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=Embryology of the Pig=
=Embryology of the Pig=
Frontispiece  
[[File:Bradley M. Patten.jpg|thumb|alt=Bradley M. Patten|link=Embryology History - Bradley Patten|Bradley Patten ( -1971)]]
Frontispiece


Reconstruction (X 17.5) showing the organ systems of a 9.4 mm. pig embryo. For explanation see figures 60 and 66.  
Reconstruction (X 17.5) showing the organ systems of a 9.4 mm. pig embryo. For explanation see figures 60 and 66.


By BRADLEY M. PATTEN
By Bradley M. Patten


Professor of Anatomy in the University of Michigan Medical School  
Professor of Anatomy in the University of Michigan Medical School




THIRD EDITION
Third Edition




WITH COLORED FRONTISPIECE
With Colored Frontispiece


AND 186 ILLUSTRATIONS IN THE TEXT (CONTAINING 412 FIGURES)  
And 186 Illustrations In The Text (Containing 412 Figures) Of Which 6 Are In Color
OF WHICH 6 ARE IN COLOR




Philadelphia : THE BLAKISTON COMPANY : Toronto


Philadelphia : THE BLAKISTON COMPANY : Toronto


Third Edition




Third Edition
Copyright, October 1948, by The Blakiston Company




Copyright, October 1948, by The Blakiston Company
By P. Blakiston's Son & Co.




BY P. Blakiston ’s Son & Co.  
Copyright, 1951, by P Blakiston's Son & Co , Inc.




Copyright, 1951, by P Blakiston’s Son & Co , Inc.
{{Patten1951 TOC}}


==Preface to Third Edition==


Preface to Third Edition
In making the revision for a new edition of this book it did not seem desirable essentially to change its form or scope. Effort has been concentrated on improving the presentation of the original subject matter and bringing it up to date, rather than on its expansion. The entire book has been reset to a greater page width which has permitted enlarging certain of the illustrations that, in earlier editions, had proved to be too greatly reduced. With the generous cooperation of the publishers several of the important plates on the cardiovascular system have been remade to a larger scale and with color. A number of new illustrations have been drawn for sections where experience has shown that students needed additional graphic assistance in interpreting their laboratory material. It is hoped that these changes will all contribute toward making the book as a whole more serviceable.




In making the revision for a new edition of this book it did not
Bradley M. Patten
seem desirable essentially to change its form or scope. Effort has been
concentrated on improving the presentation of the original subject
matter and bringing it up to date, rather than on its expansion. The
entire book has been reset to a greater page width which has permitted enlarging certain of the illustrations that, in earlier editions,
had proved to be too greatly reduced. With the generous cooperation
of the publishers several of the important plates on the cardiovascular
system have been remade to a larger scale and with color. A number
of new illustrations have been drawn for sections where experience
has shown that students needed additional graphic assistance in
interpreting their laboratory material. It is hoped that these changes
will all contribute toward making the book as a whole more serviceable.  


Bradley M. Patten
August 1048


August 1048


==Preface to First Edition==


Preface to First Edition
This book represents an endeavor to set forth in brief and simple form the fundamental facts of mammalian development. The thread of the story and the illustrations have been based on pig embryos because of their value and availability as laboratory material. But special stress has been laid on the embryological phenomena involved instead of on the details of specific conditions existing in the pig. Throughout the book, every efTort has been made to present developmental processes as dynamic events with emphasis on their sequence and significance, rather than as a series of still pictures of selected stages.




This book represents an endeavor to set forth in brief and simple
Obviously no book can deal fully with all phases of development, even in a single form, and still remain serviceable as a text. As this book is for the student, it has seemed expedient, for the sake of clearness and simplicity, to omit many things which I should like to have included. My primary aim has been to write an account in which the essentials stand out adequately interpreted and unobscured by a multiplicity of details — to lay a foundation which can be further built upon in accordance with special needs or individual desires.
form the fundamental facts of mammalian development. The thread
of the story and the illustrations have been based on pig embryos
because of their value and availability as laboratory material. But
special stress has been laid on the embryological phenomena involved
instead of on the details of specific conditions existing in the pig.
Throughout the book, every efTort has been made to present developmental processes as dynamic events with emphasis on their sequence
and significance, rather than as a series of still pictures of selected
stages.  


Obviously no book can deal fully with all phases of development,
even in a single form, and still remain serviceable as a text. As this
book is for the student, it has seemed expedient, for the sake of clearness and simplicity, to omit many things which I should like to have
included. My primary aim has been to write an account in which the
essentials stand out adequately interpreted and unobscured by a
multiplicity of details — to lay a foundation which can be further
built upon in accordance with special needs or individual desires.


Bradley M. Patten  
Bradley M. Patten
 
January 1927


January 1927


==Acknowledgment==
==Acknowledgment==
The pleasantest thing about working on this book has been the generous aid I have received from many sources. Throughout the preparation of the initial edition the encouragement, criticism, and suggestions of my colleagues, Dr. F. C. Waite and Dr. S. W. Chase, were of the greatest help. In the preparation of material and in making the illustrations for the first edition the beautifully accurate work of Miss Kathryn Toulmin was of inestimable value. In making the new drawings added in the third edition I was fortunate in securing the unusually able assistance of Mrs. Dorothy Van Eck.




The pleasantest thing about working on this book has been the
Dr. G. L. Streeter and Dr. C. H. Heuser of the Carnegie Institute allowed me free use of their extensive collection of young embryos and generously gave me many photographs made from their material. To Mrs. Charles S, Minot I am indebted for permission to use several figures from the late Professor Minot's works. The accrediting in the figure legends of these and other borrowed illustrations by no means covers my obligation to other writers. Practically the entire bibliography is a statement of indebtedness for information and ideas.
generous aid I have received from many sources. Throughout the
preparation of the initial edition the encouragement, criticism, and
suggestions of my colleagues, Dr. F. C. Waite and Dr. S. W. Chase,
were of the greatest help. In the preparation of material and in making
the illustrations for the first edition the beautifully accurate work of  
Miss Kathryn Toulmin was of inestimable value. In making the new
drawings added in the third edition I was fortunate in securing the
unusually able assistance of Mrs. Dorothy Van Eck.  


Dr. G. L. Streeter and Dr. C. H. Heuser of the Carnegie Institute
allowed me free use of their extensive collection of young embryos
and generously gave me many photographs made from their material.
To Mrs. Charles S, Minot I am indebted for permission to use several
figures from the late Professor Minot’s works. The accrediting in the
figure legends of these and other borrowed illustrations by no means
covers my obligation to other writers. Practically the entire bibliography is a statement of indebtedness for information and ideas.


I wish I might acknowledge individually the helpful services  
I wish I might acknowledge individually the helpful services rendered by many of my students, but they are too numerous. Several reconstructions which I have used directly or indirectly have been largely their work. Of even greater assistance have been their suggestions during the shaping of the work — suggestions of especial value because they were made from a point of view difficult for an instructor to appreciate without such aid.
rendered by many of my students, but they are too numerous. Several  
reconstructions which I have used directly or indirectly have been  
largely their work. Of even greater assistance have been their suggestions during the shaping of the work — suggestions of especial value  
because they were made from a point of view difficult for an instructor  
to appreciate without such aid.  


To The Blakiston Company, I am indebted for much helpful
cooperation and especially for their liberality with regard to the
number and quality of the illustrations.


No person other than my wife could have deciphered and put
To The Blakiston Company, I am indebted for much helpful cooperation and especially for their liberality with regard to the number and quality of the illustrations.
into usable form manuscript of the character I frequently turned over
to her for revision and typing. Without her generous help the preparation of the text would have been much more arduous and long
delayed.  


Bradley M. Patten


No person other than my wife could have deciphered and put into usable form manuscript of the character I frequently turned over to her for revision and typing. Without her generous help the preparation of the text would have been much more arduous and long delayed.




==Chapter  11==
Bradley M. Patten




Tlie Development of tke Circulatory System


{{Footer}}


I. The Interpretation of the Embryonic Circulation
{{Patten1951 TOC}}
 
The embryonic circulation is difficult to understand only when
the meaning of its arrangement is overlooked. If one bears in mind
certain fundamental conceptions as to the significance of the circulatory system in organic economics, and the basic morphological
principle that any embryo must go through certain ancestral phases
of organization before it can arrive at its adult structure, the changes
in the arrangement of vascular channels during the course of development form a coherent and logical story.
 
In the embryo as in the adult the main vascular channels lead to
and from the centers of metabolic activity. The circulating blood
carries food from the organs concerned with its absorption to parts of
the body remote from the source of supplies; oxygen to all the tissues
of the body from organs which are especially adapted to facilitate the
taking of oxygen into the blood ; and waste materials from the places
of their liberation to the organs through which they are eliminated.
One of the primary reasons the arrangement of the vessels in an
embryonic mammal differs so much from that in the adult, is the fact
that the emjbryo lives under conditions totally unlike those which
surround its parents. Its centers of metabolic activity are, therefore,
different; and, since the course of its main blood vessels is determined
by these centers, the vascular plan is different. No such profound
changes occur between the embryonic and the adult stages in the
circulation of a fish where embryo and adult are both living under
similar conditions.
 
The organs which in the adult mammal carry out such functions
as digestion and absorption, respiration, and excretion are extremely
complex and highly differentiated structures. They are for this reason
slow to attain their definitive condition and are not ready to become
functional until toward the close of the embryonic period. Moreover
the conditions which surround certain of the developing organs during
 
227
 
 
 
 
228
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
intra-uterine life absolutely prevent their becoming functional even
were they suflSciently developed so to do. Suppose^ the lungs, for
example, were functionally competent at an early stage of development. The fact that the embryo is reliving ancestral conditions in its
private amniotic aquarium renders its lungs as incapable of functioning as those of a man under water. Likewise the developing digestive
organs of the embryo are inaccessible to raw food materials. Further
examples are not necessary to make it obvious that were the embryo
dependent on the same organs which carry on metabolism in the adult,
development would be at an impasse.
 
An embryo must, nevertheless, solve the problem of existence
during the protracted time in which it is building a set of organs
similar to those of its parents. In the absence of a dowry of stored
food in the form of yolk, the mammalian embryo draws upon the
uterine circulation of the mother. Utilization of this source of supplies
depends on the development of a special organ which serves through
fetal life and is then discarded. The embryo takes food not into its
slowly developing gastro-intestinal tract but into its chorion, a membrane projected outside its own body and applied to the uterine wall
to form, together with it, the placenta. The nutritive materials there
absorbed from the maternal blood must be transported to the body of
the growing embryo by its own blood stream.
 
The use of food materials to produce the energy expressed in
growth depends on the presence of oxygen. For growth there must
be a means of securing oxygen and carrying it, as well as food, to all
parts of the body. Nor can continued growth go on unless the waste
products liberated by the developing tissues are eliminated. The blood
of the embryo cannot be relieved of its carbon dioxide and acquire a
fresh supply of oxygen in the primordial cell clusters which will later
become its lungs. It cannot excrete its nitrogenous waste products
through undeveloped kidneys. Its respiration and excretion, like its
absorption of food, are carried out in the rich plexus of small blood
vessels in the chorion. Here the fetal blood is separated from the
maternal, by tissues so thin that it can readily give up its waste materials to, and receive food and oxygen from, the maternal blood stream,
just as the mother’s own tissues constantly carry on this interchange
with the circulating blood. The placenta is thus the temporary
alimentary system, lung, and kidney of the mammalian embryo. The
large size of the umbilical blood vessels to the placenta is not a surprising thing — it is the entirely logical, the inevitable, expression of
the conditions under which the embryo develops.
 
 
 
THE INTERPRETATION OF THE EMBRYONIC CIRCULATION 229
 
 
The enormous chorionic blood supply during fetal life, with the
entire disappearance of this special arc of the circulation when the
organism assumes adult methods of living, is a striking example of the
determination of vascular channels by the location of functional
centers. We must not, however, overlook the fact that there are many
other centers of activity in the growing embryo less conspicuous but
equally important for its continued existence. Each developing organ
in the embryonic body is a center of intense metabolic activity.
During fetal life it must be supplied by vascular channels adequate to
care for its growth. But that is not all. Up to the time of birth each
organ has been drawing on blood furnished with food and freed of
waste materials by the activities of the maternal organism. At birth
all this must change. Each organ essential to metabolism must be
ready to assume its own active share in the process. Their vessels must
be adequate to take care not only of the needs of these organs themselves but also of the functions these organs must now take over in
maintaining the metabolism of the organism as a whole.
 
While the functional significance of the arrangement of the blood
vessels is always of importance, especially in understanding the
progressive changes in vascular plan, there is another factor which we
cannot overlook. This factor is conservative, having to do with the
things we inherit from our forebears. The goal of the embryonic
period is the attainment of a bodily structure similar to that of the
parents. Because it is so familiar, we accept with complaisance the
remarkable fact that this goal is attained with absolute regularity.
Accidents there may be, leading to defective development or malformation — but the fertilized ovum of a pig never gives rise to a cow.
The new individual will show detailed differences from its parents,
differences which are capitalized in the slow march of evolution; but
in a single generation these differences are never radical. We say
that the offspring has inherited the structure of its parents. It does
more. It inherits the tendency to arrive at its adult condition by
passing through the same sort of changes which its ancestors underwent in the countless millions of years it took their present structure
to evolve.
 
Applied to the development of the circulatory system of mammals
this means that the earliest form in which it appears will not be a
miniature of the adult circulation. The simple tubular heart pumping
blood out over aortic arches to be distributed over the body and
returned to the posterior part of the heart by a bilaterally symmetrical
venous system, in short the vascular plan which we see in young
 
 
 
230
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
mammalian embryos (Fig. 45), is essentially the plan of the circulation
in fishes. When we realize this, we are not puzzled either by the
appearance of a full complement of aortic arches, or by their subsequent disappearance to make way for a new respiratory circulation in
the lungs. We see the march of progress from a logical beginning in
ancestral conditions toward the consummation of fetal life with an
organization like that of the parent.
 
In addition to the fundamental ground plan of the circulation of
the mammalian embryo, recapitulations account for many transitory
peculiarities. The formation of a conspicuous though empty yolk-sac
with a complement of blood vessels almost as well developed as the
vitelline vessels of animals well endowed with yolk, is clearly a recapitulation of ancestral conditions. So also is the highly developed
system of venous channels in the mesonephros. If the organ itself
appears it brings with it its quota of vessels, no matter whether or not
the organ is destined to degenerate later in development.
 
Whatever peculiarities may be impressed on the course of the
circulation by the appearance of ancestral structures or by the
development of special fetal organs such as the yolk-sac and the
placenta, the main blood currents will at any time be found concentrated at the centers of activity. Changes of these main currents as one
center retrogresses and another becomes dominant, must take place
gradually. Large vessels become smaller, what was formerly an
irregular series of small vessels becomes excavated to form a new main
channel, but the circulation of blood to all parts of the body never
ceases. Even slight curtailment of the normal blood supply to any
region would stop its growth ; any marked local decrease in the circulation would result in local atrophy or malformation ; complete interruption of any important circulatory channel, even for a short time,
would inevitably mean the death of the embryo.
 
II. The Arteries
 
The Derivatives of the Aortic Arches. In vertebrate embryos
six pairs of aortic arches are formed connecting the ventral with the
dorsal aorta. The portions of the primitive paired aortae which bend
around the anterior part of the pharynx constitute the first (i.e., the
most anterior) of these aortic arches. In its course around the pharynx
the first aortic arch is embedded in the tissues of the mandibular arch
(Fig. 136, A). The other aortic arches develop later, in sequence, one
aortic arch in each branchial (gill) arch posterior to the mandibular
 
 
 
 
 
 
 
 
 
 
 
 
 
232
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
 
Worsotremnanf,
 
^ofl^aorftcarch
 
'd^aorfic arch
fyorhc sac
S^aorhcarch
A^aorhcarch
)POhnonaryarch
 
 
Right dorsal
aorta
 
 
\Ana5t0mosiS'
 
 
External carotid! artery
External carotid artery
5^ aortic arch
 
4^ aortic arch
Pulmonary arch
Pulmonary artery
 
Left dorsal aorta
 
Putmonary
Segmental arteries
 
Subclauian artery
 
 
 
Aorta
 
 
Fig. 134. Ventral aspect of vessels in the branchial region of pig embryos
of various ages. (After Heuser.) The drawings in this and the three following
figures were made directly from injected specimens rendered transparent by
treatment with wintergreen oil. A, 24 somites; B, 4.3 mm.; C, 6 mm.; D,
8 mm.; E, 12 mm.
 
 
ftemnant of
right pu/mo'
nary arch.
 
 
Pufmanaryarch
â– PuknonatyMfery
fbsienor cervical
arhny
 
'^Bmehat artery
StMnor mtercos*
 
 
SubckMan artery
 
 
 
 
 
 
234
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
arches, leaves only the ventral and dorsal aortic roots and the third,
fourth, and sixth arches to play an important r61e in the formation
of adult vessels.
 
In dealing with embryos of 9 to 12 mm. we have already seen
how the portions of the ventral aortic roots which formerly acted as
feeders to the first two arches were retained as the external carotid
arteries. These vessels, in part through the small channels left by the
disintegration of the aortic arches with which they were originally
associated, and in part through the formation of new branches to
subsequently formed structures, nourish the oral and cervical regions
(Figs. 67, 133 and 137, A, B).
 
The internal carotid arteries also, are, familiar as vessels which
arise as prolongations of the dorsal aortic roots and extend to the brain
(Fig. 67). When the portion of the dorsal aortic root which lies between arch 3 and arch 4 dwindles and drops out, the third arch is left
constituting the curved proximal part of the internal carotid artery
(Figs. 133, B, C, and 137, A~C). The part of the ventral aortic root
which, from the first, has fed the third aortic arch becomes somewhat
elongated and persists as the common carotid artery (Figs. 133, 137).
 
The fourth aortic arch has a different fate on opposite sides of the
body. On the left it is greatly enlarged and persists as the arch of the
adult aorta (Figs. 133 and 134-137). On the right the fourth arch
forms the root of the subclavian artery. The short section of the right
ventral aortic root proximal to the fourth arch persists as the innominate (brachiocephalic) artery from which both the right subclavian and the right common carotid artery arise (Fig. 133, C).
 
The sixth aortic arch changes its original relationships somewhat
more than the others. At an early stage of development branches
extend from its right and left limbs toward the lungs (Fig. 134, D, E).
After these pulmonary vessels have been established^ the right side
 
^ The details of the formation of the pulmonary arteries differ somewhat in
different mammals. In most of the forms which have been carefully studied (man,
cat, dog, sheep, cow, opossum) the pulmonary arteries maintain their original paired
condition throughout their entire length. In these forms part of the right sixth arch
is retained as the proximal portion of the adult right pulmonary artery (Fig, 133).
The pig is unusual in having its pulmonary branches fuse with each other proximally,
forming a median vessel ventral to the trachea (cf. Figs. 134, E, and 135, A). Distal
to this short median trunk the pulmonary vessels retain their original paired condition, each running to the lung on its own side of the body. Proximally the median
trunk becomes associated with the left sixth arch and the right sixth arch drops out
altogether.
 
 
 
THE ARTERIES
 
 
235
 
 
of the sixth aortic arch loses communication with the dorsal aortic
root and disappears (Fig. 135, A, B). On the left, however, the sixth
arch retains its communication with the dorsal aortic root. The
portion of it between the point where the pulmonary trunk is given
off and the dorsal aorta is called the ductus arteriosus (Figs. 133, C, and
138). During the fetal period when the lungs are not inflated the
ductus arteriosus shunts the excess blood from the pulmonary circulation directly into the aorta. The functional importance of this channel
will be more fully appreciated when we have given it further consideration in connection with the development of the heart and the
changes which take place in the circulation at the time of birth.
 
While these changes have been taking place in the more peripheral
part of the vascular channels which lead to the lungs, a fundamental
alteration has occurred in the main ventral aortic stem. Formerly a
single channel leading away from the undivided ventricle of the
primitive tubular heart, the ventral aorta now becomes divided
lengthwise into two separate channels. This division begins in the
aortic root just where the sixth arches come off, and progresses thence
toward the heart. Meanwhile, as wc shall see when we take up the
development of the heart, the ventricle has become divided into right
and left chambers. The final result of these two synchronous partitionings is the establishment of a channel leading from the right
ventricle to the lungs by way of the sixth aortic arches, and another
separate channel leading from the left ventricle to the dorsal aorta by
way of the left fourth aortic arch.
 
The Derivatives of the Intersegmental Branches of the Aorta.
In dealing with the structure of 9-12 mm. embryos comment was
made on the importance of the small intersegmental branches from
the dorsal adrta (Figs. 67 and 136, F). At that time, too, we became
familiar with some of the vessels which are derived from these branches.
The anterior appendage bud first appears at the level of the seventh
cervical intersegmental, and it is this artery which becomes enlarged
to form the subclavian. With the enlargement of the left fourth aortic
arch to form the main channel leading from the heart to the dorsal
aorta, the dorsal aortic root on the right side becomes much reduced
(Fig. 135, A-D). Caudal to the level of the subclavian it drops out
entirely. It will be recalled that the sixth aortic arch also drops out
on this side. This leaves the right subclavian communicating with the
dorsal aorta by way of a considerable section of the old dorsal aortic
root and the fourth aortic arch. In the adult, both the distal part of
 
 
 
236
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
LeffJVaorhc arch
 
 
 
 
Lehl^Qorh'carch
 
 
Optic vesich
 
 
 
 
Left 3^ aortic arch
 
 
Segmental a rtenes
 
 
Left dorsal aorta
 
 
 
 
Lefr pulmonary
arch
 
 
Left anterior cardinal i/etrt
 
Left dorsal aorta
 
 
LeftS^aorhc arci
 
 
Fig. 136. Lateral aspect of vessels in the branchial region of pig embryos
of various ages. (After Heuser.) A, 10 somites; B, 19 somites; C, 26 somites;
D, 28 somites; E, 30 somites; F, 36 somites (6 mm.).
 
this vessel (formed by the intersegmental artery) and its proximal
portion (appropriated from the old aortic arch system) pass under the
name subclavian. This accounts for the striking dissimilarities of
origin between the right and left subclavian arteries in the adult.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE ARTERIES
 
 
237
 
 
 
VtrhbmlarUry
 
 
Fig. 137. Continuation of the same series of lateral views of injected embryos.
A, 14 mm.; B, 17 mrn.; C, 19.3 mm.; D; 20.7 mm.
 
 
Cephalic to the subclavian arteries, a series of longitudinal anastomoses appear connecting the cervical intersegmentals to form the
vertebral arteries (Fig. 137). When the vertebral arteries are thus
established, all the intersegmental roots back to the subclavian drop
out, leaving the vertebral as a branch of the subclavian (Figs. 1 33, C,
 
 
 
238
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
and 137). The manner in which the vertebrals swing in to the mid-line
rostrally and become confluent with each other to form the basilar
artery, and the anastomosis between the internal carotids and the
basilar artery in the region of the hypophysis, are already familiar.
 
Caudal to the subclavian, the internal mammary artery is formed
by longitudinal anastomosing of the more cephalic of the thoracic
intersegmental arteries. Subsequent dropping out of the proximal
parts of the other intersegmentals leaves it arising from the subclavian.
Thus the steps in its origin are strikingly similar to the processes by
which the vertebral artery was established cephalic to the subclavian
(Fig. 133). Still farther caudally in the body, the intersegmental
arteries retain their original independent condition as paired branches
extending from the aorta dorsad on either side of the neural tube and
the developing spinal column (Fig. 67). Even in the adult these vessels
appear with little change in their original relations.
 
The Enteric Arteries. The first of the three enteric arteries to
appear is the anterior {superior) mesenteric. We have already followed
its origin as a pair of arteries originally called the omphalomesentcrics
which, in young embryos, extended to the surface of the yolk-sac
(Fig. 45). When the yolk-sac degenerates and the ventral part of the
body closes in, these paired channels fuse with each other to form a
median vessel situated in the mesentery and extending to the gut loop
in the belly-stalk (Fig. 66). This is now called the anterior mesenteric
artery. With the elongation and coiling of the intestine in the region
fed by it, the anterior mesenteric artery acquires many radiating
terminal branches. Its primary relations, however, remain unchanged.
 
