Development of the Olfactory Organ. — The general
development of the nasal fossa, the epithelium of which contains the
olfactory sense cells, has already been described (pp. 99 and 283),
as has also the development of the olfactory lobes of the brain
(p. 406), and there remains for consideration here merely the formation of the olfactory nerve and the development of the rudimentary
organ of Jacobson.
 
The Olfactory Nerve. — Very diverse results have been obtained by
various observers of the development of the olfactory nerve, it having
been held at different times that it was formed by the outgrowth of
fibers from the olfactory lobes (Marshall), from fibers which arise
partly from the olfactory lobes and partly from the olfactory epithelium (Beard), from the cells of an olfactory ganglion originally derived
from the olfactory epithelium but later separating from it (His),
and, finally, that it was composed of the prolongations of certain
cells situated and, for the most part at least, remaining permanently
in the olfactory epithelium (Disse). The most recent observations on
the structure of the olfactory epithelium and nerve indicate a greater
amount of probability in the last result than in the others, and the
description which follows will be based upon the observations of His,
modified in conformity with the results obtained by Disse from chick
embryos.
 
In human embryos of the fourth week the cells lining the upper
part of the olfactory pits show a distinction into ordinary epithelial
and sensory cells, the latter, when fully formed, being elongated
cells prolonged peripherally into a short but narrow process which
reaches the surface of the epithelium and proximally gives rise to
an axis-cylinder process which extends up toward and penetrates the
tip of the olfactory lobe to come into contact with the dendrites of
the first central neurones of the olfactory tract (Fig. 252). These
cells constitute a neuro-epithelium and in later stages of development retain their epithelial position for the most part, a few of them,
however, withdrawing into the subjacent mesenchyme and becoming
 
 
 
THE OLFACTORY ORGANS
 
 
 
429
 
 
 
bipolar, their peripheral prolongations ending freely among the cells
of the olfactory epithelium. These bipolar cells resemble closely
in form and relations the cells of the embryonic posterior root ganglia,
and thus form an interesting transition between these and the neuroepithelial cells.
 
The Organ of Jacohson. — In embryos of three or four months a
 
 
 
 
Fig. 252. — Diagram Illustrating the Relations of the Fibers of the Olfactory
 
Nerve.
 
Ep, Epithelium of the olfactory pit; C, cribiform plate of the'ethmoid, G, glomerulus of
 
the olfactory bulb; M, mitral cell. — (Van Gekuchten.) j
 
small pouch-like invagination of the epithelium covering the lower
anterior portion of the median septum of the nose can readily be
seen. This becomes converted into a slender pouch, 3 to 5 mm. long,
ending blindly at its posterior extremity and opening at its other end
 
 
 
430 THE ORGANS OF TASTE
 
into the nasal cavity. Its lining epithelium resembles that of the
respiratory portion of the nasal cavity, and there is developed in the
connective tissue beneath its floor a slender plate of cartilage, distinct from that forming the septum of the nose.
 
This organ, which may apparently undergo degeneration in the
adult, and in some cases completely disappears, appears to be the
representative of what is known as Jacobson's organ, a structure
which reaches a much more extensive degree of development in many
of the lower mammals, and in these contains in its epithelium sensory
cells whose axis-cylinder processes pass with those of the olfactor}
sense cells to the olfactory bulbs. In man, however, it seems to be a
rudimentary organ, and no satisfactory explanation of its function
has as yet been advanced.
 
The olfactory neuro-epithelium, considered from a comparative
standpoint, seems to have been derived from the system of lateral
line organs so highly developed in the lower vertebrates (Kupffer).
In higher forms the system, which is cutaneous in character, has
disappeared except in two regions where it has become highly
specialized. In one of these regions it has given rise to the olfactory
sense cells and in the other to the similar cells of the auditory
apparatus.
 
The Organs of Touch and Taste. — Little is yet known concerning the development of the various forms of tactile organs, which
belong to the second class of sensory organs described above.
 
The Organs of Taste. — The remaining organs of special sense
belong to the third class, and of these the organs of taste present in
many respects the simplest condition. They are developed principally in connection with the vallate and foliate papillae of the
tongue, and of the former one of the earliest observed stages has
been found in embryos of 9 cm. in the form of two ridges of epidermis, lying toward the back part of the tongue and inclined to one
another in such a manner as to form a V with the apex directed
backward. From these ridges solid downgrowths of epidermis
into the subjacent tissue occur, each downgrowth having the form
of a hollow truncated cone with its basal edge continuous with the
 
 
 
THE INTERNAL EAR 43 1
 
superficial epidermis (Fig. 253, A). In later stages lateral outgrowths develop from the deeper edges of the cone, and about the
same time clefts appear in the substance of the original downgrowths
(Fig. 253, B) and, uniting together, finally open to the surface, forming a trench surrounding a papilla (Fig. 253, C). The lateral outgrowths, which are at first solid, also undergo an axial degeneration
and become converted into the glands of Ebner (b), which open into
the trench near its floor. The various papillae which occur in the
adult do not develop simultaneously, but their number increases
with the age of the fetus, and there is, moreover, considerable
variation in the time of their development.
 
The taste-buds are formed by a differentiation of the epithelium
which covers the papillae, and this differentiation appears to stand
 
 
 
 
 
B C
 
Fig. 253. — Diagrams Representing the Development of a Vallate Papilla.
a, Valley surrounding the papilla; b, von Ebner's gland. — (Graberg.)
 
in intimate relation with the penetration of fibers of the glossopharyngeal nerve into the papillae. The buds form at various places
upon the papillae, and at one period are especially abundant upon
their free surfaces, but in the later weeks of intrauterine life these
surface buds undergo degeneration and only those upon the sides
of the trench persist, as a rule.
 
The foliate papillae do not seem to be developed until some time
after the circumvallate, being entirely wanting in embryos of four
and a half and five months, although plainly recognizable at the
seventh month.
 
The Development of the Ear. — It is customary to describe the
mammalian ear as consisting of three parts, known as the inner,
middle, and outer ears, and this division is, to a certain extent at
 
 
 
43 2
 
 
 
THE INTERNAL EAR
 
 
 
least, confirmed by the embryonic development. The inner ear,
which is the sensory portion proper, is an ectodermal structure, which
secondarily becomes deeply seated in the mesodermal tissue of the
head, while the middle and outer ears, which provide the apparatus
necessary for the conduction of the sound-waves to the inner ear,
are modified portions of the anterior branchial arches. It will be
convenient, accordingly, in the description of the ear, to accept
the usually recognized divisions and to consider first of all the
development of the inner ear, or, as it is better termed, the otocyst.
The Development of the Otocyst. — In an embryo of 2.4 mm. a
pair of pits occur upon the surface of the body about opposite the
middle portion of the hind-brain (Fig. 254, A). The ectoderm
lining the pits is somewhat thicker than is the neighboring ectoderm
 
 
 
 
 
a — B
 
Fig. 254. — Transverse Section Passing through the Otocyst (ot) of Embryos of
(A) 2.4 mm. and (B) 4 mm. — {His.)
 
of the surface of the body, and, from analogy with what occurs in
other vertebrates, it seems probable that the pits are formed by the
invagination of localized thickenings of the ectoderm. The mouth
of each pit gradually becomes smaller, until finally the invagination
is converted into a closed sac (Fig. 254, B), which separates from the
surface ectoderm and becomes enclosed within the subjacent mesoderm. This sac is the otocyst, and in the stage just described,
found in embryos of 4 mm., it has an oval or more or less spherical
form. Soon, however, in embryos of 6.9 mm., a prolongation
arises from its dorsal portion and the sac assumes the form shown in
Fig. 255, A; this prolongation, which is held by some authors to be
the remains of the stalk which originally connected the otocyst sac
 
 
 
THE INTERNAL EAR
 
 
 
433
 
 
 
with the surface ectoderm, represents the ductus endolymphaticus ,
and, increasing in length, it soon becomes a strong club-shaped
process, projecting considerably beyond the remaining portions of
the otocyst (Fig. 255, B). In embryos of about 10.2 mm. the sac
begins to show certain other irregularities of shape (Fig. 255, B, sc).
Thus, about opposite the point of origin of the ductus endolymphaticus three folds make their appearance, representing the semi
 
 
 
 
-rsc
 
 
 
Fig. 255. — Reconstruction of the Otocysts of Embryo of (A) 6.9 mm. and (B)
 
10.2 MM.
 
de, Endolymphatic duct; gc, ganglion cochleare; gg, ganglion geniculatum; gv,
 
ganglion vestibulare; sc, lateral semicircular duct. — (His, Jr.)
 
circular ducts, and as they increase in size the opposite walls of the
central portion of each fold come together, fuse, and finally become
absorbed, leaving the free edge of the fold as a crescentic canal, at
one end of which an enlargement appears to form the ampulla. The
transformation of the folds into canals takes place somewhat earlier
in the cases of the two vertical than in that of the horizontal duct, as
28
 
 
 
434
 
 
 
THE INTERNAL EAR
 
 
 
may be seen from Fig. 256, which represents the condition occurring
 
in an embryo of 13.5 mm.
 
A short distance below the level at which the canals communicate
 
with the remaining portion of the otocyst a constriction appears,
 
indicating a separation of the otocyst
into a more dorsal portion and a more
ventral one. Later, the latter begins
to be prolonged into a flattened canal
which, as it elongates, becomes coiled
upon itself and also becomes separated
by a constriction from the remaining
portion of the otocyst (Fig. 257).
This canal is the ductus cochlearis
(scala media of the cochlea), and the
remaining portion of the otocyst subsequently becomes divided by a constriction into the utriculus, with which
the semicircular ducts are connected,
and the sacculus. The constriction
which separates the cochlear duct from
the sacculus becomes the ductus reuniens, while that between the utriculus and sacculus is converted into
a narrow canal with which the ductus
endolymphaticus connects, and hence
it is that, in the adult, the connection
between these two portions of H the
otocyst seems to be formed by the
ductus dividing proximally into two
limbs, one of which is connected with
 
the utricle and the other with the saccule.
 
When first observed in the human embryo the auditory ganglion
 
is closely associated with the geniculate ganglion of the seventh
 
nerve (Fig. 255, B), the two, usually spoken of as the acustico-facialis
 
ganglion, forming a mass of cells lying in close contact with the
 
 
 
 
Fig. 256. — Reconstruction of
the Otocyst of an Embryo of
 
I3.5 MM.
 
co, Cochlea; de, endolymphatic
duct;.sc, semicircular duct. — (His
Jr.)
 
 
 
THE INTERNAL EAR
 
 
 
435
 
 
 
anterior wall of the otocyst. The origin of the ganglionic mass has
not yet been traced in the mammalia, but it has been observed that
in cow embryos the geniculate ganglion is connected with the ectoderm at the dorsal end of the first branchial cleft (Froriep), and it
may perhaps be regarded as one of the epibranchial placodes (see p.
417), and in the lower vertebrates a union of the ganglion with a
suprabranchial placode has been observed (Kupffer), this union
 
 
 
 
Fig. 257. — Reconstruction of the Otocyst of an Embryo of 20 mm., front view.
cc, Common limb of superior and posterior semicircular ducts; eg, cochlear ganglion;
co, cochlea; de, endolymphatic duct; s, sacculus; sdl, sdp, and sds x lateral, posterior and
superior semicircular ducts; u, utriculus; vg, vestibular ganglion. — {Streeter.)
 
indicating the origin of the auditory ganglion from one or more of
the ganglia of the lateral line system.
 
At an early stage in the human embryo the auditory ganglion
shows indications of a division into two portions, a more dorsal one,
which represents the future ganglion vestibular e, and a ventral one,
the ganglion cochleare. The ganglion cells become bipolar, in which
condition they remain throughout life, never reaching the T-shaped
condition found in most of the other peripheral cerebro-spinal ganglia. One of the prolongations of each cell is directed centrally to
 
 
 
43 6
 
 
 
THE INTERNAL EAR
 
 
 
form a fiber of the auditory nerve, while the other penetrates the wall
of the otocyst to enter into relations with certain specially modified
cells which differentiate from its lining epithelium.
 
â– In the earliest stages the ectodermal lining of the otocyst is
formed of similar columnar cells, but later over the greater part of
the surface the cells flatten down, only a few, aggregated together to
 
 
 
 
Fig. 258. — The Right Internal Ear of an Embryo of Six Months.
ca, ce, and cp, Superior, lateral, and posterior semicircular ducts; cr, crista acustica;
de, endolymphatic duct; Is, spiral ligament; mb, basilar membrane; ms and tnu, macula
acustica sacculi and utriculi; rb, basilar branches of the cochlear nerve. — (Retzius.)
 
form patches, retaining the high columnar form and developing hairlike processes upon their free surfaces. These are the sensory cells
of the ear. In the human ear there are in all six patches of these
sensory cells, an elongated patch {crista ampullaris) in the ampulla of
each semicircular canal (Fig. 258, cr), a round patch {macula acus
 
 
THE INTERNAL EAR 437
 
tica, mii) in the utriculus and another (ms) in the sacculus, and,
finally, an elongated patch which extends the entire length of the
scala media of the cochlea and forms the sensory cells of the spiral
organ of Corti. The cells of this last patch are connected with the
fibers from the cochlear ganglion, while those of the vestibular
ganglion pass to the cristas and maculae.
 
In connection with the spiral organ certain adjacent cells also
retain their columnar form and undergo various modifications,
 
 
 
ftfJ^\-\'
 
 
 
 
 
 
 
<&WB
 
 
 
%^,
 
 
 
 
 
 
l&i^Spfc
 
 
 
 
Fig. 259. — Section of the Cochlear Duct of a Rabbit Embryo of 55 mm.
 
a, Mesenchyme; b to e, epithelium of cochlear duct; M.t, membrana tectoria; V.s.p,
 
vein; 1 to 7, spiral organ of Corti. — (Baginsky.)
 
giving rise to a rather complicated structure whose development has
been traced in the rabbit. Along the whole length of the cochlear
duct the cells resting upon that half of the basilar membrane which is
nearest the axis of the cochlea, and may be termed the inner half,
retain their columnar shape, forming two ridges projecting slightly
into the cavity of the scala (Fig. 259). The cells of the inner ridge,
much the larger of the two, give rise to the membrana tectoria,
 
 
 
438 THE INTERNAL EAR
 
either as a cuticular secretion or by the artificial adhesion of long
hair-like processes which project from their free surfaces (Ayers).
The cells of the outer ridge are arranged in six longitudinal rows
(Fig. 259, 1-6); those of the innermost row (1) develop hairs upon
their free surfaces and form the inner hair cells, those of the next two
 
rows (2 and 3) gradually become transformed on their adjacent surfaces into chitinous
 
p /''- /^ substance and form the rods of
 
Corti, while the three outer rows
 
 
 
; (4 to 6) develop into the outer
e -^~~ __ hair cells. It is in connection
 
with the hair cells that the peripheral prolongations of the cells
of the cochlear ganglion ter™JS= d^t" 1 ™ mi «ate, and since these hair cells
 
Rabbit Embryo of Twenty-four Days, are arranged in rows extending
 
c, Periotic cartilage; ep, fibrous mem- .1 pnt j rp ] er ,a\h of the cochlear
 
brane beneath the epithelium of the canal; me ermre lengin 01 me COCIliear
 
p, perichondrium; s, spongy tissue.— (Von duct, the ganglion also IS drawn
Kolliker.) . 1 .. .
 
out into a spiral following the
coils of the cochlea, and hence is sometimes termed the spiral
ganglion.
 
