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! Online Editor &nbsp;
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| [[file:Mark_Hill.jpg|90px|left]] This 1949 third edition textbook by McEwen describes embryonic development.
<br><br>
[https://archive.org/details/06031200R.nlm.nih.gov/page/n3 1923 Edition]
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'''Modern Notes:''' [[Historic Textbooks]]
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=Vertebrate Embryology=
=Vertebrate Embryology=
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Professor Emeritus of Zoology, Oberlin College
Professor Emeritus of Zoology, Oberlin College
==Contents==
[[Book - Vertebrate Embryology (1949) 1|Part I The Germ Cells and Early Development of Amphioxus]]
* 1. Introduction
* 2. Fertilization and Early Stages in Development
* The Early Development of Amphioxus
[[Book - Vertebrate Embryology (1949) 2|Part II The Development of the Frog]]
* The Frog: from the Production of the Germ Cells through Gastrulation
* The Frog: Early or Embryonic Development Subsequent to Gastrulation
* The Frog: Later or Larval Development
[[Book - Vertebrate Embryology (1949) 3|Part III The Teleosts and Gymnophiona]]
* The Teleosts and Gymnophiona: their Segmentation and Gastrulation
[[Book - Vertebrate Embryology (1949) 3|Part IV The Development of the Chick]]
* The Chick: the Adult Reproductive Organs, and the Development of the Egg Previous to Gastrulation
* Gastrulation and Development through the First Day of Incubation
* The Chick: Development during the Second Day of Incubation
* The Chick: Development during the Third Day of Incubation
* The Chick: Development during the Fourth Day of Incubation
* The Chick: Development during the Fifth and Subsequent Days
[[Book - Vertebrate Embryology (1949) 3|Part V The Development of the Mammal]]
* The Early Development of the Mammal and its Embryonic Appenclages
* Development of the Pig to the Ten Millimeter Stage
* The Later Development of the Pig
* The Skeleton, Teeth, Hair, Hoofs and Horns
Index


==Preface to the Fourth Edition==
==Preface to the Fourth Edition==
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Oberlin College, August 15, 1923.
Oberlin College, August 15, 1923.


==Contents==


[[Book - Vertebrate Embryology (1949) 1|Part I The Germ Cells and Early Development of Amphioxus]]


1. Introduction
2. Fertilization and Early Stages in Development
The Early Development of Amphioxus
[[Book - Vertebrate Embryology (1949) 2|Part II The Development of the Frog]]
The Frog: from the Production of the Germ Cells through Gastrulation
The Frog: Early or Embryonic Development Subsequent to Gastrulation
The Frog: Later or Larval Development
[[Book - Vertebrate Embryology (1949) 3|Part III The Teleosts and Gymnophiona]]
The Teleosts and Gymnophiona: their Segmentation and Gastrulation
[[Book - Vertebrate Embryology (1949) 3|Part IV The Development of the Chick]]
The Chick: the Adult Reproductive Organs, and the Development of the Egg Previous to Gastrulation
Gastrulation and Development through the First Day of Incubation
The Chick: Development during the Second Day of Incubation
The Chick: Development during the Third Day of Incubation
The Chick: Development during the Fourth Day of Incubation
The Chick: Development during the Fifth and Subsequent Days
[[Book - Vertebrate Embryology (1949) 3|Part V The Development of the Mammal]]
The Early Development of the Mammal and its Embryonic Appenclages
Development of the Pig to the Ten Millimeter Stage
The Later Development of the Pig
The Skeleton, Teeth, Hair, Hoofs and Horns
Index
==PART IV THE DEVELOPMENT OF THE CHICK==
THE CHICK: THE ADULT REPRODUCTIVE ORGANS, AND THE DEVELOPMENT OF THE EGG PREVIOUS TO GASTRULATION
T H E Chick has long been an object of ernbryological interest, and
the study of its development has been connected with such classical
names as Malpighi (1672), Wolff (1759), and Von Baer (1828). In the
more modern era of science, moreover, workers in this field have continued to study it, until at the present time probably more details
regarding its development are known than in the case of any other animal. As will appear, however, certain points concerning the very early
stages are even yet in doubt, and are still under investigation.
Some of the reasons for the importance of this form and the study
which has been given it may be briefly indicated. In the first place the
material is usually easy to obtain and observe throughout most of the
developmental stages. Furthermore, unlike the Frog or Fish, the Chick
embryo, in common with those of other Birds as well as with those of
Reptiles, possesses certain very significant extra-embryonic membranes
and appendages. The significance of these structures lies not only in
their character and functions in the groups just cited, but also in the
fact that the same appendages and membranes occur also in the Mammals, though in a somewhat modified condition. Lastly, aside from the
features already indicated, the general development of the Chick is
more nearly mammalian than that of any of the forms previously considered.
In the following account we shall begin with a brief description of
the reproductive organs of the adult Bird.
REPRODUCTIVE ORGANS OF THE ADULT, OCDGENESIS
AND OVULATION
THE MALE
The male Bird, or Cock, possesses a pair of testes, each of which is
an ellipsoidal body about two inches long and one inch in diameter.
THE FEMALE 231
It is made up of seminiferous tubules and supporting tissue, and, as in
the case of the Frog, is rather closely attached to the dorsal wall of the
coelom by a fold of coelomic epithelium, the mesorchium. By way of
'the vase e flerentia, each testis discharges its products into its respective
Fig. 151. — Section of an ovarian ovum of the Pigeon, drawn from a preparation of
Mr. J. T. Patterson. From Lillie (Development of the Chick) . The actual dimensions of the ovum are 1.44 x 1.25 mm.
f.s. Stalk of follicle. G.V. Germinal vesicle. Gr. Granulosa. L. Latebra. p.P. Pe
ripheral protoplasm. pr.f. Primordial follicles. T h.ex. Theca externa. T hint. Theca
inter-na. Y.Y. Yellow yolk. Z.r. Zdna radiate.
vas deferens. The latter duct then leads to the cloaca, where its entrance
is marked by a papilla. There is some evidence that the sperm attain
their motility and functional capacity by the action of a testis hormone
during their passage through the vasa eflerentia (Munro, ’38).
THE FEMALE
The Ovary.—In the embryo Chick two ovaries are present, but
only the left develops. In the adult Fowl this is suspended from the
282 THE CHICK
Fig. 152.—-Reproductive organs of the Hen. (After Duval, based on a figure by
Coste). From Lillie (Development of the Chick). The figure is diagrammatic in
one respect, namely, that two ova are shown in the oviduct at different levels; normally but one ovum is found in the oviduct at a time.
1. Ovary; region of young follicles. 2 and 3. Successively larger follicles. 4. Stigmata (cicatrices), or non-vascular areas, along which the rupture of the follicles
takes place. 5. Empty follicle. 6. Cephalic lip of ostium. 7. Funnel of oviduct
.(ostium tubae abdominale‘ 5?. 0"-fin in the upper part of the oviduct. 9. The mag
num, where most, if not all, the albumen is actually secreted. 10. Albumen surrounding an ovum. 11. Ovum in portion of duct laid open to show it. 12. Germinal
disc. 13. The isthmus where the shell membrane is secreted, and possibly some thin
albumen. 14. The uterus where shell is secreted, and both layers of thin albumen
separated from remainder, producing thick albumen and chalazae (see text). 15.
Rectum. 16. Reflected wall of abdomen. 17. Anus, or external opening of cloaca.
THE FEMALE ‘ 233
body wall by the mesovarium in about the same position as the left
testis in the male. It consists of the usual vascular connective tissue
elements, or stroma, within which are imbedded ova in various stages
of growth. Each ovum is surrounded by a layer of follicle or granulosa
cells, and these in turn are encased in a sheath of the stroma called the
theca. It is sometimes customary to refer to such stages together with
their coverings as simply follicles (Fig. 151) . Normally only one ovum
matures at a time, though there may be several not many hours apart.
The Genital Tract.—-As in the case of the ovary, only the left
genital tract develops. This fact is apparently correlated with the production by Birds of fragile shelled eggs, such that the coming together
~ of two at the cloaca would be disastrous. In this connection it is of
some interest to note that although in certain species of Hawks there
are two fully developed ovaries, there is still only one genital tract
(Stanley and Witschi, ’4«0) . As regards this tract, we find that it opens
anteriorly adjacent to the ovary and posteriorly into the cloaca just
dorsal to the anus. Also it is suspended as usual from the dorsal body
wall by a mresentery-like fold of peritoneum, and in the Birds it may be
divided into three main parts as follows:
I . The Oviduct Proper. This is the anterior part and is itself divisible
into three sections:
(a) The Infundibulum or Ostium. This is a thin-walled muscular
funnel, the inner surface of which is lined by ciliated epithelium. It is
in the immediate neighborhood of the ovary, but does not directly connect with it. A '
(17) The Magnum. This is sometimes called the “ glandular portion,”
but since other parts are also glandular this is not a very good designation. The part in question is a long much convoluted tube following
immediately after the ostium. It leads into:
(c) The Isthmus. This is a shorter tube also glandular whose pos
terior end marks the termination of the oviduct proper.
II. The Uterus. This is a relatively short, dilated portion whose walls
are also glandular. It immediately follows the isthmus and leads into
the third and last main division: ‘
II I . The Vagina. This region is likewise short, but thin-walled, and
opens into the cloaca (Fig. 152).
The Ofigonia. ———The origin of the primordial germ cells and their
multiplication as oiigonia occur during the embryonic life of the Chick.
Thisearly history will therefore be dealt with later in connection with
the development of the gonadswkt the time of hatching, however, the
284 THE CHICK
oéigonia are said to have ceased to divide, and each is becoming surrounded by follicle cells preparatory to growth (Fig. 153). They may
now, therefore, be called oéicytes, or young ova, whose history from this
point onward will be taken up in more detail.
The Growth Period.
The Vitelline Membrane or Zona Radiata. — There now appears surrounding each ovum or oiicyte a membrane which is called the vitelline
membrane. Whether it is a true vitelline membrane arising entirely
from the surface of the egg itself, or whether it is secreted by the follicle cells and is therefore chorionic in character, is somewhat uncertain. As this membrane thickens slightly, it becomes pierced by minute
canals; for this reason it is also referred to sometimes as the zona radiata. Throughout these canals by way of the follicle cells the egg receives nourishment from the surrounding theca.
The Germinal Disc. —At first the nucleus occupies the center of the
oficyte, and the yolk granules are deposited in the cytoplasm around it.
This presently results in the existence of yolk-free cytoplasm only
around the periphery of the egg. This cytoplasm, however, is thicker
upon the side where the theca of the ovum is attached to the ovary; this
thickening is called the germinal disc (blastodisc) . Meanwhile the
ovum has been growing, and by the time it has become .6 mm. in diameter, the nucleus has migrated into this disc (Fig. 151).
The Deposition of Yolk. -——The growth of the ovum is largely due
to the deposits of yolk, which it appears occur in the following manner:
The nucleus, as noted, occupies at first a central position around which
the yolk begins to be formed. This‘ yolk is of a lightish color termed
white yolk, and the central mass of it which is thus deposited is known
as the latebra. Following this the peripheral layer of the protoplasm
starts to deposit around the latebra a darker colored substance, the yellow yolk. As the egg is thus enlarged, the nucleus, as indicated, leaves
its central location and takes a peripheral position, which it maintains
during subsequent growth. The result is that the yellow layer is everywhere interrupted along the path which the nucleus has taken. Along
this path there is thus left a continuous deposit of white yolk extending
from the latebra almost to the surface. It is known as the neck of the
latebra, and just beneath the blastodisc it spreads slightly to form a
plate, the nucleus of Pander (Fig. 154, B).
It should be noted that in some instances the deposit of yellow yolk
is interrupted by intermittent, usuallkthinner, layers of more white
THE FEMALE
E
Fig. 153. ——Growth stages in the oiigenesis of the Hen’s egg. From
Kellicott (Chordute Development). After Sonnenbrodt. A. Oiicyte
measuring 0.012 x 0.016 mm., the nucleus of which is 0.006 mm. in
diameter. B. Oiicyte measuring 0.018 x 0.028 mm., the nucleus of
which is 0.0105 x 0.014 mm., enclosed in follicle. C. Oiicyte measuring
0.040 x 0.045 mm., the nucleus of which is 0.020 x 0.022 mm. D. The
nucleus only, of an oiicyte measuring 5.84 x 6.16 mm., the nucleus
itself measuring 0.214 x 0.238 mm. Total view showing the small
chromosomes in the midst of a collection of chromatin nucleoli. E.
Vertical section of the nucleus only, of an oiicyte, the follicle of which
measured 37 mm. in diameter. The nucleus itself is 0.455 mm. in diameter and 0.072 mm. in greatest thickness.
c. Chromosomes. cr. Extra nuclear chromosome-like bodies. f. Follicle. m. Nuclear membrane. mf. Folds in nuclear membrane. 11. Nucleus. nu. Chromatin nucleolus. ps. Pseudo-chromosomes. .9. Centrosome. 1;. Yolk nucleus or vitellogenous body.
285
285 _ THE CHICK
Fig. 154. —— Semidiagrammatic illustration of the I-Ien’s
egg at the time of laying. From Kellicott (Chordate
Development). A. Entire “egg.” Modified from Mar»
shall. B. Vertical section through the vitellus or ovum
proper, showing the concentric layers of white and
yellow yolk. Actually there are seldom, if ever, as many
layers as this under normal conditions.
a. Air chamber. ac. Chalaziferous layer of albumen.
ad. Dense layer of albumen. af. Fluid layer of albumen.
b. Blastoderm. c. Chalaza. l. Latebra. nl. Neck of latebra. p. Nucleus f Ponder. pv. Perivitelline space. s.
Shell. smi. Innerelayer of shell membrane. Smo. Outer
layer of shell membrane. 1:. Vitellus or “yolk.” om.
Vitelline membrane. wy. Layers of white yolk. yy. Layers of yellow yolk.
yolk. This alternation was once thought to be universal, and to result
from the fact that yellow yolk was deposited during daylight and white
yolk at night (Fig. 154-, B). As indicated, however, many eggs can be
found in which no such alternation of layers exists, all the yolk aside
from the latehra and its neck being yellow. Experiment has now shown
that the diflerences in color of the layers, when they occur, are due entirely to alternating differences in the character of the food. The deeper
yellow is produced by xanthophyl, and appears in the yolk when grass
. , <3... .....
FERTILIZATION AND MEIOSIS 237
or yellow corn occurs in the diet; If this is fed periodically, it results in
an alternation of darker and lighter layers. Thus by proper feeding thick
or thin, few or numerous, layers can be produced at will. The white
yolk of the latebra and its neck, however, always occurs, and is evidently of a differentcharacter. It apparently‘ results from some influence of the nucleus, but its cause is unknown (Conrad and Warren, ’39) .
_ OVULATION, MEIOSIS, AND FERTILIZATION
During these processes the nucleus has greatly enlarged and as usual
in its enlarged form it is known as the germinal izesicle. The first maturation division is initiated about 4% hours previous to ovulation, and
is completed in about 2% hours, after which the spindle for the second
division is formed (Olsen, ’42, ’50) . At this point the large ovum still in
the ovary is grasped by the funnel shaped infundibulum or ostium. The
theca and follicle then rupture along a non-vascular line, the cicatrix,
and the egg is received into the oviduct. ’
Finally it may be noted that occasionally two eggs may mature and
be released together, in which case they are enclosed in a single shell
and form a “ double yolk egg.” While this is apparently the most usual
cause of this condition it is not the only one. Such eggs may also result either from the premature or the late ovulation of one of the
“ yolks ” (eggs), or from the picking up by the infundibulum of an
extra egg which has previously fallen into the body cavity.
FERTILIZATION AND MEIOSIS
When the egg is taken into the ostium, it is at once surrounded by
sperm which have been received from the male at a period from 24 hours
to two weeks previous to the ovulation of the ovum in question. Several
sperm enter the egg presumably, as in the Pigeon, in the neighborhood
of the hlastodisc, following which the second polar body is given off
and the egg pronucleus fuses with that of one of the sperm. Many of
the remaining sperm nuclei then degenerate, while others (supernamerary ‘nuclei or merocytes) persist for a time and produce certain phenomena to be described later in connection with segmentation.
THE HISTORY OF THE OVUM FROM FERTILIZATION
THROUGH GASTRULATION
The stages now to be described have not all been completely worked
out for the Chick. It is presumed, however, that they are somewhat
283 THE CHICK
similar to the corresponding stages in the Pigeon which have been fully
described by Patterson and Blount. Data concerning doubtful stages in
the Hen’s egg have therefore been partially supplied from the facts re:
garding the Pigeon. The points where this has been done will be noted
in passing.
v
THE APPLICATION or ALBUMEN, SHELL MEMBRANES
AND SHELL
Strictly speaking, the formation of the ovum proper is completed at
the time of ovulation, and it thus appears that what is ordinarily
spoken of as the “yolk” of the Hen’s egg is really the entire egg.
Nevertheless, in the case of the Bird, it is common usage to include
under the term egg not only the ovum proper (i.e., the “ yolk ”) but
also all its tertiary membranes, and this usage will be adhered to in the
following account:
As the yolk passes down the oviduct it takes a position such that a
line passing through the blastodisc and the center of the vegetal pole is
at right angles to the longitudinal axis of the duct at any particular
point. It then revolves slowly about the latter axis, and while so doing
receives its respective coverings from certain portions of the duct. In the
completed product these coverings of the egg or “ yolk ” are as follows:
Closely applied to the yolk comes a dense layer of albummous substance filled with fine mucin-like threads. This layer forms a thin but
firm covering, the chalaziferous membrane. At each side of the yolk opposite each end of the shell this membrane is twisted into cords, the
chalazae. Immediately outside of this chalaziferous membrane there is
said to occur a very narrow layer of thin watery albumen (Conrad and
Scott, ’38). There then comes a clear but relatively dense and wide
layer of albumen called simply dense albumen. Its density is apparently
also due to the presence of mucin. This layer in turn is surrounded by a
fairly wide layer of thin watery albumen called thin albumen which is
bounded externally by the so-called shell membrrme. The latter is a very
real and definite membrane in immediate contact with the outermost
coverings of all, the calcareous shell. The chalazae and the wide layers
of dense and of thin albumen are easily demonstrated by carefully
breaking an uncooked egg into a finger bowl. The innermost narrow
layer of thin albumen next to the chalaziferous membrane, however, is
not usually seen except by the use of more refined methods. The shell
membrane is readily detectable sticking to the inside of the shell. In a
hard-boiled egg the albumen can be more or less unwound in spiral
EGG MEMBRANES AND SHELL 289
sheets, apparently a result of the revolving of the egg in the duct during
its application. (Fig. 154, A).
The question now arises as to what parts of the genital tract listed
above are responsible for the different layers and membranes just indicated. This has been investigated by various workers, Asmundsen and
Burrnester (’36), Burmester C40), Cole (’38), Conrad and Phillips
(’38), Scott and Wai-Lan Huang (’4~l) and others. These men have
attacked the problem by removing parts of the duct to see what layers
were reduced or lacking, by studying the histology of parts of the tract
and in other ways. While the results of their investigations are not in
entire agreement on some details the following conclusions taken
largely from the discussion of Conrad and Scott (’38) are probably
very near to the truth.
Products of the Magnum. — The egg having taken about 18 minutes to pass the infundibulurn enters the magnum which it goes through
in a little short of three hours.‘ This latter region secretes all of the
thick or dense dlbumen which owes its character to numerous mucin
threads. Some (Asmundsen) claim that a little thin albumen (that of
the narrow layer?) is also secreted by the anterior part of the magnum,
but this seems to be one of the points on which there is disagreement
(see below).
Products of the Isthmus.———The egg passes through this part of
the duct in about 74 minutes, and receives here the shell membrane.
There may also be a little thin albumen secreted by this part of the
duct, though Conrad and Scott claim that almost all, if not all, of this
is produced, i.e., differentiated from other materials, while the egg, is
in the uterus. As will presently appear, however, not all the constituents
of this albumen are actually secreted in the latter organ.
Products of the Uterus.—-—The egg remains longest of all in this
region, about 20% hours, and as just suggested it is while the egg is
here that practically all of the thin albumen is -differentiated as such.
As noted, however, all of the material for this layer does not actually
originate in this part of the tract. Instead that portion of it which does
arise here consists largely of thin non-albuminous fluid and soluble
salts. This solution of salts then passes by osmosis through the already
existent shell membrane which is thereby distended. When the fluid in
question thus comes next to the dense albumen some of the protein
in the latter, other than the mucin, soon diffuses into the fluid. In this
1 Average time spent in Various parts of the duct was kindly furnished by
Dr. D. C. Warren.
290 THE CHICK
way the latter becomes albuininous, though still thin because it lacks
mucin threads.
While the egg is in the uterus there are also produced the chalazae,
chalaziferous membrane and the narrow layer of thin albumen. In this
case, however, none of the materials concerned are secreted here. The
substances for these structures are already present in the dense albumen
produced in the magnum. What happens is this: The muein fibers in the
part of the thick albumen immediately adjacent to the yolk are withdrawn from this albumen, and are concentrated against the yolk to
form the chalaziferous membrane. This concentration leaves the albumen next to the membrane without any fibers, and hence it becomes
thin, thus forming the very narrow thin layer noted as occurring in this
region. The chalazae are simply extensions of the concentration at the
two sides of the yolk. They are twisted apparently because the egg was
rotating at the time the albumen from which they are derived was laid
down, and possibly because rotation is still going on. The cause of the
separation of the mucin from the albumen is believed to ‘be mechanical,
but the process is not entirely clear. I
Finally the shell is entirely secreted by the uterus, and is known to be
substantially advanced, though not completed, after 8-10 hours within
that part of the genital tract. The source of the cuticle of the shell is uncertain, but it may be denatured protein. _
The Vagina. —- The egg probably remains only a few seconds in the
vagina before it is laid, and there is nothing added to it here.
THE PERIODICITY OF LAYING
The periodicity in the laying of eggs has been a subject of considerable investigation. Most chickens have an annual laying period of eight
or nine months, the commonest interval of rest being during the late
summer months. During the active period the Bird lays more or less
continuously at the rate of about an egg a day, if the eggs are constantly
removed. Otherwise when a suflicient number have been accumulated
the impulse to “ set ” may assert itself, and the laying ceases while a
brood is hatched and raised. From this it might be inferred that the impulse to set is dependent merely upon the accumulation of a certain
number of eggs, but the word “ may ” in the previous sentence is used
advisedly. Not every hen will set when enough eggs are accumulated.
On the other hand, the setting impulse, i.e., “ broodiness,” sometimes
asserts itself whether there are eggs or not. This is most likely to happen in the spring and early summer, i.e., during the time of year which
PERIODICITY OF LAYING 291
is the breeding season of many birds in temperate latitudes. Thus the
impulse to set is evidently due to more than the single factor of egg
accumulation. It is probably, like so many aspects of reproduction,
partly controlled by some of the endocrine glands, particularly the
pituitary, and this in turn may well be influenced by the length of day,
the temperature, or both. This irregularity in the advent of broodiness
in domestic hens is very likely the result of long selection with a View
to increasing the laying period. Even if the eggs are removed, however,
and the hen does not become broody, she does not lay one every day for
an indefinite period. Instead she lays a series of eggs on successive days,
and then skips a day, such an uninterrupted series being known as a
clutch. The eggs of a clutch, moreover, are not laid at the same time
each day. Rather the first one will be laid fairly early in the morning
of the first day, and each succeeding one about two hours later than its
predecessor on each of the following days. This continues until the last
egg of the clutch is laid around the middle of the afternoon, seldom
later. This means that after a_maximum of five or six eggs has been
laid, a day will ensue in which none is’ laid, and the hen will then begin
again in the morning of the day following.
It was formerly believed that this interruption in laying was due to
a delay in the act of laying itself. The theory was that if an egg was not
ready to be laid until late in the afternoon, the Bird would not lay it
then, but would retain it over night. Thus a day would pass with no egg
laid and the one laid the following morning would be a so-called
“ held egg.” This idea was made reasonable by the fact that there is
some difference in the degree of development of eggs, and this assumed
opportunity for prelaying incubation was supposed to account for it.
Further study, however, has rendered this theory untenable. In the first
place careful tracing of the history of eggs in the genital tract proves,
according to Scott and Warren (’36) that there are no held eggs. Instead it has been found that all eggs spend approximately 25 hours in
the genital tract with some minor variations. It is thought that these
minor variations are sufficient to account for such differences in embryonic development as are known to occur. Correlated with this near equality of time spent in the tract is the fact that each egg in a clutch is ovulated within a few minutesiof the laying of the previous one of that
clutch. These considerations would suggest that the explanation for’ the
omitted day must lie either in delay of ovulation of completely formed
eggs, or in a delay in the later growth stages of certain eggs in the
ovary.
292 THE CHICK‘
An effort to find which of the latter suppositions is true, and to deter‘mine the cause for whatever delay may occur, has been, made by subiccting the hens to variations in illumination. It has thus been found
that artificially reversing the time of illumination within the 24-hour
period will cause a corresponding reversal in the time of laying, but
this effect is delayed for about sixty hours. Also constant illumination
will cause the hens to distribute their laying more or less regularly
throughout the 24-hour period, and will make them lay more eggs to a
clutch. Clutches, however, do still occur, i.e.,. the laying is not continu»
ous. This and other data ‘led Warren and Scott (’36) to conclude that il
ilumination is responsible for normal periodicity in laying. Furthermore
since there are no held eggs the influence of the light could not be upon
the laying itself. It must be upon earlier stages in the entire process.
Finally because of the time lag before changed conditions produced
results these authors decided that the influence was also not upon ovulation, but, as intimated above, upon late stages in the growth of the
oocyte. Be this as it may, still later investigations by F raps, Neher and
Rothechild (’4-7) have shown that light is not the only environmental
factor involved. By giving or withholding food during continuous illumination it was clearly shown that this item and the accompanying
activity of obtaining it very definitely stimulate some step in the reproductive process, apparently ovulation. Also as was so thoroughly
demonstrated in the Frog, pituitary secretion seems to be the immediate
internal agent through which the external factors act.
SEGMENTATION
While the egg has been passing down the oviduct and receiving its
outer coverings, segmentation has been practically completed. As in the
Teleost and Gymnophiona eggs, this process involves only the germinal
disc (blastodisc) , which at the time of the first cleavage is about 3 mm.
in diameter and 0.5 mm. thick. It takes place in the following manner
and in the parts of the duct indicated:
The First C1eavage.~—The first cleavage furrow forms in about
the middle of the blastodisc, and extends only part way across it and
part way through it. It is completed during the passage of the magnum
(Fig. 155, A) .
The Second and Third Cleavagesp and the Accessory Cleavage. ——— As the egg enters the isthmus the second cleavage furrow begins
to form in the two existing cells; it is approximately perpendicular to
the middle of the first furrow, and is of about the same depth. There
Fig. 155.——Cleavage in the Hen’s egg. Surface views of the hlastoderm and the
inner part of the marginal periblast only. From Patterson. The anterior margin of
the blastodisc is toward the top of the page. A. Two cell stage about three hour:
after fertilization. B. Four cells, about three and one-fourth hours after fertilization.
C. Eight cells, about four hours after fertilization. D. Thirty-four cells, about four
and three-fourths hours after fertilization. E. One hundred and fifty-four cells upon
the surface; the blastoderm averages about three cells in thickness at this stage
(about seven hours after fertilization).
ac. Accessory cleavage furrows. m. Radial furrows. p. Inner part of marginal peniblaet. sac. Small cell formed by the accessory cleavage furrnra.
294 THE CHICK H N.
thus arise four cells, in each of which the furrow of the third cleavage
soon appears. These third cleavage furrows may be parallel with the
first, but their direction is quite frequently irregular. In this manner
 