The celiac artery arises from the aorta in a manner basically similar
to the origin of the anterior mesenteric artery, but at a slightly later
stage of development. The embryonic body has, therefore, become
more nearly closed ventrally and the primary dorsal mesentery has
been established. As a result the primary paired condition we expect
to see in the early stages of the formation of all of the main enteric
arteries is greatly abbreviated in the case of the celiac artery, and
almost from its first appearance it is a median vessel extending in the
mesentery toward the gastric region of the gut (Fig. 67). As development progresses it becomes extensively branched, being the main
artery which feeds the gastro-hepato-pancreatic region of the digestive
system and also the spleen which arises in its territory (Fig. 111).
 
The inferior mesenteric artery has an origin similar to that of the
celiac. It is established caudal to the anterior mesenteric artery slightly
 
 
 
THE ARTERIES
 
 
239
 
 
 
Choroid plexus of ventricle IZ
 
 
Basilar artery
 
Primordium of
occipital bone
 
Arytenoid process//^;
of larynx
 
Pharyngeal
bursa
Recurrent
laryngeal n
 
Aortic arch
 
Ductus
arteriosus
 
Dorsal aorta
 
Left common
cardinal vein
 
Lung bud
 
Adrenal gland
 
Stomach
 
Pancreas
 
Gonad
 
Gut loop in
mesentery
 
Mesonephros
 
Metanephros
 
Post cardinal vein
 
Dorsal root
ganglion
 
 
Mesocoele
 
 
Tuberoulum
posterius
 
Infundib
ulum
 
Rathke's
pocket
 
Foremen
of Monro
 
Tongue
Thyroid
gland
 
Pulmonary valves
 
 
Pericardial coelom
 
 
Septum transversum
 
Gastric branch
of vagus n.
 
Left umbilical
vein in liver
 
Dorsal
 
mesogastrium
(omental portion)
 
Gut loop in
^^^^belly-stolk
 
(umbilical) art
 
Genital tubercle
 
Nerves of
lumbosocral plexus
 
 
Fig. 138. Drawing (X 10) of parasagittal section of 15 mm. pig embryo.
The section is in a plane slightly to the left of the mid-line and passes through
the ductus arteriosus, lung bud, stomach, gonad, and metanephros.
 
 
 
240
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
later in development than the time at which the celiac appears and
is the main vessel to the posterior part of the intestinal tract (Fig. 111).
 
The Renal Arteries. The mesonephros is suppliea by many small
arteries which arise ventro-laterally from the aorta. While the metanephroi or permanent kidneys are still very small, they lie in close
proximity to the mesonephroi and are fed by small arteries which
arise from the aorta along with the mesonephric vessels (Fig. 127). The
local vessels associated with the kidneys are progressively enlarged
as the kidneys themselves grow in bulk and become the renal arteries
of the adult (Fig. 128).
 
The Arteries Arising from the Caudal End of the Aorta. The
 
main aortic trunk decreases abruptly in size where the large umbilical
(allantoic) arteries (Fig. 138) turn off into the belly-stalk. Beyond
this point the aorta is continued toward the tail as a slender median
vessel called the caudal artery (Fig. 67).
 
The posterior appendage buds arise some time after the placental
circulation has been established. The umbilical arteries are consequently of considerable size, and the small vessels which branch off
from them to feed the appendage buds are by comparison quite
insignificant. As the appendage buds grow, these small vessels grow
with them to become the external iliac arteries. When, at birth, the
placental circulation stops, the umbilical arteries are reduced to small
vessels nourishing the local tissues between their point of origin and
the umbilicus. We then know their proximal portions as the internal
iliac^ or hypogastric arteries^ and the fibrous cords which still mark their
course along the wall of the bladder (the old allantoic stalk) as the
obliterated branches of the hypogastric arteries. Thus the tables are
turned between fetal and adult life. In the fetus the external iliac
artery to the leg appears as a branch of the dominating umbilical
artery. After birth the reduced umbilical arteries under their new
name of internal iliacs, or hypogastrics, appear as branches of the
now larger external iliacs. The original umbilical root proximal to
the origin of the external iliac is called the common iliac (Fig. 150).
 
III. The Veins
 
There is a natural grouping of the veins according to their relationships which it is convenient to follow in discussing their development.
Under the term systemic veins we can include all the vessels which
collect the blood distributed to various parts of the body in the routine
of local metabolism. In young embryos these would be the cardinal
 
 
 
THE VEINS
 
 
241
 
 
veins and their tributaries, that is, the return channels of the primitive
intra-embryonic circulatory arc. In older embryos and adults the
systemic veins would include the anterior (superior) caval system
which is evolved from the anterior cardinals, and the posterior
(inferior) caval system which takes the place of the postcardinals and
their tributaries.
 
We can set apart from the general systemic circulation three
special venous arcs: the umbilical, returning the blood from the
})lacenta; the pulmonary, returning the blood from the lungs; and
the hepatic portal, carrying blood from the intestinal tract to the liver.
I'he specialized nature of the placental and pulmonary circulations
is obvious. The peculiarities of the hepatic portal system call, perhaps, for a word of explanation. Ordinarily veins^ collect blood from
local capillaries and pass it on directly to the heart. Their blood stream
is away from the organ with which they arc associated ; once collected
within a vein the blood is not redistributed in capillaries until it has
again jiassed through the heart. The portal vein arises in typical
fashion by collecting the blood from capillaries in the digestive tube.
But then, contrary to the usual procedure, its blood flows, not directly
to the heart, but to the liver where it enters a second capillary bed
and is returned by a second set of collecting vessels to the heart. With
reference to the plexus of capillaries in the liver this vein is aflferent.
Hence its designation as a portal (translated = carrying to) vein,
setting it apart from other veins which carry blood only away from
the organ with which they are associated.
 
There is a tendency among those who have done but little work on the circulation to regard any vessel which carries oxygenated blood as an artery, and any vessel
which carries blood poor in oxygen and high in carbon dioxide content as a vein.
This is not entirely correct even for the circulation of adult mammals on which the
( onception is based. In comparative anatomy and especially in embryology it is far
from being the .case. It is necessary, therefore, in dealing with the circulation of the
embryo, to eradicate this not uncommon misconception.
 
The differentiation between arteries and veins which holds good for all forms,
both embryonic and adult, is based on the structure of their walls, and on the direction of their blood flow with reference to the heart. An artery is a vessel carrying
blood away from the heart under a relatively high, fluctuating pressure due to the
pumping of the heart. Correlated with the pressure conditions in it, its walls are
heavily reinforced by elastic tissue and smooth muscle. A vein is a vessel carrying
blood toward the heart under relatively low and constant pressure from the blood
welling into it from capillaries. Correlated with the pressure conditions characteristic for it, the walls of a vein have much less elastic and muscle tissue than artery
walls, and more non-elastic connective-tissue fibers.
 
 
 
Heart
 
 
" Artt Cardinal
 
 
Developing
SubcordinolPlwu3 in
tleson.
 
 
 
SubcL
 
 
von
 
 
 
Fig. 139. Diagrams illustrating stages in the development of the systemic
veins of the pig. (After Butler.) The cardinal and omphalomesenteric veins
are shown in black, the subcardinal system is stippled, the supracardinals are
horizontally hatched, and vessels arising independently of these three systems
are indicated by small crosses,
 
A, Ground plan of the veins of a young mammalian embryo (cf. Fig. 45).
 
B, Cross-section (at level of arrow in A) showing dorso-ventral relations
of the various veins.
 
C, Diagrammatic plot of veins of mm. pig embryos.
 
D, Arrangement of veins in 6“-7 ram. pig embryos.
 
242
 
 
 
Juoulor;;,
 
 
 
 
 
 
E, Cross-section (at level of arrow in D) showing dorso- ventral relations of
vessels.
 
F, Veins in 12—13 mm. embryos.
 
G, Veins in 16-19 mm. embryos.
 
H, Veins in 22-24 mm. embryos.
 
I, Veins in 30-35 mm. embryos.
 
J, Cross-section of 17 mm, embryo at level of arrow in G.
 
K, Plan of veins in adult pig^
 
 
243
 
 
 
244
 
 
THE DEVELOPMENT OF THE CTRC.ULATORY SYSTEM
 
 
Changes in the Anterior Systemic Veins Resulting in the
Establishment of the Anterior (Superior) Vena <^va. The main
tributaries draining the anterior parts of the adult body are the
external and internal jugulars and the subclavians. In 12 mm. embryos we saw all these vessels laid down. The internal jugular is
merely the original anterior cardinal under a new name. The external jugular develops from the small branch draining the mandibular region, and the subclavian as an enlargement of one of the segmental tributaries at the level of the anterior appendage bud (Fig. 68).
 
When the appendage buds first appear, the heart lies far forward
in the body. As development progresses it is carried caudad. With
this change in the position of the heart, the common cardinal veins
(ducts of Cuvier) change their relative position in the body and come
to lie caudal to the anterior appendage buds. As a result of this
altered relation the subclavian veins from the anterior appendages,
which early in development drain into the posterior cardinals (Fig.
139, C) eventually empty into the anterior cardinals (Fig. 139, G).
 
The outstanding characteristic of the systemic venous plan of a
young embryo is its bilateral symmetry. Paired vessels from the
anterior and from the posterior parts of the body become confluent
to enter the sinus region of a simple tubular heart (Figs. 1 39, A and
144, A-C). The rerouting of the blood to enter the right side of the
heart is the all-important end toward which the mammalian venous
system is progressing throughout its development. In dealing with
local changes this basic trend should never for a moment be forgotten.
 
In the anterior systemic channels this shift to the right is accomplished very simply and directly. A new vessel forms between the
right and left anterior cardinals and shunts the left anterior cardinal
blood stream across to the right (Fig. 139, H). With the establishment
of this new channel, the part of the left anterior cardinal toward the
heart drops out (Fig. 139, I). We have now but to apply the familiar
adult names (Fig. 139, K): the new connecting vessel is the left
innominate; the old anterior cardinal between the union of the subclavian with the jugulars and the new transverse connection is the
right innominate ; from the confluence of the innominates to the heart
is the anterior vena cava. The anterior vena cava is thus composed of
the most proximal part of the right anterior cardinal and the right
common cardinal vein (duct of Cuvier). The small azygos (cervicothoracic) vein, which is the reduced posterior cardinal, indicates the
old point of transition from anterior cardinal to common cardinal.
 
 
 
THE VEINS
 
 
245
 
 
Changes in the Posterior Systemic Veins Resulting in the
Establishment of the Posterior (Inferior) Vena Cava. The changes
in the systemic veins of the posterior part of the body are much more
radical than they are anteriorly. The posterior cardinal veins which
are the primitive systemic drainage channels are associated primarily
with the mesonephroi. When the mesonephroi degenerate, it is but
natural that the posterior cardinals should degenerate with them.
Hie posterior vena cava which replaces the cardinals is a composite
vessel which gradually takes shape by the enlargement and straightening of small local channels which are, as it were, pressed into service
as the posterior cardinals degenerate.
 
The subcardinal veins initiate the diversion of the postcardinal
blood stream. The subcardinals arc established as vessels lying along
the ventro-mesial border of the mesonephros parallel with, and ventral
to, the postcardinals. Taking origin from an irregular plexus of small
vessels emptying from the mesonephroi into the posterior cardinals,
the subcardinals from their first appearance have many channels
connecting them with the postcardinals (Fig. 139, A-C). As the mesonephroi increase in size and bulge toward the mid -line the subcardinals are brought very close together. In the mid -mesonephric
region they anastomose with each other to form a large median vessel,
the subcardinal sinus, or intersubcardinal anastomosis (Figs. 60, 77,
and 139, D-F). When this sinus is established the small vessels connecting sub- and postcardinals drain into the capacious sinus rather than
toward the posterior cardinals. The result of this change very soon
becomes apparent in the disappearance of the posterior cardinals at
the level of this sinus. The blood from the posterior part of the body is
still collcctecl by the distal ends of the postcardinals but it returns to
the heart by way of the subcardinal sinus. Consequently the anterior
portions of the posterior cardinals, although they persist, are much
reduced in size (Fig. 139, F).
 
Meanwhile the increased volume of blood entering the subcardinal
sinus is finding a new and more direct route to the heart. The cephalic
pole of the right mesonephros lies close to the liver. A fold of dorsal
body-wall tissue, just to the right of the primary dorsal mesentery,
early makes a sort of bridge between these two organs. This fold is
known as the caval plica {caval mesentery) (Fig. 140). In it, as everywhere in the growing body, are numerous small vessels. Connection
of these small vessels with the plexus of channels in the liver cephalically (Fig. 140, A), and the mesonephros caudaliy (Fig. 140, B),
 
 
 
246
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSl’EM
 
 
 
' : Dorsal aorta
 
 
Posterior
cardinal vein^rV
 
Cava leaving ‘ :
mesonephros^''
 
Caval plica
hCava
 
entering liver
 
Stomach
 
Ductus
venosus
 
Gall-bladder
 
Rt umbilical
vein
 
 
 
Fig. 140. Drawings of transverse sections of 5 mm. pig embryos showing
the relations of the caval fold of the mesentery (caval plica). A, Section at
level where cephalic end of caval plica merges with liver. Here the small
vessel which is the primordium of the mesenteric portion of the posterior
vena cava enters the sinusoidal circulation of the liver. B, Section showing
merging of caudal end of caval plica with right mesonephros. At this level
the developing mesenteric portion of the posterior vena cava anastomoses
with the right subcardinal vein (cf. Fig. 139, D).
 
 
provides the entering wedge. Once a current of blood finds its way
from the mesonephros to the liver through these small vessels, enlargement of the channel proceeds with great rapidity. This new channel
becomes the mesenteric part of the inferior vena cava, (See relations of
posterior vena cava in figures 75 and 76, and portion of cava indicated
with small crosses in figure 139, D.)
 
Within the liver this new blood stream at first finds its way by
devious small channels eventually entering the sinus venosus along
with the omphalomesenteric circulation. As its volume of blood
increases it excavates through the liver a main channel which gradually becomes walled in. As this new vessel becomes more and more
definitely organized it gradually crowds toward the surface and
eventually appears as a great vein lying in a notch along the dorsal
side of the liver. This is the hepatic part of the inferior vena cava.
 
From the subcardinal sinus the most direct route to this new
 
 
THE VEINS
 
 
247
 
 
outlet is by way of the right subcardinal vein. Thus in embryos as
young as 9 to 12 mm., the formation of the posterior vena cava is well
started with its proximal portion consisting of subcardinal sinus, a
portion of the right subcardinal vein, and the new channels through
the mesentery and through the liver. (Follow this part of the cava
through figure 139 from D to K.)
 
Posterior to the level of the subcardinal sinus still amother set of
veins enters into the formation of the post-cava. These are the supracardinal veins which appear, relatively late in development, as paired
channels draining the dorsal body-wall (Fig. 139, G, J).
 
At the mid-mesonephric level, the supracardinals are diverted into
the subcardinal sinus just as happened with the postcardinals earlier
in development. Cephalic to the sinus, parts of the supracardinals
persist as the azygos group draining in a somewhat variable manner
into the reduced proximal part of the postcardinals. Caudal to the
anastomosis with the subcardinal sinus, the right supracardinal becomes the principal drainage channel of the region. Its appropriation
of the tributary vessels from the posterior appendages establishes it as
the postrenal portion of the inferior vena cava and is the last step in the
formation of that composite vessel (Fig. 139, G—K).
 
The Coronary Sinus. The ultimate fate of the left common
cardinal vein (duct of Cuvier) is a result of the shift in the course of
the systemic blood so that it all enters the right side of the heart.
Formerly returning a full half of the systemic blood stream to the
heart, the left common cardinal vein is finally left almost without a
tributary from the body. In the pig a small amount of blood usually
does continue to enter it over the left azygos (Fig. 139, K). Occasionally in the pig, and normally in most other mammals, even this is cut
off and the azygos drainage is by way of the right side to the superior
cava (dotted line in Fig. 139, K). Nevertheless the proximal part of
the old left cardinal channel is utilized. Pulled around the heart in
the course of the migration of the sinus venosus toward the right, the
left common cardinal vein lies close against the heart wall for a considerable distance (Fig. 144, D). As the heart muscle grows in bulk it
demands a greater blood supply for its metabolism. The small returning veins of this circulation find their way into this conveniently
located main vessel (Fig. 144, E). Thus even when its peripheral
circulation is cut off the left common cardinal vein still persists as the
coronary sinus into which the vessels of the cardiac wall drain (Fig.
144, F).
 
 
 
248
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
 
right
 
omplLv'
 
 
right
 
iimb.v,
 
 
umb. vein*
fused
in belly
stalk
 
 
coronary sinus
(left duct of
 
 
hepatic v,
(omph.)
 
 
 
left
 
umb.v«
 
 
Fig. 141. Diagrams showing the development of the hepatic portal circulation from the omphalomesenteric veins, and the relations of the umbilical
veins to the liver. (Adapted from several sources.)
 
A is based on conditions in pig embryos of 3-4 mm.; B, on embryos of
about 6 mm.; C, on embryos of 8-9 mm.; D, on embryos of 20 mm. and
older.
 
 
 
THE VEINS
 
 
249
 
 
The Pulmonary V eins. Phylogenetically the lungs are relatively
new structures. It is not surprising, therefore, that we find the pulmonary veins arising independently and not by the conversion of old
vascular channels. They originate as vessels which drain the various
branches of the lung buds and converge into a common trunk (Fig.
107), entering the left atrium dorsally. In the growth of the heart this
trunk vessel is gradually absorbed into the atrial wall and two or more
of its original branches open directly into the left atrium as the main
pulmonary veins of the adult (Fig. 144, D-F).
 
The Portal Vein. The blood supply to the intestines is first
established through the omphalomesenteric arteries which later
become modified to form the anterior mesenteric artery. Likewise
the drainage of the intestinal tract is provided for by vessels which
were originally the return channels of the primitive omphalomesenteric circulatory arc (Fig. 45). In dealing with 9 to 12 mm. embryos
we have already seen the primary changes which occur in these vessels.
The growing cords of hepatic tissue break up the proximal portion
of the omphalomesenteric veins into a maze of small channels ramifying through the substance of the liver (Fig. 141, A, B). But the stubs
of the omphalomesenterics persist and drain this plexus. Distal to the
liver, the original veins are for a time retained, bringing blood from
the yolk-sac and intestines to the liver. With the disappearance of the
yolk-sac and the growth of the intestines, the omphalic (yolk-sac)
portions of these veins necessarily disappear, but the mesenteric
branches persist and become more extensive concomitantly with the
increased length and complexity of the intestinal tract.
 
The original omphalomesenteric trunks into which these tributaries converge become the unpaired portal vein by forming transverse
anastomoses and then abandoning one of the original channels. The
curious spiral course of the portal vein is due to the dropping out of
the original left channel cephalic to the middle anastomosis and the
original right channel caudal to the anastomosis (Fig. 141, D).
 
The Umbilical Veins. When they are first established the umbilical (allantoic) veins are embedded in the lateral body-walls
throughout their course from the belly-stalk to the sinus venosus
(Fig. 45). As the liver grows in bulk, it fuses with the lateral body-wall.
Where this fusion occurs vessels develop connecting the umbilical
veins with the plexus of vessels in the liver (Fig. 141, B). Once these
connections are established the umbilical stream tends more and more
 
 
 
250
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
to pass by way of them to the liver, and the old channels to the sinus
venosus gradually degenerate (Fig. 141, C). ^
 
Meanwhile the umbilical veins distal to their entrance into the
body become fused with each other so that there comes to be but a
single vein in the umbilical cord (Fig. 141, C). Following this fusion
in the cord, the intra-embryonic part of the umbilical channel also
loses its original paired condition. The right umbilical vein is abandoned as a route to the liver and all the placental blood is returned
over the left umbilical vein. It is interesting to note that in spite of its
ceasing to be a through channel, part of the right umbilical vein
persists, draining the body-wall. The small blood stream it then carries
is reversed in direction, flowing back into the left umbilical (Fig. 141,
D).
 
When first diverted into the liver, the umbilical blood stream
passes through by way of a mesh work of small anastomosing sinusoids.
As its volume increases it excavates a main channel through the
substance of the liver which is known as the ductus venosus (Figs. 138
and 141, B~D). Leaving the liver, the ductus venosus becomes confluent with the hepatic veins (omphalomesenteric stubs) which drain
the maze of small sinusoids in the liver. At this point, also, the vena
cava joins the others. Thus the blood streams from the posterior
systemic circulation, from the portal circulation, and from the placental circulation all enter the heart together. Embryologically this
great trunk vessel represents the fused proximal parts of the old
omphalomesenteric veins enlarged by the placental blood from the
ductus venosus and by the systemic blood from the vena cava (Fig.
141). During its early developmental phases it is often called the
common revehent hepatic vein. In the adult or in older fetuses it is more
convenient to regard it as a part of the vena cava because, with the
cessation of the placental circulation at the time of birth, the caval
blood stream becomes the dominant one.
 
IV. The Lymphatic System
 
Important and interesting as the subject is, it has seemed expedient
to omit any account of the development of the lymphatic vessels.
Those interested in this field will find that an unusual amount of
careful work has been done on the development of the lymphatics in
pig embryos. References to some of the more important recent papers
have been included in the bibliography appended at the end of the
book.
 
 
 
BLOOD CORPUSCLES
 
 
251
 
 
V. Blood Corpuscles
 
The first blood corpuscles which appear in the circulation are
produced extra-embryonically in the blood islands of the yolk-sac
(Fig. 48). Later in development there are many blood-forming centers
within the embryo. Concerning the establishment of these centers we
find the same controversy that was commented on in connection with
the origin of blood vessels. It is maintained by some authorities that
these centers always arise from cells originally produced in the yolksac blood islands. According to this interpretation some of these blood
cells are believed to remain sufficiently undifferentiated to retain their
power of active proliferation. As such cells are carried by the circulation to various parts of the body they settle in favorable locations and
raise new families of blood corpuscles.
 
According to the local origin idea it is not necessary to account
for all the centers of blood corpuscle formation on the basis of bloodmother-cells wandering in from the yolk-sac and settling down in new
locations within the embryo. It is maintained that mesodermal cells
arising in the body have the same capacity of becoming differentiated
into blood-mother-cells as mesodermal cells which arise in the yolksac. On this interpretation blood-forming centers arise in various
parts of the body from blood-mother-cells differentiated in situ from
local mesoderm. The recent experimental work tends to indicate that
such “local origin” does occur. That this same evidence proves that
the origin of blood-forming centers from migrating cells never occurs
is by no means so clear.
 
Whatever the source of the original blood-mother-cells may be
we always find them establishing their centers of proliferation in
places where the current of the circulation is sluggish. In very young
embryos the centers of corpuscle formation are located in the maze of
small channels in the yolk-sac and the allantois. When the yolk-sac
degenerates and the allantois becomes highly specialized as part of
the chorion, these centers cease to be active and new ones are established in connection with such rich vascular plexuses as those in the
mesonephros and the liver. Still later other centers appear in the
lymphoid organs, and last of all in the bone marrow (Fig. 152).
 
The histological details of the processes involved in the production
of red blood corpuscles and the various types of white blood corpuscles
are exceedingly complex. Moreover there is by no means agreement
as to the exact manner in which these processes occur, nor as to the
 
 
 
252
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
genetic relations of one type of blood corpuscle to another. This whole
subject, other than the recognition of the multiplicity of blood-forming
centers and their shifting locations in the embryo at various phases of
development, is a special field of histogenesis entirely beyond the
scope of an elementary text.
 
VI. The Heart
 
To appreciate the significance of the changes which occur in
the growing heart one must have in mind the exigencies under which
it develops. Starting as a simple tube with the blood passing through
 
 
 
Fig. 142. Ventral aspect of the heart of the pig at various stages showing
the formation of the cardiac loop and the establishment of the primary
regional divisions of the heart. Drawn (A-E, X 30; F, X 20) from reconstructions made from series in the Carnegie Collection (A-D) and in the
Western Reserve University Collection (E, F).
 
