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Rewritten, Revised, Improved, with Many New Illustrations 



Consulting Surgeon to, and formerly Lecturer on Surgery and Anatomy at 

Middlesex Hospital, London, and Examiner in Anatomy, 

University of Durham, etc. 


Professor of Anatomy in the University of Toronto; formerly Professor of 
Anatomy, University of Michigan 

Among the American contributors will be noted : J. Playf air McMurrich, R. 
J. Terry, Irving Hardesty, G. Carl Huber, Abram T. Kerr, Charles R. 
Bardeen and Florence R. Sabih. Henry Morris, R. Marcus Gunn and 
W. H. A. Jacobson head the English contributors. 

"The ever-growing popularity of the book with teachers and students is an index 
of its value, and it may safely be recommended to all interested." — From The 
Medical Record, New York. 

The text has been completely revised. Very especial attention, in this new 
edition, has been paid to the illustrations, with the result that the teaching value 
of the book has been very materially increased. 

It contains many features of special advantage to students. It is modern, up to 
date in every respect. It has been carefully revised, and in many parts rewritten, 
and includes many new illustrations. 

Containing about 1024 Illustrations, of which many are in Colors. 
One Handsome Octavo Volume. Thumb Index. Cloth, $6.00. 
Sheep or Half Morocco, $7.00, net. Or in Five Parts, as follows, 
each part sold separately : 

PART I.— Morphogenesis. Osteology. Articulation. Index. $1.50. 

PART II.- — Muscles. Organs of Circulation. Lymphatics. Index. $2.00. 

PART III. — Nervous System. Organs of Special Sense. Index. $1.50. 

PART IV. — Organs of Digestion; of Voice and Respiration. Urinary and Repro- 
ductive Organs. Ductless Glands. Skin and Mammary Glands. 
Index. $1.50. 

PART V. — Surgical and Topograghical Anatomy. Index. $1.00. 
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With Two Hundred and Eighty-five Illustrations Several 
of which are Printed in Colors 






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



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 incorpo- 
rate 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 sug- 
gestions, and in this connection he desires especially to thank Dr. 
J. C. Watt for much generous assistance in the revision of the manu- 
script 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 experimen- 
tellen 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. 

Digitized by the Internet Archive 

in 2010 with funding from 
Columbia University Libraries 


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 con- 
stitute 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 immedi- 
ate 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 devel- 
opment 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 bibliog- 
raphies appended to each chapter make no pretensions to com- 
pleteness, 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 1 



The Spermatozoon and Spermatogenesis; the Ovum and Its Matu- 
ration and Fertilization 1 1 


The Segmentation of the Ovum and the Formation of the Germ 

Layers 3& 


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

The Development of the External Form of the Human Embryo ... 86 


The Yolk-stalk, Belly-stalk, and Fetal Membranes 107 


The Development of the Integumentary System 141 


The Development of the Connective Tissues and Skeleton . . . 153 


The Development of the Muscular System 193 





The Development of the Circulatory and Lymphatic Systems .. . . 221 

The Development of the Digestive Tract and Glands 280 


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

the Diaphragm 316 

The Development of the Organs of Respiration 331 

The Development of the Urinogenital System 338 

The Suprarenal System of Organs 370 

The Development of the Nervous System 377 

The Development of the Organs of Special Sense 427 


Post-natal Development 47° 

Index 487 





Somewhat more than seventy years ago (1839) one of the funda- 
mental 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 indi- 
vidual. 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 indica- 
tions 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 reformu- 
lated 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 

It is the province of embryology to study the mode of division of 

* 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 


the fertilized ovum and the progressive differentiation of the resulting 
cells to form the tissues, organs, and systems. But before consider- 
ing 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 coagu- 
lated 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 disin- 
tegration 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 indi- 
cate 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 felt- 
work, in the interstices of which is a more fluid material, the enchy- 
lema* 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 sub- 
jected 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. 


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 already stated that new cells arise by the division of 
preexisting ones, and this process is associated with a series of com- 
plicated 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 elon- 
gates and finally divides, each portion retaining its share of the radiat- 
ing 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 mean- 
time. 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 differ- 
ence 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 chro- 
mosomes (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 

The next stage, the metaphase (Fig. 3, A), consists of the division, 
usually longitudinally, of each chromosome, so that the cell now 

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 ap- 
proaching 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. 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 cyto- 
plasm 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 ami- 
totic 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 con- 
siderations. 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 

Second generation 

Somatic cells + germ cells 

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- 


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 germ- 
plasm. 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 repro- 
duction 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 pene- 
trate 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 con- 


E. B. Wilson: "The Cell in Development and Inheritance." Third edition. New 

York, 1900. 
O. Hertwig: "Die Zelle und die Gewebe." Jena, 1893. 







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 some- 
what 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 fila- 
ment. 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, 



which constitutes about four-fifths of the total length of the sper- 
matozoon 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. { 



Fig. 4. — Human Spermatozoon. 
1, Front view; 2, side view of the 
head; e, terminal filament; k, head; 
/, tail; m, middle-piece. — (After 

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 



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 

along each tubule in a wave-like manner and the appearances pre- 
sented at different points of the wave may be represented diagram- 
matically 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) 


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 consider- 
ably 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 sper- 
matogonia 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 prog- 
ress, 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 trans- 
formed 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 



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 there- 
fore pass into each secondary spermatocyte. These cells (Fig. 7, sc 2 ) 

Fig. 7. — Diagram Illustrating the Reduction of the Chromosomes During 

sc 1 , Spermatocyte of the first order; sc 2 , spermatocyte of the second order; sp, 


undergo division without the usual reconstruction of the nucleus and 
each of the dyads which they contain is halved, so that each sper- 
matid 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- 


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 obser- 
vations 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 chromo- 
somes divide just as do the ordinary ones, so that from each sperma- 
tocyte 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 


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 

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 repre- 
sent 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 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. 


filament grows out and soon projects beyond the limits of the cyto- 
plasm (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, 


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 sur- 
rounding 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 fur- 
nishes the material for the sheath of the axial filament, though 
this has been denied (Meves), the sheath being regarded as a differ- 
entiation 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 chromo- 
somes characteristic for the species. 


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 

The Ovum. — The human ovum is a spherical cell measuring 
about 0.2 mm. in diameter and is contained within a cavity situated 



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 


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. 


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 milli- 
meters 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 



prominence, only an exceedingly thin membrane separating the 
cavity of the follicle from the abdominal cavity. This thin mem- 
brane 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 matura- 
tion 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 de- 
velopment. Occasionally, how- 
ever, 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 preg- 
nancy; 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 Nine- 
teen Years of Age, Eight Days after 

d, Blood-clot; /, Graaffian follicle; th, 
theca. — (Kollmann.) 


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. — 

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 


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 con- 
nective tissue present and a corresponding diminution of the lutein 
cells, the corpus luteum gradually losing its yellow color and be- 
coming 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 dis- 
integrate, but, on the contrary, increase rapidly in number and be- 
come 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 


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 deriva- 
tion 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 differ- 
ences 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 con- 
tribute 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 con- 
nection 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 con- 
cerned in the production and maintenance of the modifications of 
the uterus necessary for the implantation and further development 
of the ovum. 


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 twenty- 
eight 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 degen- 
erate 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 re- 
generated 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 inter- 
menstrual stage, pregnancy not having occurred. If this be the 


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 

' 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 con- 
stituting 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 


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 



human ova, and, indeed, among mammals only with any approach 
to completeness in comparatively few forms (rat, mouse, guinea- 
pig, 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 


(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 (second- 
ary 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 

The similarity of the maturation phenomena to those of sper- 
matogenesis may be perceived trom the following diagram: 

n/"~N Spermato- 

( J cyte I 

cyte II 


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 


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 neces- 
sary 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 

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 struc- 
tures. 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 archo- 
plasm 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 archo- 
plasm sphere with its contained centrosome and is surrounded by 


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 con- 
taining 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 chromo- 
somes, 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 twenty- 
four 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 spermato- 
genesis 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 




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.) 



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 occasion- 
ally 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 com- 
pletion of this journey, even though the movement of the spermat- 
ozoon 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 


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 fertili- 
zation 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 spermat- 
ozoa 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 ma- 
jority of cases it is necessary to rely upon the date of the last men- 
struation 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 


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 approxi- 
mation, 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 super- 
fetation, 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 com- 
paratively small, and will become less and less evident as development 
proceeds, provided that the supply of nutrition to both embryos is equal. 


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," Wies- 
baden, 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 


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. 
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Ratte," Anat. Hefte, xlii, 1910. 
P. Strassmann: "Beitrage zur Lehre von der Ovulation, Menstruation und Concep- 
tion," Archiv fiir Gynaekol., lii, 1896. 
F. Villemin: "Le corps jaune considere comme glande a secretion interne," Paris, 

W. Waldeyer: "Eierstock und Ei," Leipzig, 1870. 



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 con- 
siderable 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 




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, 


D) shows a very decided difference in the size of the cells of the two 

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 segmenta- 
tion 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 



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 con- 
tinuing to form, the disk becomes divided into a large number of 
cells, those at the center being much smaller than those at the per- 
iphery. 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 cell- 


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 prac- 
tically 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 complete- 
ly 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 



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 blas- 
tula of the lower forms, but corresponds to a stage of reptilian devel- 
opment a little later than that shown in Fig. 20, since, as will be 
shown directly, the cells corresponding to the blastoderm and the 


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 flat- 
tened 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 com- 
parison, 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 



$ ^T?> ... 




'1. - 

v ■ ■ ■ , . 


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.) 


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 independ- 
ent parts during the early stages of development. That twins may be 
produced by this latter process has been abundantly shown by experi- 
mentation 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 super- 
numerary 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 


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 two- 
layered or diploblastic and later of a three-layered or triploblastic 

Just as the segmentation has been shown to be profoundly 
modified by the amount of yolk present in the ovum and by its sec- 
ondary reduction, so, too, the formation of the three primitive 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 

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 sur- 
face of the endoderm (en) on each 
side of the middle line as indicated 
in the transverse section shown in 
Fig. 24. This fold eventually be- 
comes completely constricted off 
from the endoderm and forms a 
hollow plate occupying the space between the ectoderm and endo- 
derm, 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 Meso- 
derms Pouches. 

Ch, Notochord; d, digestive cavity; 
ec, ectoderm; en, endoderm; m, medul- 
lary plate; mp, mesodermic pouch. — 



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 ecto- 
derm and endoderm (Fig. 26, mk l and mk 2 ),. evidently corresponding 
to the hollow plates formed in the same situations in Amphioxus. 


This is not, however, the only source of the mesoderm in the am- 
phibia, 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. — 

layers is known as the primitive streak. It is convenient to distin- 
guish 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 



blastoderm, the cells of which, in front of the groove, arrange them- 
selves 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 



Fig. 27. — Longitudinal Sections through Blastoderms of the Gecko, showing 

■ Gastrulation. 
ec, Ectoderm; en, secondary endoderm; en', primary endoderm; prm, prostomial meso- 
derm.— (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 



of the primary endoderm immediately below, undergo a degenera- 
tion, 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 envelop- 
ing 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 


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 



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 Endo- 
derm 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 


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 forma- 
tion 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 com- 
plete 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 proc- 
esses in man are closely similar. 



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, repre- 
sents a section, that portion of the middle layer which forms the 
roof of the amniotic cavity disappears, only the trophoblast per- 
sisting 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, 


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 prolifera- 
tion 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 


the two layers is visible. This stage may be regarded as compar- 
able 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 

■ — . 



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 



opens below into what will be the primitive digestive tract and is 
termed the neurenteric canal (Fig. t>Z, nc); it eventually closes com- 
pletely, 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 rec- 
ognize 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 



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 


Fig. 34. — Transverse Section through the Embryonic Disk of a Rabbit. 
ch, Chorda endoderm; ee, ectoderm; en, endoderm; gm, gastral mesoderm. — {After van 


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 


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 corre- 
sponds 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 princi- 
pally 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 remain- 
ing 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 


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 sense- 
organs, 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, 


a distinction which is undoubtedly convenient, though probably de- 
void of the fundamental importance which has been attributed to it 
by some embryologists. 


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. 


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. 



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 con- 
venient to consider the structure of the younger human ova at 
present known and to trace in them the appearance and develop- 
ment 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, 




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 



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 


<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 com- 
paring 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 


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 meso- 
dermal 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 chap- 
ter, 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 



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 amni- 
otic 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 excen- 
trically 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 


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 

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 



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). 


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 situ- 
ated, 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. 



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 


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 under- 
lying 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 

The Medullary Folds. — The two folds are continuous anteriorly, 
but behind they are at first separate, the anterior portion of the primi- 
tive 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 



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, Meas- 
uring 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 ecto- 
dermal layers forming the one uniting with the corresponding layers 
of the other (Fig. 46). By this process the medullary groove be- 
comes converted into a medullary canal which later becomes the 


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 


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 primi- 
tive streak, and was a plate of cells continuous at the sides with the 
primary endoderm and extending forward as far as what will eventu- 
ally 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 



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 curva- 
ture 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, be- 
tween 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 


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 extend- 
ing 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 meso- 
derm, 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 


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. — 

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 com- 
posed of mesodermic somites, lateral to this a narrow area under- 
lying 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 


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 inter- 
MEDIATE 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, 



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 • ; 


1 - -4 ; 

Fig. 50. — Transverse Section of an Embryo of 4.25 mm. at the Level of the Arm 

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 pro- 
nephros;^ 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 


cavity and only later becomes separated from it, and in lower verte- 
brates this continuity of the somite cavities with the general body- 
cavity 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 spindle- 
shaped 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 volun- 
tary 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 trans- 
formed into muscular tissue. 