inc. m _
york sc. "
CC.
, gggggnnp - uunnm: aaaaw
mp. mc. unuounn-noun using.» me. mp.
yolk sc.
cc.
.-ago. an
:2Z!’.......-. .=-..
....-.-on-...-uu.
Fig. 156.—Diagrams of vertical sections through the hlastoderm of a Hen’s egg
during cleavage stages. A. A section through an approximate 32 cell stage. B. A section through a slightly later stage where marginal cells are being added to the
original central cells. C. A section through a still later stage in which the marginal
cells have all been used up, and the extra (periblast) nuclei from some of them are
invading the periblast to form the germ wall. D. A stage just as gastrulation is
about to begin. The zones of junction and overgrowth are well marked, and the
germ wall is beginning to add cells to the original marginal cells.
ap. Approximate extent of the area pellucida, not yet marked, however, by the
thinning of the blastodermal roof. bld. Blastoderm. cc. Central cells. cp. Central
periblast. gw. Germ wall. j. Zone of junction. nmp. New marginal periblast. me.
Marginal cells. mp. Marginal periblast. acc. Original central cell region. omc.
Original marginal cell region. amp. Original marginal periblast region. sc. Seg
mentation cavity. sub. c. Subgerminal cavity. 9:. Line of separation between the
inner portion of the germ wall and the underlyingyolk. zo. Zone of overgrowth.
eight cells are formed, none of which are at first separated from the
deeper protoplasm of the disc or from that at the margin.
Before continuing the account of the regular cleavages it is now necessary to pause a moment to note certain so-called accessory cleavages.
These cleavages, which are extremely slight and transitory in the Hen’s
egg, seem to result from a few divisions of some of the supernumerary
sperm nuclei indicated above. They appear at about the four-cell stage
as faint radial furrows around the edge of the blastodisc, but by the
SEGMENTATION 295
time ten cells have formed they have completely vanished. Scattered
and degenerating sperm nuclei are sometimes observable as late as the
thirty-two-cell stage; these also, however, are presently lost sight of,
and apparently exercise no influence upon the ovum (Fig. 155) .
Fig. 157.—Vertical sections through the Chick blastoderm during the '
process of cleavage. From Kellicott (Chordate Development). After Pat- ‘
terson. A. Section through the two cell stage. B. Median section through
the thirty-two cell stage. C. Part of a longitudinal section through th
sixty-four cell stage.
b. Blastocoel or segmentation cavity. c. Central cells. i. Inner cell cut
oil by horizontal cleavage. 1. Neck of latebra. m. Marginal cells. mp.
Marginal periblast. n. Nucleus. p. First cleavage. v. Vitelline membrane.
The Central and Marginal Cells.—Subsequent to the eight-cell
condition, following the third cleavage, further furrows soon appear,
which result in the production of approximately sixteen cells. Some of
these furrows, moreover, are such as definitely to bound the outer edges
of those cells, whose protoplasm has heretofore been -continuous with
that which lay further out. Hence, there is thus created a central seg296 THE CHICK
mented area completely delineated from the unsegmented prqtoplasm
about it; the cells of this area are termed the central cells.
Cleavage then continues about the rim of this central area, producing new cells here which because of their position are called marginal
cells. These cells are for the time being unseparated both from the yolk
filled cytoplasm beneath, and from that lying still further toward the
periphery. This condition is characteristic of what is later known as the
zone of junction (see below) . As the process of cleavage goes on these
marginal cells are constantly being cut oil and added to the central
cells; meanwhile beyond them more marginal cells arise. In this manner the central segmented area is continually increasing in diameter
(Fig. 156, A; Fig. 157).
The Segmentation Cavity. —Furthermore, at the same time -that
the central cells are being defined as such by the furrows at their margins, horizontal cleavages are also taking place. These cleavages intersect the furrows which are visible from the surface, and thus cut off a
single superficial layer of the central cells from the protoplasm beneath
them. Fluid then begins to collect between this layer of cells and the
protoplasm, thus establishing a shallow space, the rudiment of the-segmenzatirm cavity.
As the egg leaves the isthmus, there have been formed in this manner approximately thirty-two cells; 9 it next enters the uterus, in which
cleavage is completed and gastrulation begun.
The Periblast and Its Segmentation.-—Further division both
horizontal and otherwise now takes place, so that the layer of central
cells, at first only one cell thick, soon acquires a thickness of several
cells; the area covered by the central and marginal cells has likewise
been increased. All the cleavage thus far indicated, however, has taken
place within the central region of the blastodisc (Fig. 156, B ) . About
the margin of this area, there remains a ring of the disc slightly darker
in color than the central portion, and about .5 mm. wide. It is still entirely unsegmented and is known as the pcriblast.
The Germ Wall and Subgerminal Cavity.” —— Presently the formation
of marginal cells about the edge of the central region reaches to the
inner margin of the ring, defined as periblast. At this point, although
the nuclei of the marginal cells continue to divide, the cytoplasmic
 