While the various changes described above have been taking
place in the otocyst, the mesoderm surrounding it has also been
undergoing development. At first this tissue is quite uniform in
character, but later the cells immediately surrounding the otocyst
condense to give rise to a fibrous layer (Fig. 260, ep), while more
peripherally they become more loosely arranged and form a somewhat gelatinous layer (s) , and still more peripherally a second fibrous
layer is differentiated and the remainder of the tissue assumes a
character which indicates an approaching conversion into cartilage.
The further history of these various layers is as follows: The inner
fibrous layer gives rise to the connective-tissue wall which supports
the ectodermal lining of the various portions of the otocyst; the
gelatinous layer undergoes a degeneration to form a lymph-like
 
 
 
THE INTERNAL EAR
 
 
 
439
 
 
 
fluid known as the perilymph, the space occupied by the fluid being
the perilymphatic space; the outer fibrous layer becomes perichondrium and later periosteum; and the precartilage undergoes
chondrification and later ossifies to form the petrous portion of the
temporal bone.
 
The gelatinous layer completely surrounds most of the otocyst
structures, which thus come to lie free in the perilymphatic space,
but in the cochlear region the conditions are somewhat different.
In this region the gelatinous layer is interrupted along two lines,
 
 
 
 
Fig. 261. — Diagrammatic Transverse Section through a Coil of the Cochlea
 
showing the relation of the scal^e.
c, Organ of Corti; co, ganglion cochleare; Is, lamina spiralis; SAT, cochlear duct; ST,
scala tympani; SV, scala vestibuli. — (From Gerlach.)
 
an outer broad one where the connective-tissue wall of the cochlear
duct is directly continuous with the perichondrium layer, and an
inner narrow one, along which a similar fusion takes place with the
perichondrium of a shelf-like process of the cartilage, which later
ossifies to form the lamina spiralis. Consequently throughout the
cochlear region the perilymphatic space is divided into two compartments which communicate at the apex of the cochlea, while below
one, known as the scala vestibuli, communicates with the space
 
 
 
440 THE MIDDLE EAR
 
surrounding the saccule and utricle, and the other, the scala tympani,
abuts upon a membrane which separates it from the cavity of the
middle ear and represents a portion of the outer wall of the petrous
bone where chondrification and ossification have failed to occur.
This membrane closes what appears in the dried skull to be an
opening in the inner wall of the middle ear, known as the fenestra
cochlea {rotunda) ; another similar opening, also closed by membrane
in the fresh skull, occurs in the bony wall opposite the utricular
portion of the otocyst and is known as the fenestra vestibuli (ovalis) .
 
The Development of the Middle Ear. — The middle ear develops
from the upper part of the pharyngeal groove which represents the
endodermal portion of the first branchial cleft. This becomes
prolonged dorsally and at its dorsal end enlarges to form the tympanic cavity, while the narrower portion intervening between this
and the pharyngeal cavity represents the tuba auditiva (Eustachian
tube).
 
To correctly understand the development of the tympanic
cavity it is necessary to recall the structures which form its boundaries. Anteriorly to the upper end of the first branchial pouch
there is the upper end of the first arch, and behind it the corresponding part of the second arch, the two fusing together dorsal to the
tympanic cavity and forming its roof. Internally the cavity is
bounded by the outer wall of the cartilaginous investment of the
otocyst, while externally it is separated from the upper part of the
ectodermal groove of the first branchial cleft by the thin membrane
which forms the floor of the groove.
 
It has been seen in an earlier chapter that the axial mesoderm
of each branchial arch gives rise to skeletal structures and muscles.
The axial cartilage of the ventral portion of the first arch is what is
known as Meckel's cartilage, but in that portion of the arch which
forms the roof and anterior wall of the tympanic cavity, the cartilage
becomes constricted to form two masses which later ossify to form the
malleus and incus (Fig. 262, m and i), while the muscular tissue of
this dorsal portion of the arch gives rise to the tensor tympani. Similarly, in the case of the second arch there is to be found, dorsal to
 
 
 
THE MIDDLE EAR 44 1
 
the extremity of the cartilage which forms the styloid process of the
adult, a narrow plate of cartilage which forms an investment for
the facial nerve (Fig. 262, VII), and dorsal to this a ring of cartilage
(st) which surrounds a small stapedial artery and represents the
stapes.
 
It has been found that in the rabbit the mass of cells from which
the stapes is formed is at its first appearance quite independent of
the second branchial arch (Fuchs), and it has been held to be a
 
 
 
 
Fig. 262. — Semi-diagrammatic Viewof the Auditory Ossicles of an Embryo of
 
Six Weeks.
i, Incus; j, jugular vein; m, malleus; mc, Meckel's cartilage; oc, capsule of otocyst;
R, cartilage of the second branchial arch; st, stapes; VII, facial nerve. — (Siebenmann.)
 
derivative of the mesenchyme from which the periotic capsule is
formed. In later stages, however, it becomes connected with the
cartilage of the second branchial arch, as shown in Fig. 262, and
it is a question whether this connection, which is transitory, does
not really indicate the phylogenetic origin of the ossicle from the
second arch cartilage, its appearance as an independent structure
being a secondary ontogenetic phenomenon. However that may
be, the stapedial artery disappears in later stages and the stapedius
 
 
 
442
 
 
 
THE MIDDLE EAR
 
 
 
P
 
 
\M
 
 
 
T
 
 
 
mi
 
 
 
muscle, derived from the musculature of the second branchial arch
and therefore supplied by the facial nerve, becomes attached to the
ossicle.
 
The three ossicles at first lie embedded in the mesenchyme
forming the roof of the primitive tympanic cavity, as does also the
 
chorda tympani, a branch of the
seventh nerve, as it passes into the
substance of the first arch on the way
to its destination. The mesenchyme
in which these various structures are
embedded is rather voluminous (Fig.
264), and after the end of the seventh
month becomes converted into a peculiar spongy tissue, which, toward the
end of fetal life, gradually degenerates, the tympanic cavity at the same
time expanding and wrapping itself
around the ossicles and the muscles
attached to them (Fig. 263). The
bones and their muscles, consequently,
while appearing in the adult to traverse the tympanic cavity, are really
completely enclosed within a layer of
epithelium continuous with that lining
the wall of the cavity, while the
handle of the malleus and the chorda
tympani lie between the epithelium of
the outer wall of the cavity and the
fibrous mesoderm which forms the
tympanic membrane.
The extension of the tympanic cavity does not, however, cease
with its replacement of the degenerated spongy mesenchyme, but
toward the end of fetal life it begins to invade the substance of the
temporal bone by a process similar to that which produces the
ethmoidal cells and the other osseous sinuses in connection with the
 
 
 
m
 
 
 
•M
 
 
 
Fig. 263. — Diagrams Illustrating the Mode of Extension of the Tympanic Cavity
Around the Auditory Ossicles.
 
M, Malleus; m, spongy mesenchyme; p, surface of the periotic
capsule; T, tympanic cavity.
The broken line represents the
epithelial lining of the tympanic
cavity.
 
 
 
THE EXTERNAL EAR 443
 
nasal cavities (see p. 175). This process continues for some years
after birth and results in the formation in the mastoid portion of the
bone of the so-called mastoid cells, which communicate with the
tympanic cavity and have an epithelial lining continuous with that
of the cavity.
 
The lower portion of the diverticulum from the first pharyngeal
groove which gives rise to the tympanic cavity becomes converted
into the Eustachian tube. During development the lumen of the
tube disappears for a time, probably owing to a proliferation of its
lining epithelium, but it is re-established before birth.
 
In the account of the development of the ear-bones given above it is
held that the malleus and incus are derivatives of the first branchial
(mandibular) arch and the stapes probably of the second. This view
represents the general consensus of recent workers on the difficult question of the origin of these bones, but it should be mentioned that nearly
all possible modes of origin have been at one time or other suggested.
The malleus has very generally been accepted as coming from the first
arch, and the same is true of the incus, although some earlier authors have
assigned it to the second arch. But with regard to the stapes the opinions have been very varied. It has been held to be derived from the first
arch, from the second arch, from neither one nor the other, but from the
cartilaginous investment of the otocyst, or, finally, it has been held to have
a compound origin, its arch being a product of the second arch while its
basal plate was a part of the otocyst investment.
 
The Development of the Tympanic Membrane and of the Outer
Ear. — Just as the tympanic cavity is formed from the endodermal
groove of the first branchial cleft, so the outer ear owes its origin to
the ectodermal groove of the same cleft and to the neighboring arches.
The dorsal and most ventral portions of the groove flatten out and
disappear, but the median portion deepens to form, at about the
end of the second month, a funnel-shaped cavity which corresponds
to the outer portion of the external auditory meatus. From the
inner end of this a solid ingrowth of ectoderm takes place, and this,
enlarging at its inner end to form a disk-like mass, comes into relation with the gelatinous mesoderm which surrounds the malleus and
chorda tympani. At about the seventh month a split occurs in the
disk-like mass (Fig. 264), separating it into an outer and an inner
 
 
 
444
 
 
 
THE EXTERNAL EAR
 
 
 
layer, the latter of which becomes the outer epithelium of the
tympanic membrane. Later, the split extends outward in the
substance of the ectodermal ingrowth and eventually unites with
the funnel-shaped cavity to complete the external meatus.
 
The tympanic membrane is formed in considerable part from
 
 
 
 
me'
 
 
 
Fig. 264. — Horizontal Section Passing through the Dorsal Wall of the
 
External Auditory Meatus in an Embryo of 4.5 cm.
 
c, Cochlea; de, endolymphatic duct; i, incus; Is, transverse sinus; m, malleus; me,
 
meatus auditorius externus; me' , cavity of the meatus; s, sacculus; sc, lateral semicircular
 
canal; sc', posterior semicircular canal; st, stapes; t, tympanic cavity; u, utriculus; 7,
 
facial nerve. — {Siebenmann.)
 
the substance of the first branchial arch, the area in which it occurs
not being primarily part of the wall of the tympanic cavity, but being
brought into it secondarily by the expansion of the cavity. The
membrane itself is mesodermal in origin and is lined on its outer
 
 
 
THE EXTERNAL EAR
 
 
 
445
 
 
 
surface by an ectodermal and on the inner by an endodermal
epithelium.
 
The auricle {pinna) owes its origin to the portions of the first and
second arches which bound the entrance of the external meatus.
Upon the posterior edge of the first arch there appear about the
end of the fourth week two transverse furrows which mark off three
tubercles (Fig. 258, A, 1-3) and on the anterior edge of the second
 
 
 
 
 
 
c
 
 
 
 
 
 
/ F
 
 
 
Fig. 265. — Stages in the Development of the Auricle.
 
A, Embryo of n mm.; B, of 13.6 mm.; C, of 15 mm.; D, at the beginning of the third
 
month; E, fetus of 8.5 cm.; F, fetus at term. — (His.)
 
arch a corresponding number of tubercles (4-6) is formed, while, in
addition, a longitudinal furrow, running down the middle of the
arch, marks ofT a ridge (c) lying posterior to the tubercles. From
these six tubercles and the ridge are developed the various parts of
the auricle, as may be seen from Fig. 265 which represents the
 
 
 
446 THE EYE
 
transformation as described by His. According to this, the most
ventral tubercle of the first arch (i) gives rise to the tragus, and the
middle one (5) of the second arch furnishes the antitragus. The
middle and dorsal tubercles of the first arch (2 and 3) unite with the
ridge (c) to produce the helix, while from the dorsal tubercle of the
second arch (4) is produced the anthelix and from the ventral one (6)
the lobule. More recent observations, however, seem to indicate
that the lobule is an accessory structure unrelated to the tubercles
and that the sixth tubercle gives rise to the antitragus, while the
fifth is either included in the anthelix or else disappears. It is
noteworthy that up to about the third month of development the
upper and posterior portion of the helix is bent forward so as to
conceal the anthelix (Fig. 265, D); it is at just about a corresponding
stage that the pointed form of the ear seen in the lower mammals
makes its appearance, and it is evident that, were it not for the forward bending, the human ear would also be assuming at this stage
a more or less pointed form. Indeed, there is usually to be found
upon the incurved edge of the helix, some distance below the upper
border of the auricle, a more or less distinct tubercle, known as
Darwin's tubercle, which seems to represent the point of the typical
mammalian ear, and is, accordingly, the morphological apex of the
pinna.
 
There seems to be little room for doubt that the otocyst belongs
primarily to the system of lateral line sense-organs, but a discussion of this
interesting question would necessitate a consideration of details concerning the development of the lower vertebrates which would be foreign to
the general plan of this book. It may be recalled, however, that the
analysis of the components of the cranial nerves described on page 415
refers the auditory nerve to the lateral line system.
 
The Development of the Eye. — The first indications of the
development of the eye are to be found in a pair of hollow outgrowths from the side of the first primary brain vesicle, at a level
which corresponds to the junction of the dorsal and ventral zones.
Each evagination is directed at first upward and backward, and,
enlarging at its extremity, it soon shows a differentiation into a
 
 
 
THE EYE 447
 
terminal bulb and a stalk connecting the bulb with the brain (Fig.
232). At an early stage the bulb comes into apposition with the
ectoderm of the side of the head, and this, over the area of con
 
 
 
Fig. 266. — Early Stages in the Development of the Lens in a Rabbit Embryo.
The nucleated layer to the left is the ectoderm and the thicker lens epithelium,
beneath which is the outer wall of the optic evagination; above and below between the
two is mesenchyme. — (Rabl.)
 
tact, becomes thickened and then depressed to form the beginning
of the future lens (Fig. 266).
 
As the result of the depression of the lens ectoderm, the outer wall
 
 
 
448
 
 
 
THE EYE
 
 
 
of the optic bulb becomes pushed inward toward the inner wall, and
this invagination continuing until the two walls come into contact,
the bulb is transformed into a double-walled cup, the optic cup, in
the mouth of which lies the lens (Fig. 268). The cup is not perfect,
however, since the invagination affects not only the optic bulb, but
also extends medially on the posterior surface of the stalk, forming
upon this a longitudinal groove and producing a defect of the ventral
wall of the cup, known as the chorioidal fissure (Fig. 267). The
groove and fissure become occupied by mesodermal tissue, and in
this, at about the fifth week, a blood-vessel develops which traverses
 
 
 
 
Fig. 267. — Reconstruction of the Brain of an Embryo of Four Weeks, showing
the Chorioid Fissure. — (His.)
 
the cavity of the cup to reach the lens and is known as the arteria
hyaloidea.
 
In the meantime further changes have been taking place in the
lens. The ectodermal depression which represents it gradually
deepens to form a cup, the lips of which approximate and finally
meet, so that the cup is converted into a vesicle which finally separates completely from the ectoderm (Fig. 268), much in the same
way as the otocyst does. As the lens vesicle is constricted off, the
surrounding mesodermal tissue grows in to form a layer between
it and the overlying ectoderm, and a split appearing in the layer
 
 
 
THE EYE
 
 
 
449
 
 
 
divides it into an outer thicker portion, which represents the cornea,
and an inner thinner portion, which covers the outer surface of the
lens and becomes highly vascular. The cavity between these two
portions represents the anterior chamber of the eye. The cavity of
the optic cup has also become filled by a peculiar tissue which represents the vitreous humor, while the mesodermal tissue surrounding
 
 
 
 
Fig. 268. — Horizontal Section through the Eye of an Embryo Pig of 7 mm.
Br, Diencephalon; Ec, ectoderm; I, lens; P, pigment, and R, retinal layers of the retina.
 
 
 
the cup condenses to form a strong investment for it, which is externally continuous with the cornea, and at about the sixth week
shows a differentiation into an inner vascular layer, the chorioid coat,
and an outer denser one, which becomes the sclerotic coat.
 