A, 7 somites; B, 13 somites; C, 17 somites; D, 25 somites; E, 3.7 mm.
(after flexion); F, 6 mm.
 
Abbreviations: At., atrium (t., right; 1., left); Bui., aortic bulb (bulbus
arteriosus); Endc., endocardial tubes; Myc., cut edge of epi-myocardium;
S.V., sinus venosus; Tr. Art., truncus arteriosus; V. ao. r., ventral aortic
roots; Vent., ventricle (r., right; 1., left); V.O.M., omphalomesenteric veins.
 
 
 
THE HEART
 
 
253
 
 
it in an undivided stream, it must become converted into an elaborately valved, four-chambered organ, partitioned in the mid-line
and pumping from its right side a pulmonary stream which is returned to the left side and pumped out again as the systemic blood
stream. And the heart cannot cease work for alteration ; there can be
no interruption in the current of blood it pumps to the growing
embryo. This is but one phase of the matter. At the end of gestation
the vascular mechanism must be prepared to function under conditions radically different from those surrounding the embryo. In spite
of the impossibility of the developing lungs being effectively exercised
under air-breathing conditions, they themselves, their blood vessels,
 
 
 
Fig. 143. Sinistral aspect of reconstructions of the pig heart.
 
A, 13 somite embryo (cf. Fig. 142, B); B, 17 somite embryo (cf. Fig.
142, C); C, 25 somite embryo (cf. Fig. 142, D); D, 3.7 mm. embryo (cf.
Fig. 142, E); E, 6 mm. embryo (cf. Fig. 142, F).
 
Abbreviations: A.C.V., anterior cardinal veins; Al.V., allantoic (umbilical) vein; Ao., aorta; At., atrium; Bui., aortic bulb; Cav. P., posterior
vena cava; Cuv. d., duct of Cuvier (common cardinal vein); Myc., cut edge
of epi-myocardium; P.C.V.; posterior cardinal vein; S.V., sinus venosus;
Sin-at., sino-atrial region of heart; Tr. Art., truncus arteriosus; V. ao. r.,
ventral aortic roots; Vent., ventricle; V.O.M., omphalomesenteric veins.
 
 
254
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
and the right ventricle which pumps blood to them, must at the
moment of birth be ready to take over the entire r^esponsibility of
oxygenating the blood. And the systemic part of the circulation as
well as the pulmonary must be prepared. Throughout intra-uterine
development the left side of the heart receives less blood from the
pulmonary veins than the right side of the heart receives from the
venae cavae. Yet after birth the left ventricle is destined to carry a
greater load than the right ventricle. It must pump through the
myriad peripheral vessels of the systemic circulation,, sufficient blood
to care for the active metabolism and continued growth of the entire
body. These are some of the situations which must be faced before the
heart can arrive at its adult condition. The manner in which they are
met is doubly interesting because they seem at first sight so difficult.
 
The Formation of the Cardiac Loop and the Establishment of
the Regional Division of the Heart. In dealing with the establishment of the circulatory system in very young embryos we saw how
the tubular heart was formed by the fusion of paired primordia
(see Chap. 5 and especially Figs. 43 and 44). The primary factor
which brings about its regional differentiation is the rapid elongation
of this primitive cardiac tube. The heart increases in length so much
faster than the chamber in which it lies that it is first bent to the right
and then twisted into a loop. Since the anterior end of the heart is
anchored in the body by the aortic roots, and the posterior end by
the great veins, it is the mid-portion of the heart-tube which, in this
process, undergoes the most extensive changes in position. This is
facilitated by the early disappearance of the dorsal mesocardium
which leaves the heart entirely free in its mid-region.
 
During the period in which the cardiac loop is being formed, the
primary regional divisions of the heart become clearly differentiated.
The sinus venosus is the thin-walled chamber in which the great veins
become confluent to enter the heart at its primary posterior end
(Fig. 144). The atrial region is established by transverse dilation of
the heart-tube just cephalic to the sinus venosus (Fig. 144).
 
The ventricle is formed by the bent mid-portion of the original
cardiac tube. As this ventricular loop becomes progressively more
extensive, it at first projects ventrally beneath the attached aortic
and sinus ends of the heart (Fig. 143, A-C). Later it is bent caudally
so that the ventricle, formerly situated cephalic to the atrium, is
brought into its characteristic adult position caudal to the atrium
(Fig. 143, D, E). Between the atrium and ventricle the heart remains
 
 
 
 
Pulmonary A
from Arch 11
 
 
Left
 
Com
 
CardV
 
 
Auricular /
 
App
 
 
Left Superior
Pulmonary V
 
 
Uft Inf Pul.V
 
Oblique Vein
Hrshall) â– 
of L Atrium
 
Great Cardiac V.
 
 
Coronary Sinus ^
 
toralTOv)
 
 
Leff Ventricle
 
 
f^XCom Card V
Vena Cava)
 
-Pul
Artery
 
 
 
Rt Pul
Veins
 
 
 
Right
Ventricle
 
Superior Vena Cava
 
Sulcus Terminalis
 
Sinus Venarum
 
Small Cardiac V
 
Inferior Vena Cava
 
Middle Cardiac Vein
 
 
Fig. 144. Six stages in the development of the heart, drawn in dorsal
aspect to show the changing relations of the sinus venosus and great veins
entering heart. (Patten: ‘‘Human Embryology,” The Blakiston Company.)
 
255
 
 
 
256
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
relatively undilated. This narrow connecting portion is the atrioventricular canal.
 
The most cephalic part of the cardiac tube undergoes least change
in appearance, persisting as the truncus arteriosus connecting the ventricle with the ventral aortic roots (Fig. 142). In very young embryos
there is a conspicuous bulge where the truncus arteriosus swings
toward the mid-line to break up in the aortic arches. This sharply
bent and somewhat dilated region is called the aortic bulb (Fig. 142,
B, C). Its location is of interest as being the place at which the paired
endocardial primordia first fused with each other (Figs. 43 and 44)
and the place at which, later in development, the division of the
truncus arteriosus into separate aortic and pulmonary roots will first
become apparent. The bulge itself soon merges into the rest of the
truncus arteriosus without giving rise to any special structure.
 
Almost from their earliest appearance the atrium and the ventricle
show external indications of the impending division of the heart into
right and left sides. A distinct median furrow appears at the apex of
the ventricular loop (Figs. 142, E, F, and 144, U, E). The atrium
meanwhile has undergone rapid dilation and bulges out on either
side of the mid-line (Fig. 144). Its bilobed configuration is emphasized by the manner in which the truncus arteriosus compresses it
mid-ventrally (Fig. 142, E, F).
 
The Partitioning of the Heart. These superficial features suggest
the more important changes going on internally. As the wall of the
ventricle increases in thickness it develops on its internal face a meshwork of interlacing muscular bands, the trabeculae carneae. Opposite
the external furrow in the ventricle these muscular bands become
consolidated as a partition which appears to grow from the apex of
the ventricle toward the atrium. This is the interventricular septum
(Fig. 147).
 
Meanwhile two conspicuous masses of peculiar, loosely organized
mesenchyme (called endocardial-cushion tissue) develop in the walls
of the narrowed portion of the heart between the atrium and the
ventricle. One of these so-called endocardial cushions of the atrio-ventricular
canal is formed in its dorsal wall (Fig. 147, A) and the other is formed
opposite, on the ventral wall. These two masses nearly occlude the
central part of the canal and thus initiate its separation into right and
left channels.
 
At the same time a median partition appears in the cephalic
wall of the atrium. Because another closely related partition is destined
 
 
 
THE HEART
 
 
257
 
 
to form here later, this one is called the first interatrial septum or
septum primum. In shape it is crescentic with its concavity directed
toward the ventricle and the apices of the crescent extending, one
along the dorsal wall, and one along the ventral wall of the atrium,
all the way to the atrio-ventricular canal where they merge, respectively, with the dorsal and ventral endocardial cushions (Fig. 147).
This leaves the atria separated from each other except for an opening
called the interatrial foramen primum.
 
While these changes have been occurring, the sinus venosus has
been shifted out of the mid-line so that it opens into the atrium to
the right of the interatrial septum (Fig. 147). The heart is now in a
critical stage of development. Its simple tubular form has been
altered so that the four chambers characteristic of the adult heart
are clearly recognizable. Partitioning of the heart into right and left
sides is well under way. But there is as yet no division of the blood
stream because there are still open communications from the right to
 
 
Carotid arch (III)
Aortic arch (IV)
Pulmonary arch (VI)
 
 
Left atrium—
Anterior cardinal
 
 
Posterior cardinal vein /
Interatrial foramen Iv
 
Septum 1 1
 
 
Truncus arteriosus
 
 
Pulmonary channel
 
 
-Aortic channel
 
 
 
interventricular
 
foramen
 
Interventricular
 
septum
 
 
Trabeculae
 
 
Valvulae venosae
 
 
/ Inferior vena cava
 
Atrio-ventricular valve
 
 
Fio. 145. Reconstruction of the heart of a 9.4 mm. pig embryo cut
open somewhat to the right of the mid-line to show its internal structure
(cf. Fig. 146).
 
 
 
 
 
 
 
 
 
 
258
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
the left side in both atrium and ventricle. A little further progress in
the growth of the partitions, however, and the two si^cs of the heart
would be completely separated. Were this to occur now, the left side
 
 
 
Fig. 146. Section (X 45) through the heart of a 9.4 mm. pig
embryo, at the level of the atrio-ventricular canals. (From the
series used in making the reconstruction appearing as figure 145.
This section may be oriented as passing horizontally through Fig.
145 at the level of the most caudal portion of the interventricular foramen.)
 
 
of the heart would become almost literally dry. For the sinus venosus,
into which systemic, portal, and placental currents all enter, opens
on the right of the interatrial septum, and not until much later do
the lungs, and their vessels develop sufficiently to return any considerable volume of blood to the left atrium. The partitions in the
 
 
 
THE HEART
 
 
259
 
 
ventricle and in the atrio-ventricular canal do progress rapidly to
completion (Figs. 147, 148, and 149), but an interesting series of
events takes place at the interatrial partition which assures an adequate supply of blood reaching the left atrium and thence the left
ventricle.
 
Just when it appears that the septum primum is going to fuse with
the endocardial cushions of the atrio-ventricular canals, closing the
interatrial foramen primum and isolating the left atrium, a new
opening is established. The more cephalic part of the septum primum
ruptures to form the interatrial for amen secundum, thus keeping a route
open from the right to the left atrium (Fig. 148).
 
At about this time the second interatrial partition makes its
appearance just to the right of the first. Also crescentic in form, this
septum secundum extends its apices along the dorsal and ventral walls
of the atrium to fuse with the septum primum near the endocardial
cushion mass which now completely divides the atrio-ventricular
 
 
 
Fig. 147. Drawings showing the initial steps in the partitioning of the
heart.
 
A, Slightly schematized drawing from reconstruction of heart of 3.7 mm.
pig embryo. The heart has been opened by a diagonally frontal cut in the
plane which can be indicated by drawing a line through the labels At. and
Vent, in figure 143, D. The dorsal portion of the heart is shown viewed from
the ventral side.
 
B, Slightly schematized drawing from a reconstruction of the heart of a
6 mm. pig embryo. The heart has been opened in a plane which can be
indicated by drawing a line through the labels At, and Vent, in figure 143, E.
The dorsal portion of the heart is shown viewed from the ventral side.
 
 
260
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
septum II
 
 
septum I
 
 
septum
 
spurium"
 
 
 
trabeculae
 
cameae
 
 
^ interatrial
 
foramen II
 
 
septum I
 
 
interatrial
â– " foramen I
^almost closed}  
 
 
a V canal
endocardial cushion
 
 
— intervent
septum
 
 
Fig, 148. Slightly schematized drawing from a reconstruction of the heart
of a 9.4 mm. pig embryo. Dorsal part of heart, interior view. The plane in
which the heart is opened can be indicated by drawing a line through the
center of the interatrial and the interventricular foramina in figure 145.
 
 
canals. But the septum secundum never becomes a complete partition.
An oval opening of considerable size persists in its center. This is the
foramen ovale (Fig. 149).
 
The newly established septum secundum and the flap-like remains
of the septum primum constitute an efficient valvular mechanism
between the two atria. When the atria are filling, some of the blood
discharged from the venae cavae into the right atrium can pass freely
through the foramen ovale by merely pushing aside the flap of the
septum primum. The inferior caval entrance lies adjacent to, and
is directed straight into, the orifice of the foramen ovale (Fig. 149).
Consequently it is primarily — some think exclusively — blood from
the inferior vena cava which passes through the foramen ovale into
the left atrium. When the atria start to contract, pressure of the blood
within the left atrium forces the flap of the septum primum against the
foramen ovale, effectively closing it against return flow into the right
atrium. Without some such mechanism affording a supply of blood
 
 
THE HEART
 
 
261
 
 
septum 11
 
 
^ interatrial foramen 11
 
 
septum I
 
 
anterior
vena cava
 
 
- pulmonary
veins
 
left atrium
 
 
coronary
 
 
 
right
 
ventricle
 
 
papillary muscle
interventricular
 
septum
 
 
Fig. 149. Schematic drawing based on dissected heart of pig fetus shortly
before birth. Interior aspect of dorsal part of heart to show valvular mechanism at foramen ovale.
 
 
to its left side, the developing heart could not be partitioned in the
mid-line ready to assume its adult function of pumping two separate
blood streams.^
 
While alf these changes have been going on in the main part of
the heart, the truncus arteriosus has been divided into two separate
channels. Reference has already been made to the start of this process
in the aortic root between the fourth and sixth arches (Fig. 133).
Continuing toward the ventricle, the division is effected by the formation of longitudinal ridges of plastic young connective tissue of the
same type as that making up the endocardial cushions of the atrioventricular canal. These ridges, called truncus ridges^ bulge progres
 
^ There are on record a few cases of a peculiar cardiac malformation in which
this valvular communication was prematurely closed and the left atrium thereby
shut off from the right. In all these cases the left side of the heart has not been sufficiently developed to support life for any length of time after birth.
 
 
262
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
sively farther into the lumen of the truncus arteriosus and finally meet
to separate it into aortic and pulmonary channels. ^ (Note shape of
lumen in Figs. 142, F, and 145.) The semilunar valves of the aorta,
and of the main pulmonary trunk (Fig. 138), develop as local specializations of these truncus ridges. Toward the ventricles from the site
of formation of the semilunar valves the same ridges are continued
into the conus of the ventricles. The fact that proximal to the valves
these ridges are, for descriptive purposes, called conus ridges should not
be allowed to obscure their developmental and functional continuity
with the truncus ridges. The truncus and conus ridges follow a spiral
course such that where the conus ridges extend down into the ventricles they meet and become continuous with the interventricular
septum. The right ventricle then leads into the pulmonary channel
and the left into the aorta. With this condition established the heart
is completely divided into right and left sides except for the interatrial
valve which must remain open throughout fetal life, until, after birth,
the lungs attain their full functional capacity and the full volume of
the pulmonary stream passes through them to be returned to the left
atrium.
 
This leaves but one of the exigencies of heart development still to
be accounted for. If, during early fetal life, before the lungs were well
developed, the pulmonary channel were the only exit from the right
side of the heart, the right ventricle would have an outlet inadequate
to develop its pumping power. For it is only late in fetal life that the
lungs and their vessels develop to a degree which prepares them for
assuming their postnatal activity, and the power of the heart muscle
must be built up gradually by continued functional activity. This
situation is met by the ductus arteriosus leading from the pulmonary
trunk to the aorta (Fig. 138). The right ventricle is not unprepared
for its adult function because it pumps its full share of the blood
throughout fetal life. Instead of all going to the lungs, however, part
of the blood pumped by the right ventricle passes by way of the ductus
arteriosus into the aorta (Fig. 150). As the lungs increase in size,
relatively more blood goes to them and relatively less goes through
the ductus arteriosus. By the time of birth enough blood is passing
through the lungs to support life, and within a short time after birth,
under the stimulus of functional activity, the lungs are able to take all
the blood from the right side of the heart and the ductus arteriosus is
gradually obliterated. The ductus arteriosus, therefore, serves during
intra-uterine life as what might be called a ‘‘compensated exercising
channel” for the right ventricle.
 
 
 
THE HEART
 
 
263
 
 
Changes in the Sinus Region. In following the story of the
development of the heart from the functional standpoint, many things
of less striking significance have been passed over. Some of these
should have a word of comment. Cephalic to the sinus orifice the
valvulae venosae which guard it against return flow fuse and are prolonged onto the dorsal wall of the atrium in the form of a ridge called
the septum spurium (false septum). This septum is, for a time, too
conspicuous to ignore, but it is of little importance in the partitioning
of the heart and soon undergoes retrogression.
 
As the heart grows it absorbs the sinus venosus into its walls so
that eventually the anterior and posterior venae cavae and the
coronary sinus all open separately into the right atrium (cf. Figs. 147,
148, and 149). Portions of the right valvula venosa are retained as the
valves of the caval and coronary orifices. In the adult heart a small
external sulcus can usually be found between the entrance of the
anterior and the posterior vena cava which records the old line of
demarcation between sinus venosus and atrium.
 
The Atrio-ventricular Valves and the Papillary Muscles. At
the point where the atrio-ventricular canals open into the ventricles
there are early indications of the establishment of valves. From the
partition in the atrio-ventricular canal and from the outer walls on
either side, masses of tissue in the shape of thick, blunt flaps project
toward the ventricle (Figs. 146 and 148). It is these masses of a primitive type of connective tissue, similar to that in the endocardial
cushions of the canal, which later become differentiated into the flaps
of the adult valves (Fig. 149). The papillary muscles and tendinous
cords which, in the adult, act as stays to these valves, arise by modification of soine of the related trabeculae carneae (cf. Figs. 146, 148,
and 149).
 
The Aortic and Pulmonary Valves. The valves which guard
the orifices of the aorta and of the pulmonary artery arise from
mesenchymal pads (Fig. 138) already mentioned as developing in
connection with the truncus septum. In early histological appearance
and in the manner in which this loose tissue gradually becomes
organized into the exceedingly dense fibrous tissue characteristic of
adult valves, they are similar to the atrio-ventricular valves. They
do not, however, develop any supporting strands comparable to the
papillary muscles and tendinous cords.
 
Course and Balance of Blood Flow in the Fetal Heart. All the
steps in the partitioning of the embryonic heart lead gradually toward
the final adult condition in which the heart is completely divided into
 
 
 
264
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
right and left sides. Yet from the nature of its living conditions it is
not possible for the tetus in utero fully to attain the adult type of
circulation. The plan of the completely divided circulation is predicated on lung breathing. In the adult the right side of the heart
receives the blood returning from a circuit of the body and pumps
it to the lungs where it is relieved of carbon dioxide and acquires a
fresh supply of oxygen. The left side of the heart receives the blood
that has just passed through the lungs and pumps it again through
ramifying channels to all the tissues of the body. In the fetus the
function of respiration is carried out in the placenta by interchange
with the maternal blood circulating through the uterus. The lungs,
although in the last part of fetal life they are fully formed and ready
to function, cannot actually begin their work until after birth. The
radical change which must inevitably take place immediately following birth in the manner in which the blood is oxygenated has led to a
widespread belief that there must be revolutionary changes in the
routing of blood through the heart and great vessels. However, as the
embryology of the circulatory system has been studied more closely
from a functional angle it is becoming increasingly clear that the
heart and the major vascular channels develop in such a manner that
the pumping load on the different parts of the heart remains balanced
at all times during fetal life. Moreover, the very mechanisms which
maintain this cardiac balance during intra-uterine life are perfectly
adapted to rebalance the circulatory load on the new postnatal basis
without involving any sudden overloading of previously inactive
parts of the vascular system.
 
To understand the changes in circulation which are so smoothly
accomplished at the time of birth it is necessary to have clearly in
mind the manner in which the way for them has been prepared
during intra-uterine life. In the foregoing account of the development
of the interatrial septal complex, emphasis was laid upon the fact
that at no time were the atria completely separated from each other.
This permits the left atrium, throughout prenatal life, to receive a
contribution of blood from the inferior cava and the right atrium by
a transseptal flow which compensates for the relatively small amount
of blood entering the left atrium of the young embryo by way of the
pulmonary circuit, and maintains an approximate balance of intake
into the right and left sides of the heart.
 
The precise manner in which this transseptal flow occurs, and
where, and to what extent the various blood streams of the fetal
 
 
 
THE HEART
 
 
265
 
 
circulation are mixed has long been a controversial subject. The recent
brilliant work of Barcroft and Barron and their co-workers has gone
far toward putting some of these old controversies into proper perspective. Their first approach was through the quantitative analysis
of blood samples drawn from various critical parts of the fetal circulation. The oxygen content of such samples has given important
evidence as to what mixing of the currents is actually taking place in
the living fetus. Later work involving the collaboration of Barclay
and Franklin utilized serial x-ray photography following the injection
of opaque material into the blood stream at various points. This
method has given further direct evidence as to the course followed by
some of the important blood currents. Synthesizing the most significant of the anatomical evidence with the newer experimental evidence, the course followed by the blood in passing through the fetal
heart may be summarized somewhat as follows. The inferior caval
entrance is so directed with reference to the foramen ovale that a considerable portion of its stream passes directly into the left atrium
(Figs. 149 and 150). Under fluctuating pressure conditions — say
following uterine contractions which send a surge of placental blood
through the umbilical vein — the placental flow may temporarily
hold back any blood from entering the circuit by way of either the
portal vein or the inferior caval tributaries (Fig. 150). For a time,
under these conditions, the left atrium would be charged almost
completely with fully oxygenated blood. Such conditions, however,
would be but temporary and would be counterbalanced 6y periods
when the portal and systemic veins poured enough blood into the
common channels to load the heart for a time with mixed or depleted
blood. The ipiportant thing physiologically is not the fluctuations,
but the maintenance of the average oxygen content of the blood at
adequate levels.
 
Compared with conditions in adult mammals, the mixing of
oxygenated blood freshly returned from the placenta with depleted
blood returning from a circuit of the body may seem inefficient. But
this is a one-sided comparison. The fetus is an organism in transition.
Starting with a simple ancestral plan of structure and living an aquatic
life, it attains its full heritage but slowly. It must be viewed as much
in the light of the primitive conditions from which it is emerging as in
comparison with the definitive conditions toward which it is progressing. Below the bird-mammal level circulatory mechanisms with
partially divided and undivided hearts and correspondingly un
 
 
266
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
 
birth.
 
 
 
THE HEART
 
 
267
 
 
separated blood streams meet all the needs of metabolism and
growth. Maintenance of food, oxygen, and waste products at an
average level which successfully supports life does not depend on
''pure currents,” although such separated currents undoubtedly
make for higher efficiency in the rate of interchange of materials.
From a comparative viewpoint, the fact that the mammalian fetus is
supported by a mixed circulation seems but natural.
 
Another significant fact is that careful measurements have shown
that the interatrial communication in the heart of the fetus at term
is considerably smaller than the inferior caval inlet. This would mean
that the portion of the inferior caval stream which could not pass
through this opening into the left atrium would eddy back and mix
with the rest of the blood in the right atrium.
 
One of the most important inferences as to the fetal circulation
based on vessel size is that the circulation through the lungs in a fetus
which is sufficiently mature to be viable is of considerable volume.
This too has now been supported by experimental work. From the
standpoint of smooth postnatal circulatory readjustments, the larger
the fetal pulmonary return becomes the less will be the balancing
transatrial flow, and the less will be the change entailed by the assumption of lung breathing. Very early in development, before the lungs
have been formed, the pulmonary return is negligible and the flow
from the right atrium through the interatrial ostium primum constitutes practically the entire intake of the left atrium. After the
ostium primum is closed and while the lungs are but little developed,
flow through the interatrial ostium secundum must still be the major
part of the blood entering the left atrium. During the latter part of
fetal life the foramen ovale in septum secundum becomes the transseptal route. As the lungs grow and the pulmonary circulation
increases in volume, a progressively smaller proportion of the left
atrial intake comes by way of the foramen ovale and a progressively
larger amount from the vessels of the growing lungs.
 