3 There are, according to Kiilliker, about forty-four cells in the blastoderm of
the Chick at this stage.
3 The ensuing description of the organization of the periblast and other later
phases of segmentation are from the accounts of Blount and Patterson, of homologous processes in the Pigeon.
SEGMENTATION 297
cleavages do not lceep pace with them. The extra nuclei (periblast
nuclei) thus produced then wander out into the region of the periblast
and convert. it into a syncytium. Some of these nuclei even move centrally for a short distance into the unsegmented protoplasm beneath
the rim of the segmentation cavity. The latter region of protoplasm thus
occupied by the extra nuclei is usually known as the central or subgenninal pcriblast (see below), to distinguish it from the strictly marginal periblast, the two regions, however, being.perfcctly continuous.
Following the above-mentioned penetration by the periblast nuclei,
what was periblast both central and marginal, is known as germ wall,
the peripheral non-nucleated cytoplasm in turn becoming periblast
(Fig. 156, D). Meanwhile, the last of the original marginal cells have
been cut off from the outlying periblast (now germ wall), and have
become continuous with, and similar in character to, the cells originally
defined as central. Vllithin the syncytial germ wall, cytoplasmic cleavage next begins to take place, and the cells which are thus produced are
added to the former marginal cells. Thus, partly by the multiplication of
the cells already in existence, and partly by the peripheral addition of
new cells arising within the wall, the central area of completely defined
cells spreads outward over the surface of the yolk‘ Upon this basis it
might be imagined that the germ wall would soon he used up, and as
regards the portion of it defined as central periblast this appears to be
true. The marginal part of the wall, however, is never exhausted during
this process of overgrowth. This is due to the fact that as fast as its inner
margin becomes nucleated and then converted into cells, a new germ
wall is created by the peripheral movement of more periblast nuclei into
the new periblast region which lies continually further out. Meanwhile,
as the cellular area is thus extended, the original segmentation cavity
likewise enlarges beneath it. This augmented central space is then often
referred to as the subgerminal cavity,‘ whose outward extension as such
ceases about the time gastrulation is completed.
The Zone of Junction and the Zone of Overgrowth.-——Beyond the
extent of the subgerminal cavity, however, the cellular area continues
to spread over the yolk. Although the actual cavity as such ceases to
expand subsequent to gastrulation, this outgrowth of the cellular region
is accompanied by an ever-widening zone, in which the newly formed
cells are nevertheless distinctly separated from the underlying yolk.
The separation is then continuous at its inner margin with the subger
4 The above distinction between segmentation cavity and subg-erxninal cavity
is frequently not adhered to, the two terms being considered synonymous.
298 THE CHICK
minal cairity. It should further be noted that at its outer edge this zone
of separation extends somewhat beyond the region where the germ
wall has been entirely organized, within its deeper portions, into cells.
in other words at the inner margin of the germ wall, the latter is already slightly separated from the yolk beneath it (Fig. 156, D, x) . In
its more peripheral part, on the other hand, the germ wall, as already
indicated, is quite continuous with the underlying yolk. Likewise, the
cells which, even in this outer zone, now cover the upper surface of the
wall as fast as it forms, are unseparated by cytoplasmic cleavage from
the unsegmented portion of the wall beneath them. Because of this lack
of separation between these superficial cells and the wall beneath them,
and also between the wall and the underlying yolk, this outer portion
of the germ wall is known as the zone of junction (Fig. 156, D). Lastly,
beyond the extreme limit of the zone of junction there exists a narrow
superficial rim of cells which extends out over the unsegmented yolk
lperihlast}, from which it is quite separate. This is called the zone of
0vergr0lL‘t/I, and, although arising from the outer edge of the zone of
junction, it seems to be maintained by the multiplication of its own
cells (Fig. 156, D).
The BZastoderm.——— It may now be added that with the appearance
of these zones the egg has become a blastula, while the entire cellular
and partially cellular area, including the zone of junction and the zone
of overgrowth, may henceforth be referred to as the blastoderm (Fig.
156, 1)). its establishment terminates the period of segmentation as
distinguished from that of gastrulation. Nevertheless, the outward extension of the blastoderm over the yolk continues for some time after
the latter process is completed. This is brought about by the steady
out-pushing of the zone of overgrowth and the germ wall, which not
only themselves increase somewhat in width (particularly the germ
wall ), but leave behind them an ever-widening area of extra-embryonic
ectoderm, mesoderm, and endoderm. The exact method by which these
cell layers are differentiated within the extra-embryonic blastoderm
will be discussed in detail later.
Before proceeding with a description of gastrulation, and the origin
of these layers, in the Bird, it is desirable to recall one point discussed in
connection with the Fish and Cymnophiona. It may he remembered
that in both the latter forms the rim of the blastoderm was homologized
with the lip of the blastopore. It was, nevertheless, indicated in the introduction that this homology is denied by some in the case of the Bird
because of the method of gastrulation in this form as about to be deTHE BLASTODERM 299
scribed. This problem will be mentioned again in that connection. One
point of functional similarity between the rim of the hlastoderm in the
Fish and Cymnophiona and that in the Bird is, however, already apparent. The process of overgrowth of the yolk, or epiboly, by the blastedermal rim, call this rim what one will, is the same in all.
ASTRULATION ‘ AND DEVELOPMENT THROUGH
THE FIRST DAY 2 OF INCUBANON
GASTRULATION
T H E problem of gastrulation in the Chick is one which has received considerable attention both by study of normal total hlastoderms
and sections, and more recently by experimental procedures. The latter
have involved removing living blastoderms and parts of blastoderms to
artificial locations, cutting them at various levels, and marking them
with vital dyes. The object has been to determine exactly what movements are taking place, where the primary layers are derived from, and
what parts of the early blastoderm give rise to specific features of the
early embryo. In spite of all this study investigators are still not in entire agreement on the answers to some of the above questions. At the risk
of satisfying no one, therefore, the writer is going to attempt to piece
together a more or less connected account. in doing so it will beneaessary to select conclusions regarding some of the moot points from different workers on the basis of what seems to us most reasonable and
likely. Statements over which there is especial disagreement will be indicated in order that the student may be aware of what is most generally accepted and what is not. It will be noted at once that the accepted
items largely concern the existence of successive stages of certain structures. Those matters under controversy, on the other hand, have mainly
to do with the interpretation of these structures, i.e., questions of their
homologies, of how they arise and what they produce. The investigators
whose accounts have been particularly consulted are Chen, Hunt,
Rawles, Ruclnick, Woodside, Pa.-steels, Peter and Spratt. The review of
the subject by Rudnick (’/-14) is especially valuable as a critical summary of the situation to that date, and the interested student is referred
to this and to articles by the other authors cited for further details.
1 Gastrulation is usually only slightly under way when the egg is layed (see
below).
1’ The term day as used in connection with the development of the Chick refers
to a period of 24- hours. ‘
i
3
FIRST DAY: GASTRULATION 301
 
.dark area within
area pel|ur.Ida=
embryonic shield
area opaca
area pellucida
primitive streak
Fig. 158. -——- Photographic surface views of early Chick
blastoderms. After Spratt. A. An unincubatcd blastoderin
of the pre-streak stage. The dark area opaca, and the lighter
area pellucida with a darker region within it, the embryonic
shield, are clearly shown. B. A blastoderm of eight hours
incubation showing the primitive streak at an early stage.
The darker embryonic shield lateral and anterior to the
streak shows clearly but is not labelled in this case.
The Area Pellucida and Area Opaca.—As gastrulation begins
the blastoderm above the subgerminal cavity becomes thinned somewhat by the outward movement of its cells. For this reason, the absence
of adhering yolk and the existence of the cavity, this central region
when viewed from above appears different from the surrounding parts.
Thus when observed upon the living egg it appears darker, while in a
stained blastoderm mounted upon a slide it is more translucent. Be302 THE CHICK
cause of this it is referred to as the area pellucida. The surrounding
parts comprising the zone of junction and zone of overgrowth on the
other hand are more whitish in the living egg, and more heavily stained
and opaque in preserved material. Therefore this surrounding region is
appropriately termed the area opaca (Fig. 158).
The Primordial. Hypoblast.—-The first step in actual gastrulation seems to he the appearance within the subgerminal cavity of a sec
A are: pellu: Na
r—-
eplblut
     
archenteron yolk
Z°"m W3“ . primordial hypoblasl: ]zg°;,r-'g°fi§',f~,
primitive streak
B
ectoderm
Fig. 159.—-«Diagrams of sections through the Chick blastoderm showing the origins of the primordial hypohlnst, the definitive endoderm and the mesoderm. A.
A median sagittal section through a very early Chick blastoderm such as is shown
in Fig. 158, A, in which the primitive streak has scarcely begun to form. The
hypohlast has just been delaminated (and, or, infiltrated) from the epiblast. The
area option at this stage consists only of the zone of junction and the zone of
overgrowth. At this stage the zone of junction is mostly, though not entirely,
identical with the germ wall. Thus it will be noted that the latter extends slightly
medially beneath the archenteric space. Later only a small part of the outer
periphery of the germ wall is thus identical with the zone of junction. B. A cross
section of the hlastoderm of a slightly later stage where the streak has formed,
and mesoderm, and perhaps definitive endoderm, is arising in connection with it
in the manner indicated in C.
ond cell layer which may be termed the primordial hypoblast. The space
between this layer and the underlying yolk then, as in the case of the
Fish, becomes the archenteron. The new layer is designated “ primordial” because it appears doubtful that it represents the final or defini.
tive hypoblast, or at least that it represents all of it. The method of its
origin is one of the disputed questions. It was formerly supposed to
originate by involution of marginal cells through a temporary interrupFIRST DAY: GASTRULATION 303
tion in the zone of junction along a small part of the hlastodermal rim.
The location of this activity if it occurred would of course represent, as
in the Fish, the dorsal blastoporal lip, and hence also as in the Fish the
future posterior region of the embryo. It has even been claimed by one
observer that an actual invagination occurs here, giving rise to a pocket
with both roof and floor, i.e., a complete archenteron (Jacobson. ’38) .
At present, however, the belief in either involution or invagination as
defined in this text is no longer entertained in the case of the Chick. Instead Peter (’38) and others seem to think that the process is rather
what we have designated as infiltration. That is to say, these workers
believe that individual cells wander in from the surface and detach
themselves within the subgerminal cavity where they eventually become
arranged to form a more or less continuous layer. lt should be noted incidentally that the sponsors of this View do not use the term infiltration,
preferring to call the inwandering of these individual cells “invagination." This, however, seems to the writer a misnomer—and confusing.-At
all events regardless of the terminology the activity is said to be as
designated.
It must further be stated that those who are agreed on the character of
the process as described are not entirely agreed on just where it takes
place. According to some (Pasteels, ’45) it occurs more or less all over
the pellucid area of the hlastoclerm. Peter, however, seems to think it
takes place mainly toward the future posterior side, especially near the
margin, with a subsequent forward movement. This would approach
more nearly the older idea of an involution from one side.
Finally it may be said that some workers (Spratt, ’46) describe the
process of hypoblast origin as one of splitting oil or delamination of
cells rather than their inwanclering (Fig. 159, A). Also at least one
investigator (Fraser, ’54«) has observed the infiltration of cells from the
epiblast at the anterior and posterior borders of the area pellucida, suggesting once more a sort of modified involution at these borders, but
without interruption of continuity in the epihlast. It is of interest to note
here that a similar problem regarding the nature of hypoblast origin
occurs in the Mammal where again some form of infiltration or delamination seems to occur. This matter will be referred to later in the
appropriate connection.
After the formation of the layer of primordial hypohlast it might be
assumed that gastrulation, as defined in this text, would be complete.
However, as noted, this hypoblast is.~prol3ably only part of the definitive
hypolalast (endoderm) , and in the Bird more than in the Frog and Fish
304 THE CHICK
it is difiicult to separate sharply the origin of the definitive hypoblast
from the origins of the mesoderm and notochord. Also the appearance
of the primitive streak, a structure previously related primarily to gastrulation, is, as we shall see, probably involved here both in the formation of definitive hypoblast, and in the origin of mesoderm and notochord. We shall therefore have to continue our discussion of these
activities more or less simultaneously as a later aspect of gastrulation.
Before proceeding with this it may be remarked that it is at about this
stage of development that the egg is usually laid. The diameter of the
entire blastoderm is approximately 3.36 mm., and that of the area pellucida about 2.16 mm. (Spratt, ’46). If unincubated it may remain in
this condition for some time. If incubation ensues before too long an interval has elapsed further development proceeds as follows:
The Primitive Streak.—The second step in gastrulation is the
development of the primitive streak whose history is as follows: Just
before the streak begins to form, about three fourths of the area pellucida_ as viewed from the surface, starts to become more darkly stain
ing and opaque toward what later proves to be its posterior side. This
is due both to a thickening of the epiblast in this region, and to the
presence of the underlying hypoblast. The part so affected is sometimes
designated as the embryonic shield, though not entirely homologous
with the region similarly named in the Fish as previously described
(Fig. 158, A ) . Presently the streak begins to appear at the posterior side
of this shield, as a still more darkly staining somewhat triangular structure with its base in Contact with the inner rim of the area opaca (Fig.
158, B). This appearance is produced by a further thickening of the
epihlast in the region concerned in a manner to be indicated below. At
first the thickened cpiblast reaches only a short distance cephalad, but
soon, as its growth is completed, its anterior end occurs at about the
middle of the pellucid area. As a result of this increase in length the
structure loses its triangular shape, and appears more as a broad band
or actual streak with a tapering and rounded anterior end. At the same
time sections reveal that from its first appearance the thickened epiblast of this band has been in intimate contact with the underlying hypoblast. A little later the hand (primitive streak) becomes still narrower, and a distinct groove develops down its middle with a little
twist or irregularity at its cephalic extremity where the groove terminatcs in a slight pit. The groove is termed the primitive groove,3 and
3 The term primitive streak is sometimes rather carelessly used to refer to both
streak and groove.
FIRST DAY: GASTRULATION 305
the pit is the primitive pic. The latter together with the surrounding
cells is called Hensen’s knot or Hensen’s node, also the primitive node
(Figs. 160, 161, 162) . The sides of the groove are sometimes designated
as the primitive folds, having nothing to do of course with the later neural folds. So far as the writer is aware no one questions the existence of
these structures as described. Again the real problem concerns the homology of the streak or groove, its origin and its functional relation to
the parts about it. Since the answer to the first of these queries depends
vnedullary told
ya vlrelllnu Intern:
   
Henna’:
knot
= blood islands at
the ma vnmlon
Fig. 160.-— Surface of the Chick hlastoderm and early embryo. A. A pre-incubation blastoderm showing the primitive streak, actually the primitive groove. B. An
18 hour blastoderm showing the beginning of the head process (notochord). C. A
24 hour blastoderm with embryo well started and the area vasculosa forming.
largely upon the answers to the last two, we shall take these latter up in
order. We shall then be prepared to return to the problem of homology.
The Origin of the Streak.--As a result of numerous marking
experiments it appears to be fairly clear that the streak originates by
the convergence of epiblast cells from the lateral regions toward the
place where the initial short “ streak” is first seen (Chen, ’32, Spratt,
’46), (Fig. 163) . This produces an aggregation of material here which
constitutes the thickening described as characteristic of this structure. It
should also be noted, as Spratt points out, that the cells thus aggregated do not pile up upon the surface of" the blastoderm, but pass inward, as he expresses it by “ invagination.” It is this process which almost at once, as previously indicated, brings them in contact with the
underlying hypoblast. After being started in this manner the lengthening of the streak occurs, according to Spratt, by the proliferation of its
cells as follows: At its front end these cells are so added as always to
be at or near the tip, as in the growing point of a plant. Posteriorly the
growth seems to be more by intussusception pushing this end backFig. 161. —— Five transverse sections through the head process and primitive streak
of a lCcIhick embryo. The head process is very short. From Lillie (Development of the
Chic’ ).
A. Through the head process, now fused to the entoderm. B. Through the primitive knot. C. Through the anterior end of the primitive groove. D. A little behind
the center of the primitive streak. E. Through the primitive plate. The total number
of sections through the head process and primitive streak of this series is 102. B is 4
sections behind A. C is 12 sections behind A. D is 59 sections behind A. E is 87
sections behind A.
Ect. Ectoderm. Enz. Entoderm. GJV. Germ wall. H.Pr. Head Process. med.pl.
Medullary plate. Mes. Mesoblast. pr.f. Primitive fold. pr.g . Primitive groove. pr.Im.
Primitive knot. pr.pl. Primitive piste.
306
.-.w
30?
Fig. 162. —— Three transverse sections of a late stage, through the head process and primitive streak of a
Chick embryo. From Lillie (Development of the Chick). A. Near the hind end of the head process. B.
Through the primitive pit. C. A short distance behind the center of the primitive streak.
BLI. Blood island. coel.Mes. Coelemic mesoblast. Ecl. Ectoderm. Ent. Entoderm. G.W. Germ-wall. med.
pl. Medullary plate. Mes. Mesoderm in area pellucida. N’ch. Notochord. pr.]. Primitive iold. pr.gr. Primitive
groove. pr.p. Primitive pit.
308 THE CHICK
ward, Accompanying, and perhaps partially caused by this movement,
the whole pellucid area changes its shape‘ from that of a circle to a pear
with the small end posterior. Finally it may be stated that this growth
of the primitive streak appears to be induced by the underlying primordial hypoblast. This is concluded from the fact that this hypoblast is at
Fig. 163.——A diagram to illustrate the movements occurring on a Chick blastedcrm during gastrulation and primitive streak formation. After Spratt. The movements are indicated by changes in the positions of carbon particles placed on the
hlastoderm at the start of the process. Horizontal rows A, B and C illustrate three
different plans of placing the particles. Vertical rows I, II and III indicate the
positions of the particles in each plan during successive stages in gastrulation.
The short horizontal lines outside the blastoderms are points of reference. Note
the general tendency of convergence toward the forming streak.
first chiefly toward the posterior of the bl-astoderm, and as it spreads
anteriorly the growth of the primitive streak follows it. There are also
other facts which support this hypothesis (Fig. 164) .
Functional Relations of the Primitive Streak.
Diflerentiation of Mesoderm, Endoderm and Ectoderm. —It is now
rather generally conceded that not only are materials moved into the
FIRST DAY: GASTRULATION 309
streak from the outlying epiblast, but they also pass through it to
specific destinations (Hunt, ’37, Spratt, ’4-6) . One of these is apparently
a layer of cells pushing out on either side of the streak between the
epiblast and the primordial hypoblast. This layer is the mesoderm. It is
also claimed that some of the cells moving through the streak pass into
and augment the previously existing primordial hypoblast (Hunt, ’37) ,
(Fig. 159, B, C). Thus this latter layer is converted into definitive hypoblast, or as it may now be called endoderm. The question as to just how
much of the endoderm owes its origin to this movement of cells through
epiblqsf A _prirnitive streak
 