The various processes resulting in the formation of the eye,
29
 
 
 
450 THE LENS
 
which have thus been rapidly sketched, may now be considered in
greater detail.
 
The Development of the Lens. — When the lens vesicle is complete,
it forms a more or less spherical sac lying beneath the superficial
ectoderm and containing in its cavity a few cells, either scattered
or in groups (Fig. 268). These cells, which have wandered into
the cavity of the vesicle from its walls, take no part in the further
development of the lens, but early undergo complete degeneration,
and the first change which is concerned with the actual formation
of the lens is an increase in the height of the cells forming its inner
wall and a thinning out of its outer wall (Fig. 269, A). These
changes continuing, the outer half of the vesicle becomes converted
into a single layer of somewhat flat cells which persist in the adult
condition to form the anterior epithelium of the lens, while the cells of
the posterior wall form a marked projection into the cavity of the
vesicle and eventually completely obliterate it, coming into contact
with the inner surface of the anterior epithelium (Fig. 269, B).
 
These posterior elongated cells form, then, the principal mass
of the lens, and constitute what are known as the lens fibers. At
first those situated at the center of the posterior wall are the longest,
the more peripheral ones gradually diminishing in length until at
the equator of the lens they become continuous with and pass into the
anterior epithelium. As the lens increases in size, however, the
most centrally situated cells fail to elongate as rapidly as the more
peripheral ones and are pushed in toward the center of the lens, the
more peripheral fibers meeting below them along a line passing
across the inner surface of the lens. The disparity of growth continuing, a similar sutural line appears on the outer surface beneath
the anterior epithelium, and the fibers become arranged in concentric layers around a central core composed of the shorter fibers.
In the human eye the line of suture of the peripheral fibers becomes
bent so as to consist of two limbs which meet at an angle, and from
the angle a new sutural line develops during embryonic life, so that
the suture assumes the form of a three-rayed star. In later life the
 
 
 
THE LENS
 
 
 
 
 
*H
 
 
 
 
 
 
 
 
 
•■■»■
 
 
 
Fig. 269.— Sections through the Lens (4) of Human Embryo of Thirty
Thirty-one Days and (B) of Pig Embryo of 36 Mu.—(Rabl.)
 
 
 
 
45 :
 
 
 
THE LENS
 
 
 
stars become more complicated, being either six-rayed or more
usually nine-rayed in the adult condition '(Fig. 270).
 
As early as the second month of development the lens vesicle
becomes completely invested by the mesodermal tissue in which
blood-vessels are developed in considerable numbers, whence the
 
 
 
 
Fig. 270.
 
 
-Posterior (Inner) Surface of the Lens from an Adult showing the
Sutural Lines. — (Rabl.)
 
 
 
investment is termed the tunica vasculosa lends (Fig. 278, tv). The
arteries of the tunic are in connection principally with the hyaloid
artery of the vitreous humor (Fig. 276), and consist of numerous
fine branches which envelop the lens and terminate in loops almost
at the center of its outer surface. This tunic undergoes degenera
 
 
the optic cup 453
 
tion after the seventh month of development, by which time the
lens has completed its period of most active growth, and, as a rule,
completely disappears before birth. Occasionally, however, it may
persist to a greater or less extent, the persistence of the portion covering the outer surface of the lens, known as the membrana papillaris, causing the malformation known as congenital atresia of the
pupil.
 
In addition to the vascular tunic, the lens is surrounded by a
non-cellular membrane termed the capsule. The origin of 'this
structure is still in doubt, some observers maintaining that it is a
product of the investing mesoderm, while others hold it to be a product of the lens epithelium.
 
It is interesting from the standpoint of developmental mechanics to
note that W. H. Lewis and Spemann have shown that in the Amphibia contact of the optic vesicle with the ectoderm is necessary for the
formation of the lens, and, furthermore, if the vesicle be transplanted to
other regions of the body of a larva, a lens will be developed from the
ectoderm with which it is then in contact, even in the abdominal region,
 
The Development of the Optic Cup.- — When the invagination of
the outer wall of the optic bulb is completed, the margins of the
resulting cup are opposite the sides of the lens vesicle (Fig. 268),
but with the enlargement of the lens and cup the margins of the
latter gradually come to lie in front of— that is to say, upon the outer
surface of — the lens, forming the boundary of the opening known
as the pupil. The lens, consequently, is brought to lie within the
mouth of the optic cup, and that portion of the latter which covers
the lens takes part in the formation of the iris and the adjacent
ciliary body, while its posterior portion gives rise to the retina.
 
The chorioidal fissure normally disappears during the sixth or
seventh week of development by a fusion of its lips, and not until
this is accomplished does the term cup truly describe the form
assumed by the optic bulb after the invagination of its outer wall.
In certain cases the lips of the fissure fail to unite perfectly, producing
the defect of the eye known as coloboma; this may vary in its extent,
sometimes affecting both the iris and the retina and forming what
 
 
 
454 THE miS AND CILIARY BODY
 
is termed coloboma iridis, and at others being confined to the retinal portion of the cup, in which case it is termed coloboma
chorioidae.
 
Up to a certain stage the differentiation of the two layers which
form the optic cup proceeds along similar lines, in both the ciliary
and retinal regions. The layer which represents the original internal portion of the bulb does not thicken as the cup increases in size,
and becomes also the seat of a deposition of dark pigment, whence
it may be termed the pigment layer of the cup; while the other layer — ■
that formed by the invagination of the outer portion of the bulb, and
which may be termed the retinal layer — remains much thicker (Fig.
268) and in its proximal portions even increases in thickness.
Later, however, the development of the ciliary and retinal portions
of the retinal layers differs, and it will be convenient to consider
first the history of the ciliary portion.
 
The Development of the Iris and Ciliary Body. — The first change
noticeable in the ciliary portion of the retinal layer is its thinning out,
a process which continues until the layer consists, like the pigment
layer, of but a single layer of cells (Fig. 271), the transition of which
to the thicker retinal portion of the layer is somewhat abrupt and
corresponds to what is termed the ora serrata in adult anatomy.
In embryos of 10.2 cm. the retinal layer throughout its entire extent
is readily distinguishable from the pigment layer by the absence in
it of all pigmentation, but in older forms this distinction gradually
diminishes in the iris region, the retinal layer there acquiring pigment and forming the uvea.
 
When the anterior chamber of the eye is formed by the splitting
of the mesoderm which has grown in between the superficial ectoderm and the outer surface of the lens, the peripheral portions of its
posterior (inner) wall are in relation with the ciliary portion of the
optic cup and give rise to the stroma of the ciliary body and of the
iris (Fig. 271), this latter being continuous with the tunica vasculosa
lentis so long as that structure persists (Fig. 278). In embryos
of about 14.5 cm. the ciliary portion of the cup becomes thrown into
radiating folds (Fig. 271), as if by a too rapid growth, and into the
 
 
 
THE IRIS AND CILIARY BODY 455
 
folds lamellae of mesoderm project from the stroma. These folds
occur not only throughout the region of the ciliary body, but also
extend into the iris region, where, however, they are but temporary
structures, disappearing entirely by the end of the fifth month. The
folds in the region of the corpus ciliare persist and produce the
ciliary processes of the adult eye.
 
Embedded in the substance of the iris stroma in the adult are
non-striped muscle-fibers, which constitute the sphincter and dila
I-Str
 
 
 
 
AE
 
 
 
CC
Fig. 271. — Radial Section through the Iris of an Embryo of 19 cm.
AE, Pigment layer; CC, ciliary folds; IE, retinal layer; I.Str, iris stroma; Pm, pupillary
membrane; Rs, marginal sinus; Sph, sphincter iridis. — (Szili.)
 
tator iridis. It has long been supposed that these fibers were differentiated from the stroma of the iris, but recent observations have
shown that they arise from the cells of the pigment layer of the optic
cup, the sphincter appearing near the pupillary border (Fig. 271,
Sph) while the dilatator is more peripheral.
 
The Development of the Retina. — Throughout the retinal region
of the cup the pigment layer, undergoing the same changes as in
 
 
 
456
 
 
 
THE RETINA
 
 
 
the ciliary region, forms the pigment layer of the retina (Fig. 272, p).
The retinal layer increases in thickness and early becomes differentiated into two strata (Fig. 268), a thicker one lying next the pigment
layer and containing numerous nuclei, and a thinner one containing
no nuclei. The thinner layer, from its position and structure,
suggests an homology with the marginal velum of the central nervous
system, and probably becomes converted into the nerve-fiber layer
 
 
 
 
'0%& B oS'
 
 
 
00 ° o o
 
 
 
o
 
 
 
o
 
 
 
Fig. 272. — Portion of a Transverse Section of the Retina of a New-born
 
Rabbit.
ch, Chorioid coat; g, ganglion-cell layer; r, outer layer of nuclei; p, pigment layer. —
 
(Falchi.)
 
of the adult retina, the axis-cylinder processes of the ganglion cells
passing into it on their way to the optic nerve. The thicker layer
similarly suggests a comparison with the mantle layer of the cord
and brain, and in embryos of 38 mm. it becomes differentiated into
two secondary layers (Fig. 272), that nearest the pigment layer
if) consisting of smaller and more deeply staining nuclei, probably
representing the rod and cone and bipolar cells of the adult retina,
 
 
 
THE RETINA
 
 
 
457
 
 
 
while the inner layer, that nearest the marginal velum, has larger
nuclei and is presumably composed of the ganglion cells.
 
Little is as yet known concerning the further differentiation of
the nervous elements of the human retina, but the history of some
of them has been traced in the cat, in which, as in other mammals,
the histogenetic processes take place at a relatively later period than
in man. Of the histogenesis of the inner layer the information is
 
 
 
 
Fig. 273. — Diagram showing the Development of the Retinal Elements.
 
a, Cone cell in the unipolar, and b, in the bipolar stage; c, rod cells in the unipolar,
and d, in the bipolar stage; e, bipolar cells; /and i, amacrine cells; g, horizontal cells;
h, ganglion cells; k, Muller's fiber; I, external limiting membrane. — (Kallius, after
Cajal.)
 
rather scant, but it may be stated that the ganglion cells are the
earliest of all the elements of the retina to become recognizable.
The rod and cone cells, when first distinguishable, are unipolar cells
(Fig. 273, a and c), their single processes extending outward from the
cell-bodies to the external limiting membrane which bounds the
outer surface of the retinal layer. Even at an early stage the cone
cells (a) are distinguishable from the rod cells (c) by their more
 
 
 
458 THE OPTIC NERVE
 
decided reaction to silver salts, and at first both kinds of cells are
scattered throughout the thickness of the layer from which they arise.
Later, a fine process grows out from the inner end of each cell, which
thus assumes a bipolar form (Fig. 2 73 , b and d) , and, later still, the cells
gradually migrate toward the external limiting membrane, beneath
which they form a definite layer in the adult. In the meantime
there appears opposite the outer end of each cell a rounded eminence
projecting from the outer surface of the external limiting membrane
into the pigment layer. The eminences over the cone cells are larger
than those over the rod cells, and later, as both increase in length,
they become recognizable by their shape as the rods and cones.
 
The bipolar cells are not easily distinguishable in the early stages
of their differentiation from the other cells with which thy are mingled, but it is believed that they are represented by cells which are
bipolar when the rod and cone cells are still in a unipolar condition
(Fig. 273, e). If this identification be correct, then it is noteworthy
that at first their outer processes extend as far as the external limiting
membrane and must later shorten or fail to elongate until their
outer ends lie in what is termed the outer granular layer of the retina,
where they stand in relation to the inner ends of the rod and cone
cell processes. Of the development of the amacrine (/", i) and
horizontal cells (g) of the retina little is known. From their position
in new-born kittens it seems probable that the former are derived
from cells of the same layer as the ganglion cells, while the horizontal cells may belong to the outer layer.
 
In addition to the various nerve-elements mentioned above, the
retina also contains neuroglial elements known as Muller's fibers
(Fig. 273, k), which traverse the entire thickness of the retina. The
development of these cells has not yet been thoroughly traced, but
they resemble closely the ependymal cells observable in early stages
of the spinal cord.
 
The Development of the Optic Nerve. — The observations on the
development of the retina have shown very clearly that the great
majority of the fibers of the optic nerve are axis-cylinders of the ganglion cells of the retina and grow from these cells along the optic
 
 
 
THE OPTIC NERVE
 
 
 
459
 
 
 
 
stalk toward the brain. Their embryonic history has been traced
most thoroughly in rat embryos (Robinson), and what follows is
based upon what has been observed in that animal.
 
The optic stalk, being an outgrowth from the brain, is at first
a hollow structure, its cavity communicating with that of the third
ventricle at one end and with that of
the optic bulb at the other. When the
chorioid fissure is developed, it extends,
as has already been described, for some
distance along the posterior surface of
the stalk and has lying in it a portion of
the hyaloid artery. Later, when the lips
of the fissure fuse, the artery becomes
enclosed within the stalk to form the arteria centralis retina of the adult (Fig.
276). By the formation of the fissure
the original cavity of the distal portion
of the stalk becomes obliterated, and at
the same time the ventral and posterior
walls of the stalk are brought into continuity with the retinal layer of the optic cup, and so opportunity is given for the passage of the
axis-cylinders of the ganglion cells along those walls (Fig. 274).
At an early stage a section of the proximal portion of the optic
stalk (Fig. 275, A) shows the central cavity surrounded by a number of nuclei representing the mantle layer, and surrounding
these a non-nucleated layer, resembling the marginal velum and
continuous distally with the similar layer of the retina. When the
ganglion cells of the latter begin to send out their axis-cylinder
processes, these pass into the retinal marginal velum and converge
in this layer toward the bottom of the chorioidal fissure, so reaching
the ventral wall of the optic stalk, in the velum of which they may
be distinguished in rat embryos of 4 mm., and still more clearly in
those of 9 mm. (Fig. 275, A). Later, as the fibers become more
numerous, they gradually invade the lateral and finally the dorsal
 
 
 
Fig. 2 74. — Diagrammatic
Longitudinal Section of the
Optic Cup and Stalk passing
through the chorioid fisSURE.
 
Ah, Hyaloid artery; L, lens;
On, fibers of the optic nerve; Os,
optic stalk; PI, pigment layer,
and R, retinal layer of the retina.
 
 
 
460 THE OPTIC NERVE
 
walls of the stalk, and, at the same time the mantle cells of the stalk
become more scattered and assume the form of connective-tissue
(neuroglia) cells, while the original cavity of the stalk is gradually
obliterated (Fig. 275, B). Finally, the stalk becomes a solid mass
of nerve-fibers, among which the altered mantle cells are scattered.
 
From what has been stated above it will be seen that the sensory
cells of the eye belong to a somewhat different category from those of the
other sense-organs. Embryologically they are a specialized portion of the
mantle layer of the medullary canal, whereas in the other organs they are
peripheral structures either representing or being associated with representatives of posterior root ganglion cells. Viewed from this standpoint,
and taking into consideration the fact that the sensory portion of the
retina is formed from the invaginated part of the optic bulb, some light
 
 
 
 
": . â– â– â– 'V ,".-â– â– '
 
 
 
Fig. 275. — Transverse Sections through the Proximal Part of the Optic Stalk
of Rat Embryos of (A) 9 mm. and (5) 11 mm. — (Robinson.)
 
is thrown upon the inverted arrangement of the retinal elements, the rods
and cones being directed away from the source of light. The normal
relations of the mantle layer and marginal velum are retained in the retina,
and the latter serving as a conducting layer for the axis-cylinders of the
mantle layer (ganglion) cells, the layer of nerve-fibers becomes interposed
between the source of light and the sensory cells. Furthermore, it
may be pointed out that if the differentiation of the retina be imagined to take place before the closure of the medullary canal — a
condition which is indicated in some of the lower vertebrates — there
would be then no inversion of the elements, this peculiarity being due to
the conversion of the medullary plate into a tube, and more especially to
the fact that the retina develops from the outer wall of the optic cup. In
 
 
 
THE VITREOUS HUMOR
 
 
 
461
 
 
 
certain reptiles in which an eye is developed in connection with the epiphysial outgrowths of the diencephalon, the retinal portion of this pineal eye
is formed from the inner layer of the bulb, and in this case there is no
inversion of the elements.
 