The intermediate cell-mass in the human embryo, as in lower 
forms, partakes of the transverse divisions which separate the individ- 
ual 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 tend- 
ency 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 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 branchio- 
meres, 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 



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 prolon- 
gations into the limb buds which invested the axial core of mesen- 
chyme 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 


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 com- 
posed 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 verte- 
bra? 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 nutri- 
tion, and hence the primary arrangement of the blood-vessels is also 
metameric, a branch passing off on either side from the main longi- 
tudinal arteries and veins to each metamere. And, further, each 
pair of muscle segments receives its own nerves, so that the arrange- 
ment 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 myo- 
tomes. 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 noto- 
chordal region of the head of lower vertebrates mesodermic somites 
are present, while anteriorly in the prechordal region there are head- 
cavities 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 


adult the original metamerism is clearly indicated in the arrange- 
ment 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 pro- 
duced 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 compara- 
ble 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 under- 
stood in connection with a study of the cranial nerves. 


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 Develop- 
ment 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. 


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, 

K. W. Zimmermann: " Ueber Kopfhohlenrudimente beim Menschen," Archiv filr 

mikrosk. Anat., liii, 1898. 



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. Pri- 
marily this depends upon the deepening of the furrow which sur- 
rounds 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 belly- 
stalk is situated, and produces a constriction of the yolk-sac, the 
portion of this structure nearest the embryonic disk becoming en- 
closed 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 




and reduces the opening through which the yolk-stalk and belly- 
stalk communicate with the embryonic tissues to a small area known 
as the umbilicus. 

In the Kromer embryo Klb (Fig. 44) this separation of the em- 
bryo 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 

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 develop- 
ment 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- 


teriorly the medullary groove has become converted into a medul- 
lary canal by the medullary folds meeting over it and fusing, but 
anteriorly it is still open. The vertex bend is well marked and 



M^' L rX. j 



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 


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.) 


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 proxi- 
mal portion may now be spoken of as the yolk-stalk. The meso- 
dermic 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 develop- 
ment 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- 


9 1 

Fig. 55. — Floor of the Pharynx 
of Embryo B, 7 mm. Long. 
Ep, Epiglottis; Sp, sinus prsecervi- 
calis; t 1 , tuberculum impar; t 2 , 
posterior portions of the tongue; 
I, II, III, and IV, branchial arches. 

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 dif- 
ferent 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 pharyn- 
geal 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 meso- 
derm 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 


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 

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 pro- 
longed outward over these structures, enclosing them in a cylindrical 
investment, the first stage of what will later be the umbilical cord 
being thus established. 



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 



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. 


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 indica- 
tions 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. 



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, some- 
what 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 


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 develop- 
ment of these regions in embryo Wt is manifest, not only from the 



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 well- 
developed 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 occu- 
pied 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 


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 degen- 
eration, 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. 



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). 


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 com- 
munications 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 mean- 
time 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), 


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 accom- 
plished by a rotation of the limbs around axes passing through the 
shoulders and hip-joints, together with a rotation about their longi- 
tudinal 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 


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 secondary- 
position 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. 



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 twenty- 
one 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 


in mm. 

between coition 


and abortion 

About 0.15 


i3- J 4 






V. B. 8.8 




V. B. 14.0 




V. B. 25.0 




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 


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 new- 
born 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. 


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. 


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, 

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, 

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 Em- 
bryonen," 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, 

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. 


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. 



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 develop- 
ment 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 cer- 
tain 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 modi- 
fications of development in the human embryo result from the inheri- 
tance 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 under- 
standing 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 con- 
stricted off from the surface of the large yolk-mass, a fold, consisting 




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, correspond- 
ing to the chorion of the mammalian embryo. The space which 
occurs between the amnion and the serosa is a portion of the extra- 
embryonic ccelom and is continuous with the embryonic pleuro- 
peritoneal cavity. 

In the ovum of the chick, as in that of the reptile, the proto- 
plasmic material is limited to one pole and rests upon the large yolk- 


mass. As development proceeds the germ layers gradually extend 
around the yolk-mass and eventually completely enclose it, the yolk- 
mass 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 yolk- 
sac 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 pleuroperi- 
toneal 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 excre- 
tory 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 dried- 
up 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 well- 
developed amnion occurs, arising, however, in a very different man- 
ner 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 


forms are greatly abbreviated in the human embryo. The envelop- 
ing 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 cell- 
mass, 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 



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 


of pregnancy — it varies in amount between one-half and three- 
fourths 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 develop- 
ment 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 dis- 
tance 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 


10 mm. in its longest diameter. The stalk, however, early under- 
goes 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 con- 
nection 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 differ- 
ences 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 



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 proc- 
ess 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 ex- 
tension 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 Sec- 

of an Embryo of 2.15 mm. 

Aa, Umbilical (allantoic) 
artery; All, allantois; am, am- 
nion; Va, umbilical (allantoic) 
vein. — (His.) 


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. 


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 em- 
bryonic 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 sub- 
sequent 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 con- 
tinuing, 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 com- 
pletely covered by embryonic ectoderm, the amnion being carried 






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. 


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 body- 
cavity 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 re- 
ceived 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 


II 9 

the wall of the uterus. Nothing is known as to how this implanta- 
tion 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 syn- 
cytial 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 




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. 


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- 



^,^^f^>r%^^ c y 




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 cap- 
sularis 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- 


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 imme- 
diate vicinity of the ovum were much swollen and apparently some- 
what 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, there- 
fore, of embryonic origin. If examined under a higher magnifica- 
tion 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 differen- 
tiated 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 plasmodi- 
trophoblast 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 



be termed the embryotroph and the type of placentation which obtains 
up to this stage and for some time longer may be termed the embryo- 
trophic 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 



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 estab- 
lished, 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 differenti- 
ated 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. 



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.) 


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, branch- 
ing 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. 


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 ad- 
vancing growth, the layer of Langhans cells disappearing except in 
small areas scattered irregularly in the villi, and the syncytium, 



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. 



Fig. 75. — Section through the Placental Chorion of an Embryo of Seven 

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 


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 connec- 
tion 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 appre- 
ciable 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 con- 
siderable portion of its thickness is expelled from the uterus, forming 
what is termed the decidua. In other words, the sloughing of the 



uterine tissue which concludes the process of menstruation is post- 
poned until the close of pregnancy, and then takes place simultane- 
ously 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 



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 occur- 
ring 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 pre- 
sents a corrugated appearance and is traversed 

Fig. 78. — Diagrammatic Sections of the Uterine Mucosa, A, in the Non- 
pregnant 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 


In sections the mucosa is found to have become greatly thick- 
ened, 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 

In addition to these changes, the epithelium of the mucosa disap- 
pears 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 

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 com- 
pressed 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 


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, 


but persists until birth, forming a part of the structure termed the 

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 embry- 
onic 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 struc- 
ture, convex on the surface next the uterine muscularis and concave 
on that turned toward the embryo, the umbilical cord being continu- 
ous 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 develop- 
ment 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 com- 
posed 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 



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.) 


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. 




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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 separa- 
tion 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 preg- 
nancy it is also of the indeciduate type. In the sheep the placentation is 


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 con- 
nective 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 approxi- 
mating 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 


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, 

L. Frassi: "Ueber ein junges menschliches Ei in situ," Arch, fiir mikr. Anal., lxx, 

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. 


F. Marchand: "Beobachtungen an jungen menschlichen Eiern," Anat.Hefte, xxi, 

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. 



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 mesen- 
chymatous 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 assum- 
ing 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 




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, 


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 under- 
lying the epidermis and showing no indications of a segmental 
arrangement. It becomes converted "principally into fibrous con- 
nective 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 


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 unseg- 
mental 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 con- 
siderable 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 (Sher- 
rington). 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 (Sherring- 
ton), 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). 


'Ts 7i\ 











Fig. 82. — Diagram 
showing the cutane- 
OUS Distribution of the 
Spinal Nerves— (Head.) 



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 Index- 
Finger 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 






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 rudi- 
mentary 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 de- 
velop 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 epi- 
trichium, 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. — Longi- 
tudinal 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.) 



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 ... 


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 


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 addi- 
tion to its basal portion of new keratinized cells, probably produced 
by the multiplication of the epidermal cells which envelop the 

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 per- 
ipheral 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 mesen- 
chyme 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, 




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 

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 sudor- 
iparous glands arise during the fifth month as solid cylindrical out- 
growths from the primary ridges 
of the epidermis (Fig. 87), and 
at first project vertically down- 
ward 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 sur- 
face, 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, ar- 
ranged 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 abdomi- 
nal 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 De- 
tached Portion of Epidermis from 
the Dorsum of the Hand. 
h, Hair follicle; s, sudoriparous gland. — 



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 appar- 
ently 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 ob- 
served is one in which they are 
represented by a circular thick- 
ening 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 corni- 
fied and are cast off, so that the 
gland area appears as a depres- 
sion 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 con- 
verted 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 col- 
umns 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.) 


the extremities of the lacteal ducts. In the areola, as the area sur- 
rounding the nipple is termed, other glands known as Montgomery' 's 
glands, also appear, their development resembling that of the mam- 
mary gland so closely as to render it probable that they are really 
rudimentary mammary glands. 



" 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 addi- 
tional 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. 


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. 


J. T. Bowen: "The Epitrichial Layer of the Human Epidermis," Anat. Anzeiger, rv 

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. 


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, 

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. 



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 com- 
pact 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 



represent the cells themselves. Fibrils and material out of which 
fibrils could be formed have undoubtedly been observed in connec- 
tive-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 




■a fmm0m^& 

■ 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 forma- 
tion of spicules extends further out into the connective-tissue mem- 
brane, radiating in all directions from the region in which it first 



developed. Later the connective tissue which lies upon either sur- 
face 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 carti- 
lage the process is somewhat dif- 
ferent. 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 calcifica- 
tion 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 mesen- 
chyme surrounding the cartilage 
has become converted into a 
periosteum (po), similar to that of membrane bone, and its osteo- 
blasts 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 



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.) 





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 calcifica- 
tion of the cartilage matrix has been extending, and as fast as the 

network of calcified trabeculse is 
formed it is invaded by the mesen- 
chyme, 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 in- 
stance, develops by several cen- 
ters, 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 ques- 
tion 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 peri- 
osteal tissue; po, periosteum. — (Szymo- 



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 pro- 
vision 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 ossifica- 
tion 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 com- 
pletely 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 peri- 
osteal 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 Ossi- 
fication Centers of 
the Femur. 

a, and b, Secondary 
centers for the great and 
lesser trochanters; d, 
diaphysis; ep, upper and 
ep', lower epiphysis. — 


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 


canaliculi, so characteristic of bone tissue. In the growth of peri- 
osteal bone not only do osteoblasts become enclosed, but blood- 
vessels also, the Haversian canals being formed in this way, and 
around these lamellae of bone are deposited by the enclosed osteo- 
blasts 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 



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 con- 
sidered, the skeleton is composed of two portions, the axial skeleton, 
consisting of the skull, the vertebrae, ribs, and sternum, developing 

fin* ' \m. : 





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 appen- 
dicular 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. 



The Development of the Vertebrae and Ribs. — The mesen- 
chyme 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 sclero- 
tome 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 por- 
tion 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 pro- 
longation dorsally between the 
myotome and the spinal cord to 
form the vertebral arch, and a 
third process laterally and ven- 
trally 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 sur- 
round 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 noto- 
chord, 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.) 


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 chondrifica- 
tion appear, one on each side of the median line, and these eventually 
unite to form a single cartilaginous body, the chondrification prob- 
ably 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 hypo- 
chordal 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 develop- 
ment will be described later. As development proceeds the cartil- 
ages 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 mesen- 
chyme 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 


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 be- 
comes 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 dis- 
appears 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 well- 
developed 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 epistro- 
pheus 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 cer- 
vical region they are short, the upper five or six being no longer than 
the transverse processes, with the tips of which their extremities 



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 

■ \;-^Z ... --■ 

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 



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 trans- 
verse 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 embry- 
onic 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 carti- 
lage which still covers the anterior and posterior surfaces of the 
vertebral body, producing disks of bone in these situations (Fig. 99, 


l6 5 

B, en and el), another appears at the tip of each spinous and trans- 
verse 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 extend- 
ing into them from the vertebral arch centers, and a similar suppres- 
sion 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 termi- 
nal 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 


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. Occasion- 
ally 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 supraster- 
nal. The fusion of the longitudinal bars gradually extends back- 
ward 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, 


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 seg- 
ments, 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. 



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. — 

Fig. 103. — Reconstruction of the Chondro- 
cranium of an embryo of 14 mm. 
as, Alisphenoid; bo, basioccipital; bs, basi- 
sphenoid; 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 rudi- 
mentary 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 medul- 
lary canal which will later become the cerebral part of the central 
nervous system. No indications of a segmental origin are to be 


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 mem- 
brane 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 condensa- 
tions 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 mesenchy- 
mal 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 fora- 
mina 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 



anatomy that the human skull is composed of a primitive unseg- 
mental chondrocranium plus four vertebrae, the latter being added 

to and incorporated with the occip- 
ital portion of the chondrocranium. 
Emphasis must be laid upon 
the fact that the cartilaginous por- 
tion 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 mesen- 
chyme in these regions being con- 
verted into fibrous connective tissue, 
which, especially in the cranial re- 
gion, assumes the form of a dense 

But in addition to the chondro- 
cranium and the vertebras incorpo- 
rated with it, other cartilaginous ele- 
ments enter into the composition of 
the skull. The mesenchyme which 
occupies the axis of each branchial 
arch undergoes more or less com- 
plete chondrification, cartilaginous 
bars being so formed, certain of 
which enter into very close rela- 
tions 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.) 



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 dia- 
grammatically in Fig. 105. 