 
 
pre-head prccess cells
germ wall
germ wall
3
primitive node primitive pit primitive streak (groove)
   
 
head process (notochord) endoderm 99"“ w°"
Fig. 164.——A diagram of a median sagittal section through the primitive streak,
A, and groove, B, and parts anterior to each, showing the origin of the head process
inotochord) according to Spratt and Fraser.
the streak, and how much to the spread of the primordial hypoblast is
one of the unanswered questions. As usual after the origin of these
layers the remaining epiblast may be called ectoderm.
Lastly, it may be noted that the process just indicated in connection
with the origin of the mesoderm and endoderm is again what we should
term a kind of infiltration. Nevertheless, as will be pointed out subsequently, it does bear some resemblance to the passage of cells around a
blastoporal lip, i.e., involution, and might help to account for the development of the groove. Also, as in the case of the inwandering of cells
from the surface into the primordial hypoblast, it has been referred to,
ambiguously the writer thinks, as “ invagination.”
The Head Process (Notochord). ——This leaves the origin of the
notochord still to be accounted for. Accompanying the above-mentioned
activities there also appears in front of the primitive streak or groove
another somewhat narrower line temporarily termed the head process
(Fig. 160, B). It begins at Hensen’s knot with which it maintains constant contact, and extends anteriorly. Sections reveal that it consists of
a line of cells somewhat like the streak, but in this case they have no
definite connection with the epihlast, now ectoderm, save at Hensen’s
310 THE CHICK
knot (Figs. 161, A; 162, .4). This head process rapidly increases in
length, and eventually undergoes histological changes to become the
notoclzord. Concerning the above statements there is no question. The
problem again arises, however, as to where the head process (notcchord) originates from, and by what method it develops. It has been
claimed that it arises by a splitting off of streak material from the epiblast in a posterior direction. Thus as the head process grows at its back
end the streak would shorten proportionally at the front end (Lillie,
’19). The streak does indeed shorten, but not proportionally. Hence it
has been claimed by others that the head process grows from cells budded off from the anterior end of the streak, and pushed forward.
Finally according to Spratt, ’47, and Fraser, ’54, the following occurs:
At first the streak, as noted, is quite short. As its substance grows anteriorly beneath the epiblast, the cells of the latter, originally just in
front of the streak, come to lie posterior to its anterior tip, i.e., somewhat behind the primitive node and pit. Some of these cells then pass
into the substance of the streak and forward within it to a point under
the node. Here they form a mass from which the head process is budded,
almost entirely posteriorly (Fig. 164, A). This means that the primitive
streak is forced to recede before it. However, according to Spratt’s evidence it does not shorten at its anterior end in the region where it is in
contact with the head process. Instead the substance of the streak is
“ pushed ” back, or at least it migrates backward. But though the streak
does not shorten at the front end, it does shorten at the back end. It
does this simply by “ dissolution ” into the ectoderm and mesoderm of
this region. As indicated in connection with one of the other theories,
however, this shortening is not quite at the same rate (i.e., proportional
to) the lengthening of the head process. Therefore Spratt suggests that
there must be some condensation of material in the shortened streak.
Eventually, nevertheless, the latter does entirely disappear, except in so
far as its remains may constitute the “ end bud ” (posterior tip) of the
embryo. Figures 164 and 165 illustrate diagrammatically the processes
supposed to be involved. This theory of head process (notochord) origin is supported by extremely careful studies based on a somewhat new
technique. Instead of the dyes previously used for marking points on
the living blastoderm, carbon particles were introduced into it, thereby
eliminating the spreading of the marks by mere dilfusion. Their movements were then kept track of in relation to certain fixed points outside
the area where the critical changes were occurring. The results seem
conclusive, but will of course have to be confirmed by other workers.
FIRST DAY: GASTRULATION 311'.
Distribution of Formative Materials in the Streak and Prestreak Blastoderrn.———In our consideration of gastrulation in the
Frog emphasis was laid on experiments indicating the distribution of
germ layer materials previous to the gastrulation process. The question
naturally arises therefore as to whether it has been 1: ssible to make
comparable pre-gastrular maps in the case of the Bird. The answer is
   
 
 