A justification of the exclusion of the optic nerve from the category
which includes the other cranial nerves has now been presented. For if
the retina be regarded as a portion of the central nervous system, it is clear
that the nerve is not a nerve at all in the strict sense of that word, but is a
tract, confined throughout its entire extent within the central nervous
system and comparable to such groups of fibers as the direct cerebellar
or fillet tracts of that system.
 
The Development of the Vitreous Humor. — It has already been
pointed out (p. 448) that a blood-vessel, the hyaloid artery, accompanied by some mesodermal tissue makes its way into the cavity
 
 
 
 
Fig. 276. — Reconstruction of a Portion of the Eye of an Embryo of 13.8 mm.
ah, Hyaloid artery; ch, chorioid coat; /, lens; r, retina. — (His.)
 
of the optic cup through the chorioid fissure. On the closure of the
fissure the artery becomes enclosed within the optic stalk and appears
to penetrate the retina, upon the surface of which its branches
ramify. In the embryo the artery does not, however, terminate
in these branches as it does in the adult, but is continued on through
the cavity of the optic cup (Fig. 276) to reach the lens, around which
it sends branches to form the tunica vasculosa lentis.
 
According to some authors, the formation of the vitreous humor
is closely associated with the development of this artery, the humor
being merely a transudate from it, while others have maintained
that it is a derivative of the mesoderm which accompanies the vessel,
and is therefore to be regarded as a peculiar gelatinous form of
 
 
 
462
 
 
 
THE VITREOUS HUMOR
 
 
 
connective tissue. More recently, however, renewed observations
by several authors have resulted in the deposition of the mesoderm
from the chief role in the formation of the vitreous and the substitution in it of the retina. At an early stage of development delicate
protoplasmic processes may be seen projecting from the surface of
the retinal layer into the cavity of the optic cup, these processes
probably arising from those cells which will later form the Miiller's
 
 
 
 
Fig. 277. — Transverse Section through the Ciliary Region of a Chick Embryo
 
of Sixteen Days.
ac, Anterior chamber of the eye; cj, conjunctiva; co, cornea; i, iris; I, lens; mc, ciliary
muscle; rl, retinal layer of optic cup; sf, spaces of Fontana; si, suspensory ligament of the
lens; v, vitreous humor. — (Angelucci.)
 
(neuroglia) fibers of the retina. As development proceeds they increase in length, forming a dense and very fine fibrillar reticulum
traversing the space between the lens and the retina and constituting
the primary vitreous humor. The formation of the fibers is especially active in the ciliary portion of the retina and it is probable that
it is from some of the fibers developing in this region that the suspensory ligament of the lens (zonula Zinnii) (Fig. 277, si) is formed
 
 
 
THE CORNEA 463
 
spaces which occur between the fibers of the ligament enlarging to
produce a cavity traversed by scattered fibers and known as the
canal of Petit.
 
A participation of similar protoplasmic prolongations from the
cells of the lens in the formation of the vitreous humor has been
maintained (von Lenhossek) and as strenuously denied. But it is
generally admitted that at the time when the hyaloid artery penetrates the vitreous to form the tunica vasculosa lentis it carries with
it certain mesodermal elements, whose fate is at present uncertain.
It has been held that they take part in the formation of the definitive
vitreous, which, according to this view, is of mixed origin, being
partly ectodermal and partly mesodermal (Van P6e), and, on the
contrary, it has been maintained that they eventually undergo
complete degeneration, the vitreous being of purely ectodermal
origin (von Kolliker).
 
The degeneration of the mesodermal elements which the latter
view supposes is associated with the degeneration of the hyaloid
artery. This begins in human embryos in the third month and is
completed during the ninth month, the only trace after birth of the
existence of the vessel being a more fluid consistency of the axis of
the vitreous humor, this more fluid portion representing the space
originally occupied by the artery and forming what is termed the
hyaloid canal {canal ofCloquet).
 
The Development of the Outer Coat of the Eye, of the Cornea, and
of the Anterior Chamber. — Soon after the formation of the optic bulb
a condensation of the mesoderm cells around it occurs, forming a
capsule. Over the medial portions of the optic cup the further
differentiation of this capsule is comparatively simple, resulting in
the formation of two layers, an inner vascular and an outer denser
and fibrous, the former becoming the chorioid coat of the adult eye
and the latter the sclera.
 
More laterally, however, the processes are more complicated.
After the lens has separated from the surface ectoderm a thin layer
of mesoderm grows in between the two structures and later gives
place to a layer of homogeneous substance in which a few cells,
 
 
 
464
 
 
 
THE ANTERIOR CHAMBER OF THE EYE
 
 
 
more numerous laterally than at the center, are embedded. Still
later cells from the adjacent mesenchyme grow into the layer, which
increases considerably in thickness, and blood-vessels also grow into
that portion of it which is in contact with the outer surface of the
lens. At this stage the interval between the surface ectoderm and
the lens is occupied by a solid mass of mesodermal tissue (Fig. 278,
co and tv), but as development proceeds, small spaces (ac) filled
 
etc
 
 
 
ec^
 
 
 
mc
 
 
 
 
Fig. 278. — Transverse Section through the Ciliary Region of a Pig Embryo or
 
23 MM.
ac, Anterior chamber of the eye; co, cornea; ec, ectoderm; I, lens; mc, ciliary muscle;
p, pigment layer of the optic cup; r, retinal layer; tv, tunica vasculosa lentis. — (Angelucci.)
 
with fluid begin to appear toward the inner portion of the mass, and
these, increasing in number and size, eventually fuse together to
form a single cavity which divides the mass into an inner and an
outer portion. The cavity is the anterior chamber of the eye, and it
has served to separate the cornea (co) from the tunica vasculosa
lentis (tv) , and, extending laterally in all directions, it also separates
from the cornea the mesenchyme which rests upon the marginal
portion of the optic cup and constitutes the stroma of the iris. Cells
arrange themselves on the corneal surface of the cavity to form a
 
 
 
THE EYELIDS 465
 
continuous endothelial layer, and the mesenchyme which forms the
peripheral boundary of the cavity assumes a fibrous character and
forms the ligamentum pectinatum iridis, among the fibers of which
cavities, known as the spaces of Fontana (Fig. 277, sf), appear.
Beyond the margins of the cavity the corneal tissue is directly continuous with the sclerotic, beneath the margin of which is a distinctly
thickened portion of mesenchyme resting upon the ciliary processes
and forming the stroma of the ciliary body, as well as giving rise to
the muscle tissue which constitutes the ciliary muscle (Figs. 277 and
278, mc).
 
The ectoderm which covers the outer surface of the eye does not
proceed beyond the stage when it consists of several layers of cells,
and never develops a stratum corneum. In the corneal region it
rests directly upon the corneal tissue, which is thickened slightly
upon its outer surface to form the anterior elastic lamina; more peripherally, however, a quantity of loose mesodermal tissue lies
between the ectoderm and the outer surface of the sclerotic, and,
together with the ectoderm, forms the conjunctiva (Fig. 277, cj).
 
The Development of the Accessory Apparatus of the Eye. — The
eyelids make their appearance at an early stage as two folds of skin,
one a short distance above and the other below the cornea. The
center of the folds is at first occupied by indifferent mesodermal
tissue, which later becomes modified to form the connective tissue
of the lids and the tarsal cartilage, the muscle tissue probably
secondarily growing into the lids as a result of the spreading of the
platysma over the face, the orbicularis oculi apparently being a
derivative of that sheet of muscle tissue.
 
At about the beginning of the third month the lids have become
sufficiently large to meet one another, whereupon the thickened
epithelium which has formed upon their edges unites and the lids
fuse together, in which condition they remain until shortly before
birth. During the stage of fusion the eyelashes (Fig. 279, h) develop
at the edges of the lids, having the same developmental history as
ordinary hairs, and from the fused epithelium of each lid there grow
upward or downward, as the case may be, into the mesodermic
3°
 
 
 
4 66
 
 
 
THE EYELIDS
 
 
 
tissue, solid rods of ectoderm, certain of which early give off numerous short lateral processes and become recognizable as the tarsal
{Meibomian) glands (m), while others retain the simple cylindrical
form and represent the glands of Moll. When the eyelids separate,
these solid ingrowths become hollow by a breaking down of their
 
 
 
 
Fig. 279. — Section through the Margins of the Fused Eyelids in an Embryo^
 
of Six Months. i "1
 
 
 
h, Eyelash; //, lower lid; m, tarsal gland; mu, muscle bundle; ul, upper lid.
Seidl.)
 
 
 
-{Schweigger
 
 
 
central cells, just as in the sebaceous and sudoriparous glands of
the skin, the tarsal glands being really modifications of the former
glands, while the glands of Moll are probably to be regarded as
specialized sudoriparous glands.
 
A third fold of skin, in addition to^the two which produce the
eyelids, is also developed in connection with the eye, forming the
plica semilunaris. This is a rudimentary third eyelid, representing
the nictitating membrane which is fairly well developed in many
of the lower mammals and especially well in birds.
 
 
 
THE LACHRYMAL GLAND 467
 
The lachrymal gland is developed at about the third month as a
number of branching outgrowths of the ectoderm into the adjacent
mesoderm along the outer part of the line where the epithelium of
the conjunctiva becomes continuous with that covering the inner
surface of the upper eyelid. As in the other epidermal glands, the
outgrowths and their branches are at first solid, later becoming hollow by the degeneration of their axial cells.
 
The naso-lachrymal duct is developed in connection with the
groove which, at an early stage in the development (Fig. 62), extends
 
 
 
 
Fig. 280. — Diagram showing the Insertions of the Lachrymal Ducts in
EMBRYOS OF 40 MM. AND 170 MM.. THE CaRUNCULA LaCRIMALIS BEING FORMED IN
 
the Latter.
 
The eyelids are really fused at these stages but have been represented as separate
• for the sake of clearness. — (Ask.)
 
from the inner corner of the eye to the olfactory pit and is bounded
posteriorly by the maxillary process of the first visceral arch. The
epithelium lying in the floor of this groove thickens toward the begining of the sixth week to form a solid cord, which sinks into the subjacent mesoderm. From its upper end two outgrowths arise which
become connected with the ectoderm of the edges of the upper and
lower lids, respectively, and represent the lachrymal ducts, and,
finally, the solid cord and its outgrowths acquire a lumen and a
connection with the mucous membrane of the inferior meatus of the
nasal cavity.
 
The inferior duct connects with the border of the eyelid some
distance lateral to the inner angle of the eye, and between its opening and the angle a number of tarsal glands develop. The superior
duct, on the other hand, opens at first close to the inner angle and
 
 
 
468 LITERATURE
 
later moves laterally until its opening is opposite that of the inferior
duct. During this change the portion of the lower lid between the
opening of the inferior duct and the angle is drawn somewhat upward, and, with its glands, forms a small reddish nodule, resting
upon the plica semilunaris and known as the caruncula lacrimalis
(Fig. 280).
 
LITERATURE.
 
G. Alexander: "Ueber Entwicklung und Bau des Pars inferior Labyrinthi der
hdheren Saugethiere," Denkschr. kais. wissench. Acad. Wien, Math.-Naturw.
Classe, lxx, 1901.
 
A. Angeltjcci: "Ueber Entwickelung und Bau des vorderen Uvealtractus der Verte
braten," Archiv fur mikrosk. Anat., xix, 1881.
F. Ask: " Ueber die Entwickelung der Caruncula lacrimalis beim Menschen, nebst
Bemerkungen iiber die Entwickelung der Tranenrohrchen und der Meibom'schen
Driisen," Anatom. Anzeiger, xxx, 1907.
 
F. Ask: "Ueber die Entwicklung der Lidrander, der Tranenkarunkel und der Nick
haut beim Menschen, nebst Bemerkungen zur Entwicklung der Tranenabfuhrungswege," Anat. Hefte, xxxvi, 1908.
 
B. Baginsky: "Zur Entwickelung der Gehorschnecke," Archiv fur mikrosk. Anat.,
 
xxviii, 1886.
I. Broman: "Die Entwickelungsgeschichte der Gehorknochelchen beim Menschen,"
 
Anat. Hefte, xi, 189S.
S. Ramon y Cajal: "Nouvelles contributions a l'etude histologique de la retine,"
 
Journ. de I' Anat. et de la Physiol., xxxii, 1896.
 
G. Cirincione: "Ueber den gegenwartigen Stand der Frage hinsichtlich der Genese
 
des Glaskorpers," Arch, fur Augenheilk., L, 1904.
A. Contino: "Ueber Bau and Entwicklung des Lidrandes beim Menschen," Arch,
fur Ophthalmol., lxvi, 1908.
 
A. Contino: "Ueber die Entwicklung der Karunkel und der plica semilunaris beim
 
Menschen," Arch, fur Ophthalmol, lxxi, 1909.
J. Disse: "Die erste Entwickelung der Riechnerven," Anat. Hefte, ix, 1897.
 
B. Fleischer: "Die Entwickelung der Tranenrohrchen bei den Saugetiere," Archiv
 
fur Ophthalmol., lxii, 1906.
H. Fuchs : " Bemerkungen iiber die Herkunft und Entwickelung der Gehorknochelchen
 
bei Kaninchen-Embryonen (nebst Bemerkungen iiber die Entwickelung des
 
Knorpelskeletes der beiden ersten Visceralbogen)," Archiv. fur Anat und Phys.,
 
Anat. Abth., Supplement, 1905.
J. Graberg: "Beitrage zur Genese des Geschmacksorgans der Menschen," Morphol.
 
Arbeiten, vn, 1898.
J. A. Hammar: "Zur allgemeinen Morphologie der Schlundspalten des Menschen.
 
Zur Entwickelungsgeschichte des Mittelohrraumes, des ausseren Gehorganges
 
und des Paukenfelles beim Menschen," Anat. Anzeiger, xx, 1901.
 
 
 
LITERATURE
 
 
 
469
 
 
 
J. A. Hammar: " Studien iiber die Entwicklung des Vorderdarms und einiger angrenz
ender Organe," Arch, fur mikrosk. Anat., Lix, 1902.
C. Heerfordt: "Studien iiber den Muse, dilatator pupilke sammt Angabe von
 
gemeinschaftlicher Kennzeichen einiger Falle epithelialer Musculatur," Anat.
 
Hefte, xiv.
J. Hegetschweiler: "Die embryologische Entwickelung des Steigbugels," Archiv
 
fur Anat. und Physiol., Anat. Abth., 1898.
 
F. Hochstetter: "Ueber die Bildung der primitiven Choanen beim Menschen,"
 
Verhandl. Anat. Gesellsch., VI, 1892.
W. His, Jr.: "Die Entwickelungsgeschichte des Acustico-Facialisgebietes beim
 
Menschen," Archiv fur Anat. und Physiol., Anat. Abth., Supplement, 1897.
A. von Kolliker: "Die Entwicklung und Bedeutung des Glaskorpers," Zeitschr.
 
fur wissensch. Zoolog., lxxvi, 1904.
P. Lang: "Zur Entwicklung des Tranenausfiihrsapparates beim Menschen," Anat.
 