The balanced atrial intake thus maintained implies a balanced
ventricular intake, and this in turn implies a balanced ventricular
output. Although not in the heart itself, we have seen that there is in
the closely associated great vessels a mechanism which affords an
adequate outlet from the right ventricle during the period when the
pulmonary circuit is developing. When the pulmonary arteries are
formed from the sixth pair of aortic arches, the right sixth arch soon
loses its original connection with the dorsal aorta. On the left, how
 
 
268
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
ever, a portion of the sixth arch persists as a large vessel connecting
the pulmonary artery with the dorsal aorta (Fig^. 138 and 150).
This vessel, already familiar to us as the ductus arteriosus, remains
open throughout fetal life and acts as a .shunt, carrying over to the
aorta whatever excess of blood the pulmonary vessels at any particular
phase of their development are not prepared to receive from the right
ventricle. As has already been pointed out, the ductus arteriosus can
be called the “exercising channel” of the right ventricle because it
makes it possible for the right ventricle to carry its full share of work
throughout development and thus to be prepared for pumping all the
blood through the lungs at the time of birth.
 
VII. The Changes in the Circulation Following Birth
 
The two most obvious changes which occur in the circulation at
the time of birth are the abrupt cutting off of the placental blood
stream and the immediate assumption by the pulmonary circulation
of the function of oxygenating blood. One of the most impressive
things in embryology is the perfect preparedness for this event which
has been built into the very architecture of the circulatory system
during its development. The shunt at the ductus arteriosus, whic ii
has been one of the factors in balancing ventricular loads throughout
development, and the valvular mechanism at the foramen ovale,
which has at the same time been balancing atrial intakes, are perfectly adapted to effect the postnatal rebalancing of the circulation.
The closure of the ductus arteriosus is the primary event and the
closure of the foramen ovale follows as a logical sequel.
 
It has long been known that the lumen of the ductus arteriosus is
gradually occluded postnatally by an overgrowth of its intimal tissue.
This process in the wall of the ductus is as characteristic and regular
a feature of the development of the circulatory system as the formation of the cardiac septa. Its earliest phases begin to be recognizable
in the fetus as the time of birth approaches, and postnatally the
process continues at an accelerated rate to terminate in complete
anatomical occlusion of the lumen of the ductus about six to eight
weeks after birth,
 
Barcroft, Barclay, and Barron have conducted an extensive series
of experiments on animals delivered by Cesarean section which,
indicate that the ductus arteriosus closes functionally far sooner than
it does anatomically. Following birth there appears to be a contraction of the circularly disposed smooth muscle in the wall of the ductus
 
 
 
THE CHANGES IN THE CIRCULATION FOLLOWING BIRTH 269
 
which promptly reduces the flow of blood through it. This reduction
in the shunt from the pulmonary circuit to the aorta, acting together
with the newly assumed respiratory activity of the lungs themselves,
aids in raising the pulmonary circulation promptly to full functional
level. At the same time the functional closure of the ductus by muscular contraction paves the way for the ultimate anatomical obliteration
of its lumen by overgrowth of intimal connective tissue. This concept
of the immediate functional closure of the ductus is so appealing on
theoretical grounds that a little extra caution in evaluating the evidence is indicated. It should be borne in mind that an initial tendency
on the part of the circular smooth muscle of the ductus to contract
does not necessarily imply a contraction sufficiently strongly and
steadily maintained to shut off all blood flow during the six to eight
weeks occupied by morphological closure. The dramatic quality of an
immediate muscular response should not cause us to forget the importance of the slower but more positive structural closure.
 
The results of increased pulmonary circulation with the concomitant increase in the direct intake of the left atrium are manifested secondarily at the foramen ovale. Following birth, as the
pulmonary return increases, compensatory blood flow from the right
atrium to the left decreases correspondingly, and finally ceases
altogether. This is indicated anatomically by a progressive reduction
in the looseness of the valvula foraminis ovalis and the consequent
diminution of the interatrial communication to a progressively narrower slit between the valvula and the septum. When equalization
of atrial intakes has occurred, the compensating one-way valve at the
foramen ovale falls into disuse, and the foramen ovale may be regarded as functionally closed.
 
Anatomical obliteration of the foramen ovale follows leisurely in
the wake of its functional abandonment. There is a considerable
interval following birth before the septum primum fuses with the
septum secundum to seal the foramen ovale. This delay is, however,
of no import because as long as the pulmonary circuit is normal and
pressure in the left atrium does not fall below that in the right, the
orifice between them is functionally inoperative. It is not uncommon
to find the fusion of these two septa incomplete in the hearts of individuals who have, as far as circulatory disturbances are concerned,
lived uneventfully to maturity. Such a condition can be characterized
as ‘‘probe potency’’ of the foramen ovale. When, in such hearts, one
inserts a probe under the margin of the fossa ovalis and pushes it
 
 
 
270
 
 
THE DEVELOPMENT OF THE CIRCULATORY SYSTEM
 
 
toward the left atrium one is, so to speak, prying behind the no longer
used, but still unfastened, interatrial door. ,
 
With birth and the interruption of the placental circuit there
follows the gradual fibrous involution of the umbilical vein and the
umbilical arteries. The flow of blood in these vessels, of course, ceases
immediately with the severing of the umbilical cord, but obliteration
of the lumen is likejy to take from three to , five weeks, and isolated
portions of these vessels may retain a vestigial lumen for much longer.
Ultimately these vessels are reduced to fibrous cords. The old course
of the umbilical vein is represented in the adult by the round ligament of the liver extending from the umbilicus through the falciform
ligament, and by the ligamentum venosus within the substance of the
liver. The proximal portions of the umbilical arteries are retained in
reduced relative size as the hypogastric or internal iliac arteries. The
fibrous cords extending from these arteries on either side of the urachus
toward the umbilicus represent the remains of the more distal portions
of the old umbilical arteries. They are known in the adult as the
obliterated branches of the hypogastric arteries, or as the lateral
umbilical ligaments.
 
Much yet remains to be learned as to the more precise physiology
of the fetal circulation and as to the interaction of various factors
during the transition from intra-uterine to postnatal conditions.
Nevertheless, with our present knowledge it is quite apparent that the
changes in the circulation which occur following birth involve no
revolutionary disturbances of the load carried by different parts of
the heart. The fact that the pulmonary circulation is already so well
developed before birth means that the changes which must occur
following birth are far less profound than was formerly believed ; and
the compensatory mechanisms at the foramen ovale and the ductus
arteriosus which have been functioning all during fetal life are entirely
competent to effect the final postnatal rebalancing of the circulation
with a minimum of functional disturbance. It is still true that as
individuals we crowd into a few crucial moments the change from
water living to air living that in phylogeny must have been spread
over eons of transitional amphibious existence. But as we learn more
about this change in manner of living, it becomes apparent that we
should marvel more at the completeness and the perfection of the
preparations for its smooth accomplishment, and dwell less on the
old theme of the revolutionary character of the changes involved.
 
 
 
==Chapter  12==
 
 
Tke Histogenesis of Bone and tlie
Development of tke Skeletal System
 
 
I. Histogenesis of Bone
 
Histologically bone belongs to the group of tissues known as the
connective and supporting tissues. In spite of their widely varying
adult conditions these.tissues are all similar in that the secreted parts,
rather than the cells themselves, carry out the functional role characteristic of the tissues. It is the secreted, fibrous portion of the binding
connective tissues which ties together various other tissues and
organs ; it is the secreted matrix of cartilage and of bone which affords
rigid support and protection to soft parts and furnishes a lever system
on which the muscles may be brought into play.
 
The cellular elements of these tissues must not be overlooked,
however, in emphasizing the functional importance of the cell
products. The cells are, so to speak, the power behind, in that they
extract the appropriate raw materials from the circulation, elaborate
them within their cytoplasm, and deposit the characteristic secretion
as an end-product. Moreover after the fiber is formed or the matrix
is laid down, it is dependent on the cells for maintenance in a healthy
active conditidn.
 
Embryologically the entire connective-tissue group arises from
mesenchymal cells. It is not surprising, in view of their closely related
functions and their derivation from a common type of ancestral cell,
that one type of connective tissue may be converted into or replaced
by another. This facility for changing the type of specialization is
sometimes referred to as plasticity.
 
The plasticity of the connective-tissue series is well exemplified
in the development of bone. Bone does not form in vacant spaces.
It is always laid down in an area already occupied by some less highly
specialized member of the connective-tissue family. The formation of
some bones begins in areas already occupied by connective tissue —
such bones are said to be intramembranous in origin, or are spoken
 
271
 
 
 
272 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
of as membrane bones. Other bones are laid down in areas already
occupied by cartilage. In this case they are said to be pndochondral in
origin, or, are called cartilage bones. It should be clearly borne in mind
that these terms apply solely to the method by which a bone develops
and do not imply any differences in histological structure, once the
bone is fully formed.
 
Likewise we should know at the outset what histologists mean
when they speak of cancellous bone and compact bone. These terms
refer not to the method of origin of the bone but to its density when
fully formed. Developmen tally all bone goes through the spongy or
cancellous stage. Some bones later become compact, others remain
cancellous. Most bones are compact in some areas and cancellous
in others.
 
The subject of bone development can be presented more simply
if we take up first the formation of primary cancellous bone intramembranously ; then the method by which this same type of spongy
bone is formed within cartilage, and finally the changes by which
cancellous bone, formed in either of the above ways, may become
secondarily compact.
 
Intramembranous Formation of Primary Cancellous Bone. In
 
an area where intramembranous bone formation is about to begin
we find an abundance of mesenchymal cells congregated and numerous small blood vessels present. The mesenchymal cells soon exhibit a
tendency to cluster together in more or less elongated groups here
and there throughout the area. If we study a group of this type which
has been aggregated for a short time we can make out the beginning
of a definite plan of organization. Near the axis of the cord delicate
fibers appear, produced by the secretory activity of the cells. As this
fibrous strand becomes more definite, the cells tend to become ranged
against it (Fig. 151, A). In so doing they retract the cytoplasmic
processes which are so characteristic of undifferentiated mesenchymal
cells and become rounded. In this stage we have essentially a connective tissue in which the fibrous strands are for the most part rather
widely separated from one another, and in which each strand has,
lined up against it^ the cells responsible for its production.
 
The actual deposition of bone matrix begins very soon after the
establishment of these primordial strands of mesenchymal cells and
fibers. In fact one usually finds the formation of bone beginning on
the older part of a strand while the strand itself is still being extended
at one end by the aggregation of more mesenchymal cells (Fig. 151,
 
 
 
HISTOGENESIS OF BONE
 
 
273
 
 
 
I'k;. 151, Formation of trabeculae of membrane bone. Projection drawings from the mandible of a pig embryo 130 mm. in length (cf. Fig. 182).
 
Abbreviations: Matrix cal., ossein matrix impregnated with calcium
salts; Matrix oss., ossein matrix not yet impregnated with calcium salts.
 
A). When the mesenchymal cells ranged against the fibrous axis of
such a strand become active in the secretion of calcareous material
they are spoken of as osteoblasts. We should not lose sight of the fact
that they are the same cells which formed the fibrous axis of the
original strands, given a new name in deference to their further
specialization and altered internal chemistry.
 
In studying the deposition of bone matrix one must bear in mind
its dual nature. The matrix consists of an organic fibrous framework
which is impregnated by a subsequent deposit of inorganic calcium
compounds. We may liken the matrix of bone to reinforced concrete*
In the making of a road or a wall, a meshwork of steel is first placed in
the forms and concrete is then poured in. The steel gives the finished
structure tensile strength and a certain amount of elasticity, the concrete gives form and hardness. So in bone the organic fibers {ossein
fibers) impart strength and resilience, while the calcium salts whh
which the fibers are impregnated give to the completed matrix body
and rigidity.
 
 
 
274 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
The two steps in the deposition of bone matrix may be demonstrated readily in areas where active bone forma^tion is going on,
owing to the fact that the presence of calcium compounds- in a tissue
markedly increases its affinity for stains. Even after most of the calcium salts have been removed from the ossein framework by treatment
of the tissue with acids (decalcification) to permit the making of
sections, the staining reaction is still apparent. This indicates that
the ossein fibers in which calcium has once been deposited are more
or less permanently changed chemically even though all the calcium
possible is subsequently removed.
 
If we look at a strand on which the osteoblasts have been active
for a time (Fig. 151, B) we see, next to the osteoblasts, a zone of bone
matrix which takes very little stain. This is the newly deposited
organic portion of the matrix as yet unimpregnated with calcium salts.
It consists of a feltwork of minute fibers so delicate and so closely
matted together that it is very difficult in ordinary preparations to see
the individual fibers at all. Slightly farther from the osteoblasts the
matrix is densely stained (Fig. 151, B). This part of the matrix has
been impregnated with calcium salts, chiefly phosphates and carbonates, and has thereby been converted into true bone matrix. The
calcium utilized by the osteoblasts in this process is brought to them
by the blood stream where it is carried in soluble form, probably in
organic linkage. It is interesting to note in this connection that the
presence of calcium and of phosphates in the blood is not in itself all
that is necessary for this process. There must be present also sufficient
vitamin D, which in some way facilitates the extraction by the osteoblasts of these raw materials from the blood and their deposition in
insoluble form as part of the bone matrix. The absence of vitamin D
from the system results in the formation of bone matrix deficient in
calcium salts and therefore lacking in rigidity — a cohdition not infrequent in pigs. Stock raisers have miscalled this condition rheumatism
but it is really the same condition known medically as rickets.
 
In the deposition of the matrix, the fibrous core of the original
strand serves as a sort of axis on which the first matrix is laid down.
When such a strand is completely invested by bone matrix, it is called
a trabecula (little beam). As the osteoblasts continue to secrete and
thereby thicken the trabecula, the accumulation of their own product
forces them farther and farther away from the axial strand about
which the first of the matrix was formed. The new matrix added is
not laid down uniformly. It is possible to make out in it markings
 
 
 
HISTOGENESIS OF BONE
 
 
275
 
 
which are suggestive of the growth rings of a tree. Apparently the
osteoblasts work more or less in cycles, depositing a succession of thin
layers of matrix. Each of these layers of the matrix is called a lamella
(Fig. 152). As the row of osteoblasts is forced back with the deposit of
each succeeding lamella, not all the cells free themselves from their
secretion. Here and there a cell is left behind. As its former fellows
 
 
Erythroblast extruding nucleus
 
Reticular connective-tissue cell j Erythroblast in mitosis
Young erythroblast
 
Normoblast
Blood vessel .
 
Fat cell --tHemocytoblast —
 
Granuloblast -
Hemocytoblast
in mitosis
 
Polykaryocyte.
 
 
 
:
 
Osteoblast ^ t
 
 
I
 
Bone cell
 
 
Bone
 
lamella
 
 
Fio. 152. A small area of bone and adjacent marrow
as seen in highly magnified decalcified sections. The
drawing has been schematized somewhat to emphasize
the relations of the cytoplasmic processes of the osteoblasts* and the bone cells so important in nutrition. In
the adjacent marrow developmental stages of various
types ofiiiijioQd cells, have been
 
 
continue to pile up new matrix, it becomes completely buried (Fig,
151, B). An osteoblast so caught and buried is called a bone cell
{osteocyte)^ and the space in the matrix which it occupies is called a
lacuna. The bone cells, thus entrapped, of necessity cease to be active
bone formers, but they play a vital part in the maintenance of the
bone already formed. They have delicate cytoplasmic processes
radiating into the surrounding matrix through minute canalidhli.
The processes of one cell come into communication with the processes
of its neighbors (Fig. 152). Thus the bone cells nearer to blood vessels
 
 
 
276 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
 
 
Fig. 153. Diagrams showing stages in establishing of a characteristic area
of primary cancellous bone by extension and coalescence of originally
separate trabeculae.
 
absorb and hand on materials to their more remote fellows which in
turn utilize these materials in maintaining a healthy condition in the
organic part of the bone matrix. It is the senescence of these cells with
the consequent lowering of their efficiency and the resultant deterioration of the ossein component of the matrix which is in part responsible
for the decreased resiliency of the bones in advanced age.
 
As the various trabeculae in an area of developing bone grow,
they inevitably come in contact with each other and fuse. Thus
trabeculae, at first isolated, soon come to constitute a continuous
system (Fig. 153). Because of its resemblance to a latticework (Latin —
cancellus), bone in thb condition, where the trabeculae are slender
and the spaces between them extensive, is known as cancellous bone.
The spaces between the trabeculae are known as marrpw spaces.
 
Endochondral Bone Formation. As the term implies, endochondral bone formation goes on within cartilage. It cannot be stated
too strongly that cartilage does not, in this process, become converted
int# bone. Cartilage is destroyed and bone is formed where the
cartilage used to be. The actual bone formation is essentially the
same as in the case of membrane bone. The phenomena of special
 
 
 
mSTCXJENESIS OF BONE
 
 
277
 
 
interest in connection with this type of bone development are those
involved in the destruction of the cartilage preliminary to the formation of bone.
 
Cartilage Formation. To trace the process logically we must start
back with the formation of cartilage. The first indication of impending
chondrogenesis is the aggregation of an exceedingly dense mass of
mesenchymal cells. This cell mass gradually takes on the shape of the
cartilage to be formed. The histogenetic changes involved are not at
first conspicuous. During the period of preliminary massing the cells
have been migrating in from surrounding regions and also increasing
the local congestion by rapid proliferation. As they are packed in
together they lose their processes and become rounded (Fig. 154, A,
1). When it seems as if no more cells could possibly be crowded in,
the course of events changes. The cells begin to separate from one
another. This is due to the fact that they have become active in
 
 
 
Fig. 154. Photomicrographs of developing cartilage. The areas photographed were from the margins of the paranasal cartilage of pig embryos
between 25 and 30 mm. in length. For location of cartilage in head see
figure 175.
 
A, Early stage showing: at (1) the massing of mesenchymal cells which
were about to be incorporated in the growing margin of the cartilage; and at
(2) an area where matrix formation is already beginning.
 
B, Slightly more advanced stage of the same cartilage showing: at (1)
increase in the amount and density of the matrix in the center of the growing
cartilage; at (2) concentration of the surrounding mesenchyme to form tire
perichondrium; and at (3) the addition of new cartilage matrix peripherally*
 
 
 
278 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
secreting. It is the accumulation of the secretion of the cells which
gradually forces them farther and farther apart unti^ they come to lie
isolated from one another in the matrix they have produced (Fig. 1 54,
A, 2). Such a method of increase in m^ss, where there are many
scattered growth centers contributing independently to the increase
in bulk of the whole, is known as interstitial growth. This interstitial
growth of young cartilage stands in sharp contrast to the appositional
growth of such rigid substances as bone or dentine or enamel where
the matrix is laid down in successive layers one upon another. Obviously interstitial growth implies plasticity of the substance produced.
Were the substance produced unyielding, the very activity of a number of growth centers within it would soon crowd those growth centers
to obliteration.
 
As the cartilage matrix is increased in amount its affinity for basic
stains becomes more marked, due probably to increase in concentration of the characteristic substance in it known chemically as chondrin.
At the same time the matrix becomes more rigid with a resultant
checking of interstitial growth. The cells continue to secrete to a
certain extent, however, as evidenced by the fact that in mature
cartilage the matrix immediately surrounding the cells becomes more
dense than the rest of the matrix. This area of denser matrix around
the lacuna in which the cell lies is known as the capsule. As the cartilage
grows older the capsules become more conspicuous and many of
them come to contain more than one cell. These nests of cells in a
common capsule are the result of cell divisions, following which the
daughter cells are held imprisoned in the original capsule of the
mother cell — further evidence of the loss of plasticity in the matrix.
 
The formation of a matrix so rigid that interstitial growth is
checked, takes place first centrally in an area of developing cartilage.
When the center has become too rigid for interstitial growth to continue, appositional growth begins to take place peripherally. While
the cartilage has been increasing in mass it has been acquiring a
peripheral investment of compacted mesenchyme. This investing
layer of mesenchyme soon becomes specialized into a connectivetissue covering called the perichondrium. The layer of the perichondrium next to the cartilage is less fibrous than the outer layer and the
cells in it continue to proliferate rapidly and become active in the
secretion of cartilage matrix. For this reason it is known as the
chondrogenetic layer of the perichondrium. It is through the activity
(rf the chondrogenetic layer that the cartilage continues to grow
 
 
 
Cartilage
 
ceil
 
 
Cart. trab. 1
 
 
 
Osteoblast
 
 
— Bone
ntotrix
 
 
Bone trabecula
 
 
Periosteum
 
 
Fig. 155. Drawing showing periosteal bud and an area of endochondral
bone formation from the radius of a 125 mm. sheep embryo. The small sketch
indicates the location of the area drawn in detail.
 
Abbreviations: Cart, eros., area from which cartilage has recently been
eroded; Cart, pre-eros., area with cartilage cells enlarged and arranged in
rows presaging erosion; Cart, trab,, remnant of cartilage matrix which has
become calcified and serves as an axis or core about which bone lamellae are
deposited to form a bone trabecula; Mes., mesenchymal cell.
 
279
 
 
 
280 HISTOGENESIS OF BONE AND DEVEI.OPMENT OF SKELETAL SYSTEM
 
 
peripherally, by apposition, long after interstitial growth has ceased
in the matrix first formed. ^
 
Cartilage Erosion. When a mass of cartilage is about to be replaced by bone, very striking changes in its structure take place. The
cells which have hitherto been secreting cartilage matrix begin to
destroy the matrix. The lacunae become enlarged and a curious arrangement of the cartilage cells becomes evident. The cells erode the
cartilage in such a manner that they become lined up in rows (Fig.
155). This process of destruction continues until the cartilage is extensively honeycombed. Meanwhile the tissue of the perichondrium overlying the area of cartilage erosion becomes exceedingly active. There is
rapid cell proliferation and the new cells, carrying blood vessels with
them, begin to invade the honeycombed cartilage (Fig. 155).
 
The Deposition of Bone. It is a striking fact that during its growth
cartilage is devoid of blood vessels, the nearest vessels to it being those
in the perichondrium. The invasion of cartilage by blood vessels
definitely determines its disintegration as cartilage, and at the same
time is the initial step in the formation of bone. For this reason the
enveloping layer of connective tissue, up to this time called perichondrium because of its relation to the cartilage, is now called
periosteum because of the relations it will directly acquire to the bone
about to be formed. This change will not be confusing if we stop to
think that both these terms are merely ones of relation, which translated mean, respectively, that tissue which surrounds cartilage, and that
tissue which surrounds bone. The important fact to bear in mind is that
this enveloping layer of tissue is of mesenchymal origin and therefore
contains cells of the stock that may develop into any of the connectivetissue family to which bone as well as cartilage belongs. When,
therefore, a mass of periosteal tissue {periosteal bud, Fig. 155) grows
into an area of honeycombed cartilage it carries in potentially boneforming cells. These cells come to lie along the strand-like remnants
of cartilage, just as in membrane bone formation osteoblasts ranged
themselves along fibrous strands. The actual deposition of bone proceeds in the same manner endochondrally as it does intramembranously. The only difference is that in one case a strand-like remnant of
cartilage serves as an axis for the trabecula, whereas in the other case
deposition begins on a fibrous strand. Extensions and fusions of the
growing trabeculae soon result in the establishment of typical cancellous bone similar to that formed intramembranously.
 
 
 
HISTOGENESIS OF BONE
 
 
281
 
 
The Formation of G)mpact Bone from Primary Cancellous
Bone. The difference between cancellous bone and compact bone is
architectural rather than histological. The fundamental composition
of the bone matrix, its lamellation, and the relations of the bone cells to
the matrix, are the same in both cases. It is the way in which the
lamellae are arranged that distinguishes these two types of bone from
each other. In cancellous bone the disposition of lamellae is such that
it leaves large marrow spaces between the trabeculae. In compact
bone there has been a secondary deposit of concentrically arranged
lamellae in the marrow spaces which greatly increases the density of
the bone as a whole.
 