By the ossification of these 
various parts three categories of 
bones are formed: (1) cartilage 
bones formed in the chondro- 
cranium, (2) membrane bones, 
and (3) cartilage bones devel- 
oping from the cartilages of the 
branchial arches. The bones 
belonging to each of these cate- 
gories are primarily quite distinct from one another and from 
those of the other groups, but in the human skull a very consid- 
erable 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 event- 
ually disappears completely, so that the adult bone, although repre- 
sented 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 connec- 
tive 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. 



studies. Nevertheless it seems advisable to retain the classification, 
indicating, where necessary, the confusion of bones of the various 

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 por- 
tion above the foramen produces 
a supraoccipital (so), which repre- 
sents 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, in- 
terparietal; so, supraoccipital. 


cranium, and even the cartilaginous partitions which divide the 
hypoglossal foramina usually disappear during the process of 

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, how- 
ever, 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 

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 begin- 
ning 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) 


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 orbito- 
sphenoids, 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 mesen- 
chyme 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 rela- 
tively 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 



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 verti- 
cal 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 por- 
tions 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.) 



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 corre- 
spond to the third and fourth bones of the typical mammalian arrange- 
ment. Furthermore, at the upper portion of the nasal wall, in front of 

the superior concha, a slight elevation, 
termed the agger nasi, is always observa- 
ble, its lower edge being prolonged down- 
ward to form what is termed the uncinate 
process of the ethmoid. This process 
and the agger together represent the up- 
permost concha of the typical arrange- 
ment, to which, therefore, the human 
arrangement may be reduced. 

A number of centers of ossifica- 
tion — the exact number is yet uncer- 
tain — 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 ossi- 
fication 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.) 



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 




Ethmoidal . 








Parts of Adult Skull; 

Basilar process. 

Squamous portion below superior nuchal 


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 form- 
ing the sides and roof of the skull in the second stage of its develop- 


ment ossifications appear, which give rise, in addition to the inter- 
parietal 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. 







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, 



Fig. i 10. — Diagram of the Ossi- 
fications of which the Maxilla 
is Composed, as seen from the 
Outer Surface. The Arrow 
Passes through the Infraor- 
bital 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 con- 
tains 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 por- 
tions 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 per- 
manently 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 



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 

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 


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 mandibu- 
lar 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), 



and ossification extends into these from the ramus to form the 
coronoid and condyloid processes. And, finally, two other inde- 
pendent 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, Ali- 
sphenoid; 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. 


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 

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 struc- 
tures 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 bran- 
chial arches may be stated in tabular form as follows, the parts repre- 
sented by cartilage which becomes replaced by membrane bone 
being printed in italics, while membrane bones which have no 
cartilaginous representatives are enclosed in brackets: 


(Palatine) . 



Spheno-mandibular ligament. 


1st arch. 


(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 sclero- 
tomes 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 

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 histologic- 
ally, 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 


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 Cen- 
ters of the Scapula. 
a, b, and c, Secondary centers for 
the angle, vertebral border, and acro- 
mion; co, center for the coracoid proc- 
ess. — (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 epi- 
physial 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, 


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, represent- 
ing 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 rela- 
tion 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 



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 re- 
mains 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, sym- 
physis, and ischial tuberosity; il, ilium; 
is, ischium; p, pubis. — (Testut.) 


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: 










f Tibiale 

\ Intermedium 







Sesamoid cartilage 


— — 


Fuses with navicular 



Carpale I 

Gr. multangular 

1 st Cuneiform 

Tarsale I 

Carpale II 

Less, multangular 

2d Cuneiform 

Tarsale II 

Carpale III 


3d Cuneiform 

Tarsale III 

Carpale IV 1 
Carpale V J 



( Tarsale TV 
I Tarsale V 


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 mesen- 
chyme 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 


sternoclavicular, the whole of the central portions of the inter- 
mediate 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 

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, 


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. 


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Anat., iv, 1905. 
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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 Car- 
tilages," 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. 


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 Primordialcran- 
iums 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. 


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. 


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 lym- 
phatics, 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 con- 
version 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 neighbor- 
ing 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 



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 

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.) 


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 mus- 
cle cells into more or less dis- 
tinct 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 

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 neighbor- 
ing cells fuse and, eventually, 
there is formed a continuous 
mass of protoplasm or syncytium in which all traces of cell bounda- 
ries 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 Heart- 
wall of a Duck Embryo of Three Days. 
— (M. Heidenhain.) 



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 (com- 
pare 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 sarco- 
plasm as it may now be termed, except immediately around the nuclei 
where, even in the adult, a certain amount of undifferentiated sarco- 
plasm 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. 

A, Central nucleus; B, new peripheral nucleus. — (Macallum.) 

geneous, but later they become differentiated into two distinct sub- 
stances 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. 


The Histogenesis of Striated Muscle Tissue.— The histo- 
genesis 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 sarco- 
plasm (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 regu- 
larly in rows and later migrating from their original central position 
to the periphery of the fibers, and, in the limb muscles, the develop- 
ment 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 vacuo- 
lated 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 divi- 
sion 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 epithe- 
lial arrangement and are indistinguishable from the somatic mesen- 
chyme which forms the axial cores of the limbs. 

The various fibers of each myotome are at first loosely arranged, 


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 obser- 
vation, 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, becom- 
ing 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 sterno- 
hyoid 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. 


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 

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 nor- 
mally 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 con- 
nective tissue compared with the amount of muscular tissue in the 
former is very striking, the inference being that these connective- 
tissue 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 


fibular lateral ligament of the knee-joint, for instance, being probably 
a degenerated portion of the peroneus longus, while the sacro- 
tuberous 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 com- 
paratively 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 con- 
sider in detail the history of each one of the skeletal muscles of the 
human body, but a statement of the general plan of their develop- 
ment 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 

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 


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 per- 
ipherally it becomes continuous with the connective tissue of the 

Fig. 120. — Embryo of 13 mm. showing the Formation of the Rectus Muscle.— 


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 


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 trans- 
versa thoracis, while in the neck these portions of some of the myo- 
tomes 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 inter- 
costals 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. 















































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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 


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 sup- 
plied by the hypoglossal nerve are also derived from myotomes, but 
they have already been considered in connection with the trunk 

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 


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 



Fig. 122. — Head of Embryos (.4) of Two Months and (B) of Three 
Months showing the Extension of the Seventh Nerve upon the Face. — 


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 

The cartilages of the fourth and fifth arches enter into the forma- 
tion of the larynx and the muscles of the corresponding branchio- 
meres constitute the muscles of the larynx, together with the remain- 
ing 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 chon- 
droglossus, belong to the fourth or fifth branchiomere, although 
the remaining muscles of this physiological set are myotomic in 

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. 






tors of 
(in part). 
Levator veli 

Muscles of 
the larynx. 








of pharynx 

(in part). 








Muscles of 



a) 3 

h-1 M 


. Masseter. 






Tensor veli 






O u 

•A <u 

Medial _ 








% I 








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 arrange- 
ment of the muscles of the higher mammalian limbs to be the result 
of a long series of progressive modifications from a primitive condi- 
tion. 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 

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 arrange- 
ment of the cutaneous nerves implies a primary metamerism of the 



dermis (see p. 143). It may mean only that the nerves, being seg- 
mental, 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, corre- 
sponding to each band upon the ventral (praeaxial) surface of the 
limb bud, there is a band similarly innervated upon the dorsal (post- 
axial) surface, since the fibers which pass to the limb from each nerve 
root sooner or later arrange themselves in praeaxial and postaxial 



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 conse- 
quence of the rotation in opposite directions which the limbs undergo 
during development (see p. 101). 



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 


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 



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 

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 


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 recog- 
nized 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 verte- 
brates, 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. 


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 

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 super- 
posed 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 



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 



the superficial layer of the palmar muscles arise, and they form with 
these the flexor digitorum sublimis. The deeper layers of the anti- 
brachial 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 correspond- 
ing 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 


of the crural flexors becomes the gastrocnemius and plantaris (Fig. 
128, A) and the deepest layer becomes the popliteus and the inter- 
osseous 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 adduc- 
tor hallucis, while the fourth and fifth layers together form the ioter- 
ossei, as in the hand, the flexor quinti digiti brevis really belonging 
to that group of muscles. 


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. 


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, 

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. 



At present nothing is known as to the earliest stages of develop- 
ment 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 

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 blood- 
vessels 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 splanch- 
nic 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 



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 sub- 

Fig. 129. — Transverse Section through the Area Vasculosa of Rabbit 
Embryos showing the Transformation of Mesoderm cells into the Vascular 

Ec, Ectoderm; En, endoderm; Me, mesoderm. — {van der Stricht.) 

At the periphery of the vascular area the vessels arrange them- 
selves 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 embry- 
onic 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 vasculariza- 
tion reaches the embryo some of the canals begin to enlarge (Fig. 




131,-4), producing arteries and veins, the rest of the network forming 
capillaries uniting these two sets of vessels, and, this process continu- 
ing, 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 termin- 
alis such as occurs in the rabbit is 
visible. Otherwise the conditions 
are probably similar to what has 
been described above, the first cir- 
culation developed being associated 
with the yolk-sac. 

It is to be noted that the capil- 
lary network of the area vasculosa 
consists of relatively wide anasto- 
mosing spaces whose endothelial 
lining rests directly upon the sub- 
stance islands (Fig. 130). In cer- 
tain of the embryonic organs, not- 
ably the liver, the metanephros 
and the heart, the network has a 
similar character, consisting of wide 
anastomosing spaces bounded by 
an endothelium which rests di- 
rectly, or almost so, upon the par- 
enchyma 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 expan- 
sion 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, Blood- 
island; Si, substance-island. — (Disse.) 



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 mesen- 
chyme 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 


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 

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. 


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 accumula- 
tion 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 dis- 
integration 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 multi- 
plication 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 



of the organs (Fig. 132). After birth the haematopoietic function of 
the liver ceases and that of the spleen becomes limited to the forma- 
tion 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 haemoglo- 
bin in their cytoplasm, their 
nuclei at the same time be- 
coming granular. Up to a 
stage at which the embryo has 
a length of about 12 mm. these 
are the only form of red cor- 
puscle in the circulation, but 
at this time (Minot) a new 
form, characterized by its 
smaller size and more deeply 
staining nucleus, makes its ap- 
pearance. These erythrocytes 
have been termed normoblasts 
(Ehrlich), although they are 
merely transition stages lead- 
ing to the formation of erythro- 
plastids 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 blood- 

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 


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 leuko- 
cytes 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 myelo- 
cytes. 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 coarse- 
ness and staining reactions of their cytoplasmic granules and by 
their reniform or constricted nucleus. They are probably deriva- 
tives of the finely granular type and it has been maintained by 
Weidenreich that their granules have been acquired by the phago- 
cytosis of degenerated erythrocytes. Finally, a third type is formed 
by the polymorphonuclear or polynuclear leukocytes (basophile cells 



of Ehrlich) (Fig. 134, d), which are to be regarded as leukocytes in 
the process of degeneration and are characterized by their irregu- 
larly lobed or fragmented nuclei, as well as by their staining 

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 mega- 
caryocytes appear to be phagocytic cells, having as their function the 
destruction of degenerated corpuscles and of the nuclei of the 


The blood-platelets have recently been shown by Wright to be 
formed from the cytoplasm of the megacaryocytes, by the constric- 
tion 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 con- 
tact in the median line. On each side of the body near the margins 
of the embryonic area a fold of the splanchnopleure appears, pro- 
jecting into the ccelomic cavity, and within this fold a very thin- 
walled 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 



cervical region of the body. At an early stage the various veins 
which have already been formed, the vitellines, umbilicals, jugulars 


Fig. 136. — Diagrams Illustrating the Formation or the Heart in the 

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 

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 



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, dia- 
phragm; dc, ductus Cuvieri; /, liver; 
v, ventricle; vj, jugular vein; vu, um- 
bilical vein. — (His.) 

Fig. 138. — Heart of Embryo of 
4.2 mm., seen from the Dorsal 

DC, Ductus Cuvieri; I A , left atrium 
rA, right atrium; vf, jugular vein; VI, 
left ventricle; vu, umbilical vein. — 

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 



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 enlarge- 
ment 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 trans- 
versely until it has assumed the 
form of a crescent whose convex- 
ity 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). 



permanent right and left atria begins with the formation of a falci- 
form 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. 



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 open- 
ing 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 independ- 
ently 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 


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. 



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 


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 separa- 
tion 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 



right side into two, flaps which project downward into the ventricular 
cavity, and an additional flap arises on each side from the lower 




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 

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 


under surfaces of these flaps the loosely arranged muscular tra- 
becular 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 repre- 
sent 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 


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 under- 
going 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 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 rela- 
tive 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 prim- 
itive 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 


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 fora- 
men 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 extra- 
embryonic splanchnic mesoderm surrounding the yolk-sac and ex- 
tends 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 



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. 



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 under- 
stood from the diagram (Fig. 
147). This arrangement repre- 
sents a condition which is per- 
manent 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 arte- 
rialized and is then collected into 
the dorsal aortae, which distrib- 
ute it to the body. But in terres- 
trial 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 con- 
tinuous, the condition repre- 
sented 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 Bran- 
chial Arch Vessels. 

a, aorta; db, aortic bulb; ec, external 
carotid; ic, internal carotid; sc, subclavian; 
I-VI, branchial arch vessels. 


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 por- 
tion 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 dis- 
appears, while the corresponding portion of the left side persists 



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 ex- 
plained 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 prox- 
imal part of the right sub- 
clavian artery, the portion of 
the ventral trunk which unites 
the arch with the aortic bulb 
becoming the innominate 

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 hypo- 
glossal 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 ar- 
tery; sc, subclavian; I- VI, aortic arch vessels. 