Anzeiger," xxxvni, 1911.
 
G. Leboucq: " Contribution a. l'etude de l'histogenese de la retine chez les mammiferes,"
 
Arch. Anat., Microsc, x, 1909.
V. von Mihalkovicz: "Nasenhohle und Jacobsonsches Organ. Eine morphologische
 
Studie." Anat. Hefte. xi, 1898.
J. L. Paitlet: "Contribution a l'etude de l'organe de Jacobson chez l'embryon
 
humain," Bibliogr. Anat., xvii, 1907.
P. van Pee: "Recherches sur l'origine du corps vitre," Archives de Biol., xix, 1902.
C. Rabl: "Ueber den Bau und Entwickelung der Linse," Zeitschrift fur wissensch.
 
Zoologie, lxiii and lxv, 1898; lxviii, 1899.
A. Robinson: "On the Formation and Structure of the Optic Nerve and Its Relation
 
to the Optic Stalk," Journal of Anat. and Physiol., xxx, 1896.
G. Speciale-Cirincione: "Ueber die Entwicklung der Tranendriise beim Menschen "
 
Arch.filr Ophthalmol., LXIX, 1908.
J. P. Schaefeer: "The Genesis and Development of the Nasolacrimal Passages in
 
Man," Amer. Journ. Anat., xm, 1912.
G. L. Streeter: "On the Development of the Membranous Labyrinth and the
 
Acoustic and Facial Nerves in the Human Embryo," Amer. Journ. of Anat.
 
vi, 1907.
N. van der Stricht: "L'histogenese des parties constituantes du neuroepithelium
 
acoustique, des taches et des cretes acoustiques et de l'organe de Corti " Arch.
 
de Biol., xxiii, 1908.
A. Szili: "Zur Anatomie und Entwickelungsgeschichte der hinteren Irisschichten
 
mit besonderer Beriicksichtigung des Musculus sphincter iridis des Menschen "
 
Anat. Anzeiger, xx, 1901.
A. Szili: "Ueber das Entstehen eines fibrillares Stutzgewebes im Embryo und dessen
 
Verhaltnis zur Glaskorperfrage," Anat. Hejte, xxxv, 190S.
F. Tuckerman: "On the Development of the Taste Organs in Man," Journal of Anat.
 
and Physiol., xxiv, 1889.
R. Versari: "Ueber die Entwicklung der Blutgefasse des menschlichen Au^es "
 
Anat. Anzeiger, xxxv, 1909.
 
 
 
CHAPTER XVII.
POST-NATAL DEVELOPMENT.
 
In the preceding pages attention has been directed principally
to the changes which take place in the various organs during the
period before birth, for, with a few exceptions, notably that of the
liver, the general form and histological peculiarities of the various
organs are acquired before that epoch. Development does not,
however, cease with birth, and a few statements regarding the
changes which take place in the interval between birth and maturity
will not be out of place in a work of this kind.
 
The conditions which obtain during embryonic life are so different from those to which the body must later adapt itself, that
arrangements, such as those connected with the placental circulation, which are of fundamental importance during the life in utero,
become of little or no use, while the relative importance of others is
greatly diminished, and these changes react more or less profoundly
on all parts of the body. Hence, although the post-natal development consists chiefly in the growth of the structures formed during
earlier stages, yet the growth is not equally rapid in all parts, and
indeed in some organs there may even be a relative decrease in size.
That this is true can be seen from the annexed figure (Fig. 281),
which represents the body of a child and that of an adult man drawn
as of the same height. The greater relative size of the head and
upper part of the body in the child is very marked, and the central
point of the height of the child is situated at about the level of the
umbilicus, while in the man it is at the symphysis pubis.
 
That there is a distinct change in the geometric form of the body
during growth is also well shown by the following consideration.
(Thoma). Taking the average height of a new-born male as 500
mm., and that of a man of thirty years of age as 1686 mm., the
 
470
 
 
 
POST-NATAL DEVELOPMENT _":
 
height of the body will have increased from birth to adolescence
 
- v _â– 
 
= 3.37 times. The child will weigh 3.1 kilos and the man
 
5::
 
66.1 kilos, and if the specific gravity of the body with the included
 
gases be taken in the one case as 0.90 and in the other as 0.93, then
 
the volume of the child's body will be 3.44 liters and that of the
 
man's 71.08 liters, and the increase in volume will be — — =20.66.
 
 
 
 
Fig. 281. — Child ast> }vL\x Drawn as of th*
" Growth of the Brain, " Contemporary Science Series
Sons.)
 
 
 
- :: ..-.-;.: .;-' S'-.:-'.;: 5r:': r ' :
 
 
 
If the increase in volume had taken place without any alteration in
the geometric form of the body, it should be equal to the cube of the
increase in height; this, however, is 3-37 s =38.27, a number wellnigh twice as large as the actual increase.
 
But in addition to these changes, which are largely dependent
 
 
 
472
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
upon differences in the supply of nutrition, there are others associated with alterations in the general metabolism of the body. Up
to adult life the constructive metabolism or anabolism is in excess
of the destructive metabolism or katabolism, but the amount of the
excess is much greater during the earlier periods of development
and gradually diminishes as the adult condition is approached.
That this is true during intrauterine life is shown by the following
figures, compiled by Donaldson:
 
 
 
Age in Weeks
 
 
Weight in Grams
 
 
Age in Weeks
 
 
Weight in Grams
 
 
o (ovum)
 
 
o . 0006
 
 
24
 
 
635
 
 
4
 
 
—
 
 
28
 
 
1,220
 
 
8
 
 
4
 
 
32
 
 
1,700
 
 
12
 
 
20
 
 
36
 
 
2,240
 
 
16
 
 
120
 
 
40 (birth)
 
 
3,250
 
 
20
 
 
28 5
 
 
 
 
 
 
 
From this table it may be seen that the embryo of eight weeks
is six thousand six hundred and sixty-seven times as heavy as the
ovum from which it started, and if the increase of growth for each
of the succeeding periods of four weeks be represented as percentages, it will be seen that the rate of increase undergoes a rapid
diminution after the sixteenth week, and from that on diminishes
gradually but less rapidly, the figures being as follows :
 
 
 
Periods of Weeks
 
 
Percentage Increase
 
 
Periods of Weeks
 
 
Percentage Increase
 
 
8-12
12-16
16-20
20-24
 
 
400
 
500
 
137
123
 
 
24-28
28-32
32-36
36-40
 
 
92
 
39
32
 
45
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
473
 
 
 
That the same is true in a general way of the growth after birth
may be seen from the following table, representing the average
weight of the body in English males at different years from birth
up to twenty-three (Roberts), and also the percentage rate of
increase.
 
 
 
Year
 
 
Number of Cases
 
 
Weight in
Kilograms
 
 
Percentage
Increase
 
 
o
 
 
45i
 
 
3-2
 
 
 
 
i
 
 
—
 
 
(10.8)
 
 
(238)
 
 
2
 
 
2
 
 
14.7*
 
 
(36)*
 
 
3
 
 
41
 
 
15-4
 
 
4.8*
 
 
4
 
 
102
 
 
16.9
 
 
9-7
 
 
5
 
 
*93
 
 
18. 1
 
 
7-i
 
 
6
 
 
224
 
 
20.1
 
 
11
 
 
7
 
 
246
 
 
22 .6
 
 
12.4
 
 
8
 
 
820
 
 
24.9
 
 
10.2
 
 
9
 
 
1,425
 
 
27.4
 
 
10
 
 
IO
 
 
1,464
 
 
30.6
 
 
"•5
 
 
ii
 
 
i,599
 
 
32.6
 
 
6-5
 
 
12
 
 
1,786
 
 
34-9
 
 
7
 
 
x 3
 
 
2,443
 
 
37-6
 
 
7-7
 
 
14
 
 
2,952
 
 
41.7
 
 
10.9
 
 
15
 
 
3,"8
 
 
46.6
 
 
11. 7
 
 
16
 
 
2,235
 
 
53-9
 
 
15-7
 
 
17
 
 
2,496
 
 
59-3
 
 
10
 
 
18
 
 
2,15°
 
 
62 .2
 
 
4.9
 
 
19
 
 
i,438
 
 
63-4
 
 
1.9
 
 
20
 
 
851
 
 
64.9
 
 
2-5
 
 
21
 
 
738
 
 
65-7
 
 
1 .2
 
 
22
 
 
542
 
 
67 .0
 
 
1.9
 
 
23
 
 
55i
 
 
67 .0
 
 
 
 
 
 
Certain interesting peculiarities in post-natal growth become
apparent from an examination of this table. For while there is a
 
* From a comparison with other similar tables there is little doubt but that the
weight given above for the second year is too high to be accepted as a good average
 
 
 
474
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
general diminution in the rate of growth, yet there are marked
irregularities, the most noticeable being (i) a rather marked diminution during the eleventh and twelfth years, followed by (2) a rapid
 
 
 
Age
 
 
 
LbsM
 
 
 
14
 
 
 
1 Z 3 * 5 6 f a 9 ID ft 12 13 14 1$ 16 17 19
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
/' V
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
/
 
 
 
 
\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
\\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
4
 
 
 
 
 
 
\\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
,;
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
i
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
**
 
 
 
 
 
 
 
 
 
 
 
 
,
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
;/
 
 
 
 
 
 
 
 
 
 
 
 
f
 
 
A
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
/
 
 
i
 
 
 
 
 
 
 
 
 
 
 
 
\
 
 
 
 
\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
/
 
 
's*
 
 
 
 
 
 
 
 
 
 
â– -'
 
 
 
 
/
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1
 
 
: v
 
 
\
 
 
^
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1 ,
 
 
 
 
y
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
s \
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
S
 
 
 
 
,.
 
 
--"
 
 
 
 
 
 
 
 
 
 
 
 
 
 
s=
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
,
 
 
 
 
 
 
 
~~
 
 
~"
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
II
 
 
i
 
 
2 3 4-5 6 7 8 9 10 11 12 13 J& I
 
 
5 /
 
 
5 17 18
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
*
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
i
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
,'i
 
 
 
 
 
 
 
 
 
 
 
 
**4
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1
 
 
*
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
'
 
'
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
\
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
-
 
-'
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1
 
 
k
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
* s
 
 
 
 
 
 
 
 
 
 
^i
 
 
y
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
V
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
-
 
._
 
 
.
 
-'
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
V
 
 
S.
 
 
y
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
p
 
 
 
 
 
 
â– v
 
 
s.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Age
 
Lbs/4
 
" 12
 
" 10
 
•■ 8
 
" 6
 
" *
' Z
 
 
 
Fig. 282. — Curves Showing the Annual Increase in Weight in (I) Boys and (II)
 
Girls.
The faint line represents the curve from British statistics, the dotted line that from
American (Bowditch), and the heavy line the average of the two. Before the sixth
year the data are unreliable. — {Stephenson.)
 
acceleration which reaches its maximum at about the sixteenth year
and then very rapidly diminishes. These irregularities may be more
 
Consequently the percentage increase for the second year is too high and that for the
third year too low.
 
It may be mentioned that the weights in the original table are expressed in pounds
avoirdupois and have been here converted into kilograms, and further the figures representing the percentage increase have been added.
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
475
 
 
 
clearly seen from the charts on page 474, which represent the curves
obtained by plotting the annual increase of weight in boys (Chart I)
and girls (Chart II). The diminution and acceleration of growth
referred to above are clearly observable and it is interesting to note
that they occur at earlier periods in girls than in boys, the diminution
occurring in girls at the eighth and ninth years and the acceleration
reaching its maximum at the thirteenth year.
 
Considering, now, merely the general diminution in the rate
of growth which occurs from birth to adult life, it becomes interesting to note to what extent the organs which are more immediately
associated with the metabolic activities of the body undergo a relative reduction in weight. The most important of these organs is
undoubtedly the liver, but with it there must also be considered the
thyreoid and thymus glands, and probably the suprarenal bodies.
In all these organs there is a marked diminution in size as compared
with the weight of the body, as will be seen from the following table
(H. Vierordt), which also includes data regarding other organs in
 
 
 
ABSOLUTE WEIGHT IN GRAMS.
 
New-born and Adult.
 
 
 
Liver
 
 
Thyreoid
 
 
Thymus
 
 
Suprarenal
Bodies
 
 
Spleen
 
 
Heart
 
 
Kidney
 
 
„ . Spmal
Brain _; .
Cord
 
 
I4I-7
1,819.0
 
 
4-85
33-8
 
 
8.15
26.9
 
 
7-05
7-4
 
 
10.6
 
163.0
 
 
23.6
300.6
 
 
23-3
3°5-9
 
 
381.0
1,430.9
 
 
5-5
39-iS
 
 
PERCENTAGE WEIGHT OF ENTIRE BODY
 
New-born and Adult.
 
 
Liver
 
 
Thyreoid
 
 
Thymus
 
 
Suprarenal
Bodies
 
 
Spleen
 
 
Heart
 
 
Kidney
 
 
Brain
 
 
Spinal
Cord
 
 
4-57
2 -57
 
 
0.16
 
0.05
 
 
0.26
0.04
 
 
0.23
 
O.OI
 
 
o-34
 
0.25
 
 
0.76
0.46
 
 
0-7S
0.46
 
 
12 .29
2 .16
 
 
0.18
0.06
 
 
 
476
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
which a marked relative diminution, not in all cases readily explainable, occurs.
 
Recent observations by Hammar render necessary some modification of the figures given for the thymus in the above table. He finds the
average weight of the gland at birth to be 13.26 grms., and that the weight
increases up to puberty, averaging 37.52 grms. between the ages of 11 and
15. After that period it gradually diminishes, falling to 16.27 g rm sbetween 36 and 45, and to 6.0 grms. between 66 and 75. Expressed in
percentage of the body weight this gives a value in the new-born of 0.42
and in an individual of 50 years of 0.02, a difference much more striking
than that shown in Vierordt's table.
 
It must be mentioned, however, that the gland is subject to much
individual variation, being largely influenced by nutritive conditions.
 
The remaining organs, not included in the tables given above,
when compared with the weight of the body, either show an increase
or remain practically the same.
 
ABSOLUTE WEIGHT IN GRAMS.
New-born and Adult.
 
 
 
Skin and Subcutaneous Tissues
 
 
Skeleton
 
 
Stomach and
Musculature , T
 
Intestines
 
 
Pancreas
 
 
Lungs
 
 
611.75
11,765.0
 
 
425-5
ii,575-°
 
 
776.5 65
28,732.0 1,364
 
 
3-5
97.6
 
 
54-i
994-9
 
 
 
PERCENTAGE OF BODY-WEIGHT.
New-born and Adult.
 
 
 
Skin and Subcutaneous Tissues
 
 
Skeleton
 
 
Musculature
 
 
Stomach and
Intestines
 
 
Pancreas Lungs
 
 
19-73
17.77
 
 
13-7
17.48
 
 
2 5-05
43-40
 
 
2 . 1
2 .06
 
 
0. 11
015
 
 
i-75
i-5o
 
 
 
From this table it will be seen that the greatest increment of
weight is that furnished by the muscles, the percentage weight of
which is one and three-fourths times as great in the adult as in the
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
477
 
 
 
child. The difference does not, however, depend upon the differentiation of additional muscles; there are just as many muscles in the
new-born child as in the adult, and the increase is due merely to an
enlargement of organs already present. The percentage weight
of the digestive tract, pancreas, and lungs remains practically the
same, while in the case of the skeleton there is an appreciable increase, and in that of the skin and subcutaneous tissue a slight
 
 
 
.
 