REEKREDK.
SNORTINIHG Of STRZAK
REGRE3$|DN
POST. BORDER
0? PSLLUCID AREA 0
n.————-———-n
00 GSIIK
Fig. 165. —- A diagram to illustrate the movements occurring in the primitive groove
(“ streak”) and parts connected with it during head process (“ chorda") formation. After Spratt. Three cells in the groove were marked by carbon particles just
before the head process started to appear as shown by the dots on the streak at the
left. As the head process forms, the location of the particles and the changes in the
parts are seen in successive stages as one passes to the right. Note what happens
to the groove as the head process lengthens.
that if one considers the existence of the primordial hypoblast as denoting the completion of gastrulation, such maps have not been made. This
is not surprising since this stage is reached prior to the laying of the
egg. However, in so far as the formation of the primitive streak is regarded as part of gastrulation, the answer is quite otherwise. Many
studies have been made of the potentialities of the various regions of the
blastoderm beginning with the late pre-streak stage, and extending on
to that of the head process. Wetzel, ’29, Rawles, ’36, Pasteels, ”37, Rudnick, ’38, most recently Spratt, ’42, and others have worked on this
problem largely by two techniques. (1) They have vitally stained or
otherwise marked the various regions of the hlastoderm in situ, and
noted the subsequent movements of the stained parts. (2) They have
isolated pieces of the blastoderm on various culture media, and observed what each piece is able to produce. Naturally, the later in de312 THE CHICK
velopment the experiments were performed, the more precise have been
the results, but also of course the further they are removed from the pregastrular situation. It is not feasible to go very deeply into this topic,
cartilage I bonemuscles»
mcaortephroa
Fig. 166. —-A diagram showing the sections into which a primitive
groove and head process stage of a Chick blastoderm was cut, and
the tissues and structures derived from the mesoderm of each isolated piece. After Rawles.
but we may present as an example of the conclusions of some of the
work on later stages one of the maps by Rawles (Fig. 166) . With reference to this map it should be stated that the results upon which it is
based were all obtained by the isolation method, and it must be admitted that this method has one weakness. Since the isolate is in a new environment the potentialities which it exhibits are not necessarily those
FIRST DAY: HOMOLOGY OF STREAK 313
it would have realized had it been left intact. In fact they are apt to be
greater, due perhaps to the removal of inhibition by neighboring parts,
or to lack of specific induction by those parts. It should be understood
that though the map selected is for mesoderm only this does not mean
that this was the only layer studied, or that the layers were transplanted
separately. The results for the different layers were merely recorded separately as a matter of convenience, and our choice of the map of this
particular layer has no special significance. As regards the conclusions,
in view of the results on earlier stages to be indicated presently, it is
perhaps noteworthy that for all layers the regions capable of producing
therhost structures were those near the center of the blastoderm, i.e.,
about Hensen’s node. It is of further interest that the left side showed
more potentialities than the right.
An example of a study of very early stages (early streak and late
pre-streak) is that of Spratt’s isolation work (742). Stated very briefly
his conclusions are essentially as follows: He finds, in substantial agreement with most others, that prospective neural plate material lies near
the center of the area pellucida. Notochord, on the other hand, is formed
from the region just behind this in about the third quarter of the pellucid area. Potential mesoderm, including heart forming material, appears to be somewhat more widely diffused both anteriorly and posteriorly. From this we see that although it has not been possible to map
prospective germ layer and organ-forming regions quite as early or as
accurately as in the case of the Amphibians, some progress has been
made. Thus it is at least evident that the materials for the nervous system, the mesoderm and notochord exist independently in more or less
separate, though overlapping, localities at the pre-streak stage, and that
they are subsequently moved into their definitive positions as the streak
develops. Whether the separation of these substances occurs still earlier, perhaps even in the unsegmented egg, as in Amphioxus and the
Arnphibia, we do not yet know.
THE HOMOLOGY OF THE PRIMITIVE STREAK
It will be recalled that the term primitive streak was used in connection with the Frog, Fish and Cymnophiona to denote the line formed
by the closed blastopore. The question now is whether the primitive
streak of the Chick is really homologous with this line, and hence represents a closed blastopore. ‘
314 THE CHICK
REASONS AGAINST HOMOLOGY OF PRIMITIVE STREAK
AND CLOSED BLASTOPORE
(1) The streak is not at any time an opening into the archenteron, as
a real blastopore is supposed to he.
(2) The origin of the primordial hypoblast at least is not related to
it, nor to its “ lips ” (sides of the groove) .
REASONS FAVORING HOMOLOGY or PRIMITIVE
STREAK AND CLOSED BIASTOPORE
(1) In the Frog and Fish it was shown that there is a convergence of
materials on the outside of the hlastula toward the forming blastopore.
Various marking experiments on the epiblast of the Chick blastoderm
show similar movements of material in its postero-lateral regions toward the forming primitive streak.
(2) In Amphioxus, the Frog, and Fish there was shown to be an involution of the materials just mentioned over the dorsal lip into the
roof and sides of the archentcron. In the Chick there is, strictly speaking, no blastopore in the region of the streak, and hence no blastoporal
lip. The streak, however, does have contact with the primordial hypohlast, and it does develop along either side of it, ridges which would
correspond to the lateral lips of a blastopore. Most important of all it
has been shown that there is a movement of material through these
ridges into the forming mesoderm, and possibly into the endoderm. In
other words as previously suggested there is a kind of “involution,” in
which the presumed homologues of the blastoporal lips are intimately
involved.
(3) In Amphioxus, the Frog, and Fish the notochord arises from
material involuted at the dorsal lip of the blastopore, and budded forward from that region. In the Chick we have seen that the notochord
originates from cells passing inward not, to be sure, through the pit,
whose anterior rim is the homologue of the dorsal blastoporal lip, but
posterior to it. Yet even here such movement is suggestive, even though
the material grows backward instead of forward to form the notochord.
(4) In Amphioxus and the Frog we have found the neurenteric canal
originating by the uniting of the neural folds over the anterior part of
the closing blastopore (primitive streak), while in the Fish Kupffer’s
vesicle, the homologue of that canal, occurs at the same location. Now
in the Chick, to be sure, there is no neurenteric canal at the anterior end
of the primitive streak. There is, however, a pit at this point which is
FIRST DAY: THE AREA OPACA 315
eventually covered by the neural folds, and in some Birds (Duck,
Goose and others) this pit does finally open to the archenteron. Thus in
these cases a neurenteric canal, incipient or actual, is formed in the
proper place if the streak be regarded as a closed blastopore.
( 5) In the Frog, certainly, and probably in the Fish, the anus forms
at the end of the closed blastopore opposite from the neurentcric canal,
the line between the two being designated as the primitive streak. We
have just seen that at least in some Birds what amounts to a neurenteric
canal forms at the anterior end of the streak. On this basis the anus
should arise at the posterior end of this structure, and apparently it does
so (Lillie, ’l9). V
( 6) In the Frog the material in and about the lip of the early blastepore is known to have remarkable inductive powers. In the Chick the
primitive streak is said by some (Woodside, ’37) to have similar powers
when transplanted beneath the epiblast of a very early primitive streak
host.
EXTENSION OF THE GERM LAYERS AND FORMATION
OF THE AREAS VASCULOSA AND VITELUNA
Up to this point the processes of gastrulation and germ layer formation have been considered only in relation to the area pellucida. It now
remains to consider what is happening in these connections in the area
opaca.
ORIGIN OF ENDODERM IN THE AREA OPACA
In connection with the origin of the primordial hypoblast before the
advent of the primitive streak, it was noted that this hypoblast arose by
the inwandering (infiltration) of cells from the surface of the blastederm, or by delamination from its under-surface. It was also said that
this probably occurs mostly about the posterior half of the blastoderm,
perhaps more especially around its margins. This hypoblast was then
supposed to be later augmented to form endoderm by infiltration of
cells through the streak. Upon this basis it is not surprising therefore
to learn that according to some accounts the endoderm of the area opaca
is derived as follows:
It is said that the nuclei from the zone of junction keep moving in toward the area pellucida. As they do so, the cytoplasm about each nucleus engulfs yolk granules, and becomes cut off from that about it to
form a definite cell. Thus the lower part of the germ wall becomes or316 THE CHICK
ganized so that toward its inner margin (the edge of the area pellucida) ,
it begins to form a cell layer. This layer is endoderm which becomes
continuous with the definitive endoderm of the area pellucida. If this
account be correct it would seem that a process which is essentially infiltration, in this case from the margins of the blastoderm, is still giving
rise to some of the endoderm, i.e., that of the area opaca. It may now be
stated that because of its subsequent history the endoderm of this area is
often referred to as yolk-sac endoderm.
THE BLOOD ISLANDS AND THETMESODERM IN THE
AREA OPACA
The Blood Islands.—Though the origin of the endoderm of the
area opaca has been described first, it actually follows slightly, both in
time and peripheral location, the formation of the mesoderm which comes
about somewhat indirectly as follows: It appears that cells from the
postero-lateral margins of the mesoderm in the area pellucida wander
into the upper part of the germ wall of the area opaca, where they also
engulf yolk granules. These cells become aggregated into small masses
in this region, and these masses presently anastomose to form a network. Throughout this network spaces or lacunae are then developed
which contain little groups of cells. Presently the walls of the lacunae
become differentiated into the flat endothelial cells characteristic of
the inner lining of blood vessels, while the cells within the lacunae be
come blood corpuscles. Because of the manner of their formation these
corpuscles are at first necessarily aggregated into groups, which appear
from the surface as darker splotches. These splotches of corpuscles, or
forming corpuscles and their surrounding endothelium, are known as
blood islands. Obviously they arise somewhat previous to the main parts
of the circulatory system with which they presently become connected
(see below).
The Mesoderm of the Area Opaca.—Coming now to the mesoderm of this region we find that it is produced by the budding off of
cells from the surface of the developing blood islands, between the islands and the overlying ectoderm. At its inner margin this mesoderm
like the endoderm becomes continuous with that occurring in the area
pellucida ( Fig. 162, C).
It remains to state that because of the indirect method of production
of this mesoderm its source as just described has been questioned by
some. Thus it has -been claimed that the blood islands, and hence the
mesoderm, come from cells originating in the zone of junction in the
FIRST DAY: THE AREA OPACA 317
same manner as the endoderm of this area. The account as we have previously given it, however, is afforded strong support by the following
fact: Patterson (’O9) has shown that where the mesoderm of the pellucid area fails to reach the germ wall no blood islands and no mesoderm develop in the area opaca. It may finally be noted that if the mesoderm of this area does arise from that in the area pellucida, as seems
most probable, then like the latter it also, though somewhat indirectly,
has its ultimate source in the primitive streak.
Though beginning in the postero-lateral regions as indicated the processes thus described are gradually working forward upon each side of
the area opaca, the proliferated mesoderm of the area pellucida keeping
pace with that which arises from the blood islands further out. Finally,
as the level of the anterior end of the head process is reached, the mesoderm of the pellucid area ceases to form. That in the area opaca, however, continues upon either side as a pair of anteriorly projecting wings,
which after proceeding somewhat beyond the future head region begin
to turn toward one another so that they eventually meet (see second
day). In the area pellucida, however, immediately in front of and
slightly to the sides of the head region, no mesoderm forms for some
time, the zone thus marked out being termed the proamnion (Fig.
160, C). Following the advent of the blood islands it soon becomes
possible to subdivide the blastoderm into further parts as follows:
The Area Vasculosa. ——-The blood vessels, having once become
formed in the area opaca, are not confined there. Very soon, especially
postero-laterally, they begin to extend into the, area pellucida, where
they unite with other vessels which have arisen in situ from the mesoderm; the entire region thus covered by them is then termed the area
vasculosa. Presently, around the outer edge of this area, its boundary begins to be clearly defined by an encircling blood vessel, the sinus ter
minalis (Fig. 160, C).
The Area Vitellina. — The remainder of the blastoderm peripheral
to the area vasculosa is termed the area vitellina, and is in turn subdivided as follows: The part at and near the blastodermal rim continues
to consist of the relatively narrow zone of overgrowth and zone of junction, and is known as the area vitellina externa. Between this area and
the area vasculosa there is then a. region which, with continued expansion of the blastoderm, soon becomes ‘rather extensive. Within it, although the germ wall is becoming occupied with yolk filled cells, these
cells have not yet become definitely organized into endoderm or blood
islands. Nevertheless this part of the wall is clearly separated from the
318 THE CHICK
epiblast above it, and is beginning to be more or less delimited from the
non-cellular yolk beneath it. The relatively broad region thus characterized is called the area vitellina interna (Figs. 167, 170, A, E ).
As has already been suggested, all of these areas, while retaining the
same relative position as regards each other, are constantly moving outward over the surface of the yolk by a process of epiboly (Fig. 167).
Fig. 167. - A. Hen’s egg at about the twenty-sixth hour of incubation, to show the
zones of the blastoderm and the orientation of the embryo with reference to the axis
of the shell. B. Yolk of hen’s egg incubated about 50 hours to show the extent of
overgrowth of the blastoderm. From Lillie (Development of the Chick). After
Duval.
a.c. Air chamber. a.p. Area pellucida. a.v. Area vasculosa. a.v.e. Area vitellina externa. a.v.i. Area vitellina interna. Y. Uncovered portion of yolk; i.e., the “yolk
blastopore" or yolk-sac umbilicus (see below, and page 362).
FURTHER HOMOLOGIES
The Margin of the Blastoderrn.——It was stated in connection
with the Fish that the margin of the blastoderm, or germ ring in that
form was entirely homologous with the blastoporal lips, and that it finally closed to form a primitive streak. It was then indicated that in the
Gymnophiona the margin of the blastopore is again the homologue of
the blastoporal lips. In this instance, however, these lips (germ ring)
become divided into two parts by the early contact of points on the lateral lips a short distance from the dorsal lip. In this manner a small
true blastopore (later a primitive streak) is formed immediately in
front of which the embryonic axis proceeds to develop. The remainder
of the blastodermal rim is then employed in covering the yolk. As it
completes this process there appears what amounts to a second or yolksac blastopore, with the closure of which the yolk is entirely enveloped.
>41! ._.......-,.-a.« W .. . .. ‘J
FIRST DAY: FURTHER HOMOLOGIES 319
The question now to be answered is what if any homologies exist between the avian primitive streak and blastodermal rim, and the blastopores of the Fish and Gymnophiona. We have already given reasons
for homologizing the primitive streak of the Chick with the streak of
less advanced forms such as the Fish and Frog in which this structure
represents the entire closed blastopore. What then of the remaining
blastodermal rim in the Bird?
In answering this let us first consider the character, and then the behavior of this rim. From what has been said it is clear that according to
present views there is no real involution at the blastodermal rim of the
Chick. Hence the epiblast and primordial hypoblast do not actually
unite along this line as at the typical lip of a blastopore. This is most
clearly true in the very early stages when the infiltration or the delamination of primordial hypoblast cells is said to occur more or less all
over the blastoderm. Even at this time, however, there is some evidence
that this process is more active about the postero-lateral margins. Later,
moreover, when the area vitellina externa has been established it has
been indicated that the origin of the cells for the endoderm of the yolk
sac, according to many, is mainly dependent upon, nuclei migrating from
the zone of junction. Thus it can be said that a kind of modified involution is after all occurring at essentially the margin of the blastoderm,
and that ectoderm and endoderm are ultimately in contact in that region. So much for the character of the margin. As to its behavior. it has
already been said that the blastoderm spreads over the yolk by the usual
process of epiboly, and this continues until finally the yolk is completely enveloped. By virtue of its method of formation the covering
thus developed consists of all three germ layers, and is called the
yolk-sac.
Upon the basis of both structure and function, therefore, it is evident
that the hlastodermal rim of the Chick bears a striking resemblance to
the blastoporal lips or germ ring of the Fish, and even more to that of
the Gymnophiona. Indeed there are only two essential differences between the rim of the blastoderm in the latter and that in the Bird. One
is the fact that in the Gymnophiona there is definite involution at one
point on the margin, while in the Bird there is not. The second difference is that in the Gymnophiona the blastoporal lips (blastodermal
rim) immediately adjacent to the region of involution soon fuse to form
a primitive streak. In the Bird, on the other hand, the primitive streak
is apparently formed by a convergence of material in the posterior part
of the blastoderm, but not from material actually in the blastodermal
320 THE CHICK
rim. In both cases the remainder of the yolk beyond the blastoderm is
temporarily uncovered, constituting the so-called yolk-sac blastopore
(Fig. 168). This is later enclosed by a yolk-sac in the Bird, and by
what virtually amounts to that in the Gymnophiona. In the Fish, of
course, the blastodermal rim is not thus divided into two parts, and
hence there is no question about the homology of all of it with a blasteporal lip. In the Fish, however, there is no endoderm in the yolk-sac.
Summary of Gastrulation Processes and Homologies in the
Chick.—We may conclude the discussion of gastrulation by summarizing the processes involved in the Chick as follows: According to
Fig. 168. —Median sagittal section. Stage of the first intersomitic groove. (Cf. Fig.
169). Owing to the bending of the primitive streak the section passes to one side of
the middle line posteriorly. From Lillie (Development of the Chick).
Ect. Ectoderm. F.G. Fore-gut. CJV. Germ-wall. H.F. Head~fold. med.pl. Anterior
end of medullary plate. Mes. Mesoderm. N’ch-l-Ent. Notochord and entoderm. Pr’a.
Proamnion. pr.kn. Primitive knot. pr.p. Primitive pit. pr.str. Primitive streak. Y.S.
Ent. Yolk-sac entoderm. '
the definitions adopted in this book they would include infiltration (i.e.,
a modified kind of involution), or (and) delamination, convergence and
epiboly. ‘
As to homologies, the primitive streak of the Bird is probably homologous with all other primitive streaks, including those in the Frog, Fish,
Gymnophiona, and, as we shall see, the Mammal. Furthermore, there is
good reason to homologize the blastodermal rim plus the primitive
streak of the Bird with the whole blastodermal rim of the Fish, though
the latter contains no endoderm. Likewise we may equally well homologize the rim of the blastoderm of the Bird minus the primitive streak
with the rim minus the streak in the Gymnophiona.
DETERMINATION OF THE EMBRYONIC AXIS
It is of course obvious that whatever fixes the position of the primitive streak determines the embryonic axis; The question therefore is
what fixes the position of the streak. We must immediately answer that,
as in the case of the Fish, we do not certainly know. However, there are
some reasonable hypotheses up to a certain point.
If a hen’s egg is allowed to rest on its side for a short time it' will he
FIRST DAY: THE EMBRYONIC AXIS
found upon opening it that
the yolk (ovum proper) has
turned so that the blastederm is uppermost. Furthermore, if the egg is fertile,
and has been incubated, the
long axis of the primitive
streak, and hence of the embryo, is sometimes exactly,
but more often roughly, at
right angles to that of the
egg shell. Lastly, it will also
be true that if the small end
of the shell is to the right
of the observer, the anterior
end of the streak, and hence
later the head end of the embryo, will usually be away
from him (Fig. 167). These
facts have long been known,
but in themselves only raise
further questions, to wit:
Why is the embryo transverse to the length of the
shell? Why is the head end
away from the observer and
why are there exceptions?
These are the crucial points.
It may be stated to begin
with that, granted one initial
321
u. r.
H. F.
\./'
Fig. 169.—Stage of first intersomitic groove
drawn from an entire mount in balsam by
transmitted light. From Lillie (Development of
the Chick).
a.c.v. Amnio-cardiac vesicle. a.o. Inner mar
gin of Area opaca. Ect. Ectoderm. Ent. Ento-_ ,
derm. H. F. Head-fold. i.s.f.l. First intersomitic
furrow. med.pl. Anterior end of medullary
plate. Mes. Mesoderm. n.g.r. Neural groove.
pr.gr. Primitive groove. Pr’a. Proamnion.
assumption, one group of known facts might account for the transverse
position, the direction of the head and the exceptions. The unproved assumption and the facts are as follows:
The assumption is that the egg passes from the ovary into the oviduct
in such a position that the blastoderm will rest against the wall of the
duct, not toward its lumen. It has been suggested by T. H. Morgan (’27)
that this might occur if the ovum is regularly more compressible in any
axis at right angles to the one vertical to the blastoderm. Granted this
initial assumption, it is then known that the blastoderm retains its position against the side of the duct as the ovum passes along it, revolving
Ent. spl. Mes. Coel. Nch. C09’
Somp.
$pl’p|.
Fig. 170.-—A. Transverse section across the axis of the embryo and the entire
blastoderm of one side. The section passes through the sixth somite of a 10s embryo,
and is intended to show the topography of the blastoderm. The regions B, C, D, E
are represented under higher magnification in the Figs. B, C, D, E. From Lillie (Development o/ the Chick).
A0. Dorsal aorta. a.u.e. Area vitellina exrerna. a.v.i. Area vitellina interna. Bl.i.
Blood island. Bl.v. Blood vessel. Cael. Coelom. GJV. Germ wall. M.0. Margin of
overgrowth. Nch. Notochord. N.F. Neural fold. Nph. Nephrotome. S. Somite. Somp.
Sammopleure. Spl’pl. Splanchnopleure. Som.Mes. Somatic layer of mesoblast. spl.
Mes. Splanchnic layer of the mesoblast. S.T. Sinus terminalis. Y.S.Em. Yolk-sac
cntoderm. ZJ. Zone of junction.
322
I
FIRST DAY: THE EMBRYONIC AXIS 323
as it goes. This means that the blastoderm traces an imaginary spiral
path around the wall of the duct. It is also known that the small end of
the shell is usually found at the leading end. Under such circumstances
Morgan further points out that the following conditions might then ensue. As the egg revolves, the two sides of the blastoderm might be under unequal pressure. This might then determine the transverse position
Fig. 171.—Median longitudinal section of the head, stage of 4 s. The section
passes through the length of one of the neural folds just behind the anterior end.
From Lillie (Development of the Chick).
a.i.p. Anterior intestinal portal. Ect. Ectoderm. Ent. Entoderm. F’ .0. Fore-gut.
H.F. Head-fold. Mes. Mesoderm. Mes.H.C. Mesohlastic head cavity. n.F. Neural
fold. or.pl. Oral plate.
of the primitive streak, its long axis lying parallel to the direction of
pressure. Furthermore, the pressure might presumably be greater on the
side toward which the egg was revolving. If so, and if the egg always
revolves in the same direction, this might determine that the anterior
end of the streak and embryo would always be on a certain side. Bartelmez (’18) has added the notion that the primitive streak axis is determined before the egg leaves the ovary. Then, if as suggested, it always passes into the duct in a certain way this might result in making
the primitive streak axis always transverse to the duct and shell. The
assumption of Bartelmez may be true, but there is no adequate proof
for it, and it seems only to push the ultimate solution further back.
ll
V
5:
it
324 THE CHICK
Morgan’s theory involves fewer unproved premises, and, due to slight
differences in direction of pressure, may account for the variations.
THE HEAD FOLD
A short distance in front of the anterior end of the head process, there
develops shortly a slight depression, and immediately posterior to this
depression a crescentic fold appears, involving both ectoderm and endoderm (Figs. 168, 169, 171). Its ends extend almost from one side of the