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, rep- 
resent the intercostal arteries, 
and still lower four pairs of lum- 
bar 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 sa- 
cral artery, which represents in 
a degenerated form the caudal 
prolongation of the aorta of 
other mammals, and, like this, 
gives off lateral branches corre- 
sponding 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 equiva- 
lent to the left, since it represents the fourth branchial vessel + a portion of the dorsal 
longitudinal trunk + the lateral segmental branch (see Fig. 142). 



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. Simi- 
larly 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 



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 ap- 
pears to be the lateral somatic 
branch of the fifth lumbar seg- 
ment, whereupon the proximal 
part of the original umbilical 
vessel degenerates and the um- 
bilical 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 in- 
ternal pudendal, which are 
distributed to the body walls 

or their derivatives, and to others, such as the vesical, inferior haemor- 
rhoidal 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, exter- 
nal iliac; G, gluteal; III, internal iliac; IP, 
internal pudic; IV, inferior vesical; Sc, scia- 
tic; U, umbilical; U', primary proximal por- 
tion of the umbilical; wd, Wolffian duct. 


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 back- 
wardly 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. 



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 connec- 
tion with the dorsal aorta, and, sending off anteriorly and posteriorly 


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, 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 



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 sub- 
clavian 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 


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 arrange- 
ment 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 prin- 
cipal 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 



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- 


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 


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 branch- 
ing 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 accom- 
panies the long saphenous nerve down the inner side of the leg, and, 



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 con- 
tinued 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- 


i ,° 











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 ante- 
rior 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 


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 hem- 
isphere unite with their fellows of the opposite side to form a longitu- 
dinal trunk, the superior sagittal sinus, lying between the two cere- 
bral 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 



drainage of the sinus. The connections which join the veins to the 
sinus become the proximal portion of the transverse sinus, the poste- 
rior 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 



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 seg- 
ments 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 


2 5 8 


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 


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 con- 
dition 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- 



sponding umbilical veins. These are represented in the belly- 
stalk 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 












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 down- 
ward 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-connec- 
tions have developed between the two vitelline veins, two of which 
pass ventral and the other dorsal to the intestine, so that the latter is 



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 

Vcd and Vcs, Right and left superior venae cavae; Vj, jugular vein;, 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 abdom- 
inal wall in which the blood now flows from above downward. The 



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 mesen- 
tery 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, hemi- 
azygos; 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 


mesenteric artery, and at the same point a cross-branch between the 
two subcardinals also develops. The portion of the right subcardi- 
al 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 degen- 
erates, 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 per- 
sist, 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 inter- 
venes 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 cross- 
connection, 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). 


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 supra- 
cardinal; 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 

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 per- 
sisting upper portion of the right posterior cardinal, while the cross- 
connection between the hemiazygos and azygos represents one of the 
originally numerous cross-connections between the supracardinals. 

The ascending lumbar veins, frequently described as the commence- 
ments of the azygos veins, are in reality secondary formations developed 
by the anastomoses of anteriorly and posteriorly directed branches of the 
lumbar veins, 


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 prox- 
imally 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 limb- 
bud 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 pri- 
mary 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 

The Pulmonary Veins. — The development of the pulmonary veins 



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 

nutrition of the embryo depends, the course of the blood is neces- 
sarily somewhat different from what obtains in the child after birth. 
Taking the placenta as the starting-point, the blood passes along the 


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 pass- 
ing 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 abdom- 
inal viscera, the greater part of it, however, passing off in the hypo- 
gastric 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 


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 distri- 
buted 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 pul- 
monary 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 mesen- 
chymal 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 



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 


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 mesen- 
teric 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 

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 espe- 
cially 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. 



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 com- 
municating with it, and the fact that some of these showed connec- 
tion 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 



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 con- 
densation 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 lym- 
phoid tissue forming around each of the branches, so that a series 
of follicles are formed, which, together with the surrounding lym- 
phatic 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.) 



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, con- 
stitute 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 



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 develop- 
ment 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 vasculariza- 
tion 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 



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 multipli- 
cation 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. 

" ' . . - 


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, connec- 
tive-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 


work on its development (Stoerk) tends, however, to disprove this 
view, and the ganglion seems accordingly to find its place among 
the lymphoid organs. 


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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. 



The greatest portion of the digestive tract is formed by the con- 
striction 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 com- 
municating 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 appear- 
ance 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 





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 cloa- 
cal membrane, the cloaca be- 
comes divided into a ventral por- 
tion, 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 rup- 
ture, however, does not occur un- 
til a comparatively late period of 
development, until after the em- 
bryo 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 sur- 
rounding ectoderm, more especially anteriorly where it forms the 
large genital tubercle (see Chapter XIII), and posteriorly where it pro- 
duces the anal tubercle. This appears as a rounded elevation on 
each side of the median line, immediately behind the cloacal mem- 
brane and separated from the root of the caudal projection by a de- 
pression, 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 


ab. Aortic bulb; h, heart; 0, auditory cap- 
sule; op, optic evagination;/>?w, pharyngeal 
membrane. — {His.) 



forward and eventually meet in front of the original anal orifice. 
From the mesoderm of the circular elevation thus produced the ex- 
ternal 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 posi- 
tion of the original cloacal membrane. 


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;, 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 differentia- 
tion of the tract into its various regions and the formation of the 
various organs found in relation with these may now be considered. 


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 maxillo- 
palatine 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 por- 
tion 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 pro- 
longation 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 



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 mem- 
brane, 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 Lip- 
groove 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 epi- 
thelium 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 


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 pharyn- 
geal 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 thick- 
enings 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 devel- 
oping beneath these, twelve additional tooth-germs are formed. 
These represent the permanent molars; their formation is much 



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 '- 



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. — 

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 tooth- 
germs 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- 


chymatous. Each of these gives rise to a definite portion of the 
fully formed tooth, the enamel organ, as its name indicates, produc- 
ing 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 sur- 
face 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 begin- 
ning 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 connec- 
tive 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 den- 
tine 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 com- 
plete investment for the dental papilla (Fig. 173), but as the ossifi- 
cation 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 



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 



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 pro- 
duces pressure upon the roots of the primary teeth. These roots 
then undergo partial absorption, and the teeth are thus loosened 


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 con- 
siderable 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. 


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 func- 
tional. 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 



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 








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 pos- 
teriorly are separated from the 
second arch by a groove which la- 
ter 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.) 


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 chondro- 
glossi. 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 develop- 
ment 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 mem- 
brane, the sensory distribution of the ninth nerve extending forward upon 
the posterior part of the anterior portion of the tongue, while a consider- 
able 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. 



has separated from the epithelium of the floor of the cheek groove 
except at its anterior end and has become embedded in the connec- 
tive 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 prob- 
ably 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 


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 

As these representatives of the various glands increase in length, 



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 ap- 
pears 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 


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 develop- 
ment 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 epi- 
thelium 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 pro- 
duce the tongue there is to be found in early stages a pair of thick- 
enings 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 forma- 
tion 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 Eusta- 
chian tubes, a collection cf lymphatic tissue takes place beneath 
the mucous membrane, forming the pharyngeal tonsil, and imme- 
diately 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. 



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 bran- 
chial 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 eventu- 
ally 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 mes- 
enchyme 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 fur- 
rows. Five pairs of these structures are developed and, in addition, 
there is a single unpaired median body. This last makes its appear- 
ance 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 im- 
pair. — (His.) 



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 depres- 
sion. As it deepens, its extremity becomes bilobed, and after the 
embryo has reached a length of 6 mm. it becomes completely sepa- 
rated 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 



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 thy- 
reoids, 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 thy- 

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 down- 
ward ventrally to the thyreoid and separating from their points of ori- 
gin, 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 

pthm 111 and pthm IV, Para thyreoids; 
sd, thyreoid; thm III, thymus; thm 7T", 
lateral thyreoid. — (Groschuff.) 



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 con- 
centric 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 


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 com- 
pletely disappear in later stages of development, but occasionally 


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 for- 
mation 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 fur- 
nished with voluntary muscles, the principal of which are the con- 
strictors 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 origin- 
ally 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 



respiration; the development of this may, however, be conveniently- 
postponed 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 consider- 
able 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 non- 
striated 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 


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 en- 
largement 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 recog- 
nizable until advanced stages of development and even in the adult 
(Mall). The first of these is at first indistinguishable from the 



pyloric portion of the stomach and can be recognized as the duo- 
denum 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 

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. — 


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 


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 trans- 
versely 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 



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 pos- 
session 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 


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, how- 
ever, 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 in- 
crease in size until it forms a conical pouch 
arising from the colon just where it is joined 
by the small intestine (Fig. 186). The en- 
largement of its terminal portion does not keep 
pace, however, with that of the portion near- 
est the intestine, but it becomes gradually 
more and more marked off from it by its lesser 
caliber and gives rise to the vermiform ap- 
pendix. 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 funnel- 
shaped, but later the proximal part, continuing to increase in diam- 
eter 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 


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. 


The Development of the Liver. — The liver makes its appear- 
ance 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 



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 


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 

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 sub- 
stance 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 



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 develop- 
ment 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 erythro- 
cytes, the liver having assumed by this time its haematopoietic func- 
tion (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 cylin- 
ders 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 cylin- 
ders which lie in close relation to branches of the portal vein (Fig. 191) , 



and thence extend throughout the neighboring cylinders, anastomos- 
ing with capillaries developing in relation to neighboring portal 
branches. As the extension so proceeds the older capillaries con- 
tinue 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 


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 hypo- 
chondrium. 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 



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 rep- 
resent the original dorsal out- 
growth, while the right ventral 
pancreas becomes the head. 

Both the dorsal and ventral 
outgrowths early become lobed, 
and the lobes becoming second- 
arily lobed and this lobation re- 
peating 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 prin- 
cipal 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. 


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 pancre- 
atic 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, blood- 
vessels penetrate. 


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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. 



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 intes- 
tine 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 ven- 
tral 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 




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 yolk- 
stalk with the trunk-cavity by pas- 
sages 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 so- 
matic mesoderm which forms the lat- 
eral 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 be- 
ginning of the sinus venosus, with the result that the ventral re- 
cesses become closed below and their continuity with the trunk- 
cavity 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. 



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 communi- 
cation; Bp, ventral parietal re- 
cess; Om, vitelline vein; Pc, peri- 
cardial cavity; Rca, dorsal pari- 
etal recess; Sv, sinus venosus; V, 
ventricle. — (His.) 



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 



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 trans- 
versum is at first almost horizontal, but later it becomes decidedly 
oblique in position, a change associated with the backward move- 
ment 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 posi- 
tion by its attachment to the wall of the digestive tract and the 
ductus Cuvieri. The anterior surface of the septum thus comes to 


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 open- 
ing of the dorsal recesses into the parietal cavity. In later stages 


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 ° 


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 des- 
tined 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). 


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 por- 
tion 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 be- 
comes 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 

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 con- 
tinued 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 


layer of the septum has reached the level of the fourth cervical seg- 
ment, 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 peri- 
toneal cavities communicate, begin to approach the dorsal wall of 
the body, with which they finally unite and so complete the separa- 
tion of the cavities. The pleural cavities continue to enlarge after 
their separation and, extending laterally, pass between the peri- 
cardium 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 tho- 
racic segment is reached the separation of the pleural and peritoneal 


3 2 3 


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 the- 
yolk-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 in- 
testine, 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 pos- 
terior surface of the diaphragm, and also to the ventral mesentery. 
When the separation is taking place, the rotation of the stomach al- 
ready 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 sur- 
faces 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.) 


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 repre- 
sents 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 com- 
mon epiploic foramen into the general peritoneal cavity. The rota- 
tion 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 


3 2 5 

lung, when this is present, and hence has been termed the infracardial 

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 at- 
tached 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 trans- 
verse 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 orig- 
inal condition is greatly modified. One of the earliest of these fusions 
takes place at the apex of the funnel, where the portion of the mesen- 
tery which passes to the tranverse colon and arches over the duo- 
denum 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 Vis- 
ceral Branches of the Ab- 
dominal Aorta in an Em- 



way the attachment of the transverse mesocolon takes the form of a 
transverse line instead of a point, and this portion of the mesentery- 
divides 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 duo- 
denum. 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 


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 retroperito- 
neal position which characterizes it in the adult. 

The descending colon, which on account of the width of its mes- 
entery 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 extend- 
ing outward, takes place, the fused layers becoming converted into 


3 2 7 

connective tissue, and this portion of the colon thus loses its mesen- 
tery 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 one- 
third 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 



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 re- 
sults (Fig. 201 B), a condition which brings it about that the omen- 
tum seems to be attached to the transverse colon and that the pan- 
creas seems to lie in the line of attachment of the transverse meso- 
colon. 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. 


By these various changes the line of attachment of the mesen- 
tery to the dorsal wall of the body has become somewhat compli- 
cated and has departed to a very considerable extent from its origi- 
nal 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 fol- 
lows: 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 prolonga- 
tion, 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. 


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. 


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. 



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 ex- 
tending almost the entire length of that portion of the digestive 
tract. As the oesophagus lengthens the lung groove remains con- 
nected with its upper portion (Fig. 182, A), and furrows which ap- 
pear 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 ven- 
tral to the oesophagus and dorsal 
to the heart and opening with the 
oesophagus into the terminal por- 
tion 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 trans- 
versum, 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, 



Fig. 202. — Portion of a Section 



A, Aorta; DC, ductus Cuvieri; L, 
lung; O, oesophagus; RP, parietal re- 
cess; VOm, vitelline vein. — (Toldt.) 

33 2 


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 pro- 
cesses 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 






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 out- 
growths 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 mono- 
podial 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. 