 
 
 
Fig. 283. — Longitudinal Section through the Sacrum of a New-born Female
 
Child.— (Fehling.)
 
diminution. The latter is readily understood when it is remembered
that the area of the skin, granting that the geometric form of the
body remains the same, would increase as the square of the length,
while the mass of the body would increase as the cube, and hence
in comparing weights the skin might be expected to show a diminution even greater than that shown in the table.
 
 
 
478
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
The increase in the weight of the skeleton is due to a certain
extent to growth, but chiefly to a completion of the ossification of
the cartilage largely present at birth. A comparison of the weights
of this system of organs does not, therefore, give evidence of the
many changes of form which may be perceived in it during the period under consideration, and attention may be drawn to some of
the more important of these changes.
 
In the spinal column one of the most noticeable peculiarities
observable in the new-born child is the absence of the curves so
characteristic of the adult. These curves are due partly to the weight
of the body, transmitted through the spinal column to the hipjoint in the erect position, and partly to the action of the muscles,
and it is not until the erect position is habitually assumed and the
musculature gains in development that the curvatures become pronounced. Even the curve of the sacrum, so marked in the adult,
is but slight in the new-born child, as may be seen from Fig. 283,
in which the ventral surfaces of the first and second sacral vertebrae look more ventrally than posteriorly, so that there is no distinct
promontory.
 
But, in addition to the appearance of the curvatures, other
changes also occur after birth, the entire column becoming much
more slender and the proportions of the lumbar and sacral vertebrae
becoming quite different, as may be seen from the following table
(Aeby) :
 
 
 
LENGTHS OF THE VERTEBRAL REGIONS EXPRESSED AS PERCENTAGES OF THE ENTIRE COLUMN.
 
 
 
Age
 
 
Cervical
 
 
Thoracic
 
 
Lumbar
 
 
New-born child
 
 
25.6
 
23-3
20.3
19.7
22 .1
 
 
47-5
46.7
 
45-6
47.2
46.6
 
 
26.8
 
 
Male 2 years
 
 
30.0
 
 
Male 5 years
 
 
34.2
 
 
Male 1 1 years
 
Male adult
 
 
33-i
31.6
 
 
 
 
 
 
 
POST-NATAL DEVELOPMENT 479
 
The cervical region diminishes in length, while the lumbar
gains, the thoracic remaining approximately the same. It may be
noticed, furthermore, that the difference between the two variable
regions is greater during youth than in the adult, a condition possibly associated with the general more rapid development of the
lower portion of the body made necessary by its imperfect development during fetal life. The difference is due to changes in the
vertebrae, the intervertebral disks retaining approximately the same
relative thickness throughout the period under consideration.
 
The form of the thorax also alters, for whereas in the adult it is
barrel-shaped, narrower at both top and bottom than in the middle,
in the new-born child it is rather conical, the base of the cone being
below. The difference depends upon slight differences in the form
and articulations of the ribs, these being more horizontal in the
child and the opening of the thorax directed more directly upward
than in the adult.
 
As regards the skull, the processes of growth are very complicated. Cranium and brain react on one another, and hence, in
harmony with the relatively enormous size of the brain at birth,
the cranial cavity has a relatively greater volume in the child than
in the adult. The fact that the entire roof and a considerable part
of the sides of the skull are formed of membrane bones which, at
birth, are not in sutural contact with one another throughout, gives
opportunity for considerable modifications, and, furthermore, the
base of the skull at the early stage still contains a considerable
amount of unossified cartilage. Without entering into minute details, it may be stated that the principal general changes which the
skull undergoes in its post-natal development are (i) a relative
elongation of its anterior portion and (2) an increase in the relative
height of the maxillae.
 
If a line be drawn between the central points of the occipital
condyles, it will divide the base of the skull into two portions, which
in the child's skull are equal in length. The portion of the skull in
front of a similar line in the adult skull is very much greater than
that which lies behind, the proportion between the two parts being
 
 
 
480 POST-NATAL DEVELOPMENT
 
5:3, against 3:3 in the child (Froriep). There has, therefore, been
a decidedly more rapid growth of the anterior portion of the skull,
a growth which is asssociated with a corresponding increase in the
dorso-ventral dimensions of the maxillae. These bones, indeed,
play a very important part in determining the proportions of the
skull at different periods. They are so intimately associated with
the cranial portions of the skull that their increase necessitates a
 
 
 
 
 
Fig. 284. — Skull of a New-born Child and of an Adult Man, Drawn as of
Approximately the Same Size. — (Henke.)
 
 
 
corresponding increase in the anterior part of the cranium, and
their increase in this direction stands in relation to the development
of the teeth, the eight teeth which are developed in each maxilla
(including the premaxilla) in the adult requiring a longer bone than
do the five teeth of the primary dentition, these again requiring a
greater length when completely developed than they do in their
immature condition in the new-born child.
 
But far more striking than the difference just described is that
in the relative height of the cranial and facial regions (Fig. 284).
It has been estimated that the volumes of the two portions have a
ratio of 8: 1 in the new-born child, 4: 1 at five years of age, and 2:1
in the adult skull (Froriep) , and these differences are due principally
to changes in the vertical dimensions of the maxillae. As with the
increase in length, the increase now under consideration is, to a
 
 
 
POST-NATAL DEVELOPMENT 481
 
ertain extent at least, associated with the development of the teeth,
hese structures calling into existence the alveolar processes which
,re practically wanting in the child at birth. But a more important
actor is the development of the maxillary sinuses, the practically
olid bodies of the maxillae becoming transformed into hollow shells,
rhese cavities, together with the sinuses of the sphenoid and frontal
>ones, which are also post-natal developments, seem to stand in
elation to the increase in length of the anterior portion of the skull,
erving to diminish the weight of the portion of the skull in front
»f the occipital condyles and so relieving the muscles of the neck of a
onsiderable strain to which they would otherwise be subjected.
 
These changes in the proportions of the skull have, of course,
nuch to do with the changes in the general proportions of the face.
3ut the changes which take place in the mandible are also imporant in this connection, and are similar to those of the maxillae in
leing associated with the development of the teeth. In the new10m child the horizontal ramus is proportionately shorter than in
he adult, while the vertical ramus is very short and joins the
Lorizontal one at an obtuse angle. The development of the teeth
if the primary dentition, and later of the three molars, necessitates
,n elongation of the horizontal ramus equivalent to that occurring
n the maxillae, and, at the same time, the separation of the alveolar
•orders of the two bones requires an elongation of the vertical ramus
f the condyle is to preserve its contact with the mandibular fossa,
,nd this, again, demands a diminution of the angle at which the
ami join if the teeth of the two jaws are to be in proper apposition.
 
In the bones of the appendicular skeleton secondary epiphysial
enters play an important part in the ossification, and in few are
hese centers developed prior to birth, while the union of the epiphyes to the main portions of the bones takes place only toward maurity. The dates at which the various primary and secondary
enters appear, and the time at which they unite, may be seen from
he following table:
 
 
 
31
 
 
 
482
 
 
 
POST-NATAL DEVELOPMENT
UPPER EXTREMITY.
 
 
 
Bone
 
 
Appearance of
 
 
Appearance of Secondary
 
 
Fusion of
 
 
Primary Center
 
 
Centers
 
 
Centers
 
 
Clavicle
 
 
6th week.
 
 
(At sternal end) 17th year.
 
 
20th year.
 
 
Scapula.
 
Body
 
 
8th week. <.
 
 
2 acromial 15th year.
 
2 on vertical border 16th year.
 
 
> 20th year.
 
 
Coracoid ....
 
 
1 st year.
 
 
 
 
15 th year.
 
 
 
 
 
 
Head 1st year.
 
 
 
 
 
 
 
 
Great tuberosity 3d year.
 
 
> 20th year.
 
 
 
 
 
 
Lesser tuberosity 5th year.
 
 
J
 
 
Humerus
 
 
â– jth week.
 
 
Inner condyle 5th year.
 
 
1 8th year.
 
 
 
 
 
 
Capitellum 3d year.
 
 
1
 
 
 
 
 
 
Trochlea 10th year.
 
 
[• 17 th year.
 
 
 
 
 
 
Outer condyle 14th year.
 
 
J
 
 
Ulna
 
 
jth week.
 
 
Olecranon 10th year.
 
 
16th year.
 
 
 
 
 
 
Distal epiphysis 4th year.
 
 
1 8th year.
 
 
Radius
 
 
jth week.
 
 
Proximal epiphysis 5th year.
 
 
17 th year.
 
 
 
 
 
 
Distal epiphysis 2d year.
 
 
20th year.
 
 
Capita turn
 
 
1st year.
 
 
 
 
 
 
Hamatum
 
 
2d year.
 
 
 
 
 
 
Triquetrum . . .
 
 
3d year.
 
 
 
 
 
 
 
 
4th year.
 
 
 
 
 
 
Multangulum
 
 
5th year.
 
 
 
 
 
 
majus.
 
 
 
 
 
 
 
 
Navicular
 
 
6th year.
 
 
 
 
 
 
Multangulum
 
 
8th year.
 
 
 
 
 
 
minus.
 
 
 
 
 
 
 
 
Pisiform
 
 
12 th year.
 
 
 
 
 
 
Metacarpals . . .
 
 
gth week.
 
 
3d year.
 
 
20th year.
 
 
Phalanges
 
 
gth-nth week.
 
 
3d~5th years.
 
 
17 th-! 8th years.
 
 
 
The dates in italics are before birth.
 
 
 
POST-NATAL DEVELOPMENT
LOWER EXTREMITY.
 
 
 
483
 
 
 
Bone.
 
 
Appearance
 
 
of
 
 
Appearance of Secondary Fusion of
 
 
Primary Center
 
 
Centers
 
 
Centers
 
 
 
 
gth week.
 
 
 
 
Crest 15th year.
 
Anterior inferior spine 15 th year.
 
 
V
 
 
 
 
 
 
â–  22d year.
 
 
Ischium
 
 
4th month.
 
 
Tuberosity 15th year.
 
 
Pubis
 
 
4th month.
 
 
Crest 1 Sth year.
 
 
 
 
Patella
 
 
Cartilage appears at 4th month, ossification in 3d year.
 
 
 
 
 
 
 
 
Head 1st year.
 
 
20 th year.
 
 
Femur
 
 
â– jth week.
 
 
J
 
 
Great trochanter 4th year.
Lesser trochanter 13 th- 14th year.
Condyle gth month.
 
 
19 th year.
1 Sth year.
2 1 st year.
 
 
Tibia
 
 
jth week.
 
 
1
 
 
Head end of gth month.
Distal end 2d year.
 
 
2ist-2 5thyear.
1 Sth year.
 
 
Fibula
 
 
Sth week.
 
 
/
\
 
 
Upper epiphysis 5th year.
Lower epiphysis 2d year.
 
 
21st year.
20th year.
 
 
Talus
 
 
jth month.
 
 
 
 
 
 
 
 
Calcaneus
 
 
6th month.
 
 
 
 
10th year.
 
 
1 6 th year.
 
 
Cuboid
 
 
A few days
after birth.
 
 
 
 
 
 
 
 
Navicular
 
 
4th year.
 
 
 
 
 
 
 
 
Cuneiforms.
 
 
1 st year.
 
 
 
 
 
 
 
 
Metatarsals
 
 
gth week.
 
 
 
 
3d year.
 
 
20th year.
 
 
Phalanges
 
 
gth-i2th week
 
 
 
 
4th-8th years.
 
 
I7th-i8th years.
 
 
 
The dates in italics are before birth.
 
So far as the actual changes in the form of the appendicular
bones are concerned, these are most marked in the case of the lower
limb. The ossa innominata alter somewhat in their proportions
after birth, a fact which may conveniently be demonstrated by considering the changes which occur in the proportions of the pelvic
diameters, although it must be remembered that these diameters
are greatly influenced by the development of the sacral curve.
Taking the conjugate diameter of the pelvic brim as a unit for comparison, the antero-posterior (dorso-ventral) and transverse diameters of the child and adult have the proportions shown in the table
on the opposite page (Fehling).
 
 
 
4 8 4
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
It will be seen from this that the general form of the pelvis in
the new-born child is that of a cone, gradually diminishing in diameter from the brim to the outlet, a condition very different from
what obtains in the adult. Furthermore, it is interesting to note
 
 
 
Diameter.
 
 
 
New-born
 
 
Adult
 
 
New-born
 
 
Female.
 
 
Female.
 
 
Male.
 
 
i .00
 
 
1 .00
 
 
1 .00
 
 
1. 19
 
 
1 .292
 
 
1 .20
 
 
0.96
 
 
1. 19
 
 
0.91
 
 
1 .01
 
 
1. 151
 
 
0.99
 
 
0.91
 
 
1.05
 
 
0.78
 
 
0.83
 
 
i-i54
 
 
0.84
 
 
 
Adult
Male.
 
 
 
(Conjugata vera .
Transverse
 
>, f Antero-posterior
 
rt 1
 
U y Transverse
 
-^ ( Antero-posterior
 
O Transverse
 
 
 
1.294
1. 18
1. 14
1 .07
 
1 -153
 
 
 
that sexual differences in the form of the pelvis are clearly distinguishable at birth; indeed, according to Fehling's .observations,
they become noticeable during the fourth month of intrauterine
development.
 
The upper epiphysis of the femur is entirely unossified at birth
and consists of a cartilaginous mass, much broader than the rather
slender shaft and possessing a deep notch upon its upper surface
(Fig. 285). This notch marks off the great trochanter from the
head of the bone, and at this stage of development there is no neck,
the head being practically sessile. As development proceeds the
inner upper portion of the shaft grows more rapidly than the outer
portion, carrying the head away from the great trochanter and forming the neck of the bone. The acetabulum is shallower at birth
than in the adult and cannot contain more than half the head of
the femur; consequently the articular portion of the head is much
less extensive than in the adult.
 
 
 
POST-NATAL DEVELOPMENT
 
 
 
485
 
 
 
It is a well-known fact that the new-born child habitually holds
the feet with the soles directed toward one another, a position only
reached in the adult with some difficulty, and associated with this
supination or inversion there is a pronounced extension of the foot
(i. e., flexion upon the leg as usually understood; see p. 102), it being
difficult to flex the child's foot beyond a line at right angles with the
axis of the leg. These conditions are due apparently to the extensor and tibialis muscles being relatively shorter and the opposing
muscles relatively longer than in the adult, and with the elongation
or shortening, as the case may be, of the muscles on the assumption
 
 
 
 
Fig. 2S5. — Longitudinal Sections of the Head of the Femur of (.4) New-born
 
Child and (B) a Later Stage of Development.
 
ep, Epiphysial center for the head; h, head; /, trochanter. — (Henke.)
 
of the erect position, the bones in the neighborhood of the anklejoint come into new relations to one another, the result being a modification of the form of the articular surfaces, especially of the talus
(astragalus). In the child the articular cartilage of the trochlear
surface of this bone is continued onward to a considerable extent
upon the neck of the bone, which comes into contact with the tibia
in the extreme extension possible in the child. In the adult, however,
such extreme extension being impossible, the cartilage upon the neck
gradually disappears. The supination in the child brings the talus
 
 
 
486 LITERATURE
 
in close contact with the inner surface of the calcaneus and with
the sustentaculum tali; with the alteration of position a growth of
these portions of the calcaneus occurs, the sustentaculum becoming higher and broader, and so becoming an obstacle in the way of
supination in the adult. At the same time a greater extent of the
outer surface of the talus comes into contact with the lateral
malleolus, with the result that the articular surface is considerably
increased on that portion of the bone. Marked changes in the form
of the talo-navicular articulation also occur, but their consideration
would lead somewhat further than seems desirable.
 
LITERATURE.
 