The essential differences between the two, and the way in which
cancellous bone may become converted into compact bone, can be
illustrated by a simple schematic diagram. Figure 156, 1, shows the
arrangement of lamellae and marrow spaces in primary cancellous
bone. The osteoblasts which have formed the trabeculae still lie along
them on the surface toward the marrow cavity. If such an area is to
IxTome compact, these osteoblasts enter on a period of renewed
activity and deposit a series of concentric lamellae in the marrow
cavity. Frequently if the marrow spaces are irregular there is a
preliminary rounding out of them by local resorption of the bone
already formed (Fig. 156, 2). This is then followed by the deposition
 
 
 
Fig. 156. Diagram showing transformation of cancellous to compact bone.
The solid lines indicate the lamellae of primary cancellous bone; the dotted
lines show the subsequently added concentric (Haversian) lamellae which
nearly obliterate the marrow spaces of cancellous bone. The sequence of
events is indicated by the numbers. Note that irregularly shaped spaces in
the cancellous bone may be rounded out by absorption before the concentric
lamellae are laid down.
 
 
 
282 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
of the concentrically arranged lamellae, sometimes called Haversian
lamellae after the man who first described them in detail (Fig. 156, 3 ).
In this process the original marrow spaces are reduced to small canals
{Haversian canals^ into which have been crowded the blood vessels
which formerly lay in the marrow cavities (Fig. 156, 4 ). These canals
maintain intercommunication with each other in the substance of the
bone, constituting a network of pathways over which the bone receives
its vascular supply. As compared with the marrow spaces of cancellous
bone, however, they are very small; and the gross appearance of a
bone which has undergone this secondary deposit of concentric
lamellae amply justifies characterizing it as ‘'compact.’’
 
II. The Development of the Skeletal System
 
In dealing with the development of the skeletal system we must
recognize at the outset that the subject is far too extensive to be
covered here with anything like completeness. It is not difficult, however, to become acquainted with the outstanding features in the
development of two or three characteristic bones, as, for example:
the sequence of events in the formation of a flat bone; the steps involved in the establishment and growth of a long bone; the way
separate ossification centers appear in a common primordial cartilage
mass and give rise to the various parts of a vertebra. Familiarity with
such type processes gives one an understanding of the factors operative in the development of the skeleton as a whole and a background
sufficient to permit ready and intelligent following up of developmental details in specific bones in which one may become interested.
 
Development of Flat Bones. The flat bones, such as the bones
of the cranium and face, are for the most part of intramembranous
origin. We are, therefore, already familiar with the early steps in their
development from our study of the histogenesis of membrane bone
(Figs. 151 and 153). After a mass of primary cancellous bone has
been laid down in a configuration which suggests that of the adult
bone being formed, there appears about this mass a peripheral concentration of mesenchyme (Fig. 157, A). This periosteal concentration
of mesenchymal tissue contains potentially bone-forming cells which
soon become active and lay down a dense layer of parallel lamellae
about the spongy center of the growing bone (Fig. 157, B). Anatomically this dense peripheral portion is known as the outer table of the
bone. The inner portion, which in the flat bones usually remains
cancellous, is called the diploii. The original mesenchymal tissue which
 
 
 
THE DEVELOPMENT OF THE SKELETAL SYSTEM
 
 
283
 
 
Periosteum
Marrow space
 
l^one trabeculae
 
 
A
 
Subperiosteal
bone lamellae
 
 
Bone trabeculae
Marrow space
 
 
Periosteum
 
 
Fig. 157. Diagrams showing the manner in which the dense peripheral
layer of a flat bone is formed by the deposition of subperiosteal lamellae about
an area of primary cancellous bone.
 
I
 
remains in the marrow spaces of the diploe develops into characteristic “red bone marrow” rich in blood-forming elements (Fig. 152).
 
The story of the growth of the mandible, a membrane bone which
starts after the manner of flat bones but which later takes on a very
elaborate shape and finally becomes largely compact, can be gleaned
by a comparative study of figures 178, 180, and 184.
 
Development of Long Bones. The long bones are characteristically of endochondral origin. The cartilage in which they are
performed is a tempiorary miniature of the adult bone. Ordinarily
there are several ossification centers involved in the formation of long
bones. The first one to appiear is that in the shaft or diaphysis. The
location of this center is shown schematically in figure 158, A, Such
 
 
 
B
 
 
 
 
284 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
b
 
A
 
 
Fig. 158, Diagrams showing liie progress of ossification in a long bone.
The stippled areas represent cartilage; the black areas indicate bone.
 
A, Primary ossification center in shaft. B, Primary center plus shell of
subperiosteal bone. C, Entire shaft ossified. D, Ossification centers have
appeared in the epiphyses. E, Entire bone ossified except for the epiphyseal
cartilage plates.
 
details as the cartilage erosion which preceded its appearance and
the manner in which the deposit of bone was initiated have already
been considered (Fig. 155). Our interest now is in the relation of such
an endochondral ossification center to other centers, and to the bone
as a whole.
 
Almost coincidently with the beginning of bone formation within
the cartilage the overlying periosteum begins to add bone externally
(Fig. 158, B). In view of the fact that the bone-forming tissue carried
into the eroded cartilage arose from the periosteum, this activity of
the periosteum itself is not surprising. Moreover we have already
encountered this same phenomenon of periosteal bone formation in
the outer table of flat bones.
 
The formation of bone which starts at about the middle of the
shaft soon extends toward either end until the entire shaft is involved
(Fig. 158, C), leaving the two ends {epiphyses) still cartilage. Toward
the end of fetal life ossification centers appear in the epiphyses. The
number and location of these epiphyseal centers vary in different long
bones. There is always at least one center in each epiphysis and there
may be two or more. Not uncommonly there are two centers in one
epiphysis and one in the other, as illustrated in figure 158, D.
 
Between the bone fojcmed in the diaphysis and that formed in the
epiphysis there persists a mass of cartilage known as the epiphyseal
plaie which is of vital importance in the growth in length of the bone.
 
 
 
 
 
THE DEVELOPMENT OF THE SKELETAL SYSTEM
 
 
285
 
 
We should expect from the rigidity of bone matrix that interstitial
growth could not account for its increase in length. This was long ago
demonstrated experimentally by exposing a developing bone and
driving into it three small silver pegs, two in the shaft and one in the
epiphysis. The distance between the pegs being recorded, the incision
was closed and development allowed to proceed until a marked
increase had occurred in the length of the bone. On again exposing
the pegs, the two in the shaft were found to be exactly the same
distance apart as when they were driven in, but the distance between
the pins in the shaft and that in the epiphysis had increased by an
amount corresponding to the increase in length of the bone. This
indicates clearly that the epiphyseal plates constitute a sort of temporary, plastic union between the parts of the growing bone. Continued
increase in the length of the shaft is accomplished by the addition of
new bone at the cartilage plate. These epiphyseal plates persist during
the entire postnatal growth period. Only when the skeleton has
acquired its adult size do they finally become eroded and replaced
by bone which joins the epiphyses permanently to the diaphysis.
 
As the bone increases in length there is a corresponding increase
in its diameter. The manner in which this takes place is also susceptible of experimental demonstration. If madder leaves, or some of the
alizarin compounds extracted from them, be fed to a growing animal,
the bone formed during the time the feeding is continued is colored
red. If the madder is discontinued, bone of normal color is again
formed ; but the color still remains in the bone laid down while madder was being added to the diet. Thus it is possible, by keeping a record
of alternate f>eriods of feeding and withholding madder and comparing these records with the resulting zones of coloration in a bone, to
obtain very accurate information on the progress of bone growth and
resorption. Applied to the development of long bones this method
shows their increase in diameter to be due to continued appositional
growth beneath the periosteum. As the bone is added to peripherally
there is a corresponding resorption centrally. This central resorption
results in the formation of a cavity in the axis of the long bone which
is called the marrow canal (Fig. 158, C). With the further increase
in the diameter of a bone, its marrow canal becomes correspondingly
enlarged. A significant mechanical fact might be cited in this connection. 'Engineers have determined that the strongest rod which can be
made from a given weight of steel is obtained by molding it into
tubular form. The development of an essentially tubular shaft by
 
 
 
286 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
progressive increase in the size of the marrow cavity gjivcs a long bone
maximum strength with minimum weight.
 
The Formation of the Vertebrae. The development of the vertebrae is of interest to the student primarily because it exemplifies so
excellently a fundamental embryological phenomenon — the origin
of separate parts from an undifferentiated primordial tissue mass,
and the subsequent association of these parts to form an organized
structure. In studying young embryos we traced the history of the
mesodermic somites through their early differentiation. It will be
recalled that from the ventro-mesial face of each somite there arises
a group of mesenchymal cells called collectively a sclerotome (Fig.
42). These cells migrate from either side toward the mid-line and
become aggregated about the notochord. From these masses of cells
the entire vertebral column is destined to arise.
 
The first significant change which takes place in these primordial
masses is the clustering of sclerotomal cells derived in part from each
of the two adjacent somites into groups which are located opposite
the intervals between the myotomes. In studying series of transverse
sections this arrangement is easy to overlook unless the density of the
cells about the notochord is carefully noted in passing from section
to section. It shows very clearly, however, in frontal sections (Fig. 159).
Each of these cell clusters is the primordium of the centrum of a vertebra. Once formed they rapidly become more dense and more definitely
circumscribed (Fig. 160). Soon after the centrum takes shape, paired
mesenchymal concentrations extending dorsally and laterally from
the centrum establish the primordia of the neural arches and of the
ribs (Fig. 161).
 
 
 
Fig. 159. Semi-schematic coronal sections through the dorsal
region of young embryos to show how the vertebrae became intermyotomal in position. Note that the primordium of a centrum is
formed by cells originating from the sclerotomes of both the
adjacent pairs of somites.
 
 
 
THE DEVELOPMENT OF THE SKELETAL SYSTEM
 
 
287
 
 
 
Fig. 160. Transverse section from pig cm])ryo of 17 mm. cut at the level of the
lungs to show the structures in the dorsal body-wall. (After Minot.)
 
 
mantle layer ependymal
 
 
marginal layer
of eord
 
neural arch
 
 
of cord
 
 
layer


 
[[Category:Draft]]
eympathetic ganghon
anterior cardinal vein
 
 
 
left left duct
erihre fetdium vt Ctteter
 
 
Fig. 161 . Transverse section of 20 mm. pig embryo cut at the level of the lungs
to show the developing vertebra and ribs. (After Minot.)
 
 
 
288 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
 
Fig. 162. Transverse section from 40 mm. pig embryo cut at the level of the
lungs to show the developing vertebra and ribs.
 
 
The Stage in w^hich the various parts of the vertebrae arc sketched
in mesenchymal concentrations, is frequently spoken of as the blastemal stage. It is rapidly followed by th^ cartilage stage. Conversion to
cartilage begins in the blastemal mass first in the region of the centrum
and then chondrification centers appear in each neural and each
costal process (Fig. 161). These spread rapidly until all the centers
fuse and the entire mass is involved (Fig. 162). The cartilage miniature of the vertebra thus formed is at first a single piece showing no
lines of demarcation where the original centers of cartilage formation
became confluent, and no foreshadowing of the separate parts of
which it will be made up after the cartilage has been replaced by
bone. Shortly before ossification begins the rib cartilage becomes
separated from the vertebra, but the vertebra itself remains in one
piece throughout the cartilage stage (Fig. 162).
 
The locations of the endochondral ossification centers which
appear in a vertebral cartilage are indicated schematically in figure
163. It readily can be seen how the spreading of these centers of bone
formation will establish the conditions which exist in an adult vertebra. The median ossification center gives rise to the centrum. The
centers in the neural processes extend dorsally to complete the neural
arch. The spinous process in most of the vertebrae is formed by a
prolongation of these same centers to meet dorsal to the neural canal.
 
 
 
 
Fig. 164. Diagram of four types of vertebrae indicating the parts derived
from the different ossification centers shown in figure 163. The part formed by
the median center in centrum is cdncentrically ringed; the parts arising from
the costal centers are stippled; parts derived from the lateral centers in the
neural arches are indicated in line-shading,
 
289
 
 
 
 
 
 
 
290 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
In forms such as the pig the spinous processes of tl>e more anterior
thoracic vertebrae are very long. In these vertebrae additional ossification centers appear in the spinous process and fuse with those in the
 
Scapula Humerus
 
 
 
Radius Mandible
 
& ulna
 
 
Fig. 165. Photograph (X 1/^ showing the
ossification centers which have appeared in pig
embryos of 35 mm. This and the two following
figures were made by photographing in transmitted light embryos in which all the uncalcified
tissues had been rendered transparent by treatment with potassium hydroxide and glycerine.
 
neural processes. The transverse processes with which the tubercles
of the ribs articulate are formed by the lateral extension of the
primary ossification centers in the neural processes. These same centers
extend ventrally also, and meet the centrum (cf. Figs. 163 and 164).
 
 
Scapula
/ Humerus
 
 
 
Fic. 166. Photograph (X IM) showing the progress of ossification in the
skeleton of a 65 mm. pig embryo.
 
 
 
Fig. 167. Photograph {X showing the extent of ossification in the skeleton of a 90 nun. pig embryo.
 
 
 
292 HISTOGENESIS OF BONE AND DEVELOPMENT OF SKELETAL SYSTEM
 
 
The shaft of the rib is formed by extension of its primary ossification center (Fig. 163). After birth, secondary epiphyseal centers
appear in the tubercle and head of the rib. These centers are separated from the shaft by persistent cartilage plates in the manner
described in discussing the development of long bones. Fusion of the
secondary epiphyseal centers with the shaft of the rib does not take
place until the skeleton has acquired its adult dimensions.
 
The foregoing discussion has been based on a thoracic vertebra in
which the relations of the rib to the vertebra show most clearly.
All the vertebrae have the costal element represented, although it is
greatly reduced and modified in other regions than the thoracic. A
study of figure 164, in which the components of vertebrae from the
cervical, thoracic, lumbar, and sacral regions are schematically
indicated, will make the homologies apparent. With these homologies
in mind it is sufficiently evident, without going into further detail,
how all these vertebrae arise by a process similar to that described
for the thoracic vertebrae.
 
The Progress of Ossification in the Skeleton as a Whole. It
 
would carry us beyond the scope of this book to take up the development of specific bones. Each has its own story involving the formation
of the connective tissue or the cartilage mass which precedes it ; local
erosion centers if it be preformed in cartilage; number, location, and
time of appearance of ossification centers; growth in length and
diameter; development of epiphyses; time of fusion of epiphyses and
diaphysis; and finally the development of muscle ridges and articular
facets. Without entering into a discussion of details of this sort, it is
possible nevertheless to follow the general progress of ossification in
the skeletal system as a whole. Embryos which have been treated with
potassium hydroxide and then cleared in glycerine clearly show the
various ossification centers. In such preparations the areas where
calcium salts have been deposited stand out white in reflected light
and opaque in transmitted light. ^Figures 165-167, which are photographs of preparations of this type, can be used to trace the history
of the more important bones. It should perhaps be stated explicitly
that these figures arc included primarily to give a general view of the
progress of ossification and secondarily to afford a readily available
source of reference for following up points of interest that may arise.
It is not a profitable use of the student’s time to attempt to memorize
the ossification centers which have appeared in embryos of any given
age.
 
 
 
==Chapter  13==
 
 
Tke Development of tke Face and Jaws
and tke Teetk
 
 
I. The Face and Jaws
 
The Stomodaeum. In studying the early development of the
digestive tract we saw that the primitive gut first appeared as a
cavity which was blind at both its anterior and posterior ends (Fig.
37). Its opening in the future oral region is established by the meeting
of an ectodermal depression, the stomodaeum, with the cephalically
growing anterior end of the gut. The stomodaeal depression, even
as late as the time the oral plate ruptures and establishes communication between the anterior end of the gut and the outside world, is very
shallow (Fig. 40). The deep oral cavity characteristic of the adult is
formed by the forward growth of structures about the margins of the
stomodaeum. Some idea of the extent of this forward growth can
be gained from the fact that the tonsillar region of the adult is at
about the level occupied by the stomodaeal plate when it ruptures.
The growth of the structures bordering the stomodaeum, then, not
only gives rise to the superficial parts of the face and jaws, but actually
builds out the walls of the oral cavity itself.
 
The Jaws. Because the face of a young embryo is pressed against
the thorax it is difficult to study unless the entire head is cut off and
mounted separately. Preparations of this kind observed under a dissecting microscope by strong reflected light show the surface configuration of the facial region very clearly. The most conspicuous landmarks
are the stomodaeal depression, which in view of its fate we may now
call the oral cavity, and the olfactory pits. In embryos as small as
7 mm. most of the structures which take part in the formation of the
face and jaws are already clearly distinguishable (Fig. 168). In the
mid-line cephalic to the oral cavity is a rounded overhanging prominence known as the Jrontal process. On either side of the frontal process
are horseshoe-shaped elevations surrounding the olfactory pits. The
 
293
 
 
 
294
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
median limbs of these elevations are known as the nap-medial processes
and the lateral limbs are called the naso-lateral processes.
 
Growing toward the mid-line from the cephalo-lateral angles of
the oral cavity are the maxillary processes. In lateral views of the
head (Figs. 31 and 32) it will be seen that the maxillary processes
and the mandibular arch merge with each other at the angles of the
mouth. Thus the structures which border the oral cavity cephalically
are: the unpaired frontal process in the mid-line, the paired nasal
processes on either side of the frontal, and the paired maxillary
 
 
 
Fig. 168. Face of 7 mm. pig embryo photographed
X 15. Note especially the unmistakably paired character of the thickenings which later fuse in the mid-line
to complete the mandibular arch.
 
 
processes at the extreme lateral angles. From these primitive tissue
masses the upper jaw and the nose are derived.
 
The caudal boundary of the oral cavity is less complex, being
constituted by the mandibular arch alone. In very young embryos
(Fig. 168) the origin of the mandibular arch from paired primordia
is still clearly evident. Appearing first on either side of the mid-line
are marked local thickenings due to the rapid proliferation of mesenchymal tissue. Until these thickenings have extended from either side
to meet in the mid-line there remains a conspicuous mesial notch.
With their fusion, the arch of the lower jaw is completed (Figs. 1 69172).
 
 
 
THE FACE AND JAWS
 
 
295
 
 
In 10-12 mm. embryos (Fig. 169) very marked progress can be
seen in the development of the facial region. The maxillary processes
are much more prominent and have grown toward the mid-line,
crowding the nasal processes closer to each other. The nasal processes
have grown so extensively that the frontal process between them is
completely overshadowed (cf. Figs. 168 and 169). The growth of the
 
 
 
Fig. 169. Face of 11 ,5 mm. pig embryo photographed
X 12. Fusion of the right and left components of the
mandibular arch is practically complete. Both the
medial and lateral limbs of the horseshoe-shaped nasal
processes have undergone conspicuous enlargement.
Note especially the approximation of each naso-medial
process to the maxillary process of the same side.
 
 
medial limbs of the nasal processes has been especially marked and
they appear almost in contact with the maxillary processes on either
side.
 
The groundwork for the formation of the upper jaw is now well
laid down. Its arch is completed by the fusion of the two nasomedial processes with each other in the mid-line, and with the
maxillary processes laterally (Fig. 170), The premaxillary bones
carrying the incisor teeth are formed, later, in the part of the upper
jaw which is of naso-medial origin. The maxillary bones, carrying all
 
 
 
296
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
the upper teeth posterior to the incisors, are developed in the part of
the arch arising from the maxillary processes.
 
Nasal Chambers. The olfactory pits have by this time become
much deepened, not only by the growth of the nasal processes about
them, but also by extension of the original pits themselves which soon
break through into the oral cavity (Figs. 93 and 97, C). We may now
 
 
lateral
process
 
naso*‘ medial
process
 
tojngue
 
hyomandibular
 
cleft
 
hyoid arch
 
 
Fig. 170. Face of 16 mm. pig embryo photographed X 10. The
naso-medial processes have fused with the maxillary processes on
either side, and with each other in the mid-line, thus completing
the arch of the upper jaw.
 
speak of the external openings of the nasal pits as the nostrils {external
nares) and their new openings into the oral cavity as posterior nares or
nasal choanae. The septum of the nose is formed by fusion in the midline of the original naso-medial processes'; the upper part of the
bridge of the nose is derived from the frontal process; and the alae
of the nose arise from the naso-lateral processes (Fig. 172).
 
Naao-Iacrimal Duct. Where the naso-lateral process and the
maxillary process meet each other there is formed iot a time a well
 
 
 
THE FACE AND JAWS
 
 
297
 
 
 
Fir;. 171. Face of 17.5 mm. pig embryo photographed X 10. The originally separate processes have now largely lost their identity in the series of
fusions which have taken place in the formation of the face.
 
 
marked groove, which extends to the mesial angle of the eye (Fig. 169).
This is known as the naso4acrimal groove. It soon closes over superficially (Fig. 171), and it is usually stated that the deep portion of the
original groove is converted into a tube, the naso-lacrimal duct^ or
tear duct, wjhich drains the fluid from the conjunctival sac of the eye
into the nose. Recently Politzer has maintained that the nasolacrimal duct arises as an independent epithelial downgrowth from
the conjunctival sac which follows closely along the line of closure of
the old naso-optic furrow.
 
Tongue. While these changes are going on externally, the tongue
is being formed in the floor of the mouth. Anatomically the tongue is
usually described as consisting of a freely movable part called its body,
and a less freely movable portion, called its root, by which it is
attached in the oro-pharyngeal floor. The body of the tongue arises
from a small median elevation, the tuberculum impar^ and paired lateral
lingual primordia. These elevations appear very early in development
on the inner face of the first branchial (mandibular) arch (Fig. 173,
B). The tuberculum impar grows slowly and is soon crowded in on
 
 
 
298
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
by the more rapidly growing lateral lingual primordia which form
the great bulk of the body of the tongue (Fig. 173, c5.
 
 
 
Fig. 172. Face of 21.5 rum. pig embryo
photographed X 10. The characteristic features
of the adult face are even at this early stage
clearly recognizable. The regions of the upper
jaw and nose which have arisen from originally
distinct primordia are differentiated by shading.
Vertical hatching indicates origin from frontal
process; stippling, from naso-lateral processes;
small crosses, from naso-medial processes; horizontal hatching, from maxillary processes. The
entire lower jaw is derived from the mandibular
arch.
 
 
Arising in the pharyngeal floor at the bases of the second and
third branchial arches is an elevation known as the copula (i.e., yoke)
 
 
THE FACE AND JAWS
 
 
299
 
 
because of the way it joins these arches together (Fig. 173, B). The
copula, supplemented by some tissue from the adjacent basal portions of branchial arches 2, 3, and 4, gives rise to the root of the
tongue.
 
All the various elevations which thus take part in the formation
 
 
- Branchial arch i
 
-Branchial arch 2
 
'Branchial arch 3
.Branchial arch 4
-Glottis
 
 
Lateral lingual anlagt
 
Copula
 
Epiglottis
 
Glottis
 
 
Branchial arch i
Lateral lingual anlage
 
Branchial arch $
 
Branchial arch 4
 
Arytenoid ridge
 
c
 
Fio. 173. Dissections of pig embryos made to expose the floor of the
mouth and show the development of the tongue. (After Prentiss.) A, 7 mm.;
B, 9 mm.; C, 13 mm. (All figures X 12.)
 
 
B
 
 
 
Tuherculum impof
 
Branchial arch 2
 
Epiglottis
 
Glottis
 
 
Lateral lingual anlage
Tuherculum impar
 
Epiglottis
Arytenoid ridge
 
 
Branchial arch i
Tuherculum impar
 
Branchial arch 2
 
Branchial arch 3
Branchial arch 4
 
Arytenoid ridge
 
 
 
300
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
 
THE FACE AND JAWS
 
 
301
 
 
of the tongue must be thought of as composed of an outer covering
and the underlying mesodermal tissue which causes the covering to
bulge into the lumen. The covering tissue arises in situ from the lining
of the branchial arches involved. The sensory innervation of the surface of the tongue is, therefore, just what one would expect from the
basic relations of the cranial nerves to the branchial arches. The
epithelium of the body of the tongue gets its sensory supply from the
lingual branch of the mandibular division of the trigeminal (V) nerve
(Fig. 97, A, B), and from the chorda tympani branch of the seventh
nerve. The root of the longue receives its sensory fibers from the glossopharyngeal (Fig. 94) and vagus nerves.
 