area pellucida to the other. The crest of this fold is not raised perpendicularly to the surface, but extends forward so that it overhangs the depression indicated above. It is the head fold, and its anterior edge
marks the anterior end of the embryo. The lateral and posterior limits
of the embryo are not distinguishable until much later. \
THE FORE—GUT
From the method of its formation, the cavity within the head fold is
necessarily lined by endoderm which is co-extensive with the endoderm
of the archenteric cavity posterior to it. It is the anterior portion of the
future fore-gut, the portion which may be said to represent the pharyngeal region. It is a broad, flattened cavity, and as suggested, opens posteriorly into the extensive archenteric space over~lying the yolk. The region of this wide opening is known as the anterior intestinal portal. The
endoderm on the antero-ventral side of the fore-gut soon fuses with the
ectoderm below it in a limited region to form the oral plate (Fig. 171) ;
elsewhere between the ectoderm and endoderm of this vicinity, there are
scattered mesoderm cells, i.e., mesenchyme.
DIFFERENTIATION DF THE EMBRYONIC MESODERM IN
THE AREA PELLUCIDA
THE SOMITES AND LATERAL PLATES
The lateral sheets of mesoderm of the area pellucida now become
thickened along either side of the head process and primitive streak
The ridges thus formed are known as the vertebral or segmental plates,
while the remaining lateral portions of the sheets are called the lateral ’
plates. Just in front of the anterior end of the primitive streak a transverse fissure now appears in each of the vertebral plates. The region of
the plates immediately anterior to these fissures then constitutes the first
pair of.sorr_zitcs; they remain continuous anteriorly with the mesoderm
FIRST DAY: SOMITES, LATERAL PLATES 325
of the head region (Fig. 172) . Slightly behind the first pair of fissures
a second pair develops, and the part of the vertebral plates between
the first and second pairs of fissures is the second pair of somites. The
exact number of somites, and correlated development, varies consider
€. 0.
F. G.
Fig. 172.—Chick embryo with three pairs of somites (about 23
hours). Dorsal view. From Lillie (Development of the Chick).
zz.c.v. Amnio-cardiac vesicle. a.a. Inner margin of area opaca. F .G.
Fore-gut. N’ch. Notochord. n.F. Neural fold. pr.gr. Primitive groove.
31, .92, 3;. First, second, and third somites. .
ably, especially in the early stages, due to the breed of hen, the condition of the egg at laying, the precise temperature and other factors.
At the end of 24- hours, however, there are usually from three to six
of them——often about four——lying anterior to the primitive streak
and hence upon either side of the head process, i.e., the rudiment of the
notochord. The first four pairs of these somites later disappear, being
included in the posterior part of the head.
326 THE CHICK
The Nephrotome. —A narrow strip of each lateral plate immediately adjacent to the somites serves, as it were, to unite them to the main
part of the plate. It is known as the nephrotome, and later gives rise to
the excretory organs.
THE COELOM
Within the lateral sheets, which for a time remain connected with the
somites by means of the nephrotomes, horizontal splits now develop.
They occur first in the anterior portions and gradually spread elsewhere. Of the two sheets thus formed, the one next to the ectoderm is the
somatic or parietal mesoderm (somazopleure) , and that next to the endoderm the splanchnic or visceral mesoderm (splanchnopleure) . The
space between them is the coelom (Fig. 170).
THE RUDIMENT OF THE PERICARDIAL CAVITY
In the region of the head fold, the coelomic spaces on each side push
toward each other. By so doing, they finally work their way in between
the ectoderm and endoderm just at the bend where these two layers pass
up from the depressed area under the fold on to its ventral surface. At
the end of 24 hours, the walls of the opposite spaces have met each other
and fused, so that the spaces themselves are separated only by a thin
layer of mesoderm. This process tends to separate the ectoderm and the
endoderm by pushing.the latter further back, and thus increasing the
length of the fore-gut. These in-pushing portions of the coelom are
the amnio-cardiac vesicles, and they represent the rudiment of the pericartlial cavity (Figs. 172, 183).
THE NERVOUS SYSTEM
Among the most conspicuous features of the early embryo is the rudiment of the central nervous system. This system first appears in the following manner:
THE MEDULLARY OR NEURAL PLATE
Beginning almost at the anterior limit of the head fold the ectoderm
above and along each side of the head process is thickened somewhat;
this thickening is the medullary plate. Posteriorly, the lateral portions
of the plate extend also along each side of the primitive streak (groove).
while the central portion merges with the ectoderm of the groove.
FIRST DAY: THE NEURAL TUBE 327
THE MEDULLARY GROOVE AND MEDULLARY FOLDS
Presently a depression appears running down the middle of the medullary plate above the head process, and on each side of this depression, the lateral portions of the plate rise up as two parallel ridges. The
depression is, of course, the medullary or neural groove, while the
ridges are the medullary or neural folds (Fig. 172). Approximately at
the anterior end of the plate, the ends of the folds meet one another.
However, because of the fact that they are already quite close together,
this meeting does not form an extensive transverse ridge as in the Frog.
Posteriorly, the folds do not at first reach quite to the region of the
first somite, but before the end of the day they have extended backward
to about the anterior end of the shortened primitive streak.
THE NEURAL TUBE
The parallel medullary folds now bend toward one another until
their crests meet and fuse a little distance posterior to the anterior limit
of the head fold, in the region of the future rnid-brain. As in the case of
the Frog, a continuation of this fusion results in the formation of a
thick-walled tube, whose roof, sides, and floor are derived from the inner walls of the medullary folds and from the groove; it is the neural
tube and its cavity of course is the neural canal. As in the Frog, also,
there occurs shortly after the fusion of the folds, a separation between
their inner (neural) and outer walls, the latter reconstituting above the
tube a continuous layer of ectoderm.
These processes continue both anteriorly and posteriorly until the
tube is entirely closed in. During the closure, however, the usual anterior and posterior openings into the neural canal persist. The former
is the neuropore, corresponding to the structure of that name in the
forms previously studied; this opening is closed during the first day. It
should also be noted that because of the protrusion of the folds in this
region, they extend forward slightly beyond the anterior limit of the
fore-gut (Fig. 172). Later, as growth proceeds, this region is actually
carried over the anterior end of the embryo on to the ventral side (see
below under flexures). Posteriorly fusion takes place more rapidly,
keeping pace with the extension of the medullary folds. Because of the
greater distance to be traversed, however, the process in this direction
is not completed until some time later. The completion at this end is
marked by the disappearance of the primitive streak (Fig. 173).
328
THE CHICK
8.0.8.
op. Ves.
ceph. Mes.
F. G.
V. o. m.
s. 2.
n. T.
s. T.
N'ch.
Fi . 173.—-Chick embryo with seven pairs of somites
(alxaout 26-27 hours). Dorsal view. From Lillie (Development of the Chick).
a.c.s. Anterigr cerebral suture; i.e., line of fusion of
neural folds ‘here. ceph.Mes. Cephalic mesoderm. F.G.
Fore-gut. N’ch. Notochord. n.T. Neural tube. op.Ves. Op
..tic vesicle. Pr’-a. Proamnion. pr.str. Primitive streak.
3.2,.-r.7. Second and seventh somitee. V.a.m. 0mphaIomesenteric (vitelline) vein.
Fig. 174.—Transverse section
Am.I". Amniotic fold. A0. Aorta. Coel. Coelom.
362). My. Myotome. My’c. Myocoel. N’ch. Nolochord.
somite. Scler. Sclerotome. V.c.p. Posterior cardinal vein.
W
n
D
through the twentieth somite of a 29 s embryo.
Derm. Dermalome. Gn.
N.Cr. Neural crest. Nep/z.T
Ganglion.
Wolfiian duct.
1
L5.
Nephrogenous tissue. 3.20.
From Lillie (Development of the Chick).
Lateral limiting sulcus (see page
Twentieth
FIRST DAY:
THE NEURAL TUBE 329
330 THE CHICK.
THE NEURAL CRESTS
At the same time that fusion of the folds is occurring, cells are proliferated between the outer and inner layers of each fold, just in the
region of its crest. Thus, as fusion takes place, these cells form a band
along either side of the dorsal part of the neural tube between it and
the surface ectoderm. These bands are the neural crests, which at this
time are united with one another across the dorsal surface of the tube
(Fig. 174).
THE OPTIC VESICLES
Anterior to the first point of fusion, the neural tube is broadened
somewhat. This is the region of the future optic vesicles.
SUMMARY OF THE CONDITION AT THE END OF THE
FIRST DAY OF INCUBATION ‘
I. THE MESODERMAL STRUCTURES
About four pairs of somites are present, lying in front of the primitive knot and connected with the mesoderm of the respective lateral
plates by the longitudinal nephrotomal bands.
The lateral mesoderm extends throughout the area pellucida except
in the region of the proamnion, and together with the endoderm is being differentiated in the area opaca. In the latter area, the formation of
this layer has progressed anteriorly until a pair of wing-like extensions
are level with the tip of the head fold. Also in the area pellucida this
mesoderm has been split into two sheets, the somatopleure and splendinopleure, with the coelomic space between them, and this process is
spreading into the area opaca. Beneath the fore-gut, the walls of the
amnio-cardiac portions of the coelorn have just met each other, and the
rudiment of the pericardial cavity is thus indicated in this region.
ln connection with the formation of the mesoderm, blood vessels and
corpuscles have started to appear in the area opaca and area pellucida,
transforming both into the area vasculosa. The latter is beginning to be
bounded by the sinus terminalis.
4 Degree of development, including somite number, as noted, varies considerably,
especially through 48 hours of incubation. and the hour or stage conditions designated in this text do not exactly agree with the carefully obtained results of Hamburger and Hamilton, 51. However, they are believed to correspond well with
those indicated on the slides sold by most of the Biological Supply companies.
FIRST DAY: SUMMARY 331
Outside the area vasculosa is an area consisting only of partially differentiated germ wall, the zone of junction, and the zone of over-xmwth
the area vitellina. 5 ’
II. THE HEAD FOLD AND THE FORE—GUT
The head fold has formed and in the process has given rise to the an.
terior or pharyngeal portion of the fore-gut.
III. THE RUDIMENTS OF THE NERVOUS SYSTEM
The medullary folds have appeared in the region in front of the primitive knot and have fused for a short space at their anterior ends_ The
neural crests have begun to appear, and the rudiments of the optic vesicles are also indicated.
10
HE CHICK: DEVELOPMENT DURING THE SECOND
DAY OF INCUBATION
GENERAL APPEARANCE
TH E embryos of the higher vertebrates, including Reptiles, Birds
and Mammals, all develop in a more or less confined space, i.e., either
within an egg shell or within the uterus. Also, in the early embryonic life,
almost the anterior half of the organism in these forms is occupied by the
brain which is growing very rapidly. Not only is this true, but the dorsal
part of the mid-brain is growing with disproportionate rapidity, and this,
combined with the confining space, causes a very marked bending of the
entire anterior region. This bending presently leads also to a turning of
the head end (torsion), and finally of the whole embryo, upon its side,
as described below. Thus though the bending and turning are basically
due to changes in the brain, and will be described in terms of that
structure, it is convenient to do it under the heading of general external
features.
FLEXURES AND TORSION
The Cranial Flexure. ——The first bend, and one previously noted
in connection with the brain of the Frog, is the cranial flexure. In the latter animal it was the only marked flexure of the brain, and had nothing
to do with development in a confined space. Indeed the curve of this region of the brain was rather in part the remains of a portion of the original curvature of the egg. In the Chick and other higher animals the
cranial flexure does not have this origin, but it does involve exactly the
same regions of the brain, and the front of the embryo; i.e. it involves
the fore-brain region which is bent down anterior to the notochord.
This flexure begins at about thirty hours, and by the end of the day the
bending is so great that the morphologically dorsal side of the midbrain is actually the most anterior part of the embryo. The morphologically anterior side of the fore-brain, on the other hand, faces posteriorly
so that this part of the embryo almost touches the heart (Figs. 175,
176). Finally, it should be noted that, as in the Frog, this flexure, in so
SECOND DAY: LIMB BUDS
far as it concerns the brain, is permanent, and is the only one of
those indicated at this time which
is so.
The Cervical F1exure.——By
the end of the day another broad
curvature is evident, extending
through the region of the hindbrain and back into the trunk. This
is the cervical flexure, and has no
counterpart in the Amphibian.
The Lateral Rotation or
Torsion. —— Finally as a result of
both these flexures the front of the
embryo would be thrust deep into
the yolk were it not for a lateral
twist which begins at the anterior
end. By 48 hours it has progressed
posteriorly about as far as the
back end of the cervical flexure,
i.e., approximately to the thirteenth somite. It is called the lateral ratation or torsion, and eventually results in turning the entire
embryo over so that it lies upon
its left side (Fig. 176) .1 It
should be clearly understood in
this connection that the terms dorsal, ventral and lateral in the present and following descriptions are
used in their morphological sense.
Thus dorsal will always refer to
the side of the embryo upon which
Fig. 175. —Chicl: embryo with twenty
pairs of somites (about 4-3 hours). Dorsal view. From Lillie (Development of
the Chick).
A.o.m. Vitelline artery. au.P. Auditory pit. Cr.Fl. Cranial flexure. D.C.
Ductus Cuvieri. Dienc. Diencephalon.
M esenc. Mesencephalon. M etenc. Metencephalon. Myelenc. I and 2. Anterior
and posterior divisions of the myelencephalon. 0p.Ves. Optic Vesicle. Ph.
Pharynx. pr.str. Primitive streak. s.2.s.5.,
etc. Second, fifth, etc., somites. Telenc.
Telencephalon. Vel.tr. Velum transversum. I/en. Ventricle.
the nerve cord and notochord occur, and ventral will refer to the opposite side regardless of how the embryo lies.
LIMB BUDS
No limb buds are ordinarily visible at 48 hours. Nevertheless, if tissue from the locations where they would later appear is transplanted to
1 Occasional embryos are found lying upon the right side. Apparently this does
not prevent subsequent normal development.
334 THE CHICK
Fig. 176. ——Chick embryo with twentyseven pairs of somites (about 48 hours).
From Kellicott (Chardatc Developmerm. After Lillie.
a. Auricle. am. Posterior margin of
amniotic folds. c. Carotid loop. cf. Cranial flexure (cervical flexure also shown,
see p. 333). d. Diencephalon. dC. Ductus Cuvieri. g1, g2, g3. Visceral clefts. i.
Isthmus. 1. Lens. ma. Mandibular arch.
ms. Mesencephalon. mt. Metencephalon.
a. Otocyst; to right of otocyst is ganglion of VII and VIII cranial nerves. r.
Retinal layer. S2, 510, 520. Second, tenth,
and twentieth somites. L. Tail-bud. 1;.
Vemricle. va. Vitelline artery. vv. Vite]line vein. 1, 2, 3. First, second, third aortic arches. V. Ganglion V cranial nerve.
other locations it will produce
there either a wing or a hind limb
depending upon its source. Furthermore, the dorso-ventral and
antero-posterior axes of these
transplanted tissues will not have
been altered, i.e., such potential
limb tissue (anlage) transplanted
in an inverted position will produce an inverted limb. Thus it appears that the destiny of this tissue has already been rather completely determined. It will not
only form a limb, but a limb of
a particular type which retains all
its original axes (Hamburger,
’38).
THE SOMITES
When last mentioned, the somites were described as masses of
mesoderm connected with the lateral plates by means of the nephrotomes. During the second 24
hours the connection between
nephrotome and somite is obliterated throughout the greater part
of the embryo; the number of
pairs of the latter increases to 27,
and beginning at the anterior end
the development of each of the
sornites proceeds in the following
manner:
THE MYOTOMES AND
THE CUTIS PLATES
Each somite is at first composed
of an outer layer of comparatively
dense cells surrounding an inner
mass of mesenchyme, the latter
SECOND DAY: THE FORE-GUT 335
comparable to a myocoel, so far as one exists (Fig. 170, B). Presently,
however, the denser layer of cells on the side of the somite next to the
nerve cord and notochord largely disappears, leaving the latter structures in direct contact with the mesenchymatous mass indicated above.
At the same time the dense layer upon the dorsal and outer side of the
somite becomes thicker. The dorsal portion of this outer layer is the rudiment of the myotome, while the more lateral and ventral portion is the
cutis plate or dermatome. Before the second day has passed, the dorsal
or myotomal portion of the above plate of cells begins to turn sharply
upon itself and grow downward between the mesenchyme and the cutis
plate. Thus a double layer of cells begins to be fonned consisting of the
cutis plate on the outside and the myotomal plate on the inside (Fig.
1 74) .
THE SCLEROTOME
The mesenchyme which now begins gradually to surround the notochord and the ventro-lateral region of the nerve cord is the rudiment of
the sol otome.
THE ALIMENTARY TRACT
THE F ORE—GUT
The Stomodaeum.—--During the first day it was noted that the
antero-ventral end of the fore-gut came in contact with the ectoderm at a
point on the ventral side of the head fold to form the oral plate. Now,
as the result of the downward flexure of the head and also of the pushing forward of the mandibular arches (see below), the central region of
the plate becomes relatively depressed to form a pit lined by ectoderm.
It is the beginning of the stomodaeum, and by a continuation of the
above process it presently acquires a considerable depth.
Rathke’s Pocket. — From the antero-dorsal wall of the stomodaeum
a small diverticulum now appears growing anteriorly along the morphologically ventral side of the posterior portion of the fore-brain
which has been bent down in front of it. It is called Rathke’s pocket,
and is destined to become the anterior part of the hypophysis or pituitary. (See the footnote on this under the Frog.)
The Visceral Pouches and Arches.
The Pouches. —— ln the anterior or pharyngeal portion of the fore-gut,
a series of vertical folds of the endodermal wall begin to push outtaward the ectoderm on each side of the head. These are the visceral
336 THE CHICK
pouches, and they develop in regular order, the most anterior pair appearing first. The first pair are known as the first visceral or hyomandibular pouches, and the remaining pairs, of which there are three, as
the second, third, and fourth visceral (“ branchial ”) pouches. They
decrease in size posteriorly, the last pair being relatively small. The
first pair of pouches, i.e., the hyomandibulars, fuse with the corresponding ectodermal invaginations (visceral furrows) only at their dorsal
ends, while the second and third pairs fuse with their respective furrows throughout their lengths, except for a slight interruption in their
upperhalves. The point of fusion of the first pouch now becomes perforated as the first or spiracular cleft. The fusion of the fourth pair of
pouches and furrows, and the perforation at the points of fusion of the
second and third pairs to form actual visceral clefts, occurs later (Figs.
176 and 194).?
The Arches. -—~ Anterior and posterior to each pouch the mesenchyme
becomes thickened to form the visceral arches. The arch in front of the
first or hyomandibular pouch is the first visceral or mandibular arch,
and the one between it and the second pouch is the second visceral_or
hyoid arch. The remainder are simply the third, fourth, and fifth visceral (“ branchial ”) arches, and they appear in the same order as the
pouches; the fifth and last arch is hardly more than a transitory vestige.
Presently, blood vessels and nerves pass into the arches, as will be indicated later.
It should be noted in passing, that though these pouches and arches
correspond to the similarly developed structures in the Frog, in this
case no gills ever appear in connection with any of them. The term visceral rather than branchial is therefore more aptly applied to them all.
The Thyroid. -—-This begins to develop near the end of the second
day as a small thickening in the middle of the floor of the pharynx, between the ventral ends of the second pair of visceral arches. Before the
end of the day it has become slightly evaginated so as to form a shallow depression in the pharyngeal floor (Fig. 184) .
2 According to a recent investigator (Dudley, ‘42) there are actually six visceral
pouches in the Chick embryo, but the last two are very early merged with the
fourth to form what this author calls the “fourth visceral complex,” the “sixth
pouch ” component later forming the post-branchial body (see below). As will he
noted later, others have regarded the primordial lung outgrowths as fifth visceral_
pouches. It appears to the present writer that these are all somewhat forced
attempts to make the situation in the Bird square more nearly with that in some.
of the lower Chordates. Whether either the lung outgrowths or the rudimentary
structures referred to by Dudley really represent any visceral pouches or not, is,
the writer believes, still open to considerable question.
SECOND DAY: THE HIND—GUT 337
The Respiratory Systems--Late the second day a longitudinal
groove, with a pair of slight posterior expansions, appears in the floor of
the pharynx caudal to the visceral pouches. lt is the beginning of the
larynx, the trachea, and the lungs, and thus represents the start of the
entire respiratory system. In this connection it may be recalled that according to one View the lung primordia of the Frog are to be homologized with a hypothetical seventh pair of gill pouches. It is therefore
of interest to find that in this case the above expansions which later develop into the lung primordia of the Chick are similarly homologized
by some with a fifth pair of visceral pouches. (See, however, preceding
footnote.) ”
The Liver. —Just at the posterior limit of the fore-gut behind the
pharyngeal region, there appear at this time two slight antero-ventrally
directed evaginations of the endoderm whose development is said to depend on Contact with the veins ( cardiac primordial in this region {W illier, and Rawles, ’3] T). The diverticula are not of course suspended in
space. but pushed forward into the mass of splanchnic mesoclerm (ventral nzesentery) which unites the gut and the ductus venosus in this vicinity. One of the diverticula is a little in advance of the other both in
position and in time of appearance. lt presently pushes forward so as to
lie just dorsal to the point of union of the vitelline veins (see below),
while the other, at this period, is barely distinguishable. These two
diverticula represent the rudiments of the liver.
THE MID—GUT
There is little indication of any real mid-gut during the secondday,
but rather merely a wide enteric space overlying the yolk. The beginning of folds along the sides of the embryo continuous with the lateral
margins of the head fold suggests, however, the manner in which this
portion of the gut will be formed.
THE HIND—GUT
The Posterior Intestinal Portal and Anal Plate. —— At the close
of the second day the hind-gut begins to develop in connection with a
tail fold very similar to the head fold. There is thus formed a posteriorly
directed cavity lined by endoderm, and lying beneath the remains of the
primitive streak. It is the hind-gut, and opens anteriorly into the wide
enteric space overlying the yolk (rudiment of the mid-gut). As in the
case of the fore-gut, the region of this opening is termed an intestinal
portal—in this instance, the posterior intestinal portal. There is li338 THE CHICK
nally one further resemblance between fore- and hind-guts in that at the
end of the latter the endoderm comes in contact with the ectoderm and
fuses with it. This point of fusion  at the posterior end of the primitive streak. and marks the location of the future anus. lt is termed the
anal plate or eloacal membrane. Besides these points of resemblance,
there are now to he noticed Certain important differences as follows
(Fig. 177):
T.B. tf
Eat. all. t.
Fig. 177.~—Median longitudinal section through the hind end of an embryo of
about 21 s. From Lillie (Development of the Chick}.
an.p[. Anal plate. an.!. Anal tube I’l1ixirl-grill. Ect. Ectoderm. Ent. Endoderm. files.
Mesoderm. p.IT.p. Posterior intestinal portal. T.B. Tail-bud. t.f.So’pl. Tail iold.in the
zomatopleurc and ectoderm. t.f.Sp‘pl. Tail fold in the splanchnopleure and endoem.
The Ventral Mesentery.—lt has been stated that the hind-gut
is formed in connection with a tail fold, just as the fore-gut is formed
in connection with the head fold, and in a general way this is true. In
the case of the tail fold, however, there is this difference. The endoderm
is folded in to form the hind-gut and the intestinal portal, but in this
case the ectoderm follows this infolding much more slowly than it did
in the case of the head fold. Thus it happens that the hind-gut arises
before there is any very marked indication of a tail fold on the surface
of the blastoderm. For this reason the anal plate, unlike the oral plate,
remains dorsal for some time, and is only gradually carried around onto
the ventral surface (Fig. 177) . ‘
Furthermore, this lagging behind of the ectodermal portion of the
fold necessarily means that there is a gap between the two cell layers;
this gap in the case of the tail fold is filled by mesoderm. Presently lateral extensions of the embryonic coelom press back into this region
upon either side, but for a time they do not meet one another. Thus
there is left a median mesodermal mass extending from the ventral side
   