C. Aeby: "Die Altersverschiedenheiten der menschlichen Wirbelsaule." Archiv fur
 
Anal, und Physiol., Anat. Abth., 1879.
W. Camerer: " Utersuchungen iiber Massenwachsthum und Langen wachsthum der
 
Kinder," Jahrbuchfiir Kinderheilkunde, xxxvi, 1893.
H. H. Donaldson: "The Growth of the Brain," London, 1895.
H. Fehling: "Die Form des Beckens beim Fotus und Neugeborenen und ihre Bezie
hung zu der beim Erwachsenen," Archiv fur Gynakol., x, 1876.
H. Friedenthal: " Das Wachsthum des Korpergewichtes des Menschen und anderer


Saugethiere in verschiedenen Lebensaltern," Zeit. allgem. Physiol., ix, 1909.
J. A. Hammar: "Ueber Gewicht, Involution und Persistenz der Thymus im Post
fotalleben des Menschen," Archiv fur Anat. und Phys., Anat. Abth., Supplement,


1906.
Somatic cells + germ cells, etc.
W. Henke: " Anatomie des Kindersalters," Handbuch der Kinder krankheiten (Cerhardt) ,


Tubingen, 1881.
C. Hennig: "Das kindliche Becken," Archiv fur Anat. und Physiol., Anat. Abth., 1880.
C. Huter: "Anatomische Studien an den Extremitatengelenken Neugeborener und


Erwachsener," Archiv fur patholog. Anat. und Physiol., xxv, 1862.  
It is evident, then, while the somatic cells of each generation die at their appointed time and are differentiated anew for each generation from the germ cells, the latter, which may be termed collectively the germ-plasm, are handed on from generation to generation without interruption, and it may be supposed that this has been the case ab initio. This is the doctrine of the continuity of the germ-plasm, a doctrine of fundamental importance on account of its bearings on the phenomena of heredity.
W. Stephenson: "On the Relation of Weight to Height and the Rate of Growth in


Man," TheLancet, 11, 1888.
R. Thoma: " Untersuchungen iiber die Grosse und das Gewicht der anatomischen


Bestandtheile des menschlichen Korpers," Leipzig, 1882.  
It is necessary, however, to fix upon some link in the continuous chain of the germ-plasm as the starting-point of the development of each individual, and this link is the fertilized ovum. By this is meant a germ cell produced by the fusion of two units of the germplasm. In many of the lower forms of life (e.g., Hydra and certain turbellarian worms) reproduction may be accomplished by a division of the entire organism into two parts or by the separation of a portion of the body from the parent individual. Such a method of reproduction is termed non-sexual. Furthermore in a number of forms (e. g., bees, Phylloxera, water-fleas) the germ cells are able to undergo development without previously being fertilized, this constituting a method of reproduction known as parthenogenesis. But in all these cases sexual reproduction also occurs, and in all the more highly organized animals it is the only method that normally occurs; in it a germ cell develops only after complete fusion with another germ cell. In the simpler forms of this process little difference exists between the two combining cells, but since it is, as a rule, of advantage that a certain amount of nutrition should be stored up in the germ cells for the support of the developing embryo until it is able to secure food for itself, while at the same time it is also advantageous that the cells which unite shall come from different individuals (cross-fertilization), and hence that the cells should retain their motility, a division of labor has resulted. Certain germ cells store up more or less food yolk, their motility becoming thereby impaired, and form what are termed the female cells or ova, while otners discard all pretensions of storing up nutrition, are especially motile and can seek and penetrate the inert ova; these latter cells constitute the male cells or spermatozoa. In many animals both kinds of cells are produced by the same individual, but in all the vertebrates (with rare exceptions in some of the lower orders) each individual produces only ova or spermatozoa, or, as it is generally stated, the sexes are distinct. It is of importance, then, that the peculiarities of the two forms of germ cells, as they occur in the human species, should be considered.
H. Vierordt: "Anatomische, Physiologische und Physikalische Daten und Tabellen,"


Jena, 1893.
==Literature==
H. Welcker: "Untersuchungen iiber Wachsthum und Bau des menschlichen


Schadels," Leipzig, 1862.
{{Ref-Wilson1900}}


O. Hertwig: "Die Zelle und die Gewebe." Jena, 1893.




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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.

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   McMurrich 1914: General 1 Spermatozoon - Spermatogenesis - Ovum - Fertilization | 2 Ovum Segmentation - Germ Layer Formation | 3 Medullary Groove - Notochord - Somites | 4 Embryo External Form | 5 Yolk-stalk - Belly-stalk - Fetal Membranes Organogeny 6 Integumentary System | 7 Connective Tissues - Skeleton | 8 Muscular System | 9 Circulatory - Lymphatic Systems | 10 Digestive Tract and Glands | 11 Pericardium - Pleuro-peritoneum - Diaphragm | 12 Respiration | 13 Urinogenital System | 14 Suprarenal System | 15 Nervous System | 16 Organs of Special Sense | 17 Post-natal | Figures
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This is a draft version of McMurrich's 1914 embryology textbook.



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Book Review (1913)

The Development of the Human Body: A Manual of Human Embryology. By J. Playfair McMurrich. Philadelphia, P. Blakiston's Son and Company, fourth edition, 1913, pp. 8 + 495, 285 figures, $2.50 net.

A new edition of McMurrich's Embryology has just appeared. The general character of the previous editions is retained, particular attention being given to the development of organs and less space devoted to the early stages of the embryo. Parts of the book have been re-written and other parts revised. The numerous typographical errors so conspicuous in the third edition have been eliminated. The volume is of pocket size with flexible binding. The large, clear type has been retained. Perhaps the most important feature of the book is the author's clearness of expression. If embryology is to be an aid to anatomy it would appear desirable to find some royal road to this science, which is at present perhaps more difficult than adult anatomy. The student is usually bewildered by serial sections unless guided by a very clear presentation of the subject. McMurrich has achieved this end at the expense of many details, and no doubt rightly. Yet, the large mass of facts contained in this small book seems remarkable. A few passages are ambiguous if not erroneous, and may deserve mention. McMurrich describes the observations of Will on gastrulation in the Gecko, but transposes Will's terminology of primary and secondar}^ endoderm. If gastrulation in vertebrates means anything, certainly the formation of endoderm by invagination is the primitive process and is 'primary' from the standpoint of phylogeny, whereas delamination is a secondary method of formation although it does occur earliest in the development of mammalian embryo. On page 60 the author describes the origin of the mesoderm as follows : "the layer of enveloping cells splits into two concentric layers, the inner of which seems to be mesodermal in its nature and forms a layer lining the trophoblast," and again on page 110: "the extra-embryonic mesoderm, instead of growing out from the embryo to enclose the yolk sac, splits off directly from the enveloping layer." And on page 55: "a splitting of the enveloping layer has occurred, so that the wall of the ovum is now formed of three layers, an outer one which may be termed the trophoblast, a middle one which probably is transformed into the extra-embryonic mesoderm of later stages, though its significance is obscure, and an inner one which is the primary endoderm." I know of no primate, however, in which it has been demonstrated that any 102 BOOK REVIEW 103 part of the mesoderm is delaminated from the trophoblast. McMurrich supposes a separate origin of the extra-embryonic mesoderm. He may be correct, since the origin of the mesoderm is unknown in man. The relation of bloodvessels to endoderm in the liver anlage is described on page 308 as follows: "Shortly after the hepatic portion has been differentiated, its substance becomes permeated by numerous blood vessels (sinusoids) and so divided into anastomosing trabecular" Since these blood vessels arise by a breaking up of the vitelline veins, which are present before the liver anlage appears, it might seem more logical to say that the liver invades the vitelline veins than that the veins invade the liver. McMurrich describes the separation of the coelomic cavities as they occur in the rabbit. The separation of the pericardium from the other cavities is simpler in man in that there are no openings ventral to the viteline veins that have to be closed. This is described in Keibeland Mall's Handbook (vol. 1, p. 526) as follows: "In the rabbit the pericardial coelom ends in two dorsal and two ventral recesses, all four of which connect subsequently with the peritoneal coelom. However, only the dorsal recesses break into the peritoneal coelom in the human embryo." A description of the simpler condition in the human embryo might be preferable in a textbook. Finally, there is a point which is of no evident importance in embryology, but since McMurrich has brought it up, we will consider it very briefly. He describes protoplasm as having a visible reticular or possibly alveolar structure. This reticular ' theory' is based on fixed material, although is it well known that fixing fluids produce the same appearance in clear gelatine jelly or clear albuminous fluids. Formalin and osmium tetroxide do not produce a reticular structure in gelatine jelly and protoplasm fixed with them appears homogeneous. Butschli observed an alveolar structure in 'living' protoplasm under certain conditions, but this appearance seemsto be exceptional or possibly abnormal. With the ultra-microscope of Siedentopf and Zigsmondy, particles smaller than the largest molecules may be made very evident. With this instrument the protoplasm of erythrocytes, and both nucleus and cytoplasm of frog erythrocytes, appear absolutely homogeneous, until injured by abnormal conditions. When transferred from the normal medium to even so good an imitation as Ringer's solution coagulations begin to appear. The numerous deutoplasmic granules in many cells prevent a universal application of this method. To sum up: McMurrich's Embryology may be considered as a very convenient and desirable text book for medical students. It might also be used as a brief reference book provided the subject matter be verified by looking up the literature cited at the end of each chapter. J. F. McClendon.

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The Development of the Human Body - A Manual of Human Embryology

J. Playfair McMurrich
J. Playfair McMurrich (1859 – 1939)

By

J. Playfair McMurrich, A. M., Ph. D., Ll. D.


Professor Of Anatomy In The University Of Toronto

Formerly Professor Of Anatomy In The University Of Michigan

Fourth Edition, Revised And Enlarged

With Two Hundred and Eighty-five Illustrations Several of which are Printed in Colors

Philadelphia

P. Blakiston's Son & Co.


1012 Walnut Street

1914

Copyright, 1913, By P. Blakiston's Son & Co.

Book Review by Frederic T. Lewis - Anatomical Record 1 (1906) 
The Development of the Human Body by J. Playfair McMurrich. Third edition. P. Blakiston's Son & Co., Philadelphia, 1907. X + 528 pages, 277 figures. $3.00

McMurrich's "Development of the Human Body" is the best brief didactic American text-book of embryology. In its third edition it has been thoroughly revised, but is no larger than before. The Basle nomenclature has been introduced, but here and there are found such rejected terms as lymph follicles, discus proligems, and uterus masculinm. The word anlage does not occur. In English we may say- that "' an organ arises (or begins) as an outgrowth " rather than "' the anlage of the organ is an outgrowth " ; and that " the liver at first is a diverticulum " rather than " the anlage of the liver is a diverticulum." Apparently no new terms have been proposed.

The following changes and additions are of interest. In man the number of chromosomes is 24 (Duesberg) instead of 16. The studies of Heape on menstruation in lower mammals, of Marshall on the internal secretion of the ovaries, of Kirkham and of Gerlach showing that the mouse has a second polar body overlooked by Sobotta, and of several authors in favor of the epithelial origin of lutein cells have been added. Hubrecht's term trophohlast, corresponding with epiblast and hypoblast, is replaced by Minot's trophoderm, corresponding with ectoderm, etc.; it is described as a modified ectoderm adapted for implantation rather than as a distinct germ layer. The description of implantation has been modified with reference to Rejsek's study of Sperinophilus and Doria's account of a young human embryo. Assheton's and Peebles' experimental studies of growth in the chick's blastoderm have been substituted for the concrescence theory, which is not mentioned in the third edition. The existence of a primary chordal canal opening at Hensen's knot is now admitted (p. 60) ; in view of this change the secondary chordal canal (p. 100) should be more sharply defined.

In connection with the development of the skeleton, a new figure is introduced illustrating the relation of vertebrae to segments; ]\Iairs work on the ossification centers of the interparietal and maxilla, and Fawcett's on the pterygoid process have been cited.

Red blood corpuscles are described as discs. Since photographs of the cup-shaped form have not yet been published, two are insei-ted in this review. They represent a few corpuscles of the foi'm in question among a large number of shadows from which the luemoglobin has disappeared. It is improbable that IJetterer is correct in regarding the cup as being only apparent and due to the distribution of haemoglobin. Such cupshaped appearances as are shown in the photograph are characteristic of circulating and of well-preserved blood. They appear in embryos wlien the nuclei of the erythroblasts are eliminated.

Of the origin of blood plates it is said that " the most plausible suggestion is that they are the fragmented nuclei of broken-down leucocytes." The fine investigation of James 11. Wright is not mentioned.

There are many clianges in the chapter on blood-vessels. A new diagram shows the questionable fifth aortic arch as equivalent to the other five. Six aortic arches require the presence of five pharyngeal pouches, yet according to McMurrich, there are but four. The fifth arch is however, said to be " rudimentary." Mall's important work on the cerebral veins in man replaces Selzer's description of these veins in the guinea-pig, but the complex story of development is rendered confusing by a misplacement of figures, Fig. 151 B (p. 271) being younger than Fig. 151 A. The supracardinal veins of Huntington and McClure are described and figured. These include the dorsal limbs of the loop formed by the cardinal veins around the ureter, and other more anterior vessels of a different origin; the anterior portion is shown in the figures of the cat, but not of the rabbit. Tlie loop around the ureter is well known from the studies of Tloehstetter. His diagrams (Hertwig's Handhuch, Vol. 3, p. 142) show the extent of the loop more accurately than McMurrich's figure, and correctly indicate the relation of the genital veins to its ventral limb (compare also Anat. Anz., Vol. 25, p. 271). If the term supracardinal vein could be restricted in its application, so as to be synon}anous with dorsal limh of the ureteric loop, it would be more readily adopted. The veins of the limbs are described at greater length than in the previous edition, but without the necessary figures.

Photographs of human red blood corpuscles within a blood vessel. Fixing reagent unknown. Weigert's stain. Those retaining their haemoglobin are cup-shaped. A, X 315 diams. B, X 630 diams.


In the account of the lymphatic vessels interpretations rather than observations are considered, and thus the " discordant " element is emphasized. A figure of a section through the jugular lympli sac, such as Professor Sabin has published, is much needed, and reconstructions are better than diagrams. Sabin's studies of lymph glands and ]\rairs work on the spleen have been incorporated. The spleen has been transferred from the chapter on mesenteries to that on the circnlatorv system, and as a result of Stoerk's investigations, the coccygeal gland is placed with the lymphoid organs.

The description of the entodermal tract now includes Flint's conclusion that the hmg in the pig grows by lateral branching, and that the suppression of the left eparterial bronchus and the development of the right infracardiac bronchus are correlated with the position of the aortic arch and heart respectively.

Tandler's record of the pronephros in human embryos up to 20 mm. is noted, and the development of the ovary and testis liavc been rewritten with reference to Allen's work. Presumably McMurrich hesitates to accept the determination of the large cells appearing in the entoderm and supposedly migrating into the sexual glands, as germ cells; he does not refer to them.

The suprarenal glands liave been redescribed, with a figui'e and references to Wiesel's studios. Tliey are placed in a chapter witli the carotid glands and Zuckerkandl's organs.

Under the nervous system the bearing of Harrison's experiments on the interpretation of sheath cells and on the neurone theory is recorded. Streeter's reconstruction of the otocyst of a 20-mm. human embryo replaces that of His of a similar stage. Fuch's observation in the rabbit that the stapes is at first separate from the second branchial cartilage is regarded as an ontogenetic condition and not of general significance.

The large number of changes in this edition of McMurrich's book reflect the progress of embryology during the last three years. Students will like the small size of the volume, and teachers will appreciate Professor McMurrich's estimate of the value of recent researches.