There is a general tendency for the individual branches of the 
various sets to be arranged in regular succession and for their develop- 
ment 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 un- 
represented in the left lung, and is pecu- 
liar in that it lies behind the right pul- 
monary artery (Fig. 203, C), or in the 
adult, after the recession of the heart, 
above it, whence it is termed the epar- 
terial 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 ven- 
tral bronchus in the left lung, but at 
the same time it affords opportunity for 
an excessive development of the corre- 
sponding 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 sec- 
ond 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 some- 
times produces a well-marked infra-cardiac lobe in the right lung. 

Fig. 204. — Diagram of the 
Final Branches of the Mam- 
malian Bronchi. 

A, Atrium; B, bronchus; S, 
air-sac. — (Miller.) 



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 enlarge- 
ments are separated from one another by only very thin partitions 
of mesenchyme, in which the pulmonary vessels form a dense net- 
work. The final branching of each ultimate bronchus or bronchiole 
results in the formation at its extremity of from three to five enlarge- 
ments, 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 origi- 
nal 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 epi- 
thelial lining of the trachea and 
bronchi remain columnar or cu- 
bical in form and become ciliated 
at about the fourth month, but 
those of the epithelium of the air- 
sacs 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 Em- 
bryo of Twenty-eight Days, Seen 
from Behind and Above, the Dorsal 
Wall of the Pharynx being Cut 

co, Cornicular, and cu, cuneiform tu- 
bercle; Ep, epiglottis; T, unpaired por- 
tion of the tongue. — (Kallius.) 


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 pulmon- 
ary 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., 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, 


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 car- 
tilages 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 glosso- 
laryngeal 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, 


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 subclav- 
ian artery, while the left curves beneath the arch of the aorta (see 
Fig. 149). 


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. 



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 develop- 
ment, 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 


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 




embryo, although its duct, the pronephric or Wolffian duct, undergoes 
complete development and plays an important part in the develop- 
ment 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 repro- 
ductive 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 




Fig.^2o8. — Transverse Section through Chick Embryo of about Thirty-six 


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 


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 em- 
bryos 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 exist- 
ence 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 glomer- 
ulus lying free in the coelomic cavity (Fig. 209, e.g.). Internal 



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 participa- 
tion 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 en- 
tirely unconnected with either the 
coelomic epithelium or the 
Wolffian duct. Later the cords 
become connected with the cce- 
lomic epithelium and acquire a 
lumen, and near the coelomic end 

of the tubule, at a region corresponding to the chamber of a pro- 
nephric 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 Em- 
bryo of Three Days. 

ao, Aorta; gl, glomerulus; gr, genital 
ridge; mes, mesentery; ml, mesonephric 
tubule; vc, cardinal vein; Wd, Wolffian 
duct. — (Mihalkovicz.) 



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 seg- 
ments and in the nephrogenic cord as far back as the region of the 
third or fourth lumbar segment, where the cord is partially inter- 
rupted. This interruption marks the dividing line between the meso- 
nephric 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 



(Fig. 211). Each is attached to the dorsal wall of the body by a dis- 
tinct 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 out- 
growth lies lateral to the posterior portion of the Wolffian ridge, 



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 form- 
ing 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 

t, Urinary tubule; Ur, ureter; v, renal am- 
pulla. — (Haycrqft.) 



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 meta- 
nephric 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 


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 elon- 
gate (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 


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 invag- 
ination 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 ante- 
rior one containing the bladder. With the formation of this trans- 









V s> 













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.) 


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 thicken- 
ing into the substance of the Wolffian ridge, displacing the mesoneph- 
ric tubules to a greater or less extent. These columns are com- 
posed of two kinds of cells: (i) smaller epithelial cells with a rela- 
tively 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 por- 
tions 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 rete- 
cords, 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 



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 albu- 
ginea. By its development the sex-cords are separated from the 



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 — 


■- 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 

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 belong- 
ing to the anterior part of the mesonephros. Since like the sex- 
cords, they have by this time separated from the epithelium that 


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 in- 
terstitial 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 con- 
nective-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 mean- 
time 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 sex- 
cells and epithelial cells (Fig. 217). The invasion of the stroma 
continuing, these spheres or cords (Pfluger's cords) become divided 



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 folli- 
culi, which collects at one side of 
the follicle and eventually forms 
a considerable portion of its con- 
tents. 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 num- 
ber of follicles, but sooner or later these follicles undergo degenera- 
tion so that shortly after birth practically no traces of the cords re- 
main. 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 func- 
tional 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. 

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 epithe- 
lium; d and e, spherical masses; /, pri- 
mary follicle; g, blood-vessel. — (From 
Gegenbaur, after Waldeyer.) 


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 develop- 
ment proceeds in one of two directions according as the genital 
ridge becomes a testis or an ovary, the Wolffian body in part under- 
going 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 


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 out- 
growth 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 recto- 
uterine 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 


the lower pole of that organ, whence it extends to the anterior ab- 
dominal 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 posi- 
tion 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 vesico- 
uterine 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 ter- 
minate blindly at the ends nearest the ovary, but at the other ex- 
tremity, 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, there- 
fore, 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 


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 histo- 
logical 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 semi- 
circular 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 



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 


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, epi- 
didymis; 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. 



Indifferent Stage. 

Male. Female. 

Genital ridge J 


Fimbria ovarica. 

Ovarian ligament. 
Round ligament. 

Wolffian body < 

Globus major of epididymis. 


Vasa aberrantia. 


Wolffian ducts. . . . • 

Body and globus minor of 

Vasa deferentia. 
Seminal vesicles.' 
Ejaculatory ducts. 

Collecting tubules of epo- 

Canal of Gartner. 

Miillerian ducts . . . < 

Sessile hyatid. 
Uterus masculinus. 

Fallopian tubes. 



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 per- 
sisting 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 con- 
nection 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 compart- 
ments. 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 Uro- 
genital Sinus. — So far the relations of the lower ends of the urino- 
genital ducts have not been considered in detail, although it has been 



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 



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 repre- 
sents 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 


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 uro- 
genital 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 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 well- 
marked 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 


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 appear- 
ance 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 


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 differenti- 
ates 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. 



Urinary bladder. 

Urinary bladder. 

Proximal portion of urethra. 


Bulbo-urethral glands. 

Bulbo-vestibular glands. 

Urogenital sinus .... 

The rest of the urethra. 


Genital tubercle. . . . 



Genital folds 

Prepuce and integument of 


Labia minora 

Genital swellings... . 


Labia majora. 

It is stated above that the layers which compose the walls of the scro- 
tum are identical with those of the abdominal wall. This may be seen in 
detail from the following scheme: 


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 hermaph- 
roditism 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 dis- 
tance 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 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 elonga- 
tion of the vaginal processes into the substance of the genital swell- 
ings, 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 guber- 
nacula. 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 


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 

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 scro- 
tum. 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 degen- 
eration 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. 



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, 

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 


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, 

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. 



To the suprarenal system a number of bodies of peculiar struc- 
ture, 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 sym- 
pathetic 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 supra- 
renal 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 struc- 
tures (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 pro- 
liferations of the ccelomic epithelium, which project into the sub- 
jacent 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 




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 


& Sr ns 

tern J „-.-— 



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 pro- 
nephros. The fact that in some mammals the cortical (interrenal) cells are 


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 chrom- 
affine 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- 

... ■ / 

'•'§'''•. ' . . .■/. 



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 neigh- 
boring 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 



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 med- 
ullary 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 

The Intercarotid Ganglia. — These structures, which are fre- 


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 

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 repre- 
sent 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 im- 
portance 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 signifi- 
cance 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 



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- 


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. 


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. 



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 con- 
sisting of branched prolongations of the syncytial cytoplasm. This 



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 



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 



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 mesen- 
chymal connective tissue. That is to say, they are formed from the 
peripheral portions of the cytoplasm of the neuroglial and ependy- 
mal 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 


OqQ q ®», 


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 syncy- 
tium from the territory of one neuroglial cell to another, many of 
those, indeed, arising in connection with the ependymal cells extend- 
ing 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 


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 nerve- 
roots (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 recog- 
nized in sections and termed the 
neural crest (Fig. 229, A). When 
the lips of the groove fuse to- 
gether 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 out- 
ward over the surface of the 
canal (Fig. 229, C). Since this 
proliferation is most active in the 
regions of the crest which corre- 
spond 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 axis- 
cylinder, 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.) 


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 per- 
ipherally 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 develop- 
ment 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 grow- 
ing 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 


effect that the nerve fibers first appear as chains of cells and that the axis- 
cylinders, being differentiated from the cytoplasm of the chains, are really 
multicellular products. Many difficulties stand in the way of the ac- 
ceptance 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 com- 
posed 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 de- 
finite 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 

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 


(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 differ- 
entiated portion elongates at a much slower rate than the surround- 
ing tissues and so appears to recede up the spinal canal, until its 


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 indi- 
cated by the dorsal longitudinal fissure in the adult cord, the central 
canal of which corresponds to the ventral portion only of the embry- 
onic cavity. While this process has been going on both the roof- 
and 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 



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 


3 86 


A " 





1 — H 



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 

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 fas- 
ciculi, is the last to develop, appearing to- 
ward the end of the ninth month of fetal 

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 differ- 
entiation 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 evag- 
inations, 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 fore- 
brain, the remainder of the vesicle giving rise to what is known as 

Fig. 232. — Reconstruction of 
the Brain of an Embryo of 2.15 


h, Hemisphere; i, isthmus; m, 
mesencephalon; mf, mid-brain flex- 
ure; mt, metencephalon ; myl, myel- 
encephalon; nf, nape flexure; ot, otic 
capsule; op, optic evagination; t, 
diencephalon. — (His.) 



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 mid- 
brain 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 por- 
tion which is known as the medulla oblongata, or, to retain a con- 
sistent 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 un- 
derstood 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 




Medulla oblongata (I) . 

/ Pons (II 1). 

^ Cerebellum (II 2). 

SBrachia conjunctiva (III). 
Cerebral peduncles (posterior 
portion) . 

2nd Vesicle Mesencephalon 

Cerebral peduncles (anterior por- 
tion) (IV 1). 
Corpora quadrigemina (IV 2). 

1st Vesicle < 



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. 


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-/ 




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 mid- 
brain, 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 


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 asso- 
ciated 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 mesen- 
chyme dips, and, as the groove deepens in later stages, the mesen- 
chyme 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 roof- 
plate which forms the floor of the groove. 

In embryos of about 9 mm. the differentiation of the dorsal 



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 soli- 
tary 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- 


39 1 

ders of the neuroblasts of the ventral zone, and later, on the establish- 
ment of the olivary bodies, other fibers, descending from the cere- 
bellum, 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 Development of the Metencephalon and Isthmus. — Our knowl- 
edge of the development of the metencephalon, isthmus, and mesen- 
cephalon 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 struc- 
ture 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 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 lamel- 
late structure characteristic of 
the cerebellum. 

The histogenetic develop- 
ment of the cerebellum at first 
proceeds along the lines which 
have already been described 
as typical, but after the devel- 
opment 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 ve- 
lum 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 ob- 
longata 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 con- 
junctiva 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. 


Nuclei of termination of sen- 
sory roots of cranial nerves. 
Pons nuclei. 


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 pos- 
sesses for a time a capacious cavity. Later, however, it increases in 
size less rapidly than adjacent parts and its walls thicken, the roof- 
and 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 com- 
parison 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. 



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 thin- 
walled, 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 pos- 
terior 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 al- 
most reaches the dermis, forming a 
hollow evagination of the brain roof 
known as the pineal process. The dis- 
tal extremity of this process enlarges to 
a sac-like structure which later be- 
Fig. 238.— Transverse Section comes lobed, and, by an active pro- 

of the Diencephalon of an Em- Hferation of the cells lining the cavi- 
bryo 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 en- 
tire structure, body and stalk, constitutes what is known as the 

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 trans- 
parent 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 



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 evi- 
dence 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 be- 
comes reduced to a thin membrane 
upon the surface of which blood- 
vessels, developing in the surround- 
ing mesenchyme, arrange them- 
selves 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 

/, ., 




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 mid- 
brain 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. 

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 ven- 
tricle, 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 


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 con- 
verted into a mass of solid epithelial cords embedded in a mesen- 
chyme 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 injun- 
dibidum, 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 roof- 
plate. From the more ventral portion of each dorsal zone there is 


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 


— •■/ 

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 



it forms the roof of the interventricular foramen, is prolonged out 
upon the dorsal surface of each hemisphere, and, becoming invag- 
inated, 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 

Fig. 241. 