The skeletal muscle that makes up the main mass of the tongue
beneath the mucosal covering is derived from mesodermal cell masses
that are believed to migrate into the pharyngeal floor from the myotomes of the occipital somites. Ontogenetically, in mammalian embryos this migration is exceedingly difficult to trace, for the cells of
myotomal origin early mingle indistinguishably with the local mesenchymal cells. Nevertheless the way the hypoglossal nerve (XII),
which is the cranial nerve arising at the level of these occipital myotonies, grows in with the developing lingual muscles (Figs. 93, 94,
and 97, A, B) and innervates them, furnishes strong circumstantial
evidence for this interpretation of tongue muscle origin and migration.
 
Palate. Coincidently also the palatal shelf is being formed in the
upper jaw and separating off the more cephalic portion of the original
stomodaeal chamber. Since it is into this cephalic portion of the cavity
that the nasal pits break through (Figs. 93 and 97, C), the formation
of the palatal shelf in effect prolongs the nasal chambers backwards
so they open eventually into the region where the oral cavity becomes
continuous with the pharynx.
 
The palate as well as the arch of the upper jaw is contributed to
by both the naso-medial processes and the maxillary processes. From
the premaxillary region a small triangular median portion of the
palate is formed (Fig. 174). The main portion of the palate is derived
from the maxillary processes. From them shelf-like outgrowths arise
 
Fig. 1 74. Photographs (X 6) of dissections of pig embryos made to expose
the roof of the mouth and show the development of the palate. A, 20.5 mm.;
B, 25 mm.; C, 26.5 mm.; D, 29.5 mm.
 
The diagrams of transverse sections are set in to show the relations before
(E) and after (F) the retraction of the tongue from between the palatine
processes.
 
 
 
 
302
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
on either side and extend toward the mid-line (Fig. 174, A— G). When
these palatal shelves first start to develop, the tongue lies between
them (Fig. 174, E). As development progresses the tongue drops down
(Fig. 174, F); the palatal shelves are extended toward the mid -line
and finally fuse with each other medially and with the premaxillary
process anteriorly to complete the palate (Fig. 174, D). At the same
time the nasal septum grows toward the palate and becomes fused
to its cephalic face (Figs. 174, F, and 178). Thus the separation of
 
 
 
 
Fig. 175. Transverse section of snout of 28 mm. pig
embryo (X 12). The area included in the rectangle is
shown in detail in the following figure.
 
 
right and left nasal chambers from each other is accomplished at the
same time that the nasal region as a whole is separated from the oral.
 
II. The Development of the Teeth
 
The Dental Ledge. Local changes leading toward tooth formation can be made out in the jaws of embryos as small as 1 5 mm. or
even less. By the time a size of 28-30 mm. has been attained, a definite
thickening of the oral epithelium can readily be seen on both the
upper and the lower jaw. This band of epithelial cells which pushes
into the underlying mesenchyme around the entire arc of each jaw
is known as the labi<Hlental ledge {labio-dental lamina) (Figs. 175 and
176). Shortly after its first appearance, cross-sections show this ledge
 
 
 
THE DEVELOPMENT OF THE TEETH
 
 
303
 
 
of epithelial cells to be differentiating into two parts, a more distal
part which by its ingrowth marks off the elevation which is to become
 
 
 
Fig. 176. Drawing (X 130) showing labio-dental ledge of
28 mm. pig embryo. For location of area represented see preceding figure.
 
 
 
Fig. 177. Drawing (X 130) showing differentiation of the
labio-dental ledge into labio-gingival lamina and dental ledge.
 
The ingrowth of the labio-gingival lamina initiates the separation of the lip from the gum (gingiva). From the dental ledge a
series of local bud-like outgrowths are formed, each of which
gives rise to the enamel cap of a tooth.
 
The region shown is the same as that in the preceding figure
but from a slightly older (37 mm.) embryo.
 
the lip from that which is to become the gum, and a more proximal
part which is destined to grow into the gum and give rise to the
enamel-forming organs of the teeth. The part of the original labio
 
 
 
304
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
dental ledge which separates the lip from the gum (gingiva) is known
as the labio-gingival lamina^ and the part of the original ledge which is
to take part in tooth formation is known as the denial ledge or dental
lamina (Fig. 177).
 
Enamel Organs. Soon after the dental ledge is established, local
buds arise from it at each point where a tooth is destined to be formed.
Since these cell masses give rise to the enamel crown of the tooth they
are termed enamel organs. As would be expected, the enamel organs
 
 
nasal bone
 
 
cartilage of
 
nasal septum
 
 
nasal
 
chamber
 
 
nasal process
 
of premaxilla
 
 
 
dental papilla
enamel organ
 
 
labiogingival
 
lamina
 
Meckel’s cartilage
 
 
tooth germ
 
mandible
 
 
Fig. 178. Drawing (X 10)tDf a transverse .section of the snout
of a 71 mm. pig embryo. The area included in the rectangle is
shown in detail in the following figure.
 
 
for the milk teeth are budded off from the dental ledge first, but the
cell clusters which later give rise to the enamel of the permanent teeth
are formed at a surprisingly early time (Fig. 180). They remain
dormant, however, during the growth period of the milk teeth and
begin to develop actively only after the jaws have enlarged sufficiently
to accommodate the permanent dentition.
 
The histogenetic processes involved in the formation of milk teeth
and permanent teeth are essentially the same. It is, therefore, sufficient
to trace them in the case of the milk teeth only, keeping in mind that
the same process is repeated later in life in the formation of the permanent teeth.
 
In a section of the developing mandible which cuts the dental ledge
 
 
THE DEVELOPMENT OF THE TEETH
 
 
305
 
 
at a point where an enamel organ is being formed, the shape of the
enamel organ suggests that of an irregularly shaped, inverted goblet,
the section of the dental ledge appearing somewhat like a distorted
stem (Fig. 178). The epithelial cells lining the inside of the goblet early
take on a columnar shape. Because they constitute the layer which
secretes the enamel cap of the tooth, they are called ameloblasts (enamel
 
 
 
formers) (Fig. 179). The outer layer of the enamel organ is made up
of closely packed cells which are at first polyhedral in shape but which
soon, with the rapid growth of the enamel organ, become flattened.
They constitute the so-called outer epithelium of the enamel organ (Fig.
179). Between the outer epithelium and the ameloblast layer is a
loosely aggregated mass of ceils called collectively, because of their
characteristic appearance, the enamel pulp or the stellate reticulum
(Fig. 179).
 
 
306
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
 
Fig. 180. Developing tooth from lower jaw of a 120 mm. pig embryo
(X 14). The small sketch including half of the tongue (left) and part of the
lip (right) gives the relations of the region drawn. The area in the rectangle
is shown in detail in the following figure.
 
 
 
Fig, 181. Projection drawing (X 350) of segment of enamel organ and
adjacent pulp from a 120 mm. pig embryo to show ameloblast and odontoblast layers. For location of area represented see preceding figure.
 
 
 
THE DEVELOPMENT OF THE TEETH
 
 
307
 
 
The Dental Papilla. Inside the goblet-shaped enamel organ
there is caught a mass of mesenchymal cells which are said to constitute the dental papilla (Fig. 179). The cells of the dental papilla
proliferate rapidly and soon form a very dense aggregation. The outer
cells of this mass are destined to secrete the dentine of the tooth and
the inner cells to give rise to the pulp of the tooth.
 
A little later in development the enamel organ begins to assume
the shape characteristic of the crown of the tooth it is to lay down
(Fig. 180). At the same time the outer cells of the dental papilla
take on a columnar form similar to that of the ameloblasts (Fig. 181).
They are now called odontoblasts (dentine formers) because they are
about to become active in secreting the dentine of the tooth.
 
In the central portion of the dental papilla vessels and nerves arc
beginning to make their appearance so that the picture is already
suggestive of the condition seen in the pulp of an adult tooth. Meanwhile the growth of the dental papilla toward the gum has crowded
the stellate reticulum of the enamel organ in the crown region so it is
nearly obliterated (Fig. 180). This brings the ameloblasts of this region
much closer to the many small blood vessels which lie in the surrounding mesenchyme. The approach of the ameloblasts to the neighboring
vascular supply would appear to be significant, since it is precisely
here at the tip of the crown where the ameloblasts first begin to secrete
enamel (Fig. 182).
 
By the time the enamel organ has been well established the dental
ledge has lost its connection with the oral epithelium, although traces
of it can still be identified in the mesenchyme at the lingual side of the
tooth germ (Fig. 180). The cluster of cells which is destined to give
rise to the enamel organ of the permanent tooth of this level can be
seen budding off from the ledge close to the point from which the
enamel organ of ffie milk tooth arose (Figi >^).
 
Formation of Dentine. With these preparatory developments
complete, the tooth-forming structures are, so to speak, ready to go
about the fabrication of dentine and enamel. As is the case with bone,
enamel and dentine are both composed of an organic basis in which
inorganic compounds are deposited. We may use the same comparison
that was used in describing bone : that of the familiar use in construction operations of a steel meshwork into which concrete is poured, the
steel giving the finished structure some degree of elasticity and increasing the tensile strength while the concrete gives body and solidity.
In the case of such hard structures in the body as bone, dentine, and
 
 
 
308
 
 
DEVELOPMENT OF FACE, JAWS AND lEETH
 
 
pr^ordiutn enamel organ
' permanent tooth
 
 
odontoblast layer
pulp of tooth
 
 
 
enamel pulp (stellate reticulum)
periosteum of alveolar socket
outer epithelium of enamel organ
 
 
Fig. 182. Developing tooth from lower jaw of a 130 mm. pig embryo
(X 30). The small sketch gives the relations of the regions drawn. The area
in the rectangle is shown in detail in the following figure.
 
 
blood vessel dentine enamel blood vessel in
 
1 i, 1 mesenchyme
 
 
 
fiber lirocess of enamel organ
 
 
Fio, 183. Projection drawing (X 350) of small segment of developing
 
 
THE DEVELOPMENT OF THE TEETH
 
 
309
 
 
enamel, these interlacing organic strands in the matrix give the tissue
its resilience and tensile strength, and the calcareous compounds
deposited in the organic framework give form and hardness.
 
Although bone, dentine, and enamel are similar in having both
organic and inorganic constituents in their matrix they are quite
different in detail, both as to composition and microscopical structure.
Bone has approximately 45 per cent of organic material while dentine
has but 30 per cent and adult enamel 5 per cent or less. There is also
considerable difference in the kind and proportion of inorganic compounds present in each. Structurally they are totally unlike. Bone
matrix is formed in lamellae and has cells scattered through it.
Dentine is formed without lam^llation and has its cellular elements
lying against one face and sending long processes into tubules in the
matrix. Enamel is prismatic in structure and the cells which form
it lie against its outer surface while it is being deposited, but are
destroyed in the eruption of the tooth.
 
The first dentine is deposited against the inner face of the enamel
organ, the odontoblasts drawing their raw materials from the small
vessels in the pulp and secreting their finished product toward the
enamel organ. It is significant in this connection that in an active
odontoblast the nucleus, which is the metabolic center of the cell, has
gravitated toward the source of supplies and come to lie in the extreme
pulpal end of the cell (Fig, 183). Also, the end of the odontoblast
toward the enamel organ, where the elaborated product of the cell
is being accumulated preparatory to its extrusion, can be seen to
take the stain especially intensely. Although our knowledge of intracellular chemistry is as yet exceedingly fragmentary and we do not
know the exact chemical nature of the product in this stage, the staining reaction is clearly indicative of the presence of calcium compounds
of some sort.
 
If attention is turned now to the recently formed dentine, two
zones distinctly different in staining reaction can be seen. The zone
nearer the cells is pale, taking but little stain (Fig. 183). This zone
consists of the recently deposited organic part of the matrix not as yet
impregnated with calcareous material. The zone nearer the enamel
organ will be found, by contrast, very intensely stained. This is the
older part of the dentine matrix which has had the organic framework
impregnated with calcareous material.
 
As the odontoblasts continue to secrete additional dentine matrix
the accumulation of their own product inevitably forces the cell
 
 
 
310
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
layer back, away from the material previously deposited. Apparently
strands of their cytoplasm become embedded in the material first laid
down and are then pulled out to form the characteristic processes of
the odontoblasts known as the dentinal fibers (Fig. 183). As the layer
of secreted material becomes thicker and the cells are forced farther
from the material first deposited, these dentinal fibers become progressively longer. Even in adult teeth where the dentine may be as much
as 2 mm. in thickness they extend from the odontoblasts which line
the pulp chamber to the very outer part of the dentine. These dentinal
fibers are believed to be concerned with maintaining the organic
portion of the dentine matrix in a healthy condition. When the pulp
is removed from a tooth, taking with it the odontoblasts, we know
that the dentine undergoes degenerative changes which involve,
among other things, increase in brittleness. This would seem to be
attributable to the degeneration of the organic framework of a matrix
no longer nourished by the odontoblasts.
 
Formation of Enamel. While the dentine is being laid down by
the cells of the odontoblast layer, the enamel cap of the tooth is being
formed by the ameloblast layer of the enamel organ. As was the case
with the odontoblasts, the active cells of the ameloblast layer are
columnar in shape and their nuclei, too, lie in the ends of the cells
toward the source of supplies, in this case the small vessels in the
adjacent mesenchyme (Fig. 183). The amount of organic material
laid down as the framework of enamel is much less than is the case
with either bone or dentine, and it is therefore more difficult to make
out its precise character and arrangement. It is, nevertheless, possible
to see in decalcified sections, delicate fibrous strands projecting from
the tips of the ameloblasts into the areas of newly formed enamel
(Fig. 183). It seems probable that these strands {Tomes'' processes) are
in some way involved in the formation of the organic matrix of
enamel. The problem of tracing the relations of Tomes’ processes to
the organic framework of enamel is greatly complicated by the fact
that where the ameloblasts have deposited calcium compounds the
calcium has rendered the organic part of the matrix so avid in its
affinity for stains that it is not possible to discern fine structural details
because of the very density of the resulting coloration (Fig. 183). This
reaction of the tissue to stains persists even after the inorganic calcium
compounds have been removed by decalcification, indicating that
the organic framework itself has been chemically altered by the
calcium deposited in it.
 
 
 
THE DEVELOPMENT OF THE TEETH
 
 
311
 
 
In spite -of these difficulties in getting at the exact nature and
arrangement of the organic matrix of enamel, it is quite possible to
see the genesis of its fundamental prismatic structure. Each amcloblasl
builds up beneath itself a minute rod or prism of calcareous material.
These prisms are placed with their long axes approximately at right
angles to the dento-enamel junction. Collectively these enamel prisms
form an exceedingly hard cap over the crown of the tooth which in
its structural arrangement suggests a paving of polygonal bricks laid
on end. There is sufficient difference in the rate at which the different
 
 
growth lines
 
in enamel
 
 
pulp chamber
 
growth lines
in dentine
 
 
root canal
 
 
cementum
 
 
oral epithelium
 
 
osteolilasts
 
of periosteum
of alveolus
 
 
connective
tissue ftbers
 
 
cementoblasts
( from
dental sac)
 
 
blood vessels
and nerves to pulp
 
 
Fig. 184. Schematic diagram showing the topography of a tooth and its
relations to the bone of the jaw. The numbered zones indicate empirically
the sequence of deposition of the dentine and enamel. The so-called growth
lines in the dentine and enamel follow the general contours indicated by
the dotted lines in the figure but arc much more numerous.
 
 
 
312
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
ameloblasts work so that in actively growing enamel the surface is
jagged and irregular due to the varying extent to ^<^hich the different
prismatic elements have been calcified (Fig. 183).
 
Both enamel formation and dentine formation begin at the tip
of the crown and progress toward the root of the tooth (Figs. 182 and
184). But the entire crown is well formed before the root is much more
than begun. The progressive increase in the length of the root is an
important factor in the eruption of the tooth, for as the root increases
in length the previously formed crown must move closer to the surface
of the gum. Even when the crown of the tooth begins to erupt the
root is still incomplete, and it does not acquire its full length until the
crown has entirely emerged.
 
The Formation of Cementum. The so-called cementum of the
tooth is virtually a bone encrustation of its root. No cementum is
formed until the tooth has acquired nearly its full growth and its
definitive position in the jaw. But the first indications of specialization
in the tissue destined to give rise to it can be seen long before the
cementum itself appears.
 
Outside the entire tooth germ, between it and the developing bone
of the jaw, there occurs a definite concentration of mesenchyme.
The concentration becomes evident first at the base of the dental
papilla and extends thence crownwards about the developing tooth,
which it eventually completely surrounds.
 
This mesenchymal investment is known as the dental sac (Fig. 182).
In the eruption of the tooth the portion of the dental sac over the
crown is destroyed, but the deeper portion of the sac persists and
becomes closely applied to the growing root. At about the time the
tooth has acquired its final position in the jaw, the cells of the dental
sac begin to form the cementum. Histologically and chemically
cementum is practically identical with subperiosteal bone. When we
consider the manner of origin of the dental sac and of the periosteum
of the bone socket (alveolar socket) in which the root of the tooth lies,
and see how they arise side by side from the same sort of tissue, this
seems biit natural. The dental sac is essentially a layer of periosteal
tissue facing the root of the tooth and back to back with the periosteal
tissue of the alveolar socket (Fig. 182).
 
The Attachment of Tooth in the Jaw. The attachment of the
tooth in its socket is brought about by the development, between the
dental sac and the periosteum of the tooth socket, of an exceedingly
tough fibrous connective tissue. As the periosteum of the alveolus
 
 
 
THE DEVELOPMENT OF THE TEETH
 
 
313
 
 
adds new lamellae of bone to the jaw on the one side, and the dental
sac adds lamellae of cementum to the root of the tooth on the other,
the fibers of this connective tissue are caught in the new lamellae.
Thus the tooth comes to be supported by fibers which are literally
calcified into the cementum of the tooth at one end and into the bone
of the jaw at the other (Fig. 184). The mechanism involved is precisely
the same as that which occurs in the burying of tendon fibers in a
growing bone, where the buried ends of the fibers are known as the
penetrating fibers of Sharpey.
 
Replacement of Deciduous Teeth by Permanent Teeth. The
 
replacement of the temporary or “milk” (deciduous) dentition by the
permanent teeth is a process which varies in detail for each tooth.
The general course of events is, however, essentially similar in all
cases. The enamel organ of the permanent tooth arises from the
dental ledge near the point of origin of the corresponding deciduous
tooth (Fig. 180). With the disappearance of the dental ledge, the
permanent tooth germ comes to lie in a depression of the alveolar
socket on the lingual side of the developing deciduous tooth (Fig. 185).
 
When the jaws approach their adult size the hitherto latent primordia of the permanent teeth b<-gin to go through the same histo
 
 
Fio. 185. Photomicrograph (X 5) of upper jaw of 160 mm. pig embryo
showing the milk cuspids just breaking through the gum.
 
 
314
 
 
DEVELOPMENT OF FACE, JAWS AND TEETH
 
 
 
Fig. 186. Photomicrograph (X 6) of section through the jaw of a puppy
showing a deciduous tooth nearly ready to drop out and the developing
permanent tooth deeply embedded in the jaw below it. The space about the
crown of the permanent tooth was occupied in the living condition by
enamel. Fully formed enamel, being approximately 97 per cent inorganic
in composition, is almost completely destroyed by the decalcification with
acids which must be carried out before such material can be sectioned.
(From a preparation loaned by Dr* S. W. Chase.)
 
 
THE DEVELOPMENT OF THE TEETH
 
 
315
 
 
genetic changes we have already traced in the case of the temporary
teeth. As the permanent tooth increases in size, the root of the deciduous tooth is resorbed and the permanent tooth comes to lie underneath
its remaining portion (Fig. 186). Eventually nearly the entire root
of the deciduous tooth- is destroyed and its loosened crown drops out,
making way for the eruption of the corresponding permanent tooth.
 
 
 
 
BiLlio grapky
 
 
Although this docs not purport to be a complete bibliography
on the development of the pig, I have tried to make it comprehensive.
At least one reference has been included on every phase of the subject
concerning which I could find published information. Under each
of the main subject headings some of the articles referred to have
extensive bibliographies of the literature in their special field. But,
with the exception of a few outstanding contributions, no papers have
been included which are merely of historical interest. For such articles
reference should be made to the exhaustive bibliographies compiled
by Minot (1893) and by Keibel (1897). Furthermore, the list is largely
restricted to contributions based directly on pig embryos. Exception
to this rule has been made in favor of a few general articles which
are of especial assistance in acquiring a perspective on some phase of
the subject. Also, a number of articles based on other forms have been
included when no work appeared to have been done on corresponding
phases of development in the pig. It is hoped that such a list of
selected references will furnish a starting point for following up any
desired line of inquiry without involving one in a discouraging
multiplicity of titles.
 
Texts and Manuals
 
Arey^ L. i?., 19^6. Developmental Anatomy. Saunders, Philadelphia, 5th Ed., ix &
 
616 pp.
 
Baumgartner, W, J,, 1924, Laboratory Manual of the Foetal Pig, Macmillan, New
York, vii & 57 pp.
 
Boyden, E. A., 1936. A Laboratory Atlas of the 13-mm. Pig Embryo. (Prefaced by
younger stages of the chick embryo.) The Wistar Institute Press, Philadelphia,
iv & 104 pp.
 
Hamilton, W. J., Boyd, J. D., and Mossman, H. W., 1945. Human Embryology.
 
Williams and Wilkins, Baltimore, viii & 366 pp.
 
Hertwig, O., 1901-07. Handbuch der Vergleichenden und experimentellen Entwicklungslehre der Wirbeltiere. (Edited by Dr. Oskar Hertwig and written by
numerous collaborators.) Fischer, Jena.
 
Huettner, A, F., 1941. Fundamentals of Comparative Embryology of the Vertebrates.
Macmillan, New York, xiv & 416 pp,
 
Jordan, H. E,, and Kindred, J. E., 1948. A Textbook of Embryology. Appleton, New
York, 5th Ed., xiv & 613 pp.
 
 
317
 
 
 
 
318
 
 
BIBLIOGRAPHY
 
 
Keihel^ F., 1897. Normentafeln zur Entwicklungsgeschichte der Wirbelthiere. I. Des
Schweines. Fischer, Jena, 114 S.
 
Martin^ P., 1912. Lehrbuch der Anatomic der Haustiere. ^chickhard & Ebner,
Stuttgart. (Bd. 1, Allgemeine und vergleichende Anatomic mit Entwicklungsgeschichte, xii & 811 S.)
 
Minot ^ C, S., 1893. A Bibliography of Vertebrate Embryology. Memoirs, Boston
Soc. Nat. History, Vol. 4, pp. 487-614.
 
Mtnot, C, S., 1911. A Laboratory Textbook of Embryology. The Blakiston Company,
Philadelphia, 2nd Ed., xii & 402 pp.
 
Needham^ J'., 1931. Chemical Embryology. Macmillan, New York, xxii & 2021 pp.
 
Patten^ B. A/., 1929. Early Embryology of the Chick. The Blakiston Company, Philadelphia, 3rd Ed., xiii & 228 pp.
 
Patten, B. M., 1946. Human Embryology. The Blakiston Company, Philadelphia,
XV & 776 pp.
 
Stssori, S., 1921. The Anatomy of the Domestic Animals. Saunders, Philadelphia,
2nd Ed., 930 pp.
 
Wetss, P.y 1939. Principles of Development. A Text in Experimental Embryology.
Holt, New York, xix & 601 pp.
 
IVieman, H. L., 1930. An Introduction to Vertebrate Embryology. McGraw-Hill,
New York, xi & 411 pp,
 
Windle, W, F., 1940. Physiology of the Fetus. Saunders, Philadelphia, xiii & 249 pp.
 