Frederic T. Lewis.

Contents

Introduction

Part I. - General Development.


CHAPTER I. The Spermatozoon and Spermatogenesis; the Ovum and Its Maturation and Fertilization

CHAPTER II. The Segmentation of the Ovum and the Formation of the Germ Layers

CHAPTER III. The Medullary Groove, Notochord, and Mesodermic Somites

CHAPTER IV. The Development of the External Form of the Human Embryo

CHAPTER V. The Yolk-stalk, Belly-stalk, and Fetal Membranes


Part II. - Organogeny.


CHAPTER VI. The Development of the Integumentary System

CHAPTER VII. The Development of the Connective Tissues and Skeleton

CHAPTER VIII. The Development of the Muscular System

CHAPTER IX. The Development of the Circulatory and Lymphatic Systems

CHAPTER X. The Development of the Digestive Tract and Glands

CHAPTER XI. The Development of the Pericardium, the Pleuro-peritoneum, and the Diaphragm

CHAPTER XII. The Development of the Organs of Respiration

CHAPTER XIII. The Development of the Urinogenital System

CHAPTER XIV. The Suprarenal System of Organs

CHAPTER XV. The Development of the Nervous System

CHAPTER XVI. The Development of the Organs of Special Sense

CHAPTER XVII. Post-natal Development


Preface to the Fourth Edition

The increasing interest in human and mammalian embryology which has characterized the last few years has resulted in many additions to our knowledge of these branches of science, and has necessitated not a few corrections of ideas formerly held. In this fourth edition of this book the attempt has been made to incorporate the results of all important recent contributions upon the topics discussed, and, at the same time, to avoid any considerable increase in the bulk of the volume. Several chapters have, therefore, been almost entirely recast, and the subject matter has been thoroughly revised throughout, so that it is hoped that the book forms an accurate statement of our present knowledge of the development of the human body.


To several colleagues the author is indebted for valuable suggestions, and in this connection he desires especially to thank Dr. J. C. Watt for much generous assistance in the revision of the manuscript and for undertaking the correction of the proof-sheets.


In addition to the works mentioned in the preface to the first edition as of special value to the student of Embryology, mention should be made of the Handbuch der vergleichenden mid experimentellen Entwickhmgslehre der Wirbeltiere edited by Professor Oscar Hertwig and especially of the Manual of Human Embryology edited by Professors F. Keibel and F. P. Mall. University of Toronto.


Preface to the First Edition

The assimilation of the enormous mass of facts which constitute what is usually known as descriptive anatomy has always been a difficult task for the student. Part of the difficulty has been due to a lack of information regarding the causes which have determined the structure and relations of the parts of the body, for without some knowledge of the why things are so, the facts of anatomy stand as so many isolated items, while with such knowledge they become bound together to a continuous whole and their study assumes the dignity of a science.


The great key to the significance of the structure and relations of organs is their development, recognizing by that term the historical as well as the individual development, and the following pages constitute an attempt to present a concise statement of the development of the human body and a foundation for the proper understanding of the facts of anatomy. Naturally, the individual development claims the major share of attention, since its processes are the more immediate forces at work in determining the conditions in the adult, but where the embryological record fails to afford the required data, whether from its actual imperfection or from the incompleteness of our knowledge concerning it, recourse has been had to the facts of comparative anatomy as affording indications of the historical development or evolution of the parts under consideration.


It has not seemed feasible to include in the book a complete list of the authorities consulted in its preparation. The short bibliographies appended to each chapter make no pretensions to completeness, but are merely indications of some of the more important works, especially those of recent date, which consider the questions discussed. For a very full bibliography of all works treating of human embryology up to 1893 reference may be made to Minot's Bibliography of Vertebrate Embryology, published in the "Memoirs of the Boston Society of Natural History," volume iv, 1893. It is fitting, however, to acknowledge an especial indebtedness, shared by all writers on human embryology, to the classic papers of His, chief among which is his Anatomie menschlicher Embryonen, and grateful acknowledgments are also due to the admirable text-books of Minot, O. Hertwig, and Kollmann.


Anatomical Laboratory, University of Michigan.



Introduction

Somewhat more than seventy years ago (1839) one of the fundamental principles of biology was established by Schleiden and Schwann as the cell theory. According to this, all organisms are composed of one or more structural units termed cells, each of which, in multicellular organisms, maintains an individual existence and yet contributes with its fellows to the general existence of the individual. Viewed in the light of this theory, the human body is a community, an aggregate of many individual units, each of which leads to a certain extent an independent existence and yet both contributes to and shares in the general welfare of the community.


To the founders of the theory the structural units were vesicles with definite walls, and little attention was paid to their contents. Hence the use of the term "cell" in connection with them. Long before the establishment of the cell theory, however, the existence of organisms composed of a gelatinous substance showing no indications of a definite limiting membrane had been noted, and in 1835 a French naturalist, Dujardin, had described the gelatinous material of which certain marine organisms (Rhizopoda) were composed, terming it sarcode and maintaining it to be the material substratum which conditioned the various vital phenomena exhibited by the organisms. Later, in 1846, a botanist, von Mohl, observed that living plant cells contained a similar substance, upon which he believed the existence of the cell as a vital structure was dependent, and he bestowed upon this substance the name protoplasm, by which it is now universally known.


By these discoveries the importance originally attributed to the cell-wall was greatly lessened, and in 1864 Max Schultze reformulated the cell theory, defining the cell as a mass of protoplasm, the presence or absence of a limiting membrane or cell-wall being immaterial. At the same time the spontaneous origination of cells from an undifferentiated matrix, believed to occur by the older authors, was shown to have no existence, every cell originating by the division of a preexisting cell, a fact concisely expressed in the aphorism of Virchow - omnis cellula a cellula.


Interpreted in the light of these results, the human body is an aggregate of myriads of cells,* - i. e., of masses of protoplasm, each of which owes its origin to the division of a preexistent cell and all of which may be traced back to a single parent cell - a fertilized ovum. All these cells are not alike, however, but just as in a social community one group of individuals devotes itself to the performance of one of the duties requisite to the well-being of the community and another group devotes itself to the performance of another duty, so too, in the body, one group of cells takes upon itself one special function and another another. There is, in other words, in the cell-community a physiological division of labor. Indeed, the comparison of the cell-community to the social community may be carried still further, for just as gradations of individuality may be recognized in the individual, the municipality, and the state, so too in the cell-community there are cells; tissues, each of which is an aggregate of similar cells; organs, which are aggregates of tissues, one, however, predominating and determining the character of the organ; and systems, which are aggregates of organs having correlated functions.

  • It has been estimated that the number of cells entering into the composition of the body of an adult human being is about twenty-six million five hundred thousand millions!


It is the province of embryology to study the mode of division of the fertilized ovum and the progressive differentiation of the resulting cells to form the tissues, organs, and systems. But before considering these phenomena as seen in the human body it will be well to get some general idea of the structure of an animal cell.


This (Fig. i), as has been already stated, is a mass of protoplasm, a substance which in the living condition is a viscous fluid resembling in many of its peculiarities egg-albumen, and like this being coagulated when heated or when exposed to the action of various chemical reagents. As to the structure of living protoplasm little is yet known, since the application of the reagents necessary for its accurate study and analysis results in its disintegration or coagulation. But even in the living cell it can be seen that the Fig. i. - Ovum of New-born protoplasm is not a simple homogeneous ?^ IL r ? WI n TH Follicle - cells ~ substance. What is termed a nucleus is usually clearly discernible as a more or less spherical body of a greater refractive index than the surrounding protoplasm, and since this is a permanent organ of the .cell it is convenient to distinguish the surrounding protoplasm as the cytoplasm from the nuclear protoplasm or karyoplasm.


The study of protoplasm coagulated by reagents seems to indicate that it is a mixture of substances rather than a simple chemical compound. Both the cytoplasm and the karyoplasm consist of a more solid substance, the reticulum, which forms a network or feltwork, in the interstices of which is a more fluid material, the enchylema* The karyoplasm, in addition, has scattered along the fibers of its reticulum a peculiar material termed chromatin and usually contains embedded in its substance one or more spherical bodies termed nucleoli, which may be simply larger masses of chromatin or bodies of special chemical composition. And, finally, in all actively growing cells there is differentiated in the cytoplasm a peculiar body known as the archo plasm sphere, in the center of which there is usually a minute spherical body known as the centrosome.

  • It has been observed that certain coagulable substances and gelatin, when subjected to the reagents usually employed for "fixing" protoplasm, present a structure similar to that of protoplasm, and it has been held that protoplasm in the uncoagulated condition is, like these substances, a more or less homogeneous material. On the other hand, Biitschli maintains that living protoplasm has a foam-structure and is, in other words, an emulsion.


It has been already stated that new cells arise by the division of preexisting ones, and this process is associated with a series of complicated phenomena which have great significance in connection with some of the problems of embryology. When such a cell as has been described above is about to divide, the fibers of the reticulum in the neighborhood of the archoplasm sphere arrange themselves so as to form fibrils radiating in all directions from the sphere as a center, and the archoplasm with its contained centrosome gradually elongates and finally divides, each portion retaining its share of the radiating fibrils, so that two asters, as the aggregate of centrosome, sphere and fibrils is termed, are now to be found in the cytoplasm (Fig. 2, A). Gradually the two asters separate from one another and eventually come to rest at opposite sides of the nucleus (Fig. 2, C). In this structure important changes have been taking place in the meantime. The chromatin, originally scattered irregularly along the reticulum, has gradually aggregated to form a continuous thread (Fig. 2, A), and later this thread breaks up into a definite number of pieces termed chromosomes (Fig. 2, B), the number of these being practically constant for each species of animal. In man the number has been placed at twenty-four (Flemming, Duesberg) , but the recent observations of Guyer indicate that it is probably twenty-four in the female and twenty-two in the male. The significance of this difference in the two sexes will be considered in connection with the fertilization of the ovum (p. 32).


As soon as the asters have taken up their position on opposite sides of the nucleus, the nuclear reticulum begins to be converted into a spindle-shaped bundle of fibrils which associate themselves with the astral rays and have lying scattered among them the chromosomes (Fig. 2, C). To the figure so formed the term amphiaster is applied, and soon after its formation the chromosomes arrange themselves in a circle or plane at the equator of the spindle (Fig. 2, D) and the stages preparatory to the actual division, the prophases, are completed.


The next stage, the metaphase (Fig. 3, A), consists of the division, usually longitudinally, of each chromosome, so that the cell now contains twice as many chromosomes as it did previously. As soon as this division is completed the anaphases are inaugurated by the halves of each chromosome separating from one another and approaching one of the asters (Fig. 3, B), and a group of chromosomes, containing half the total number formed in the metaphase, comes to lie in close proximity to each archoplasm sphere (Fig. 3, C). The spindle and astral fibers gradually resolve themselves again into the reticulum and the chromosomes of each group become irregular in shape and gradually spread out upon the nuclear reticulum so that •two nuclei, each similar to the one from which the process started,


Fig. 2. - Diagrams Illustrating the Prophases of Mitosis. - (Adapted from E. B. Wilson.)


Fig. 3. - Diagrams Illustrating the Metaphase and Anaphases of Mitosis.: - (Adapted from E. B. Wilson.) are formed (Fig. 3, D). Before all these changes are accomplished, however, a constriction makes its appearance at the surface of the cytoplasm (Fig. 3, C) and, gradually deepening, divides the cytoplasm in a plane passing through the equator of the amphiaster and gives rise to two separate cells (Fig. 3, D).


This complicated process, which is known as karyokinesis or mitosis, is the one usually observed in dividing cells, but occasionally a cell divides by the nucleus becoming constricted and dividing into two parts without any development of chromosomes, spindle, etc., the division of the cell following that of the nucleus. This amitotic method of division is, however, rare, and in many cases, though not always, its occurrence seems to be associated with an impairment of the reproductive activities of the cells. In actively reproducing cells the mitotic method of division may be regarded as the rule.


Since the process of development consists of the multiplication of a single original cell and the differentiation of the cell aggregate so formed, it follows that the starting-point of each line of individual development is to be found in a cell which forms part of an individual of the preceding generation. In other words, each individual represents one generation in esse and the succeeding generation in posse. This idea may perhaps be made clear by the following considerations. As a result of the division of a fertilized ovum there is produced an aggregate of cells, which, by the physiological division of labor, specialize themselves for various functions. Some assume the duty of perpetuating the species and are known as the sexual or germ cells, while the remaining ones divide among themselves the various functions necessary for the maintenance of the individual, and may be termed the somatic cells. The germ cells represent potentially the next generation, while the somatic cells constitute the present one. The idea may be represented schematically thus:

First generation

Somatic cells + germ cells

II Second generation

Somatic cells + germ cells

II Third generation


Somatic cells + germ cells, etc.


It is evident, then, while the somatic cells of each generation die at their appointed time and are differentiated anew for each generation from the germ cells, the latter, which may be termed collectively the germ-plasm, are handed on from generation to generation without interruption, and it may be supposed that this has been the case ab initio. This is the doctrine of the continuity of the germ-plasm, a doctrine of fundamental importance on account of its bearings on the phenomena of heredity.


It is necessary, however, to fix upon some link in the continuous chain of the germ-plasm as the starting-point of the development of each individual, and this link is the fertilized ovum. By this is meant a germ cell produced by the fusion of two units of the germplasm. In many of the lower forms of life (e.g., Hydra and certain turbellarian worms) reproduction may be accomplished by a division of the entire organism into two parts or by the separation of a portion of the body from the parent individual. Such a method of reproduction is termed non-sexual. Furthermore in a number of forms (e. g., bees, Phylloxera, water-fleas) the germ cells are able to undergo development without previously being fertilized, this constituting a method of reproduction known as parthenogenesis. But in all these cases sexual reproduction also occurs, and in all the more highly organized animals it is the only method that normally occurs; in it a germ cell develops only after complete fusion with another germ cell. In the simpler forms of this process little difference exists between the two combining cells, but since it is, as a rule, of advantage that a certain amount of nutrition should be stored up in the germ cells for the support of the developing embryo until it is able to secure food for itself, while at the same time it is also advantageous that the cells which unite shall come from different individuals (cross-fertilization), and hence that the cells should retain their motility, a division of labor has resulted. Certain germ cells store up more or less food yolk, their motility becoming thereby impaired, and form what are termed the female cells or ova, while otners discard all pretensions of storing up nutrition, are especially motile and can seek and penetrate the inert ova; these latter cells constitute the male cells or spermatozoa. In many animals both kinds of cells are produced by the same individual, but in all the vertebrates (with rare exceptions in some of the lower orders) each individual produces only ova or spermatozoa, or, as it is generally stated, the sexes are distinct. It is of importance, then, that the peculiarities of the two forms of germ cells, as they occur in the human species, should be considered.

Literature

Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.

O. Hertwig: "Die Zelle und die Gewebe." Jena, 1893.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   McMurrich 1914: General 1 Spermatozoon - Spermatogenesis - Ovum - Fertilization | 2 Ovum Segmentation - Germ Layer Formation | 3 Medullary Groove - Notochord - Somites | 4 Embryo External Form | 5 Yolk-stalk - Belly-stalk - Fetal Membranes Organogeny 6 Integumentary System | 7 Connective Tissues - Skeleton | 8 Muscular System | 9 Circulatory - Lymphatic Systems | 10 Digestive Tract and Glands | 11 Pericardium - Pleuro-peritoneum - Diaphragm | 12 Respiration | 13 Urinogenital System | 14 Suprarenal System | 15 Nervous System | 16 Organs of Special Sense | 17 Post-natal | Figures


McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.


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