-Median Longi- 
tudinal 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 


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 hemis- 
pheres, 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 ■(_... < 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 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 pro- 
duced by the calcarine persists 
to form the calcar {hippocam- 
pus 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 sur- 
face 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 



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 


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 



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 sub- 
stance 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 

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 develop- 
ment of these three sets of fibers and especially those forming the 
corpus callosum, the dorsal portion of the lamina terminalis be- 
comes enlarged so as to form a triangular area extending between 
the two cerebral hemispheres (Fig. 245), the corpus callosum form- 



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 forma- 
tion, is a completely closed cav- 
ity and is not lined with epen- 
dymal tissue of the same nature 
as that found in the other ven- 

Owing to the very consider- 
able size reached by the trian- 
gular area whose history has just 
been described, important 
changes are wrought in the ad- 
joining portions of the mesial 
surface of the hemispheres. Be- 
fore the development of the area 
the gyrus dentatus and the hip- 
pocampus extend forward into 
the anterior portion of the hem- 
ispheres (Fig. 244), but on ac- 
count of their position they be- 
come encroached upon by the 
enlargement of the corpus callo- 
sum, 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 Sec- 
tion or the Brain of an Embryo of 
Four Months. 

c, Calcarine fissure; ca, anterior com- 
missure; cc, corpus callosum; cf. chorioidal 
fissure; dg, dentate gyrus; fm, interven- 
tricular foramen; h, hippocampal fissure; 
po, parieto-o c c i p i t a 1 fissure. — (Mihal- 



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 lobe- 
like 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 rhinen- 
cephalon, but in man they 
remain smaller, and although 
they are at first hollow, con- 
taining 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 struc- 
ture, the olfactory bulb and stalk, while its proximal portion gives 
rise to the olfactory tracts, the trigone, and the anterior perforated 

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 in- 
nermost 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 

ac, Anterior commissure; cc, corpus callo- 
sum; dg, dentate gyrus;/, fornix; i, infundib- 
ulum; mc, intermediate mass; si, septum 
pellucidum; vi, velum interpositum. — (Mihal- 



origin of the other layers recognizable in the adult cortex, nor is it 
known whether any migration, similar to what occurs in the cere- 
bellar 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 develop- 
ment 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 axis- 
cylinders 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 neuro- 
blasts 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 out- 
ward, 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 


terminate in the median ventral portion of the body as the anterior 
cutaneous nerve. The splanchnic branch forms a ramus communi- 
cans 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 adja- 
cent 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 forma- 
tion 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 verte- 
brate morphology, the metamerism of the head. That the meta- 
merism 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 dis- 
location, as it were, of the two roots had occurred. 

The results obtained from investigation along this line have not, 


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 com- 
ponents occur which are not represented in the nerves of the spinal 

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 meso- 
derm 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 repre- 
sented 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 em- 
bryonic 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 



and in the substance of the formatio reticularis, and known as the 
lateral motor nuclei; and (3) a series of nuclei in which afferent nerve- 
fibers terminate, situated still more laterally in the floor of the ven- 
tricle 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 Hypo- 
glossal (XII) Nerves. — (His.) 

mixed nerve, but those from all the ventral motor nuclei form inde- 
pendent 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 





Ventral Motor 

Lateral Motor 











































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 undoubt- 
edly belongs to ah entirely different category from the other periph- 
eral 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 


fact which indicates the equivalency of these latter with the fibers 
which arise from the neuroblasts of the anterior horns of the spinal 

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 belong- 
ing 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 



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 


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, hyomandib- 
ular 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 prac- 
tically 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 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 pala- 
tine 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 sense- 
organs 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 Mam- 
malia, 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 



forms, and may require some modifications when the components 
have been subjected to actual observation: 






































































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 anter- 
iorly they will be found to change their position somewhat as the 
medulla is reached and eventually come to lie in the reticular forma- 
tion, 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 


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 pla- 
codes, are developed from the ectoderm along a line corresponding 
with the level of the auditory invagination, while on a line corre- 
sponding 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 

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 relation- 
ships with one another. The various pairs are serially homologous neither 



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 meso- 
dermic 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 corre- 
sponding 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 verte- 
brates, 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 meso- 
dermal tissues. 

To understand the development of the sympathetic system it 
must be remembered that it consists typically of three sets of gan- 
glia. One of these is constituted by the ganglia of the ganglionated 
cord (Fig. 249, GC), the second is represented by the ganglia of the 







Fig. 248. — Transverse Section through an Embryo Shark (Scyllium) of ii mm., 


Ch, Notochord; E, ectoderm; G, posterior root ganglion; Gs, sympathetic ganglion; .1/, 

spinal cord. — (Onodi.) 



prevertebral plexuses (PVG), such as the cardiac, solar, hypogas- 
tric, 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 axis- 
cylinder 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 Sympa- 
thetic 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 com- 
municantes (Fig. 249, WR), which pass from the spinal nerves to the 
adjacent ganglia of the ganglionated cord, some of them terminat- 
ing 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 



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 proxi- 
mal 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 sym- 
pathetic 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 


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 

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 be- 
comes 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 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 


plexus and have separated somewhat from the ganglionated cord, 
the fiber bundles which unite the upper mass with the cord represent- 
ing 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, pul- 
monary 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 down- 
ward the coeliac plexus, which originally occupied a position opposite 
the region of the ganglionated cord from which the splanchnic nerves 

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. 


C. R. Bardeen: "The Growth and Histogenesis of the Cerebrospinal Nerves in 
Mammals," Amer. Journ. AnaL, 11, 1903. 


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. 


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. 
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filr mikrosk. Anat., xxvn, 1886. 

G. Retzius: "Das Menschenhirn," Stockholm, 1896. 

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Archiv filr Entwicklungsmechanik, v, 1897. 
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Region of the Human Embryo," Amer. Journ. Anat., iv, 1904. 
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xvii, 1909. 



Like the cells of the central nervous system, the sensory cells 
are all of ectodermal origin, and in lower animals, such as the earth- 
worm, for instance, they retain their original position in the ecto- 
dermal 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 repre- 
sented 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 with- 
drawn from the surface receive stimuli only through overlying cells, 
and in certain cases these transmitting cells are not specially differ- 
entiated, 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 



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 forma- 
tion 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 epithe- 
lium (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 

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 develop- 
ment retain their epithelial position for the most part, a few of them, 
however, withdrawing into the subjacent mesenchyme and becoming 



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 neuro- 
epithelial 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 


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 


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, dis- 
tinct 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 

The Organs of Touch and Taste. — Little is yet known con- 
cerning 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 prin- 
cipally 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 epi- 
dermis, 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 


superficial epidermis (Fig. 253, A). In later stages lateral out- 
growths 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, form- 
ing a trench surrounding a papilla (Fig. 253, C). The lateral out- 
growths, 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 glosso- 
pharyngeal 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 


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 meso- 
derm. 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 



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 endolymph- 
aticus three folds make their appearance, representing the semi- 


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 



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 sub- 
sequently becomes divided by a con- 
striction 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 re- 
uniens, while that between the utri- 
culus 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 



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 ecto- 
derm 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 gang- 
lia. One of the prolongations of each cell is directed centrally to 

43 6 


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. 

■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 hair- 
like 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- 


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, 





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, 


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 be- 
come transformed on their ad- 
jacent 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 per- 
ipheral 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 

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 some- 
what 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 



fluid known as the perilymph, the space occupied by the fluid being 
the perilymphatic space; the outer fibrous layer becomes peri- 
chondrium 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 compart- 
ments which communicate at the apex of the cochlea, while below 
one, known as the scala vestibuli, communicates with the space 


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 tym- 
panic cavity, while the narrower portion intervening between this 
and the pharyngeal cavity represents the tuba auditiva (Eustachian 

To correctly understand the development of the tympanic 
cavity it is necessary to recall the structures which form its bound- 
aries. Anteriorly to the upper end of the first branchial pouch 
there is the upper end of the first arch, and behind it the correspond- 
ing 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. Simi- 
larly, in the case of the second arch there is to be found, dorsal to 


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 

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 







muscle, derived from the musculature of the second branchial arch 
and therefore supplied by the facial nerve, becomes attached to the 

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 pecu- 
liar spongy tissue, which, toward the 
end of fetal life, gradually degener- 
ates, 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 tra- 
verse 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 



Fig. 263. — Diagrams Illus- 
trating the Mode of Exten- 
sion of the Tympanic Cavity 
Around the Auditory Ossicles. 

M, Malleus; m, spongy mesen- 
chyme; p, surface of the periotic 
capsule; T, tympanic cavity. 
The broken line represents the 
epithelial lining of the tympanic 


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 ques- 
tion 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 opin- 
ions 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 rela- 
tion 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 



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 


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 



surface by an ectodermal and on the inner by an endodermal 

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 


/ 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 for- 
ward 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 

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 concern- 
ing 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 out- 
growths 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 



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 

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 sepa- 
rates 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 



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 repre- 
sents 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 ex- 
ternally 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, 


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 con- 
tinuing, a similar sutural line appears on the outer surface beneath 
the anterior epithelium, and the fibers become arranged in concen- 
tric 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 




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 : 


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 cover- 
ing the outer surface of the lens, known as the membrana papil- 
laris, causing the malformation known as congenital atresia of the 

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 prod- 
uct 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 Am- 
phibia 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 


is termed coloboma iridis, and at others being confined to the reti- 
nal portion of the cup, in which case it is termed coloboma 

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 inter- 
nal 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 pig- 
ment 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 ecto- 
derm 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 


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- 



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 dif- 
ferentiated 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 



the ciliary region, forms the pigment layer of the retina (Fig. 272, p). 
The retinal layer increases in thickness and early becomes differen- 
tiated 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 



Fig. 272. — Portion of a Transverse Section of the Retina of a New-born 

ch, Chorioid coat; g, ganglion-cell layer; r, outer layer of nuclei; p, pigment layer. — 


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, 



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 

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 


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 min- 
gled, 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 horizon- 
tal 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 gang- 
lion cells of the retina and grow from these cells along the optic 



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 ar- 
teria 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 con- 
tinuity with the retinal layer of the op- 
tic 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 num- 
ber 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 fis- 

Ah, Hyaloid artery; L, lens; 
On, fibers of the optic nerve; Os, 
optic stalk; PI, pigment layer, 
and R, retinal layer of the retina. 


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 repre- 
sentatives 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 

": . ■■■'V ,".-■■' 

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 im- 
agined 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 



certain reptiles in which an eye is developed in connection with the epiphy- 
sial 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, accom- 
panied 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 



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 substitu- 
tion 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 in- 
crease 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 espe- 
cially 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 sus- 
pensory ligament of the lens (zonula Zinnii) (Fig. 277, si) is formed 


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 pene- 
trates 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, 



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 




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 


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 con- 
tinuous 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 per- 
ipherally, 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 

4 66 


tissue, solid rods of ectoderm, certain of which early give off numer- 
ous 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.- 



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 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 hol- 
low 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- 


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 begin- 
ing of the sixth week to form a solid cord, which sinks into the sub- 
jacent 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 open- 
ing 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 


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 up- 
ward, and, with its glands, forms a small reddish nodule, resting 
upon the plica semilunaris and known as the caruncula lacrimalis 
(Fig. 280). 


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 Tranenabfuhr- 
ungswege," 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. 



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. 
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fur Anat. und Physiol., Anat. Abth., 1898. 

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Verhandl. Anat. Gesellsch., VI, 1892. 
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Menschen," Archiv fur Anat. und Physiol., Anat. Abth., Supplement, 1897. 
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Anzeiger," xxxvni, 1911. 

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Arch. Anat., Microsc, x, 1909. 
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Studie." Anat. Hefte. xi, 1898. 
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humain," Bibliogr. Anat., xvii, 1907. 
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Zoologie, lxiii and lxv, 1898; lxviii, 1899. 
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to the Optic Stalk," Journal of Anat. and Physiol., xxx, 1896. 
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Arch.filr Ophthalmol., LXIX, 1908. 
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Man," Amer. Journ. Anat., xm, 1912. 
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Acoustic and Facial Nerves in the Human Embryo," Amer. Journ. of Anat. 

vi, 1907. 
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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 dif- 
ferent from those to which the body must later adapt itself, that 
arrangements, such as those connected with the placental circula- 
tion, 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 develop- 
ment 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 



height of the body will have increased from birth to adolescence 

- v _■ 

= 3.37 times. The child will weigh 3.1 kilos and the man 


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 

- :: ..-.-;.: .;-' 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 well- 
nigh twice as large as the actual increase. 

But in addition to these changes, which are largely dependent 



upon differences in the supply of nutrition, there are others associ- 
ated 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 

















40 (birth) 



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 percent- 
ages, 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 











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 


Number of Cases 

Weight in 























18. 1 








22 .6 






















x 3 











11. 7 











62 .2 













1 .2 



67 .0 




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 



general diminution in the rate of growth, yet there are marked 
irregularities, the most noticeable being (i) a rather marked diminu- 
tion during the eleventh and twelfth years, followed by (2) a rapid 




1 Z 3 * 5 6 f a 9 ID ft 12 13 14 1$ 16 17 19 

/' V 


























: v 



1 , 


s \ 












2 3 4-5 6 7 8 9 10 11 12 13 J& I 

5 / 

5 17 18 















* s 
















" 12 

" 10 

•■ 8 

" 6 

" * 
' Z 

Fig. 282. — Curves Showing the Annual Increase in Weight in (I) Boys and (II) 

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 rep- 
resenting the percentage increase have been added. 



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 interest- 
ing to note to what extent the organs which are more immediately 
associated with the metabolic activities of the body undergo a rela- 
tive 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 


New-born and Adult. 








„ . Spmal 
Brain _; . 












New-born and Adult. 










2 -57 










12 .29 
2 .16 




which a marked relative diminution, not in all cases readily explain- 
able, occurs. 

Recent observations by Hammar render necessary some modifica- 
tion 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 s- 
between 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. 

New-born and Adult. 

Skin and Sub- 
cutaneous Tissues 


Stomach and 
Musculature , T 






776.5 65 
28,732.0 1,364 



New-born and Adult. 

Skin and Sub- 
cutaneous Tissues 



Stomach and 

Pancreas Lungs 



2 5-05 

2 . 1 
2 .06 

0. 11 


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 



child. The difference does not, however, depend upon the differen- 
tiation 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 in- 
crease, 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 diminu- 
tion even greater than that shown in the table. 



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 pe- 
riod 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 hip- 
joint 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 pro- 
nounced. 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 verte- 
brae look more ventrally than posteriorly, so that there is no distinct 

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) : 






New-born child 


22 .1 




Male 2 years 


Male 5 years 


Male 1 1 years 

Male adult 



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 pos- 
sibly associated with the general more rapid development of the 
lower portion of the body made necessary by its imperfect develop- 
ment 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 compli- 
cated. 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 de- 
tails, 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 


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 


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 impor- 
ant in this connection, and are similar to those of the maxillae in 
leing associated with the development of the teeth. In the new- 
10m 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 epiphy- 
es to the main portions of the bones takes place only toward ma- 
urity. 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: 





Appearance of 

Appearance of Secondary 

Fusion of 

Primary Center 




6th week. 