l
SECOND DAY: THE HEART 339
of the hind-gut backward and upward to the underside of the lagging
ectoderm. That portion in contact Wiitll the gut may be referred to as
splanchnic, and that in contact with the ectoderm as 50ma;gc_ The two
portions are continuous, however, and together are known as the ventral
mesenteri-' of the hind-gut.
 
B
Fig. 178.—Ventral views of the head ends of Chick embryos. From Lillie (Development of the Chick). A. Embryo with five pairs of somitcs (about 23 hours). B.
Embryo with seven pairs of somites (about 25 hours).
a.c.v. Amnio-cardiac vesicle. a.i.p. Anterior intestinal portal. End’c.s. Endocardial
septum. F .0. Fore-gut. Ht. Heart. M }*’C. Myocardium. N’ch. Notochord. N’ch.T. Anterior tip of nomchord. n.F'. Neural fold. op.Ves. Optic vesicle. p.C. Pericardial cav
ity (amnio-cardiac vesicles). Pr’a. Proamnion. 32.54. Second and fourth mesodermal
somites. V .o.m. Omphalomesenteric vein.
THE CIRCULATORY SYSTEM
THE HEART
The Origin and the Formation of the Enclothelial Lining. —
While blood vessels and corpuscles have been developing from the germ
wall in the area opaca, vessels have also begun to form in the area pellucida. These latter vessels, which are in direct continuity with ‘chose already formed, also arise from blood islands, though these islands are
slightly different from those of the area opaca. They are merely aggregations of cells, apparently detached from the splanchnic mesoderm,
and the vessels into which they develop are temporarily entirely devoid
340 THE CHICK
of corpuscles. Erythrocytes, however, are soon supplied from the area
opaca, and also by cells buddecl from the posterior ends of the dorsal
aortae (Danchakoii, ’07). Thus from the cell aggregates, as indicated,
rudiments of two large vessels (the omphalomesenzeric or vitelline veins)
Fig. 179. ——~ Sections through Chick embryos showing particularly the formation of
the heart. pericardial cavity. and pharynx. From Kellieott (Chordate Developrmnzt).
After Lillie. A. Just posterior to the anterior intestinal portal of a Chick with seven
pairs of somitcs (about 25 hours). B. Section just anterior to A. C. Through the
heart of an embryo with ten pairs of somites (about 29 hours).
am. Axial mesodermal thickening. (:0. Lateral dorsal aorta. ebc. Exocoelom. cc.
Ectoderm. en. Endoderru. hb. Hind-brain. 17. Blood islands. 17]). Anterior intestinal
portal. my. Mayocardium (muscular layer of heart). n. Notochord. nc. Nerve cord.
p. Pharynx. pc. Pericardial cavity (atnnio-cardiac vesicles). 3. Endothelial septum.
so. Somatic mesorlerm. sp. Splanchnic mesoderm. th. Cardiac entlothelium. 11. Area
vasculosa. um. Ventral mesocardium. w. Germ wall. y. yolk~sac endodcrm.
soon appear in the area pellucida (Fig. 178) : Each rudiment rests upon
one of the ventro-lateral walls of the fore-gut, between it and the median-lateral wall of the respective amnio-cardiac vesicle from which it
has arisen (Fig. 179, A) .3 The anterior portions of these rudiments then
form the ehdothelial lining of the heart in the following manner:
It is to be recalled thatthe amnio-cardiac vesicles have already become fused beneath the fore—gut, just in front of the endodermal wall
3 The evidence of this figure would seem to indicate that the vessels are derived
from the walls of the gut rather than from those of the vesicles, and some authorities hold this to he the case. In view, however, of the origin of the other blood
vessels of this area from the mesoclerm, it seems more likely that the latter derivation is the true one.
i
a—u—an.m..._..,..
SECOND DAY: THE HEART 341
which marks its posterior limit (Fig. 178, A). The fusion now progresses posteriorly, as it does so pushing back and closing in the ventralateral gut walls against which the veins indicated in the preceding paragraph are resting. Thus as these walls come together the anterior ends
of the above mentioned vessels are likewise brought together side by
side beneath the newly formed gut, and as this occurs they fuse with one
another to form a single vessel with a median partition. This partition
soon disappears, and the single median tube which remains is t: e endothelial lining of the rudimentary heart (Figs. 178, B and 179, B, C).
The Myocardium of the Heart. — The median walls of the amniocardiac vesicles which now lie against each side of the endothelial tube
presently press in above and below it, and fuse with each other. Thus
the tube is completely surrounded by mesoderm which forms the myocardium or muscular element of the heart, and its covering the visceral
pericardium.
The Mesocardia.———The above fusion leaves the endothelial tube
and its myocardium suspended from the mesodermal covering of the
ventral wall of the fore-gut, or pharynx, by a double layered sheet of
mesoderm (ventral mesentery) here termed the dorsal mesocardium.
Ventrally also a similar sheet attaches the tube to the underlying
splanchnic mesoderm. The latter quickly disappears, and the former
does so later, except at the anterior and posterior ends of the heart (Fig.
179, C).
The Pericardial Cavity and Parietal Pericardium. —With the
fusion and disappearance of the median walls of the amnio-cardiac vesicles, it is clear that their cavities have become a single space which surrounds the heart. This space is the pericardial cavity, and its walls constitute the rudiments of the greater part of the parietal pericardium.
Postero-laterally, however, the pericardium is still incomplete, and
hence the above cavity continues to communicate in this direction with
the general coelom.
The Rudiments of the Atria, Ventricles, Bulbus and Truncus
Arteriosus. — In connection with the description of the development
of the Frog heart it was noted that the development of all Vertebrate
hearts is essentially similar. This similarity has already become apparent as between the Frog and Chick in that the hearts of both start with
the fusion of two vessels to form a tube. Further similarities will now
reveal themselves in the transformations of this tube in the Chick to
form the adult organ.
342 THE CHICK
As in the Frog, the straight tuhe first increases in length, and, its
‘ends being fixed, its middle hows laterally to the right (Figs. 180 and
181). The broad apex of the how is then drawn ventrally, and usually
slightly posteriorly, while the whole tube is at the same time thrown
into a loop. (These terms of direction it should here be recalled are being used in the morphological sense regardless of the rotation of the
embryo onto its side.) Again as in the Frog, the loop which has been
produced in the originally straight tube lies to the right of the median
line. This means that the posterior limb of the loop extends ventrally,
op. Ves.
VII -VIII
an. F.
V. o. rn.
5.4.
a. i. p.
Fig. 180. —— Ventral view of the anterior end of ti Chick embryo
with sixteen pairs of somites (about 38 hours). From Lillie
(Development of the Chick).
(I.i.p. Anterior intestinal portal. au.P. Auditory pit. B.a. Bulbus arteriosum F.B. Fore-brain. Inf. lnfundihulum. op.Ves. Optic vesicle. 0r.p1. Oral plate. Pr’am. Proamnion. 3.4-. Fourth
somite. Tr.a. Truncus arteriosus. v.Ao. Ventral aorta. Ven. Ven- '
tricle. V.o.m. Omphalomesenteric (vitelline) ‘vein. V II—-VIII.
Acustico-facialis ganglion.
and as suggested, usually slightly posteriorly. The middle part then
curves laterally toward the right, where it passes into the ascending limb
which extends dorsally, anteriorly and medially back into the median
plane (Figs. 108, 176) . It now remains to indicate the parts of the future
heart which the various regions of this loop are destined to form. Beginning at the posterior end the region where the posterior limb starts
to descend will comprise the atria. The apex of the loop and a small
portion of the descending and ascending limbs will become the ventricles. The larger part of the anterior ascending limb will become the
bulbu: and truncus arteriosua.
SECOND DAY: BLOOD VESSELS 343
As regards the functioning of the Chick heart, the first indications of
it have been found to occur about the twenty-ninth hour of incubation,
and as in the Frog, long before any innervation. T he contractions begin along the right side of the heart tube in the future ventricular region, and then spread to the left. As the atrial region forms behind the
ventricular, the contractions also extend to it, and finally to the sinus
venosus. As in the case of the Frog, experimental transections of the
heart tube show that the inherent rate of contraction increases as one
passes posteriorly. Also the most posterior region at any given stage
acts as the pacemaker, while the older anterior regions gradually lose
the power of automatic contraction. Thus the rate for the whole heart
is slowly stepped up and is finally set by the sinus, which is ultimately
incorporated into the right atrium ( Patten and Kramer, ’33, Barry,
’42). Later on following innervation the rate of heat is of course partially under nervous control.
THE BLOOD VESSELS OF THE EMBRYO
The Arteries.
The Dorsal Alarms and Their Branches. Along each side of the embryo, just at the inner margin of the pellucid area, two vessels now develop. These are the dorsal aortae (Fig. 181, A). Anteriorly each is
continued into a vessel differentiated in the mesenchynie on either side
of the head.l’osteriorly they give elf branches between the somites (segmental arteries) , and finally leave the sides of the embryo at about the
level of the seventeenth somite to pass out into the general vascular network as the vitellinc arteries. Near the end of the second day the two
dorsal aortae fuse with one another in the region above the heart, forming for a short distance a single dorsal vessel.
Development of the Aortic Arches. -——-The truncus arteriosus
at first runs anteriorly a short distance, this short relatively horizontal
extension being called a ventral aorta. It is. however, merely a continuation of the truncus, and is presently so incorporated with it that there
is no distinction. At its anterior end this short extension of the truncus
divides into two vessels which extend still further forward in the pharyngeal floor. They also are frequently called ventral aortae. As will
presently appear, however, their proximal portions really constitute the
proximal parts of the first pair of aortic arches (Figs. 180. 176). Somewhat anterior to the oral plate each of these vessels bends sharply upward to join the respective dorsal aorta, this bend being termed the
344 THE CHICK
Fig. 181.~—Chick embryo with 12 pairs of somites (about 33 hours). From Lillie
(Development of the (Jhiclr). A. Dorsal view of entire embryo. B. Ventral view of
anterior end.
A.C.S. Anterior cerebral suture. a.z'.p. Anterior intestinal portal. A0. Dorsal aorta.
F.C. Fore-gut. H.B. Hind-brain. Ht. Heart. M.B. Mid-brain. op.Ves. Optic vesicle
or.pl. Oral plate. pr.slr. Primitive streak. 82 S12. Second and twelfth somites. v.Ao.
Ventral aorta. V.o.m.. Omphalomesenteric vein.
carotid loop. Meanwhile, as previously indicated, the visceral pouches
and arches have been forming, and in the arches certain blood vessels
have been developing on each side as follows:
In the first place the single or common ventral aorta has, as pre-'
dicted, become incorporated into the truncus whose wide dorsal end
now terminates directly beneath the visceral arches. While this has been
SECOND DAY: BLOOD VESSELS 345
occurring each first or mandibular arch has pushed ventrad. As a result
of this the proximal part of each of the separate ventral aortae comes to
lie within about the ventral four-fifths of the respective mandibular
arch. Thus, as suggested above, this part of each ventral aorta comes
to form the proximal portion of each first aortic arch. The more distal
fifth of each first aortic arch which will lie within the corresponding
distal fifth of the mandibular arch, remains for the time being incomplete. The proximal four-fifths of this vessel is, however, still connected
with the dorsal aorta by way of the remaining anterior tip of the respective ventral aorta and carotid loop as previously indicated (Fig.
176). The actual completion of the distal portion of the first aortic arch
so that this artery lies entirely within the‘ mandibular arch apparently
does not occur until the third day, and wiil be described when that stage
is reached. The development of the remaining aortic arches is more
straightforward. The second aortic arches develop in the second visceral or hyoid arches, and the third aortic arches develop in the third visceral arches. These last pairs arise as buds from the dorsal aortae which
grow almost directly ventrad through the arches to -join the dorsal end
of the truncus.
The Veins and the Lateral Mesocardia. —— As has been indicated
above, the endothelial portion of the heart is formed by the growing together of two large vessels (omphalomescnteric veins) . It now remains
to state that this union continues for a short distance posterior to the
atrial rudiments. The most anterior part of this continuation is somewhat dilated and is known as the sinus venosus, while slightly further
back it receives the name of ductus venosus. The most anterior portion
of the sinus venosus is sometimes regarded as part of the heart proper,
because later it is involved in the development of the right atrium. At
this stage, however, it may best be considered as a part of the venous
system.
During the second day there develops in the mesenchyme on each
ventro-lateral side of the brain a vessel which runs posteriorly as far as
the level of the heart. These are the anterior cardinal veins. Meantime
there has occurred on each side of the embryo a fusion of the lateral
body wall with the posterior part of the sinus venosus. Thus a pair of
septa have been formed each of which passes somewhat diagonally laterally and dorsally from the posterior part of the sinus to the respective
body wall. These are called the lateral mesocarclia, and within each of
them develops a rather large vein, the ductus Cuvicri (Figs. 176; 182, C).
Each ductus Cuvieri connects ventrally with the sinus venosus and dorFig. 182.——Diagrams of the circulation in the Chick embryo and area vasculosa.
From Kellicott (Chordate Development). The vascular network of the area vasculosa is omitted for the most part. A. Anterior and central parts of the embryo and
vascular area at about thirty-eight hours (sixteen pairs of somites). Viewed from
beneath. After Popofi. B. Median and anterior parts of vascular area and embryo at
about seventy-two hours. (twenty-seven pairs of somizes; the number is usually
nearer to 36 at. this age). Viewed from beneath. After Popoff. C. The main vascular
trunks of the fourth day. After Lillie (modified).
a. Dorsal aorta. aa. Aortic arches (first and second in A, second, third, and fourth
in C) . ac. Anterior cardinal vein. al. Allantois. au. Atrium. b. Bulbus arteriosus. 11G.
Ductua Cuvieri. dv. Ductus venosus. cc. External carotid artery. 1:. Heart. ic. Internal
carotid artery. la. Lateral dorsal aorta. It). Left anterior vitelline vein. p. Anterior
intestinal portal. pc. Posterior cardinal vein. u. Posterior vitelline vein. ru. Right
anterior vitalline vein. t. Sinus terminalis. tr. enoue trunks oi the area vasculosa.
v. Venn-icle. va. Vitelline artery. w. Vitelline or omphalomesentcric vein (in this
region really lateral vitellinc vein) . .
546
SECOND DAY: CIRCULATION 347
sally with the posterior end of the respective anterior cardinal vein.
From this point of union still another Vein grows posteriorly along each
side of the body. These veins are known as the posterior cardz'.vzals tl7i;:.
182, C ).
THE‘ EXTRA:-EMBRYONIC BLOOD VESSELS
Extension of the Area Vasculosa and the Mesoderm. —~By
about the end of the second day the two anterior wings of the area vasculosa, and the extra-embryonic mesoderm and entoderm which accompany them, have bent toward one another and have fused in front of the
proaxnnion; The area vasculosa, therefore, now entirely surrounds the
latter region, and is itself completely encircled by the sinus terrninalis,
which has been referred to above (Fig. 182, A, B ) . Meanwhile, certain
veins and arteries have extended from the embryo into the vascular area,
as follows:
At the posterior end of the ductus venosus, the union of the vessels
which form it terminates, and each passes outward into the area pellucida. At this point the y are known as the vitelline or omphalomesenteric
veins. Upon coming into this region each of the veins turns e.nteriorly
and runs past the head around the inner boundaries of the approaching
wings of the area vasculosa. Hence these extensions are known as the
right and left anterior vizelline veins. First by a system of capillaries,
but presently directly, each of these veins then becomes connected with
the anterior extremities of the sinus terrninalis. It thus happens that as
the vascular wings meet one another, the sinus terminalis not only be
comes complete, but the ends of the two anterior vitelline veins also
' meet and form one vessel (Fig. 182). At the proximal ends of these
veins each gives rise during this period to a slight lateral outgrowth the beginnings of the lateral vizelline veins.
The vitelline arteries, already referred to, extend out into the lateral
portions of the area vasculosa some distance back of the vitelline veins,
i.e., by the end of the day at about the twentieth somite.
THE CIRCULATION AS ESTABLISHED ON SECOND DAY
It will now be seen that with the establishment of the capillary network within the area vasculosa, and the formation of the arches connecting the ventral and dorsal aortae within the embryo, a complete
system of circulation has been made possible. The further development
of this system will be described as it occurs.
348 THE CHECK
THE NERVOUS SYSTEM
THE MAIN DIVISIONS OF THE EARLY BRAIN
Early on the second day of incubation a slight constriction appears
just back of the optic vesicles, marking the posterior boundary of the
fore-brain or prosencephalon. Presently this is followed somewhat further back by another constriction which marks the posterior limit of
Fig. 183. ——-Median sagittal section through the head end of a Chick with 18 pairs
of somites labout 4-0 hours). From Lillie (Development of the Chick).
tt.i.p. Anterior intestinal portal. Aa. Dorsal aorta. At. Atrium. E.E.B.C. Exocoelom
(extra-ernliryonic body cavity). F.B. Fore-brain. H.B. Hinv.l~l)rain. H .F.Am. Head~fold
of amnion. Inf. lnfundibulum. Isth. Isthmus. M.B. Mid-brain. N’r:h. Notochord. or.pl.
Oral plate (oral membrane) . P.C. Pericardial cavity. Ph. Pharynx. Pr’a. Proamnion.
pr’n.g. Preoral gut. Retzopt. Optic recess. S.V. Sinus venosus. Tr.a. Trnncus arte
riosus. Vcn. Ventricle.
the mid-brain or mesencephalon. The part posterior to this is the hindbmin or rhombencephalon which passes insensibly into the region of the
spinal cord. The posterior limit of the hind-brain, however, may be fixed
in a general way at this time by the position of the fourth somite (Figs.
181, 183). lt should again be noted that the cranial and cervical flexurcs
are especially concerned with the brain. As suggested, however, because
that organ occupies so large a part of the anterior of the embryo at this
stage these flexures affect the whole organism in this region and were
therefore described under general appearances.
THE FORE—BRAIN OR PROSENCEPHALON
Its Extent.—-—0n the posterior wall, i.e., on the floor of that part
of the brain where the cranial flexure is most pronounced, at the end of
the slightly bent notochord, is an invagination. It is directed antero
..‘,,t-.-yw
SECOND DAY: THE‘ FORE—BRAIN 34.9
ventrally into the cavity of the brain, and is called the tuberculium. posterius (F ig. 184). On the opposite or anterior wall of the brain a little
below the level of this evagination is the slight, but broad, constriction