Zeitzschmann, 0,, 1923-24. Lehrbuch dcr Entwicklungsgeschichte der Haustiere.
R. Schoetz, Berlin, 542 S,
 
General Articles and Articles of Significance for the
Interpretation of Developmental Processes
 
Alexander, J., 1944. The Gene — A Structure of Colloidal Dimensions. Chapt. 37,
pp. 808-819, in “Colloid Chemistry,” Vol. V, edited by Jerome Alexander,
Reinhold Publishing Corp., New York.
 
Allen, W. F., 1918. Advantages of sagittal sections of pig embryos for a medical
embryology course. Anat. Rec., Vol. 14, pp. 183-191.
 
Barth, L. G., 1944. Colloid Chemistry in Embryonic Development. Chapt. 39, pp.
851-859, in “Colloid Chemistry,” Vol. V, edited by Jerome Alexander, Reinhold Publishing Corp., New York.
 
Chambers, R., 1944. Some Physical Properties of Protoplasm. Chapt. 41, pp. 864-875,
in “Colloid Chemistry,” Vol. V, edited by Jerome Alexander, Reinhold
Publishing Corp., New York.
 
Conklin, E, G., 1914. The cellular basis of heredity and development. Pop. Sci.
Monthly, Vol. 85, pp. 105-133. '
 
Hartman, C. G., 1 931 . Development of the egg as seen by the physiologist. Sci. Monthly,
Vol, 33, pp, 17-28.
 
Medley, 0. F., 1926. Quantitative study of growth of certain organs in pig fetus. Bull.
Med. Coll. Va., Vol. 23, pp. 19-36.
 
Holtfreter, J., 1947. Changes of structure and the kinetics of differentiating embryonic
cells. Jour. Morph., Vol. 80, pp. 57-92.
 
Holtfreter, J., 1947. Observations on the migration, aggregation and phagocytosis of
embryonic cells. Jour. Morph., Vol. 80, pp. 93-111.
 
 
 
THE SEX ORGANS AND GAMETOGENESIS
 
 
319
 
 
Kingsbury, B. F,, 1926. On the so-called law of anteroposterior development. Anat.
Rec., Vol. 33, pp. 73-87.
 
Lewis, W, H., 1947. Mechanics of invagination. Anat. Rec., Vol. 97, pp. 139-156.
 
Spemann, H., 1927. Organizers in animal development. Proc. Roy. Soc. London,
Ser. B, Vol. 102, pp. 177-187.
 
Spemann, H., 1938. Embryonic Development and Induction. Yale Univ. Press,
New Haven, xii & 401 pp.
 
The Sex Organs and Gametogenesis
 
Allen, E., 1923. Ovogenesis during sexual maturity. Am. Jour. Anat., Vol. 31, pp.
439-481.
 
Allen, E., Kountz, IV. B., and Francis, B. F., 1925. Selective elimination of ova in the
adult ovary. Am. Jour. Anat., Vol. 34, pp. 445-468.
 
Bascom, K. F., 1925. Quantitative studies of the testis. I. Some observations on the
cryptorchid testes of sheep and swine. Anat. Rec., Vol. 30, pp. 225-241.
 
Bascom, K. F., and Ouerud, H. L., 1925. Quantitative studies of the testicle. II. Pattern
and total tubule length in the testicles of certain common mammals. Anat. Rec.,
Vol. 31, pp. 159-169.
 
Blandau, R. J., 1945. The first maturation division of the rat ovum. Anat. Rec.,
Vol. 92, pp. 449-457.
 
Corner, G. W., 1917. Maturation of the ovum in swine. Anat. Rec., Vol. 13, pp. 109
112 .
 
Comer, G. IV., 1919. On the origin of the corpus luteum of the sow from both granulosa and theca interna. Am. Jour. Anat., Vol. 26, pp. 117-183.
 
Evans, H. M., and Swezv, 0., 1929. Ovogenesis and the normal follicular cycle in adult
mammalia. Mem. LTniv. Cal., Vol. 9, pp. 119-224.
 
Everett, N. B., 1945. The present status of the germ-cell problem in vertebrates. Biol.
Rev., Vol. 20, pp. 45-55.
 
Gould, H. N., 1923. Observations on the genital organs of a sex intergrade hog. Anat.
Rec., Vol. 26, pp, 241-261.
 
Hargitt, G. T., 1925-30, The formation of the sex glands and germ cells of mammals.
Jour. Morph. & Physiol., Vols. 40, 41, 42, 49.
 
Hartman, C. G., 1926. Polynuclear ova and polyovular follicles in the opossum and
other mammals, with special reference to the problem of fecundity. Am. Jour.
Anat., Vol. 37, pp. 1-52.
 
Hill, R. T., Allen, E., and Kramer, T. C., 1935. Cinemicrographic studies of rabbit
ovulation. Anat. Rec., Vol. 63, pp. 239-245.
 
Kellicott, W. E,, 1913. A Textbook of General Embryology. Holt, New York, v &
376 pp.
 
Kiipfer, M., 1920. Beitrage zur Morphologic der weiblichen Geschlechtsorgane bei
den Saugetieren. Ueber das Auftreten gelber Korper am Ovarium des domestizierten Rindes und Schweines. Vierteljahrsschrift d. Naturf. Gesellsch., Zurich,
Bd. 65, S. 377-433.
 
Latta, J, S., and Pederson, E. S,, 1944. The origin of ova and follicle cells from the
germinal epithelium of the ovary of the albino rat as demonstrated by selective
intravital staining with India ink. Anat. Rec., Vol. 90, pp. 23-35,
 
 
 
320
 
 
BIBLIOGRAPHY
 
 
Morgan^ T. H.j 1926. The Theory of the (iene. Yale Univ. Press, New Haven, xvi &
343 pp.
 
Fainter, T. S,, 1922. Studies in mammalian spermatogenesis. J(/Ur. Exp. Zodl., Vols.
35, 37, 39. Jour. Morph. & Physiol., Vol. 43.
 
Parker, G. H., and Bullard, C., 1913. On the size of litters and the number of nipples
in swine. Proc, Am. Acad. Arts & Sciences, Vol. 49, pp. 399-426.
 
Patten, W., 1925. Life, evolution and heredity. Sci. Monthly, Vol. 21, pp. 122-134.
 
Pincus, G., and Enzrnann, K. V,, 1937. The growth, maturation and atresia of ovarian
eggs in the rabbit. Jour. Morph., Vol. 61, pp. 351-383.
 
Pltske, E. C., 1940. Studies on the influence of the zona pellucida in atresia. Jour.
Morph., Vol. 67, pp 321-349.
 
Schmaltz-, B., 1911. Die Slrucktur der Ge.schlechtsorgane der ITaussaugetiere. P. Parey,
Berlin, xii & 388 S.
 
Smith, J, T., and Kettcrmgharn, R. C., 1937-38. Rupture of the graafian follicles.
Part I. Am. Jour. Obs. & Gyn., Vol. 33, pp. 820-827. Part IT Am. Join. Obs.
& Gyn., Vol. 36, pp. 45.3-460.
 
Stein, K. F., and Allen, E., 1942. Attempts to stimulate proliferaticm of the germinal
epithelium of the ovary. Anat. Rec., Vol. 82, pp. 1 -9.
 
Thanhoffer, L. de, 1934. The structure of the graafian follicle as revealed by microdissection. Zeilschr. f. Anat. u. Entwg., Bd. 102, S. 402-408.
 
Warwick, B. L., 1925. The eflect of vasectomy on swine. Anat. Rec., Vol. 31, pp.
19-21.
 
Wihon, E. B., 1925. The Cell in Development and Heredity. Macmillan, New York,
3rd Ed., ix & 1232 pp.
 
Young, W. C , 1929. A study of the function of the epididymis. I. Is the attainment
of full spermatcjzoon maturity attributable to some specific action of the epididymal secretion? Jour. Morph. & Physiol., Vol. 47, pp. 479-495.
 
Young, W. C., 1929. A study of the function of the epididymis. II. The importance of
an aging process in sperm for the length of the period during which fertilizing
capacity is retained by sperm isolated in the epididymis of the guinea-pig. Jour.
Morph. & Physiol., Vol. 48, pp. 475-491.
 
Young, W. C., 1931. A study of the function cT the epididymis. III. Functional changes
undergone by spermatozoa during their passage through the epididvmis and
vas deferens in the guinea-pig. Jour. Exp. Biol., Vol. 8, pp. 151-162.
 
The Sexual Cycle, Fertilization, Sex Determination
 
Allen, E., 1926. The ovarian follicular hormone: a study of variation in pig, cow, and
human ovaries. Proc. Soc. Exp. Biol. & Med., Vol. 23, pp. 383-387.
 
Allen, E,, Danforth, C. H., and Daisy, ErA,, 1939. Sex and Internal Secretions. Williams
and Wilkins, Baltimore, 2nd Ed., xxxvi & 1346 pp.
 
Allen, E., and Daisy, E. A., 1927. Ovarian and placental hormones. Physiol. Reviews,
Vol. 7, pp. 600-650.
 
Ampolsky, Z)., 1928. Cyclic changes in size of muscle fibers of the fallopian tube of
the sow. Am. Jour. Anat., Vol. 40, pp. 459-469.
 
Blandau, R., and Money, W. L., 1944. Observations on the rate of transport of spermatozoa in the female genital tract of the rat. Anat. Rec., Vol. 90, pp. 25.5-260.
 
 
 
THE SEXUAI, CYCLE, FERTILIZATION, SEX DETERMINATION 321
 
 
Burns, R. K., Jr., 1938. Hormonal control of sex differentiation. Am. Nat., Vol. 72,
pp. 207-227.
 
Comstock, R. E., 1939. A study of the mammalian sperm cell. I. Variations in the
glycolytic power of spermatozoa and their relation to motility and its duration.
Jour. Exp. Zool., Vol. 81, pp. 147—164.
 
Corner, G. W., 1915. The corpus luteum of pregnancy as it is in swine. Carnegie Inst.,
Contrib. to E-mbryoL, Vol. 2, pp. 69-94.
 
Corner, G. W., 1917. Variations in the amount of phosphatids in the corpus luteum of
the sow during pregnancy. Jour. Biol. Chem., Vol. 29, pp. 141-143.
 
Corner, G. W , 1919. On the origin of the corpus luteum of the sow from both granulosa and theca interna. Am. Jour. Anal., Vol. 26, pp. 117-183.
 
Cornet, G. W., 1921. Cyclic changes in the ewaries and uterus of swine, and their
relations to the mechanism of implantaticm. Carnegie Inst., Contrib. to Embryol.,
Vol. 13, pp. 117-146.
 
Corner, G. IT., 1923. Cyclic variation in uterine and tubal contraction waves. Am.
Jour. Anat., Vol. 32, pp. 345-351.
 
Corner, G IT., 1928. Physiolcjgy of the corpus luteum. I. The efTect of \Try early
ablation of the corpus lutcnun upon embryos and uterus. Am. Jour. Physiol.,
Vol. 86, pp. 74-81.
 
Coiner, G IV., 1942. The Hormones in Human Reproduction. Princeton Univ. Press,
Princeton, xix & 265 pp.
 
Corner, G. IT., and Allen, IT, M., 1929. Physiology of the corpus luteum. II. Production
(3f a special uterine reac tion (progestational proliferation) by extracts of the
cwpus luteum. Am. Jour. Physiol., Vol. 88, pp. 326-339.
 
Coinn, G. IT., and Allen, IT. A/., 1929. Physiology of the corpus luteum. III. Normal
growth and implantation of embryos after very earlv ablation of the ovaries,
under the influence of extracts of the corpus luteum. Am. lour. Physiol., Vol. 88,
pp. 340-346.
 
Cornn, G. IT, and Amsbaugh, A. E., 1917. Oestrus and ovulation in swine. Anat. Rec.,
Vol. 12, pp. 287-291.
 
Corner, G. IT., and Snyder, F. F., 1922. Observations on the structure and function
of the uterine ciliated epithelium in the pig, with reference to certain clinical
hypothesep. Am. Jour. Obs. & Gyn., VcjI. 3, pp. 358-366.
 
Crew, F. A. E., 1925. Prenatal death in the pig and its effect upon the sex-ratio.
Proc. Roy. Soc. Edinburgh, V^ol. 46, pp. 9-14.
 
Hammond, J., 1934. The fertilisation of rabbit ova in relation to time: A method of
controlling the litter size, the duration of pregnancy and the weight of the young
at birth. Jour. Exp. Biol., Vol. 11, pp. 140-161.
 
Hartman, C. G., 1929. The homology of menstruation. J. A. M. A., Vol. 92, pp. 19921995.
 
Hartman, C. G., and Squier, R. R., 1931. The follicle-stimulating effect of pig anterior
lobe on the monkey ovary. Anat. Rec., Vol. 50, pp. 267-273.
 
Keye, J. D., 1923. Periodic variations in spontaneous contractions of uterine muscle
in relation to the oestrous cycle and early pregnancy. Bull. Johns Hopkins Hosp.,
Vol. 34, pp. 60-63.
 
Lewis, L. L., 1911. The vitality of reproductive cells. Agric. Exp. Sta. Oklahoma,
Bull. No. 96.
 
 
 
322
 
 
BIBLIOGRAPHY
 
 
Lillie^ F. /?., 1919. Problems of Fertilization. Univ. Chicago Press, xii & 278 pp.
 
Loeb^ Z.., 1923. The mechanism of the sexual cycle, with special reference to the corpus
luteum. Am. Jour. Anat., Vol. 32, pp. 305-343. ^
 
Markee, J. E., Pasqualetti^ R. A., and Hinsey, J. C., 1936. Growth of intraocular endometrial transplants in spinal rabbits. Anat. R^c., Vol. 64, pp. 247-253.
 
Marshall^ F, H. A., 1922. The Physiology of Reproduction. Longmans, Green & Co.,
London, 2nd Ed., xvi & 770 pp.
 
McKenzie^ F. F, 1926. The normal oestrous cycle in the sow. Univ. Missouri Coll.
Agric. Exp. Sta. Res. Bull., Vol. 86, pp. .S-41.
 
Papanicolaou^ G. A'"., 1923. Oestrus in mammals from a comparative point of view.
Am. Jour. Anat., Vol. 32, pp. 285-292.
 
Parker^ G. IL, 1931. Passage of sperms and of eggs through oviducts in terrestrial
vertebrates. Phil. Trans. Roy. Soc. London, Ser. B, Vol. 219, pp. 381-419.
 
Phillips, R. W., and Andrews, F, j\., 1937. The speed oi travel of ram spermatozoa.
Anat. Rec., Vol. 68, pp. 127-132.
 
Pincus, G., 1936. The Eggs of Mammals. Macmillan, New York, ix & 160 pp.
 
Schott, R. G., and Phillips, R. W., 1941. Rate of sperm travel and time of ovulation in
sheep. Anal. Rec,, Vol. 79, pp. 531-540.
 
Secktnger, D, L., 1923. Spontaneous contractions of the Fallopian tube of the domestic
pig with reference to the oestrous cycle. Bull. Johns Hopkins Hosp., Vol. 34,
pp. 236-239.
 
Snyder, F. F., 1923. Changes in the Fallopian tube during the ovulation cycle and
early pregnancy. Bull. Johns Hopkins Hosp., Vol. 34, pp. 121-125.
 
Soderwall, A, L., and Blandau, R. J., 1941. The duration of the fertilizing capacity of
spermatozoa in the female genital tract of the rat. Jour. Exp. Zool , Vol. 88,
pp. 55-64.
 
Stockard, C. /?., 1923. The general morphological and physiological importance of
the oestrous problem. Am. Jour. Anat., Vol. 32, pp. 277-283.
 
Surface, F. M., 1908, Fecundity of swine. Biometrika, Vol. 6, pp. 433-436.
 
Swingle, W, W., 1926. The determination of sex in animals. Physiol. Rev., Vol. 6,
 
pp. 28-61.
 
Toothill, M. C., and Toung, W. C.,\93\. The time consumed by spermatozoa in passing
through the ductus epididymidis of the guinea-pig as determined by means of
India-ink injections. Anat. Rec., Vol. 50, pp. 95-107.
 
Wilson, K. M., 1926. Histological changes in the vaginal mucosa of the sow in relation
to the oestrous cycle. Am. Jour. Anat., Vol. 37, pp. 417-432.
 
Witschi, E., 1932. Physiology of embryonic sex differentiation. Am. Nat., Vol. 66,
pp. 108-117.
 
Cleavage, Germ Layer Formation^ and the Structure of Young
 
Embryos
 
Assheton, R., 1899. The development of the pig during the first ten days. Quart. Jour.
Micr. Sci., Vol. 41, pp. 329-359.
 
Clark, R, T,, 1934. Studies on the physiology of reproduction in the sheep. II. The
cleavage stages of the ovum. Anat. Rec., Vol. 60, pp. 135-159.
 
Conklin, E, G., 1909. The application of expieriment to the study of the organization
and early differentiation of the egg. Anat. Rec., Vol. 3, pp. 149-154.
 
 
 
CI.EAVAGE, GERM LAYERS, YOUNG EMBRYOS
 
 
323
 
 
Defrise^ A.^ 1933. Some observations on living eggs and blastulae of the albino rat.
Anat. Rec., Vol. 57, pp. 239-250.
 
Green^ W, W., and Winters, L. Af., 1946. Cleavage and attachment stages of the pig.
Jour. Morph., Vol. 78, pp. 305-316.
 
Heuser, C\ H., and Streeter, G. L., 1 929. Early stages in the development of pig embryos,
from the period of initial cleavage to the time of the appearance of limb-buds.
Carnegie Inst., Contrib. to Embryol., Vol. 20, pp. 1-29.
 
Keibel, F., 1891. Uebcr die Entwicklungsgeschichte des Schweines. Anat. Anz.,
Bd. 6, S. 193-198.
 
Keibel, F., 1894. Studien zur Entwicklungsgeschichte des Schweines (Sus scrofa
domesticus). I. Morph. Arbeiten, Bd. 3, S. 1-139.
 
Keibel, F., 1895. Ueber einige Plattenmodelle junger Schwein-embryonen. Verhandlungen d. Anat. Ges., Erganzungsheft Bd. 10, Anat. Anz., S. 199-201.
Keibel, F., 1896. Studien zur Entwicklungsgeschichte des Schweines (Sus scrofa
domesticus). II. Morph. Arbeiten, Bd. 5, S. 17-168.
 
Kingsbury, B, F., 1920. I'he developmental origin of the notochord. Science, N. S.,
Vol. 51, pp. 190-193.
 
Kingsbury, B. F., 1924a. The developmental significance of the notochord (Chorda
dorsalis). Zeitschr. f. Morphologic u. Anthropologic, Vol. 24, pp. 59-74.
Kingsbury, B. F,, 1924b. The significance of the so-called law of cephalocaudal differential growth. Anat. Rec., Vol. 27, pp. 305-321.
 
Kingsbury, B, F., 1926. On the so-called law of antero-posterior development. Anat.
Rec., Vol. 33, pp. 73-87.
 
Lewis, F. T., 1902. The gross anatomy of a 12-mm. pig. Am. Jour. Anat., Vol. 2,
 
pp. 211-226.
 
Lewis, W, H,, and Gregory, P. W,, 1929. Cinematographs of living developing rabbiteggs. Science, Vol. 69, pp. 226-229.
 
Lewis, W. H., and Hartman, C, G., 1933. Early cleavage stages of the egg of the monkey
(Macacus rhesus). Carnegie Inst., Contrib. to Embryol., Vol. 24, pp. 187-201.
Lewis, W. PL, and Wright, E, S,, 1935. On the early development of the mouse egg.
 
Carnegie Inst., Contrib. to Embryol., Vol. 25, pp. 113-144.
 
Macdonald, E., and Long, J, A., 1934. Some features of cleavage in the living egg of
the rat.^Am. Jour. Anat., Vol. 55, pp. 343-361.
 
Nicholas, J. S., and Hall, B. V., 1942. Experiments on developing rats. II. The development of isolated blastomeres and fused eggs. Jour. Exp. Zooi., Vol. 90, pp.
441-459.
 
Patten, B. M,, and Philpott, R., 1921 . The shrinkage of embryos in the processes preparatory to sectioning. Anat. Rec., Vol. 20, pp. 393-413.
 
Pincus, G,, 1939. The comparative behavior of mammalian eggs in vivo and in vitro.
IV. The development of fertilized and artificially activated rabbit eggs. Jour.
Exp. Zodl., Vol. 82, pp. 85-131.
 
Robinson, A,, 1892. Observations upon the development of the segmentation cavity,
the archenteron, the germinal layers, and the amnion in mammals. Quart.
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THE CIRCULATORY SYSTEM
 
 
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332
 
 
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THE DIGESTIVE AND RESPIRATORY SYSTEMS
 
 
333
 
 
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334
 
 
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THE DUCTLESS GI.ANDS
 
 
335
 
 
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336
 
 
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CONNECTIVE TISSUES, SKELETAL, AND MUSCULAR SYSTEMS 337
 
 
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338
 
 
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TWINS, DOUBLE MONSTERS, ANOMALIES
 
 
339
 
 
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phosphatase in the developing tooth. Jour. Cell. & Comp. Physiol., Vol. 26,
pp. 25-33.
 
Bevel andei ^ G.^ and Johnson^ P. T., 1946. The histochemical localization of glycogen in
the developing tooth. Jour. Cell. & Comp. Physiol,, Vol. 28, pp, 129-137.
 
Bild^ T., 1902. Die Entwickelungsgeschithte des Zahnsystems bei Sus domesticus und
das Verhaltnis der Lippenfurchenanlage zur Zahnleiste. Anat. Anz., Bd, 20, S.
401-410.
 
Chase, S, W., 1932. Histogenesis of the enamel. Jour. Am. Dental Assn., Vol. 19, pp.
1275-1289.
 
Glasstone, S., 1935. The development of tooth germs in vitro. Jour. Anat., Vol. 70,
 
pp. 260-266.
 
Ilampp, E. G., 1940. Mineral distribution in the developing tooth. Anat. Rec., Vol.
77, pp. 273-291.
 
Held, //., 1926. Uber die Bildung des Schmelzgewebes. Zeitschr. f. mikr. Anat. P'orsch.,
Bd. 5, S. 668-687.
 
Ihrsch, M., 1921. Der Liickzahn von Sus domesticus, ein Beitrag zur Entwicklungsgeschichte des Gebisses von Sus domesticus und zur Kenntnis des Wesens der
Dentitionen. Anat. Anz., Bd. 54, S. 321-330.
 
Jasswom, G., 1924. Uber die Histogencse der Dentingrundsubstanz der Saugetiere.
Arch. f. mikr. Anat., Bd. 102, S. 291-310.
 
Morse, A , and Creep, R. 0 , 1947. Alkaline glycerophosphatase in the developing
teeth of the rat. Its localization and activity characteristics as influenced by pH
of the substiate and length of incubation time. Anat. Rec., Vol. 99, pp. 379-395.
 
Rein, G , 1882. Untersuchungen iiber embryonale Entwicklungsge.schichte der Milchdriise. Arch. f. mikr. Anat., Bd. 20, S. 431-501 und Bd. 21, S. 678-694
 
Saundeis, J, B, de C, M., Nuckolls, J., and Ftisbie, H. F., 1942. Amelogenesis. A histologic study of the development, formation and calcification of the enamel in the
molar tooth of the rat. Jour. Am, Coll. Dentists, Vol. 9, pp. 107-136.
 
Schmidt, V,, 1925. Studien fiber die Histogencse der Haut und ihrer Anhangsgebilde
bei Saugetieren und beim Menschen. I. Die Histogencse des Hufes bei Schweineembryonen. Zeitschr, f. mikr. Anat. Forsch., Bd. 3, S. 500-557.
 
Schour, /., and Steadman, S. R., 1935. The growth pattern and daily rhythm of the
incisor of the rat. Anat. Rec., Vol. 63, pp. 325-333.
 
Schultze, O., 1892. Ueber die erste Anlagc des Milchdrfisenapparates. Anat. Anz.,
Bd. 7, S. 265-270.
 
Thoms, H., 1896. Untersuchungen fiber Bau, Wachsthum und Entwicklung des Hufes
der Artiodactylen, insbesondere des Sus scrofa. Deutsche Thieraerzliche Wochenschr., Jahrgang. 4, S. 379-383.
 