(At sternal end) 17th year. 

20th year. 



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. 



■jth week. 

Inner condyle 5th year. 

1 8th year. 

Capitellum 3d year. 


Trochlea 10th year. 

[• 17 th year. 

Outer condyle 14th year. 



jth week. 

Olecranon 10th year. 

16th year. 

Distal epiphysis 4th year. 

1 8th year. 


jth week. 

Proximal epiphysis 5th year. 

17 th year. 

Distal epiphysis 2d year. 

20th year. 

Capita turn 

1st year. 


2d year. 

Triquetrum . . . 

3d year. 

4th year. 


5th year. 



6th year. 


8th year. 



12 th year. 

Metacarpals . . . 

gth week. 

3d year. 

20th year. 


gth-nth week. 

3d~5th years. 

17 th-! 8th years. 

The dates in italics are before birth. 






Appearance of Secondary Fusion of 

Primary Center 



gth week. 

Crest 15th year. 

Anterior inferior spine 15 th year. 


■ 22d year. 


4th month. 

Tuberosity 15th year. 


4th month. 

Crest 1 Sth year. 


Cartilage appears at 4th month, ossification in 3d year. 

Head 1st year. 

20 th year. 


■jth week. 


Great trochanter 4th year. 
Lesser trochanter 13 th- 14th year. 
Condyle gth month. 

19 th year. 
1 Sth year. 
2 1 st year. 


jth week. 


Head end of gth month. 
Distal end 2d year. 

2ist-2 5thyear. 
1 Sth year. 


Sth week. 


Upper epiphysis 5th year. 
Lower epiphysis 2d year. 

21st year. 
20th year. 


jth month. 


6th month. 

10th year. 

1 6 th year. 


A few days 
after birth. 


4th year. 


1 st year. 


gth week. 

3d year. 

20th year. 


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 con- 
sidering 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 com- 
parison, the antero-posterior (dorso-ventral) and transverse diame- 
ters of the child and adult have the proportions shown in the table 
on the opposite page (Fehling). 

4 8 4 


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 diam- 
eter from the brim to the outlet, a condition very different from 
what obtains in the adult. Furthermore, it is interesting to note 








i .00 

1 .00 

1 .00 

1. 19 

1 .292 

1 .20 


1. 19 


1 .01 

1. 151 









(Conjugata vera . 

>, f Antero-posterior 

rt 1 

U y Transverse 

-^ ( Antero-posterior 

O Transverse 

1. 18 
1. 14 
1 .07 

1 -153 

that sexual differences in the form of the pelvis are clearly distin- 
guishable at birth; indeed, according to Fehling's .observations, 
they become noticeable during the fourth month of intrauterine 

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 form- 
ing 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. 



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 ex- 
tensor 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 ankle- 
joint come into new relations to one another, the result being a modi- 
fication 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 


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 becom- 
ing 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. 


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, 

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. 
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. 
H. Vierordt: "Anatomische, Physiologische und Physikalische Daten und Tabellen," 

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

Schadels," Leipzig, 1862. 


After-birth, 137 
After-brain, 387 
Agger nasi, 176 
Allan tois, 113, 361 
Alveolo-lingual glands, 294 

groove, 290 
Amitotic division, 7 
Amnion, 108, 109 
Amniotic cavity, 54 
Amphiarthrosis, 188 
Amphiaster, 4 
Angioblast, 221 
Annulus of Vieussens, 233 
Anterior commissure, 405 
Anthelix, 446 
Antitragus, 446 
Anus, 282 
Aortic arches, 243 

bulb, 231 

septum, 236 
Archenteron, 48, 280 
Archoplasm sphere, 4 
Arcuate fibers, 391 
Areas of Langerhans, 313 
Arrectores pilorum, 147 
Arteries, 240 

anterior tibial, 253 

aorta, 244 

branchial, 242 

carotid, 243 

centralis retinae, 459 

cceliac, 246 

common iliac, 245 

epigastric, 250 

external iliac, 247, 253 
maxillary, 243 

femoral, 254 

hyaloid, 448 

hypogastric, 247, 268 

inferior mesenteric, 246 

innominate, 244 

intercostal, 245 

internal mammary, 250 
maxillary, 242 
spermatic, 246 

Arteries, interosseous, 251 

Ungual, 243 

lumbar, 245 

median, 251 

middle sacral, 245 

peroneal, 254 

popliteal 253 

posterior tibial, 255 

pulmonary, 243 

radial, 253 

renal, 246 

saphenous, 253 

sciatic, 253 

subclavian, 245 

superficial radial, 251 

superior intercostal, 248 
mesenteric, 246 
vesical, 247 

temporal, 242 

ulnar, 251 

umbilical, 116, 241, 247 

vertebral, 248 

vitelline, 119, 223 
Articular capsule, 188 
Ary-epiglottic folds, 335 
Arytenoid cartilages, 336 
Aster, 4 
Atresia of duodenum, 306 

of pupil, 453 
Atrial septum, 233 
Atrio-ventricular valves, 238 
Auerbach, plexus of, 420 
Auricle, 445 
Axis cylinder, 378 


Bartholin, glands of, 362 
Belly-stalk, 68, 114 
Bile capillaries, 309 
Bladder, 359 
Blastoderm, 42 
Blastopore, 48, 54, 57 
Blastula, 39 
Blood, 224 

islands, 222 


4 88 


Blood platelets, 229 

vessels, 221 
Body cavity, 48 
Bone, development of, 154 

growth of, 157 
Bone-marrow, 156 

atlas, 162, 165 

axis, 165 

carpal, 184, 187, 482 

clavicle, 183, 482 

coccyx, 166 

conchae, 176 

epistropheus, 162, 165 

ethmoid, 174 

femur, 186, 483, 484 

fibula, 186, 483 

frontal, 178 

humerus, 184, 482 

hyoid, 182 

ilium, 186, 483 

incus, 179, 440 

innominate, 185, 483 

interparietal, 172 

ischium, 186, 483 

lachrymal, 178 

malleus, 179, 440 

mandible, 180 

maxilla, 179 

metacarpal, 185, 482 

metatarsal, 188, 482 

nasal, 178 

occipital, 170, 172 

palatine, 179 

parietal, 178 

patella, 186, 483 

periotic, 169, 176 

phalanges, 185, 188, 482, 483 

premaxilla, 179 

pubis, 186, 483 

radius, 184, 482 

ribs, 162, 165 

sacrum, 165 

scapula, 183, 482 

sphenoid, 173 

stapes, 441 

sternum, 166 

suprasternal, 166 

tarsal, 187, 483 

temporal, 176 

tibia, 186, 483 

turbinated, 175 

ulna, 184, 482 

vertebrae, 160, 164, 478 

vomer, 175 

zygomatic, 178 
Brachia conjunctiva, 394 

Brain, 386, 475 
Branchial arches, 90, 97 

clefts, 90 

epithelial bodies, 294, 295 

fistula, 91 
Branchiomeres, 81 
Bronchi, 333 

Bucconasal membrane, 283 
Bulbo-urethral glands, 362 
Bulbo-vestibular glands, 362 
Burdach, fasciculus of, 385 
Bursa omentalis, 324 

Caecum, 301, 305 

Calcar, 403 

Canal of Cloquet, 463 

of Gartner, 357 

of Nuck, 365 

of Petit, 463 
Canalized fibrin, 128 
Capillaries, 224 
Cartilages of Santorini, 336 

of Wrisberg, 336 
Caruncula lacrimal is, 468 
Cauda equina, 384 
Caul, 112 
Cell, 1, 3 

division, 4 

theory, 1 
Centrosome, 4 
Cerebellum, 392 
Cerebral aqueduct, 395 

convolutions, 402 

cortex, 407 

hemispheres, 398 

peduncles, 394 
Cheek groove, 291 
Chin ridge, 100 
Chondrocranium, 169, 172 
Chorda canal, 57 

dorsalis, 75 

endoderm, 75 
Chorioid coat, 449, 463 

plexus, 389, 397, 401 
Chorioidal fissure of brain, 401 

of eye, 448, 453 
Chorion, 67,118 

frondosum, 124 

laeve, 124 
Chorionic villi, 123 
ChromafSne organs, 370 
Chromatin, 3 
Cnromosomes, 4 

accessory, 15 

reduction of, 14, 30 



Ciliary body, 454 

ganglion, 424 

muscle, 465 
Cisterna chyli, 270 
Cleft palate, 284 

sternum, 168 
Clitoris, 363 
Cloaca, 280, 360 
Cloacal membrane, 287 
Cloquet, canal of, 463 
Coccygeal ganglion, 275 
Ccelom, 48, 78 
Collateral eminence, 404 
Colliculus seminalis, 357 
Coloboma, 453 
Colon, 303 
Conjunctiva, 465 
Connective tissues, 153 
Cornea, 449, 464 
Corniculate cartilages, 336 
Corona radiata, 21, 353 
Coronary sinus, 232 
Corpora mamillaria, 398 

quadrigemina, 395 
Corpus albicans, 24 

callosum, 405 

luteum, 23 

striatum, 400 
Corti, spiral organ of, 437 
Cowper, glands of, 362 
Cranial nerves, 409 

sinuses, 255 
Cricoid cartilage, 336 
Cuneiform cartilages, 336 
Cutis plate, 80 
Cytoplasm, 3 
Cyto-trophoblast, 122 


Darwin's tubercle, 446 
Decidua basalis, 132 

capsularis, 121, 131 

reflexa, 121 

serotina, 132 

vera, 130 
Decidual cells, 131, 137 
Dendrites, 379 
Dental groove, 285 

papilla, 285 

shelf, 285 
Dentate gyrus, 403 
Dermatome, 80 
Descent of ovary, 365 

of testis, 366 
Diaphragm, 320 

Diarthrosis, 188 
Diencephalon, 387, 396 
Discus proligerus, 19, 353 
Double monsters, 46 
Duct of Santorini, 312 

of Wrisberg, 312 
Ductus arteriosus, 244, 268 

Botalli, 244 

choledochus, 307, 308 

cochlearis, 434 

Cuvieri, 257 

ejaculatorius, 355 

endolymphaticus, 433 

reuniens, 434 

venosus, 260 
Duodenum, 302, 303, 306 


Ear, 431 

Ebner, glands of, 431 
Ectoderm, 48 
Embryo, age of, 102 

external form, 86 

growth of, 472 
Embryonic disc, 54 
Embryotroph, 123 
Enamel organ, 285 
Enchylema, 3 
Endocardium, 229 
Endoderm, 43 
Enveloping layer, 42 
Ependymal cells, 377 
Epiblast, 48 

Epibranchial placodes, 417 
Epidermis, 141 
Epididymis, 354 
Epiglottis, 335 
Epiphyses, 156 
Epiphysis cerebri, 396 
Epiploic foramen, 324 
Episternal cartilages, 166 
Epitrichium, 141 
Eponychium, 145 
Epoophoron, 356 
Erythrocytes, 225 
Erythroplastids, 226 
Eustachian tube, 294, 440 

valve, 234 
Extrauterine pregnancy, 22 
Eye, 446 
Eyelids, 465 

Fallopian tubes, 357 
Fasciculus communis, 414 



Fasciculus of Burdach, 385 

of GoU, 385 

solitarius, 414 
Fenestra cochleae, 440 

ovalis, 440 

rotunda, 440 

vestibuli, 440 
Fertilization of ovum, 31 
Fetal circulation, 266 
Fibrinoid, 128 
Fifth ventricle, 406 
Filum terminale, 384 
Fimbria, 405 

_ ovarica, 357 
Foliate papillae, 431 
Fontana, spaces of, 465 
Foramen caecum, 296 

of Winslow, 324 

ovale, 233, 240 
Fore-brain, 386 
Formatio reticularis, 390 
Fornix, 405 
Frontal sinuses, 176 
Funiculus cuneatus, 385 

gracilis, 385 
Furcula, 294 

Gartner, canals of, 357 
Gall bladder, 307, 308 
Ganglionated cord, 422 
Gastral mesoderm, 50, 62 
Gastrula, 48 
Geniculate bodies, 398 
Genital folds, 363 

ridge, 338, 349 

swelling, 363 

tubercle, 363 
Germ cells, 7 

layers, 47, 60 

plasm, 8 
Giraldes, organ of, 354 
Glands of Bartholin, 362 

bulbo-urethral, 362 

bulbo-vestibular, 362 

of Cowper, 362 

of Ebner, 431 

Meibomian, 466 

of MoU, 466 

salivary, 292 

tarsal, 466 
Goll, fasciculus of, 385 
Graafian follicle, 19 
Great omentum, 324 
Groove of Rosenmiiller, 295 

Gubernaculum testis, 356 
Gynaecomastia, 151 


Haematopoietic organs, 225 
Haemolymph nodes, 273 
Hairs, 146 
Hare lip, 100, 179 
HassalPs corpuscles, 298 
Haversian canals, 158 
Head cavities, 79 

process, 56, 69 
Heart, 229, 475 
Helix, 446 
Hensen's node, 56 
Hermaphroditism, 365 
Hind-brain, 387 
Hippocampus, 402 
Hyaloid canal, 463 
Hydatid of Morgagni, 355 

stalked, 359 
Hydramnios, 112 
Hymen, 357 
Hyperthelia, 151 
Hypertrichosis, 148 
Hypoblast, 48 
Hypochordal bar, 161 
Hypophysis, 399 
Hypospadias, 365 
Hypothalamic region, 398 

Implantation of ovum, 119 

Infracardial bursa, 345 

Infundibulum, 399 

Inguinal canal, 367 

Inner cell mass, 44 

Insula, 404 

Interarticular cartilages, 189 

Intercarotid ganglion, 373 

Intermediate cell mass, 77 

Interrenal organs, 370 

Interventricular foramen, 400 

Intervertebral fibro-cartilage, 162 

Intestine, 301, 476 

Iris, 454 

Isthmus cerebri, 387, 392 


Jacobson, organ of, 429 

Joints, 188 

Jugular lymph sac, 286 




Karyokinesis, 7 
Karyoplasm, 3 
Kidney (see Metanephros) , 343, 475 

Labia majora, 363 
minora, 363 

Lachrymal gland, 467 

Lamina terminalis, 399 

Langerhans, areas of, 313 

Langhans cells, 126 

Lanugo, 147 

Larynx, 334 

Lateral thyreoids, 299 

Lens, 447, 450 

Lesser omentum, 324 

Leukocytes, 227 


broad, of uterus, 349, 356 
coraco-humeral, 216 
coronary, of liver, 321 
falciform, of liver, 321 
fibular lateral, of knee, 200 
flavan, 162 

inguinal, 349, 355, 357 
interspinous, 162 
of the ovary, 358 
pectinatum iridis, 463 
round, of liver, 268 
round, of uterus, 358 
sacro-tuberous, 200 
spheno-mandibular, 180 
suspensory of lens, 462 

Limbs, 90, 100 

Lip-ridge, 100 

Lips, 284 

Liver, 306, 475 

Lungs, 331, 476 _ 

Luschka's ganglion, 275 

Lymphatics, 268 

Lymph nodes, 272 
sacs, 268, 270 

Lymphocytes, 227, 273 


Mammary gland, 148 
Mandibular process, 92 
Mastoid cells, 443 
Maturation of ovum, 28 
Maxillary antrum, 176 

process, 92 
Meckel's cartilage, 171, 179 

diverticulum, 113, 305 

Mediastina, 322 
Medulla oblongata, 387 
Medullary canal, 73, 88 

folds, 70, 72 

groove, 70 

sheath, 382 
Megacaryocytes, 228 
Meibomian glands, 466 
Meissner, plexus of, 420 
Membrana pupillaris, 453 

reuniens, 81 

tectoria, 437 
Membrane bone, 154 
Menstruation, 26 
Mesamceboids, 222 
Mesencephalon, 387, 395 
Mesenchyme, 61 
Mesenteriole, 327 
Mesentery, 323 
Mesocardium, 316 
Mesocolon, 326 
Mesoderm, 48 

somatic, 78 

splanchnic, 78 

ventral, 77 
Mesodermic somites, 72, 76 
Mesogastrium, 324 
Mesonephros, 341 
Mesorchium, 367 
Mesothelium, 61 
Metamere, 83 
Metanephros, 343 
Metencephalon, 387, 392 
Mid-brain, 387 
Middle ear, 440 
Milk ridge, 148 
Mitosis, 7 
Moll, glands of, 466 
Montgomery's glands, 150 
Morgagni, hydatid of, 355 
Morula, 43 
Mouth cavity, 283 
Mtillerian duct, 347 
Muscle plates, 80 

arrectores pilorum, 147 

biceps femoris, 216 

branchiomeric, 206 

chondroglossus, 208 

ciliary, 465 

coccygeus, 204 

constrictor of pharynx, 208, 299 

cranial, 205 

curvator coccygis, 204 

depressors of hyoid, 202 

digastric, 206 

dilatator iridis, 455 



Muscles, dorsal, 200 

eye, 205 

facial, 206 

gastrocnemius, 215, 219 

geniohyoid, 202 

genioglossus, 202 

glosso-palatinus, 208 

hyoglossus, 202 

hyposkeletal, 202 

intercostal, 202 

laryngeal, 208 

latissimus dorsi, 198 

levator ani, 204 

limb, 210 

longus capitis, 202 
colli, 202 

lumbrical, 218 

masseter, 206 

mylohyoid, 206 

obliqui abdominis, 202 

occipito-fron talis, 198, 206 

omohyoid, 198 

pectorals, 216 

perineal, 204 

peroneus longus, 216 

platysma, 206 

pronator quadratus, 216 

psoas, 202 

pterygoids, 206 

pyramidalis, 202 

rectus abdominis, 199, 202 

sacro-spinalis, 199, 204 

scaleni, 202 

serrati posteriores, 199 

serratus anterior, 199 

skeletal, 197 

soleus, 215, 219 

sphincter ani, 204 
cloacae, 205 
iridis, 455 

stapedius, 206, 441 

sternohyoid, 198 

sternomastoid, 198, 202, 208 

styloglossus, 202 

stylohyoid, 206 

stylopharyngeus, 208, 299 

temporal, 206 

tensor tympani, 206, 440 
veli palati, 206 

transversus abdominis, 202 
thoracis, 202 

trapezius, 198, 202, 208 
Muscle tissue, 193 
Myelencephalon, 387, 389 
Myelin, 382 
Myelocytes, 227 
Myoblasts, 195 

Myocardium, 229 
Myotome, 80, 198 


Nails, 144 
Nape bend, 90 
Nasal pit, 99 

process, 99 
Naso-lachrymal duct, 467 
Nephrogenic cord, 342 
Nephrostome, 340 
Nephrotome, 80 
Nerve components, 410, 413 

roots, 380 

auditory, 415 

cranial, 409 

hypoglossal, 412 

olfactory, 428 

optic, 458 

recurrent, 337 

spinal, 408 

accessory, 416 

splanchnic, 424 
Nerve tissue, 377 
Neural crest, 380 
Neurenteric canal, 58, 69, 73 
Neuroblasts, 378 
Neuroglia cells, 378, 379 
Neuromeres, 418 
Neurone theory, 382 
Nitabuch's stria, 135 
Non-sexual reproduction, 8 
Normoblasts, 226 
Notochord, 74 
Nuck, canal of, 365 
Nucleoli, 4 
Nucleus, 3 


(Esophagus, 299 
OEstrus, 27 
Odontoblasts, 287 
Olfactory lobes, 406 

organ, 428 
Olivary body, 390 
Omentum, 324 
Oocyte, 29 
Optic cup, 448, 453 

recess, 399 
Oral fossa, 88, 99, 280 
Organ of Giraldes, 354 

of Jacobson, 429 

of Rosenmuller, 356 



Organs, 2 

of taste, 430 

of Zuckerkandl, 374 

Osteoblasts, 154 

Osteoclasts, 158 

Otocyst, 432 

Otic ganglion, 424 

Ovary, 352 

descent of, 365 

Ovulation, 21, 26 

Ovum, 19 

fertilization of, 31 
implantation of, 119 
maturation of, 28 
segmentation of, 38 

Palate, 283 
Pancreas, 311, 476 
Paradidymis, 354 
Paraphysis, 397 
Parathymus, 299 
Parathyreoid bodies, 297 
Paroophoron, 356 
Parotid gland, 292 
Parovarium, 356 
Parthenogenesis, 8 
Penis, 364 

Pericardial cavity, 317, 318 
Perineal body, 362 
Perionyx, 145 
Periosteum, 155 
Periotic capsule, 169, 176 
Peritoneum, 323 
Petit, canal of, 463 
Pfliiger's cords, 352 
Pharyngeal bursa, 294 

membrane, 280 

tonsil, 294 
Pharynx, 294 

Pharyngo-palatine arches, 283 
Pineal body, 396 
Pinna, 445 
Pituitary body, 399 
Placenta, 133, 137 

accessory, 126 

embryotrophic, 123 

haematrophic, 123 

prsevia, 133 
Placentar infarcts, 135 
Plasmodi-trophoblast, 122 
Plasmodium, 122 
Pleurae, 322 

Pleuro-peritoneal cavity, 78, 320 
Plica semilunaris, 466 

Polar globules, 30 
Polycaryocytes, 228 
Polymastia, 151 
Polyspermy, 34 
Pons, 392 

flexure, 389 
Post- anal gut, 281 
Post-natal development, 470 
Posterior lymph sac, 270 
Precaudal recess, 281 
Precoracoid, 189 
Prepuce, 364 
Primitive groove, 56, 69 

streak, 50, 69 
Processus globularis, 99 
Pronephric duct, 339 
Pronephros, 339 
Pronuclei, 31 
Procestrum, 27 
Prostate gland, 362 
Prostomial mesoderm, 50, 58 
Protoplasm, 2 
Proto vertebrae, 77 


Rathke's pouch, 285, 399 

Rauber's covering layer, 44 

Rectum, 281 

Red nucleus, 395 

Reduction of chromosomes, 14, 30 

Restiform body, 391 

Rete cords, 349 

ovarii, 354 

testis, 352 
Retina, 455 

Retroperitoneal lymph sac, 270 
Rhinencephalon, 407 
Rosenmuller, groove of, 295 

organ of, 356 

Sacculus, 434 
Sacral bend, 90 
Salivary glands, 291 
Santorini, cartilages of, 336 

duct of, 312 
Sarcode, 1 
Scala tympani, 440 

vestibuli, 439 
Sclerotic coat, 449, 463 
Sclerotome, 80 
Scrotum, 364 



Sebaceous glands, 147 
Segmentation of ovum, 38 
Semicircular ducts, 433 
Semilunar valves, 239 
Seminiferous tubules, 352 
Septum pellucidum, 406 

primum, 233 

secundum, 233 

spurium, 232 

transversum, 318, 320, 323 
Sertoli cell, 14 
Sex cells, 349 

cords, 349 
Sexual reproduction, 8 
Sinusoid, 223 
Sinus, coronary, 232 

pocularis, 355 

praecervicalis, 97 

terminalis, 222 

venosus, 230 
Situs inversus viscerum, 46 
Skin, 141, 476 
Skull, 168, 479 
Socia parotidis, 291 
Solitary fasciculus, 390 
Somatic cells, 7 
Spaces of Fontana, 465 
Spermatic cord, 367 
Spermatid, 14 
Spermatocyte, 14 
Spermatogenesis, 13 
Spermatogonia, 14 
Spermatozoon, 11 
Sphenoidal cells, 176 
Spheno-palatine ganglion, 424 
Spinal cord, 383, 475 

nerves, 408 
Spiral organ of Corri, 437 
Spleen, 274, 475 
Stomach, 301 
Sublingual ganglion, 424 

gland, 293 
Submaxillary ganglion, 424 

gland, 292 
Substance islands, 222 
Sudoriparous glands, 148 
Sulcus Monroi, 396 
Superfetation, 36 
Suprabranchial placodes, 417 
Suprarenal bodies, 370, 475 

accessory, 372 
Supratonsillar- fossa, 295 
Suture, 188 

Sympathetic nervous system, 418 
Synchondrosis, 188 
Syncytium, 122 
Systems, 2 

Tail filament, 94 
Tarsal glands, 466 
Taste, organs of, 430 
Teeth, 285 
Tegmentum, 394 
Telencephalon, 386, 398 
Testis, 350 

descent of, 366 
Thalami, 397 
Thebesian valve, 234 
Thoracic duct, 271 
Thymus gland, 297, 476 
Thyreoid cartilage, 335 

gland, 296, 475 
Thyreo-glossal duct, 296 
Tissues, 2 
Tongue, 289 
Tonsils, 295 
Touch, organs of, 430 
Trachea, 334 
Tragus, 446 
Trophoblast, 55 
Tuba auditiva, 440 
Tubae uterinse, 357 
Tuber cinereum, 398 
Tuberculum impar, 289 
Tunica vaginalis testis, 367 

vasculosa lentis, 452 
Tween-brain, 387 
Twin-development, 46 
Tympanic cavity, 442 

membrane, 443 


Ultimo-branchial bodies, 299 
Umbilical cord, 92, 116 
Umbilicus, 86 
Urachus, 115, 361 
Ureter, 344 
Urethra, 361 
Urogenital sinus, 360 
Uterovaginal canal, 349 
Uterus, 357, 359 

masculinus, 355 
Utriculus, 434 

prostaticus, 355 


Vagina, 357 
Vaginal process, 365 
Vallate papillae, 430 



Vas deferens, 355 

anterior cardinal, 255 
tibial, 265 

ascending lumbar, 264 

azygos, 264 

basilic, 265 

cephalic, 265 

emissary, 259 

external jugular, 258 

hemiazygos, 264 

hepatic, 262 

inferior vena cava, 263 

innominate, 258 

internal jugular, 255 

jugulo-cephalic, 265 ■ 

limb, 265 

long saphenous, 265 

portal, 261 

posterior cardinal, 255 

primary fibular, 265 
ulnar, 265 

pulmonary, 265 

renal, 263 

subcardinal, 262 

superior vena cava, 258 

supracardinal, 263 

suprarenal, 263 

umbilical, 116, 260 

vitelline, 223, 259 
Velum, anterior, 394 

interpositum, 397 

marginal, 378 

Velum, posterior, 389 
Ventricular septum, 236 
Vermiform appendix, 305 
Vernix caseosa, 112, 147 
Vertex bend, 86 
Vesicula seminalis, 355 
Vieussens, annulus of, 233 
Villi, chorionic, 123 
intestinal, 305 
Vitreous humor, 449, 461 
Vulva, 363 


Wharton's jelly, 118 
Winslow, foramen of, 324 
Wirsung, duct of, 312 
Witch milk, 151 
Wolffian body, 341, 354 

duct, 339, 354 

ridge, 338 
Wrisberg, cartilage of, 336 

Yolk sac, 86, 112 

stalk, 86, 90, 112 

Zona pellucida, 21 
Zuckerkandl, organ of, 374 


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