Z^itzschmann, 0., 1924. Die Entwicklung des Systems der ausseren Haut. (b) Die
Haare. (Schwein.) Lehrbuch der Entwicklungsgcschichte der Haustierc, S.
186-194.
 
Twins, Double Monsters, Anomalies
 
Baumgartner, W, J,, 1928. A double monster pig — Cephalothoracopagus monosymmetros. Anat. Rec., Vol. 37, pp. 303-316.
 
Berge, S,, 1941. The inheritance of paralysed hind legs, scrotal hernia and atresia ani
in pigs. Jour. Heredity, Vol. 32, pp. 271-274.
 
 
 
340
 
 
BIBLIOGRAPHY
 
 
Bishopy Af., 1921. The nervous system of a two-headed pig embryo. Jour. Comp.
Neur., Vol. 32, pp. 379-428.
 
Bishops M., 1923. The arterial system of a two-headed pig embfyo. Anat. Rec., Vol.
26, pp. 205-222.
 
Carey, E,, 1917. The anatomy of a double pig, Syncephalus thoracopagus, with
especial consideration of the genetic significance of the circulatory apparatus.
Anat. Rec., Vol. 12, pp. 177-192.
 
Chidester, F. /?., 1914. Cyclopia in mammals. Anal. Rec., Vol. 8, pp. 355-366.
 
Cornel, G, W., 1921. Abnormalities of the mammalian embryo occurring before implantation. Carnegie Inst., Contrib. to Embryol., Vol. 13, pp. 61-66.
 
Corner, G. W., 1922. The morphological theory of monochorionic twins as illustrated
by a series of supposed early twin embryos of the pig. Bull. Johns Hopkins Hosp.,
Vol. 33, pp. 389-392.
 
Corner, G. W., 1923. The problem of embryonic pathology of mammals with observations upon intra-uterine mortality in the pig. Am. Jour. Anat., Vol. 31, pp.
523-545.
 
Denison, H., 1908. Notes on pathological changes found in the embryo pig and its
membranes. Anat. Rec., Vol. 2, pp. 253-256.
 
Dutta, S. K., 1930. Notes on the cyclopian eye and other deformities of the head in a
pig. (Sus cristatus Wagn.) Allahabad Univ. Studies, Sci. Sec., Vol. 7, pp. 53-103.
 
Fitzpatrick, F, L,, 1928. The dissection of an abnormally developed foetal pig, with
notes on the possible origins of such “freaks.” Proc. Iowa Acad. Sci., Vol. 35,
pp. 319-325.
 
Hughes, W., 1927. Sex-intergrades in foetal pigs. Biol. Bull., Vol. 52, pp. 121-136.
 
Jordan, H. E., Davis, J. S., and Blackford, S. D., 1923. The operation of a factor of
spatial relationship in mammalian development, as illustrated by a case of
quadruplex larynx and triplicate mandible in a duplicate pig monster. Anat.
Rec., Vol. 26, pp. 311-318.
 
Kingsbury, B, F., 1909. Report of a case of hermaphroditism (H. Verus lateralis) in
Sus scrofa. Anat. Rec., pp. 278-281.
 
Nordby, J. E., 1929. Congenitad skin, ear, and skull defects in a pig. Anat. Rec., Vol.
42, pp. 267-280.
 
Poklman, A, G., 1919. Double ureters in human and pig embryos. Anat. Rec., Vol.
15, pp. 369-373.
 
Schwalbe, E., 1906-1913. Die Morphologic dcr Missbildungen des Menschen und der
Tiere. Fischer, Jena, Vol. 1, xvi & 230 pp., Vol. 2, xx & 410 pp., Vol. 3, Abt. 1,
270 pp., Abt. 2, 858 pp., Anhang, 266 pp.
 
Streeter, G. L., 1924. Single-ovum twins in the pig. Am. Jour. Anat., Vol. 34, pp.
183-194.
 
Thuringer, J, M,, 1919. The anatomy of a diccphalic pig (Monosomus diprosopus).
Anat. Rec., Vol. 15, pp. 359-367.
 
JVarkany, J., and Roth, C, B., 1948. Congenital malformations induced in rats by
maternal vitamin A deficiency. II. Effect of varying the preparatory diet upon
the yield of abnormal young. Jour. Nutrition, Vol. 35, pp* 1-11.
 
Williams, S. R., and Rauch, R. W., 1917. The anatomy of a double pig (Syncephalus
thoracopagus). Anat. Rec., Vol. 13, pp. 273-280.
 
 
 
Index
 
 
To facilitate the use of this book in connection with otheis in which the terminology
may differ somewhat, many synonyms which were not used in the text have been put into
the index and cross-referenced to the alternative terms used in this book; for example,
Wolffian body, a teim not used in this text, is frequently applied to the mesonephros. It
appears m the index thus: Wolffian body (= mesonephros, q.v.).
 
Both figure and page references are given in the index. The figure references arc preceded by the letter /.
 
 
Abdominal pregnancy, 217
Abducens nerve, 169
Accessory nerve, 172
Acoustic ganglion, /92, 117, 169
Acoustico-facialis ganglion ( = early undifferentiated condition of ganglia of
7th and 8th cranial nerves), 169
Action system, 141
 
Adrenal, /1 00, /106, /1 27, /1 28, /1 38, 223
After-birth, 105
Alae of nose, 296
Alar plate of neural tube, 1 57
Allantois, circulation of, /45,/51, 99
formation of,/30,/37, 99
function of, 106
relations of,/49,/55, 103
Alveolar periosteum, 312
Ameloblasts,/179-183, 305, 310
Amnion, formation of,/25,/37,/50, 97
function ofi 96
relations of,/49, 97
Amniotes, 96
Amniotic folds, 97
Ampulla of ductus deferens, /3, 8
Anal plate ( = cloacal plate, q.v.)
Anamniotes, 96
Angioblast, 87 footnote
Animad pole, 37
 
Anterior intestinal portal, /37, 74
Anterior neuropore, /29, 71
Anus,/118,/132, 210, 225
Aorta, see arteries
Aortic arches, sec arteries
Aortic bulb, 256
Aortic chromaffin body, 223
Appendage buds, /3 1-34, 65, 109
 
 
Appendix, of epididymis, /1 24, 213
of testis, /1 24, 217
Appositional growth, 278
Aqueduct of Sylvius, 158
Archenteron, 43
Areola, 105
 
Arterial circle (of Willis), 131
Arteries, allantoic, /45,/51, 89, 132, 240
aorta, dorsal, /67, /1 33-1 37, 88, 131,
235, 240
 
aoita, ventral, /45, /1 33, 88, 235
aortic arches, /45, /1 33-1 37, 88, 129,
 
233
 
basilar, /66, /67, /133, /137, 131, 238
brarhio-cephalic,/133, 234
carotid, common, /1 33, /1 37, 234
carotid, external, /67, /1 33, /1 37, 130,
 
234
 
carotid, internal, /66, /133, /137, 130,
234, 238
caudal, 240
 
cervical, intersegmental,/66,/133,/l37,
130, 235
 
coeIiac,/67,/lll, 132, 240
ductus arteriosus /1 33, /ISO, 235, 262,
268
 
hypogastric, 240, 270
iliacs,/66, /150, 240
innominate, 234
internal mammary, 238
interscgmcntal,/67,/133, 130, 235
mesenteric, ant., /67, /1 11, 132, 238
mesenteric, post»,/lll, 238
omphalomesenteric, /45, 89, 131, 238
pulmonary, /67, /1 33-1 37, 130, 234,
267
 
 
341
 
 
 
342
 
 
INDEX
 
 
Arteries — {Continued)
 
renal, /1 27, /1 28, 240
speimatic, /1 27-1 30
 
subclavian, /67, /133-135, 131, 234,
235
 
umbilical (allantoic), /66, /1 50, 89, 132,
240
 
vertebral,/66,/67,/137, 131, 237
vitelline, /45, 89, 132
Arytenoid process of larynx, /1 38
Astrocytes, 150
Atresia of ovarian follicles, 20
Atrio-ventricular canal, 126, 256
Atrium, 126, 254
 
Auditory, ganglion, see acoustic nerve, 1 69
vesicle, /36,/61, 117, 169
 
Basal plate of neural tube,/91, 157
Belly-stalk, /49, 97
Bicornate uterus, 216
Bile duct, common, 185
Bladder (urinary), /118, /124, /125, /130,
209
 
Blastocoele, j\S, 41
Blastocyst, /1 7, 41
 
elongation of, /1 8, 45
riastodermic vesicle ( == blastocyst, q.v.)
Blastodisc ( — embryonic disc, q.v.)
Blastorneie, 38
Blastula, 41
 
Blood cells, formation of,/48,/152, 90, 251
Blood islands, /48, 89, 251
Blood vessels, formation of, /48, 87
see also arteries and veins
Body axis, 60
Body cavity, see coelom
Body folds, /38, 72
Body-stalk, sec belly-stalk
Bone, cancellous, 272
cells, /1 51, /1 52, 275
compact, 272
endochondral, 272
histogenesis of, 271
intramembranous, 272
lacuna, 275
lamellae, /1 51, 275
marrow, /1 52, 251
matrix, /1 51, 273
trabeculae, /1 51, /1 53, 274
Bone, formation of,
 
compact from cancellous, 281
 
 
Bone — '{Continued)
 
endochondral, /1 55, 276
flat, /1 57, 282 '
 
intramem branous, /1 5 1 , /1 53, 272
long, /1 58, 283
 
primary cancellous, /1 51, /153, 272,
281
 
Bowman’s capsule, /1 16, 203
Brachial plexus, 116
Brain, formation of, /36, 69
neuromeric structure of, 69
regional differentiation of,/87, 1 S4
ventricles of, /88, 156, 160
3-vesicle stage, /36, 70
5-vesicle stage, /60, 110
Branchial arches (= gill arches, q.v.)
 
Bridge of nose, 296
 
Broad ligaments of uterus, 222
 
Bronchi, 189
 
Bulbo-urethral gland, /3, /1 24, 9, 213
 
Clalcification
 
of bone, 274
of teeth, 309
 
Calyces of renal pelvis, /1 19
Canal, atrio-ventncular, 126
Haversian, /1 56, 282
inguinal, /1 29, 222
of Gartner, 217
pleural,/! 10
pleuro- peritoneal, 195
semicircular, 169
Canalicuii, /152
Cancellous bone, /1 53, 276
Capsule, glomerular, /1 21, 203
of Bowman, /1 21 , 203
of cartilage cells, 278
Cardiac loop, /1 42, 125, 254
Cardinal vessels, see veins
“Cartilage bone,” 272
Cartilage, erosion, /1 55, 280
formation, /1 54, 277
matrix, 278
Caval plica, /1 40, 245
Cecum, /102, 122, 182
Cementoblasts, /1 84
Cemcntum,/184, 312
Central canal of spinal cord,/86, 151
Centrum, see vertebrae
Cephalic region, differentiation of, 61
mesoderm of, 1 91
 
 
 
INDEX
 
 
343
 
 
Cephalic region — (Continued)
precocity of, 54
Cerebellar peduncles, 157
Cerebellum, /80,/87, 157
Cerebral aqueduct, 158
Cerebral cortex, 145, 162
Cerebral ganglia, see ganglia, cranial
Cerebral hemispheres, /87, 162
Cerebral peduncles, /1 00, 158
Cerebro-spinal paths, 142
Cervical flexure, 66
Cervical sinus, /32, /1 69, 109
Cervix of uterus, 216
Choanae, of nose, 296
Chondrin, 278
Chondrogenesis, 276
Chondrogenctic layer, 278
Chorda dorsalis ( = notochord, q.v.)
 
Chorion, /55-57, 103
Chorionic vesicle, /54, 103
Chorionic villi, /57
Choroid fissure, of eye,/39, 117
Choroid plexus, anterior (of 3rd ventricle),
/106, 158, 162
 
lateral (of 1st and 2nd ventricles), /1 00.
 
162
 
posterior (of 4th ventricle), /65, /99,
/1 38, 156
 
Chromaffin tissue, 223
Chromosomes, sex, 23
species number of, 21
Circle of Willis, /1 33, 131
Circulation, changes in at birth, 268
early embryonic, /45,/51, 92
hepatic portal, 241
interpreta^tion of embryonic, 227
placental,/45,/51,/55, 93, 263
pulmonary, 262, 267
vitelline, /45, /1 41, 93, 249
Circulatory arcs, 92
Cleavage, /1 2, /1 3, /1 4, 37
Cleavage cavity, see blastococle
aitoris,/132, 225
Cloaca, /65, /1 18, 120, 209
Cloacal plate (membrane), /37, 209
Cochlea, 169
Coelom, /1 09
 
abdominal, /1 11, 194
diflferentiation of, /1 08, 193
formation of,y20, f22,/108, 51, 120, 189
intra- and extra-embryonic, 52, 190
 
 
Coelom — (Continued)
 
partitioning of, /1 11-1 13, 194
pericardial, /26, /44, /1 09, /1 10, /111,
53, 87, 195
peritoneal, /1 10, 189
pleural,/! 11, /1 13, 194
thoracic, 194
 
Colliculi, inferior, suptTior (lobes of corpora
quadngemma), /80, /87, 157, 167
Colon (large intestine), /1 02, 182
Columns of spinal cord, /8 6, 154
Commissural ganglion, /92, 172
Components of spinal nerve, 151
Concrescence, /20, 46
Coordinating centers, 145
Copula, /41
Cord, spinal, 71, 115
 
and reflexes, /80, 142
histogenesis of, /8 1-8 3, 147
white and grey matter of,/86, 151
Cord, umbilical, 250
Corona radiata, 18
Coronary sinus, sec veins
Corpora quadrigemma, /87, 157
Corpus albicans, 27
Corpus hacmorrhagicurn, 26
Corpus luteum, formation of,/6, 24
in pregnancy, /1 0, 26
significance of, 33
Corpus striatum, 1 63
 
Cowper’s gland (= bulbo-urethral gland,
q.v.)
 
Cranial flexure, 66
 
Cranial ganglia, see ganglia
 
Cranial nerves, see nerves
 
Crura cerebri (= cerebral peduncles, q.v.)
 
Cumulus oophorus, 19
 
Cutis plate, see dermatome
 
Cystic duct,/104,/105, 184
 
Decalcification, 274
Deciduous placenta, 103
Deitcr’s nucleus, /80, 144
Dental ledge, /1 75-1 80, 302
Dental papilla, 307
Dental sac, /1 82, 312
Dentinal fiber /1 83, 310
Dentine, /1 83, 307
Dermatome, /42, 81
 
Deutoplasm, effect of on cleavage, /1 2, 37
in pig ovum, 39
 
 
 
344
 
 
INDEX
 
 
Diaphragm, /1 00, /1 12, 194
Diaphragmatic ligament of mesonephros,
/1 23, 218
 
Diaphysis, of long bone, 283
Dicnccphalon, /60, 110, 158
Diestrum, 29
 
Diocoele ( =« lumen of diencephalon)
Dio-mesencephalic boundary, 110
Dio-telcncephalic boundary, 110
Diploe, of bone, 282
Diploid number of chromosomes, 22
Dorsal, aorta, see arteries
flexure, 66
 
mesentery, /1 08, /1 11, 192
mesocardium,/43,/144, 87, 254
nerve roots, see nerves, spinal
root ganglia, see ganglia, spinal
Duct of, Cuvier ( = common cardinal
vein, q.v.)
 
Santorini, 186
Wirsung, 186
 
Ductus, arteriosus, /1 33, /1 38, /1 50, 235,
262, 268
 
choledochus ( = common bile duct, q.v.)
deferens, /3, /1 18, /1 24, 9, 213
endolymphaticus, 117
venosus, see veins
Duodenum, /1 00, /1 02, 184
 
Ear, external, /33,/34, 62
internal, 62, 117, 169
middle, 118, 170
Ectoderm, 59, 98
 
derivatives of,/27
Efferent ductules, 213
Egg nests, 1 5
 
Ejaculatory duct, /3, /1 24, 9, 213
Embryonic disk, /1 6, /1 9, 45, 60
Enamel, /183, 310
Enamel organ, /1 79-1 82, 303
Enamel prisms, 311
 
Endocardial cushion of A-V. canal, /39,
/147,/148, 126, 256
Endocardial cushion tissue, 256
Endocardial primordia,/43,/44, 85
Endocardium, 85
Endolymphatic duct, 117
Endothelium, origin of vascular, /48, 87
Entoderm, 59
 
derivatives of, /27
formation of, /1 6, 43
 
 
Eparterial bronchus, /1 07
Ep>endymal cells, 150
Ependymal layer of c^d,/81, 147
Epicardium, 85
 
Epididymis, /3, /1 24, /1 29, 213
 
Epi-myocardium,/43,/44, 85
 
Epiphyseal cartilage plates, 284
 
Epiphyseal ossification centers, 284
 
Epiphyses of long bones, /1 58, 284
 
Epiphysis, of diencephalon, /1 06, 110, 158
 
Epiploic foramen, 181
 
Epo6phoron,/125, 217
 
Equation division, 22
 
Erythroblasts, /1 52
 
Esophagus, /64,/72, 80, 119, 179
 
Estrus, 29
 
Estrus cycle, /9, 29
 
Eustachian tube, 118, 175
 
Evolution, 198, 229
 
Exocoelom, 52
 
Extra-embryonic coelom, see coelom
Extra-embryonic membranes, /49, 94
Eye, 63, 116, 167
 
Face, development of, /1 68-1 72, 293
Facial nerve, /92, 169
Falciform ligament, /1 1 1 , 193
Fallopian tube (= uterine tube, q.v.)
Fasciculi, see columns of spinal cord
Fertilization, /1 1, 34
Flexion, 65, 107
Floor plate of neural tube, 156
Fetal membranes, see extra-embryonic
membranes
 
Follicle, ovarian, /6,/7, 17
Foramen, ovale, 260, 267, 268 see also
interatrial
 
Foramen of Monro, /88, /1 38, 160
Foramen of Winslow, 181
Fore-brain, see prosencephalon
Fore-gut, /26,/37, 74
Fossa ovalis, 269
 
Fovea cardiaca (= ant. intestinal portal,
q.v.)
 
Frontal lobe, 163
 
Frontal process, /1 68-1 72, 293
 
Froriep’s ganglion, /92, 172
 
Gall-bladder, /46, /103-105, /1 40, 80, 119,
184
 
Gametes, /8, 9, 12, 15
 
 
 
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.

Patten 1951: 1 Foreword to the Student | 2 Reproductive Organs - Gametogenesis | 3 Sexual Cycle | 4 Cleavage and Germ Layers | 5 Body Form and Organs | 6 Extra-Embryonic Membranes | 7 Embryos 9-12 mm | 8 Nervous System | 9 Digestive - Respiratory and Body Cavities | 10 Urogenital | 11 Circulatory System | 12 Bone and Skeletal System | 13 Face and Jaws | Bibliography
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This historic 1951 embryology of the pig textbook by Patten was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.


By the same author: Patten BM. The Early Embryology of the Chick. (1920) Philadelphia: P. Blakiston's Son and Co.

Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381


Modern Notes

pig

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Embryology of the Pig

Bradley M. Patten
Bradley Patten ( -1971)

Frontispiece

Reconstruction (X 17.5) showing the organ systems of a 9.4 mm. pig embryo. For explanation see figures 60 and 66.

By Bradley M. Patten

Professor of Anatomy in the University of Michigan Medical School


Third Edition


With Colored Frontispiece

And 186 Illustrations In The Text (Containing 412 Figures) Of Which 6 Are In Color


Philadelphia : THE BLAKISTON COMPANY : Toronto


Third Edition


Copyright, October 1948, by The Blakiston Company


By P. Blakiston's Son & Co.


Copyright, 1951, by P Blakiston's Son & Co , Inc.


Patten 1951: 1 Foreword to the Student | 2 Reproductive Organs - Gametogenesis | 3 Sexual Cycle | 4 Cleavage and Germ Layers | 5 Body Form and Organs | 6 Extra-Embryonic Membranes | 7 Embryos 9-12 mm | 8 Nervous System | 9 Digestive - Respiratory and Body Cavities | 10 Urogenital | 11 Circulatory System | 12 Bone and Skeletal System | 13 Face and Jaws | Bibliography

Preface to Third Edition

In making the revision for a new edition of this book it did not seem desirable essentially to change its form or scope. Effort has been concentrated on improving the presentation of the original subject matter and bringing it up to date, rather than on its expansion. The entire book has been reset to a greater page width which has permitted enlarging certain of the illustrations that, in earlier editions, had proved to be too greatly reduced. With the generous cooperation of the publishers several of the important plates on the cardiovascular system have been remade to a larger scale and with color. A number of new illustrations have been drawn for sections where experience has shown that students needed additional graphic assistance in interpreting their laboratory material. It is hoped that these changes will all contribute toward making the book as a whole more serviceable.


Bradley M. Patten

August 1048


Preface to First Edition

This book represents an endeavor to set forth in brief and simple form the fundamental facts of mammalian development. The thread of the story and the illustrations have been based on pig embryos because of their value and availability as laboratory material. But special stress has been laid on the embryological phenomena involved instead of on the details of specific conditions existing in the pig. Throughout the book, every efTort has been made to present developmental processes as dynamic events with emphasis on their sequence and significance, rather than as a series of still pictures of selected stages.


Obviously no book can deal fully with all phases of development, even in a single form, and still remain serviceable as a text. As this book is for the student, it has seemed expedient, for the sake of clearness and simplicity, to omit many things which I should like to have included. My primary aim has been to write an account in which the essentials stand out adequately interpreted and unobscured by a multiplicity of details — to lay a foundation which can be further built upon in accordance with special needs or individual desires.


Bradley M. Patten

January 1927

Acknowledgment

The pleasantest thing about working on this book has been the generous aid I have received from many sources. Throughout the preparation of the initial edition the encouragement, criticism, and suggestions of my colleagues, Dr. F. C. Waite and Dr. S. W. Chase, were of the greatest help. In the preparation of material and in making the illustrations for the first edition the beautifully accurate work of Miss Kathryn Toulmin was of inestimable value. In making the new drawings added in the third edition I was fortunate in securing the unusually able assistance of Mrs. Dorothy Van Eck.


Dr. G. L. Streeter and Dr. C. H. Heuser of the Carnegie Institute allowed me free use of their extensive collection of young embryos and generously gave me many photographs made from their material. To Mrs. Charles S, Minot I am indebted for permission to use several figures from the late Professor Minot's works. The accrediting in the figure legends of these and other borrowed illustrations by no means covers my obligation to other writers. Practically the entire bibliography is a statement of indebtedness for information and ideas.


I wish I might acknowledge individually the helpful services rendered by many of my students, but they are too numerous. Several reconstructions which I have used directly or indirectly have been largely their work. Of even greater assistance have been their suggestions during the shaping of the work — suggestions of especial value because they were made from a point of view difficult for an instructor to appreciate without such aid.


To The Blakiston Company, I am indebted for much helpful cooperation and especially for their liberality with regard to the number and quality of the illustrations.


No person other than my wife could have deciphered and put into usable form manuscript of the character I frequently turned over to her for revision and typing. Without her generous help the preparation of the text would have been much more arduous and long delayed.


Bradley M. Patten


Cite this page: Hill, M.A. (2026, October 15) Embryology Book - Embryology of the Pig. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig

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© Dr Mark Hill 2026, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G


Patten 1951: 1 Foreword to the Student | 2 Reproductive Organs - Gametogenesis | 3 Sexual Cycle | 4 Cleavage and Germ Layers | 5 Body Form and Organs | 6 Extra-Embryonic Membranes | 7 Embryos 9-12 mm | 8 Nervous System | 9 Digestive - Respiratory and Body Cavities | 10 Urogenital | 11 Circulatory System | 12 Bone and Skeletal System | 13 Face and Jaws | Bibliography
Retrieved from ‘https://embryology.med.unsw.edu.au/embryology/index.php?title=Book_-_Embryology_of_the_Pig&oldid=299731’
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