referred to above as marking the posterior boundary of the fore-brain.
This boundary may now be more accurately defined as a plane passing
from the tuberculum posterius on the posterior wall to the mid-point of
the broad constriction on the anterior wall. This mid-point marks also
the position of the future posterior commissure (see fourth day).
Fig. 184.——— Optical sagittal section of the head of an embryo of 22-23 s. The heart
is represented entire. From Lillie (Development of the Chick).
Atr. Atrium. B.a. Bulbus arteriosus. Cr.Fl. Cranial fiexure. Dienc. Diencephalon.
Hyp. Rathlce’s pocket, rudiment of anterior hypophysis. Inf. lnfundilmlum. Md.
Mantlilmlar arch. Melenc. Metencephalon. Myelenc. Myelencephalon. or.pl. Oral
plate. Pr’o.C. Preoral gut. Th. First indication of thyroid. Rec.opt. Optic recess.
Telcnc. Telencephalon. T.p. Tuherculum posterius. V.tr. Velum transversum.
Parts of the Fore—brain.
The Infundibulum.——Just ventral to the tuberculum posterius, a
small posteriorly directed evagination now appears lying slightly be
neath the anterior end of the notochord. It is the beginnirg of the in-
frmdibulum, the future posterior part of the pituitary (Fig. 184).
The Region of the Optic V esicles.——Ventral to the infundibulum,
but still on the posterior wall, is a thickening, the rudiment of the future
optic chiasma (not noticeable in Fig. 184) , while immediately ventral to
this thickening is a small evagination, the optic recess. From this recess
the hollow optic vesicles have grown out on either side, and as they have
grown their proximal parts have been constricted, as in the case of the
350 THE CHICK
Frog, to form the optic stalks. Below the optic recess, the posterior wall
begins to curve anteriorly onto the present ventral surface. This region
is relatively thin and is known as the lamina. zermirzaiis. Within it the
torus transversus is scarcely visible as yet.
The Cerebral Hem.ispheres.—Near the end of the second day the
sides of the fore-brain just dorsal to the lamina terminalis begin to push
out as the future cerebral hemispheres. Their cavities will be theylateral
ventricles opening into the cavity of the fore-brain or third ventricle,
through the foramina of Monro.
The Velum Transversum and Region of E piphysis. —— Beyond the region of the lamina terminalis on the antero—ventral side of the forehrain, we come to a portion of the wall which is slightly depressed. it is
known as the velum trcrnsversum. Further dorsal to this point on approx
imatcly the anterior surface may he found, also, the suggestion of an
outpushing; it marks the general region from which the epiphysis
(pineal gland) later (fourth day) arises. This brings us to the slight but
broad constriction mentioned above as indicating the location of the
future. posterior commissure, and the limit of the fore-brain.
The Divisions of the Fore—brain. ———As in the case of the Frog, it
is customary to divide the fore-brain into two parts, which with the aid
of the above lanclmarks may now be easily defined. That part of the
fore-brain which lies vcntro-anterior to a plane passing from the pos
,terior wall just ventral to the optic recess to the anterior wall slightly
anterior to the middle of the velum transversum is the telenceplzalon.
The remaining portion of the fore-brain, whose posterior limit is defined above, is then the diencephalon. The cerebral hemispheres arise
from the former.
THE MID—BRAIN OR MESENCEPHALON
The anterior boundary of the rnesencephalon ‘coincides with the posterior boundary of the diencephalon, marked by the broad constriction previously referred to. The posterior boundary may be defined as a
transverse plane passing from the postero-ventral wall or floor just
above and behind the tuberculum posterius, upward to about the middle
of another rather broad constriction on the antero-dorsal wall (Fig.
184). The roof of the mid-brain, moreover, is growing so rapidly in
connection with the cranial flexure, that it soon arches outward as the
most anterior region of the embryo. Other parts of the mesencephalon
have not appeared, and will, therefore, be described later as they
arise.
SECOND DAY: SPINAL CORD, NEURAL CRESTS 351
THE HIND-BRAIN OR RI-IOMBENCEPHALON
Its Extent. —The hind-brain lies entirely dorsal to the notochord,
and extends from the constriction marking the boundary of the midhrein posteriorly into the spinal cord. Its posterior boundary, as stated
above, can be defined only as that part opposite the fourth somite_ A5
in the case of the mid-brain, the parts of the hind-brain are not yet discernible, and will be indicated when they appear.
The Divisions of the Hind-brain.——The divisions of the hindbrain are also difficult to define at this early stage. We may say, however, that the anterior division is relatively short, and is known as the
nzretencephalon. The remainder of the brain constitutes the posterior di~
vision known as the myelencephalon. The cavity which extends through
both is called the fourth ventricle.
THE SPINAL CORD AND ETS NEIURAL CRESTS,
The Cord. —~As fast as the neural tube is formed by the fusion of
the neural folds, its central canal tends to become compressed laterally
and elongated dorso-ventrally. Its lateral walls also gradually thicken,
and at the end of the second day these walls consist chiefly of two sorts
of cells. First, there are elongated cells extending from the central canal out to its outer walls. These are the cells originally lining the canal,
now known as ependymal cells, and their function is that of support.
Secondly, among the ependymal cells and near the central canal are numerous rounded cells known as germinal cells. They later give rise to
neuroblasts or primitive nerve cells, and also probably to more supporting elements termed glia cells. It has recently been claimed (Barron,
’46) that some of the germinal (indifferent) cells are stimulated to become neuroblasts by contact with growing dendrites of other neuroblasts already partially differentiated.
The Neural Crests and Rudimentary Spinal Ganglia. — As indicated in the /previous chapter, the neural crests when first formed are
simply bands of cells which extend along the dorso-lateral walls of the
neural tube, on either side between it and the ectoderm. As was also
stated, these bands or crests are at first fused with one another dorsally.
By the end of the second day, however, in the older (i.e., anterior) portion of the tube, this dorsal fusion has been obliterated. In this region
there have also appeared in the crests successive enlargements, which
presently become separated from one another to form a series of rudimentary spindl ganglia. There is one of these ganglia for each somite,
352 THE CHICK
except for those of the head region, opposite whose somites the crests
disappear. The spinal ganglia at this time contain both neuroblasts and
indifferent cells.
THE CRANIAL GANGLIA
The neural crests of the head region anterior to the somites do not
disappear. but also form enlargements which separate and take part in
the formation of certain of the cranial ganglia. Parts of these ganglia,
however, are placodal in origin, and surprisingly, according to some
authors some of them even contain endodermal elements as indicated
below. By the end of the second day the ganglionic rudiments are visible, beginning at the anterior end, in the following positions:
The V Nerve Gang1ion.—~The ganglion for the V or trigeminal
nerve is somewhat anterior to the dorsal end of the first or mandibular
Ztl‘t".l1. At the end of the second day it usually appears merely as at Clark
patch in this region (Fig. 176), but later (see third day) it acquires
distimxtly the form of an inverted Y. Apparently most, or all, of this
ganglion is derived from crest material tYntema, 314-) .
The VII and VIII Nerve Ganglia. —-The ganglia for these
nerves form a single mass, the acustico-faciallis ganglion. It lies at this
time just antero-ventral to the auditory sac (see below) ; i.e._, it is above
and slightly in front of the dorsal end of the second or hyoid an-h.
Though unlabeled, it is shown in Figure 176 in the position indicated.
Jones, ’-42 has claimed that part of the VII ganglion is derived from the
dorsal wall of the first visceral pouch, an unusual source of nerve tissue
since the pouch is of course endoderm. Later study (Yntema, 7&4‘), however, seems to show that the origin is, as might be expected, partly crest
and partly placode. The geniculate portion is thought to come from the
placode, which, though closely associated with a pouch, is definitely not
part of it, while the remainder of the facial nerve ganglionic complex is
from the crest. The VIII ganglion appears to be entirely placodal.
The IX and X Nerve Gang1ia.—The IX and X nerve ganglia
arise together, but at the end of the second day they begin to become
separated. The former, or glossopharyngeal ganglion, is then situated
above the dorsal end of the third visceral arch while the latter, or vagus
ganglion, lies above the ends of the fourth and fifth visceral arches.
These ganglia are not visible in Figure 176. As to their sources, it appears that both contain some crest material, while it has again been
claimed by both Winiwarter, ’39 and Jones, ’42 that material for the
L
SECOND DAY: THE EYE 353
petrosal portion of IX and the jugulare part of X are from't'he second
and third visceral pouches respectively. It seems most probable, however, that, as in the case of the VII nerve ganglion, difficulty in separating the ectodermal and endodermal elements has led to error and that
only “ adjacent ectoderm,” i.e., placode, is involved. A diagram of the
location and form of the cranial gan- '
glia viewed from above early on the
second day is given in Figure 185.
ORGANS OF SPECIAL SENSE
THE EYE
The Optic Stalks, the Uptic Cup
and the Choroid Fissure. —The
optic vesicles, it will be recalled, are
hollow out-pushings from the forehrain with which they remain connected by constricted regions known
as the optic stalks (Fig. 186). These
stalks are the so-called “ optic nerves,”
though as will appear, the real optic
nerves develop later. It is to be noted
that the above constriction has occurred in such a manner that each
stalk connects with its vesicle near the
ventral side of the latter, rather than
at its center. Invagination of the outer
wall of the vesicle now occurs, oblit
Fig. 185. — Diagram of the cephal_ . ic neural crest of a chick of about
crating its original cavity, and con» 12 somiles From Lillie (Develop
ment of the Chick). After Wilhelm
vetting it into the two-layered optic His at Auditory Sm 3. 50mm,"
cup, with the optic stalk attached to
its ventral edge. The walls of the cup on either side of the point where the
stalk is attached now grow outward, i.e., toward the ectoderm, but their
ventral edges do not quite meet one another. Thus a fissure is left in the
ventral side of the cup extending from its edge inward to the optic
stalk. This, as in the Frog, is the choroid fissure. Meanwhile the rim of
the cup bounding its aperture, the pupil,’ becomes slightly constricted.
The invaginated or outer wall of the vesicle has now necessarily become
354 THE CHICK
the inner wall of the cup, and will, therefore, be referred to as the inner
wall in future discussion. It is the rudiment of the nervous layer of the
retina (see Chapter ll).
The Development of the Lens. —Before the above invagination
of each optic vesicle occurred, the vesicle had pushed out far enough to
‘touch the surface ectoderm. When this happened, the ectoderm at the
point of contact began to thicken, and when the invagination of the vesicle took place, this thickened ectodermal wall also invaginated. Thus
a hollow thick-walled sac was formed resting just within the rim of the
Fig. 186. — Diagrams of sections through the eye of the Chick embryo at the end
of the second day. From Kellicott (Chordate Development). After Lillie. The dorsal
margin is toward the top of the page in A and B. A. Eye as viewed directly. B. Vertical section through the line x—cf, in A. C. Horizontal section through the line y—y in A.
cf. Choroid fissure. co. Cavity of primary optic vesicle. ec. Superficial ectoderm
of head. i. Inner or nervous layer of the retina. l. Lens. 0. Outer or pigmented layer
of optic cup. 01. Opening of lens sac from surface of head. pc. Posterior (vitreous)
chamber of eye.-s. Optic stalk, continuous with the floor and lateral wall of the
diencephalon.
optic cup. This is, of course, the rudiment of the lens; at the end of the
second day it has not quite detached itself from the outer ectoderm.
As in the case of the Amphibian, it has been shown that the optic cup
has the power to induce lens formation in ectoderm which would not
otherwise form it. Thus optic vesicles or cups from embryos up to the
4.0-somite stage (fourth day) will induce lenses when transplanted to
young hosts (primitive streak to’ eight somites). In a host older than
four somites, however, the transplant will produce positive results only
when implanted as far anterior as the potential head or neck region. In
any case actual contact of the cup with the ectoderm seems necessary to
effect induction. Also as in the Amphibian, the new lens may come from '
cells of the optic cup itself as well as from the host ectoderm (Alexander, ’37) , and the inductive process is a gradual one (McKeehan, ’54).
3
3
l
5.
l
E
.oa.......« ~ e
.-
SECOND DAY: THE EXCRETORY SYSTEM 355
THE EAR
The sensory part of the ear begins as a thickening of the ectoderm on
the side of the head above and slightly posterior to the dorsal end of
the hyoid arch. This thickening presently starts to invaginate, thus forming a depression -— the auditory pit. During the second day the process
of invagination continues, and is soon accompanied by an approximation of the anterior and posterior lips of the pit. Near the end of the
second day the ventral lip also takes part in the closure by moving dorsally, and thus the pit is transformed into a small mouthed sac. It is the
auditory sac or otocyst (Fig. 176).
THE URINOGENITAL SYSTEM
Because of their close connection in the adult, the excretory and reproductive systems are, as usual, considered under a common heading.
Their development, however, is largely separate, and must, therefore,
he so treated. Of the two systems, only certain parts of the excretory
appear during the second day.
THE EXCRETORY SYSTEM
The excretory system of the Chick in common v",'.Li that of other Amniota consists of three separate parts, the pronephros, mesonephros, and
metanephros. These parts develop in the order named, and the first two
have largely disappeared by the close of embryonic life; only the last
remains functional as the permanent excretory organ of the adult. During the second day the pronephros develops, and near its close the mesonephros has just begun to appear.
The Pronephros.——The pronephros is vestigial in character, and
only appears typically from the tenth to the fifteenth somites. Rudiments of it, however, are sometimes found as far forward as the fifth
somite. In the more posterior region indicated, its development is as
follows:
The Pronephric Tubules. —— In the dorso-lateral portion of the nephrotome opposite the posterior end of each somite a thickening occurs,
and from it a cord of cells grows outward and upward for a short distance (Fig. 187, pr’n. 1). At the same time the nephrotome becomes
detached from the somite. These lateral outgrowths are termed the pronephric tubules, though they usually do not acquire any lumen. Some356 THE CHICK
times, however, a slight lumen is present in the proximal part of the
tubule (Fig. 187, pr’n. 2), and it opens into the coelom as a rudimentary nephrostome. It is also said that degenerate glonwrztli (or more
properly glomi) sometimes develop later on the coelomic wall opposite
the nephrostomal mouths (Lillie).
The Pronephric and W olfiian Ducts. — The distal part of each of the
above cell cards or “ tubules ” presently bends posteriorly and grows in
Fig. 187.-——A. Transverse section through the twelfth somite of a 16s embryo.
From Lillie (Development of the Chick). B. Three sections behind A to show the
nephmstoine of the same pronephric tubule.
A0. Aorta. CC. Central canal. Coel. Coelom. E.E.B.C'. Extra-emllryonic coelom
Iexocoelnm 1. .lIs'c/L. Mesenchyme. N’c}1. Notoclmrd. n.Cr. Neural crest. .’V’.st Nephrostome. n.T. Neural tube. pr'n. 1,2. Distal and proximal divisions of pronephric tubule.
$.12. Twelfth sornite. Sa’pl. Somatopleure. Spl’pl. Splanchnopleure. V.c.p. Posterior
cardinal vein.
this direction until it comes in contact with the tubule following it. In
this manner, a continuous backwardly directed cord of cells is formed
which connects with each successive tubule. Finally, the bent portion of
the last cell cord continues to grow posteriorly between the nephrotomal mass and the body wall. As will appear subsequently, the anterior
end of this backward growing rod of cells is the rudiment of the pronephric duct. and its more posterior portion, the rudiment of the mesonephric or Wolflian duct. Before the end of the second day, indeed, the
anterior or pronephric section of the rod has acquired a lumen, thus becoming a real duct.
SECOND DAY: THE AMNION 357
The Mesonephros. —— The mesonephros corresponds to the organ of
the same name which functions as the permanent excretory organ of the
Frog. In the Chick, however, as indicated above, this excretory function
continues only during a part of embryonic life. The antericr end of the
inesonephros slightly overlaps the posterior end_of the pronephric region, but its development here is rudimentary, the organ acquiring its
typical form only from the twentieth to the thirtieth somites. During the
close of the second day it begins to appear in the following manner,
development progressing posteriorly.
The Primary Mesonephric Tu‘ou1es.——The nephrotome in the
region indicated becomes separated both from the somites and the lateral plate. It then lies just ventro-medially to the rod of cells which is
to become the Woliiian duct. Above this duct the posterior cardinal vein
presently appears, while between the nephrotome and the median line
of the embryo runs the dorsal aorta. The nephrotorne is thus between
the aorta and the future Woliiian duct (Fig. 174«). Presently in the
neighborhood of each somite, there appear in this nephrotomal band
two or more spherical condensations. Then beginning at the anterior
end of the band each of these condensed spheres starts to acquire a cavity, each vesicle thus formed being the rudiment of a mesonephric tubule
and a Malpighian. body. The more ventral spheres in each somite are
the first thus to become vesicular, and they are the rudiments of the socalled primary mesonephric tubules as distinguished from the others.
(See next chapter, Fig. 207.)
T AMNION AND OTHER EXTRA-EMBRYONIC STRUCTURES
From the embryological point of view all Vertebrates belong to one
of two classes; i.e., the Anamniota or the Amniota. The former group
includes Amphibians and Fishes, while the latter includes Reptiles,
Birds, and Mammals. The Amniota. as the name implies. are those
which possess an amnion, while the Anamniota are those which lack it.
Amphioxus, the Frog, and Fish have been studied as representatives of
the latter class, and we are now studying the Chick as an example of
the former or Amniote group. The amnion begins to form on the second day of the Chick’s incubation, but is not completed until about the
fourth day. In order to make the structure of this organ more clear, however, it seems best to describe its entire development, together withthat
of certain other extra-embryonic organs and membranes.
358 . THE CHICK
THE AMNION IN PROCESS OF DEVELOPMENT
Development during the Second Day. —- During the second day
a fold in the blastoderm occurs just in front of the head of the embryo
in the region of the proamnion. Since there is as yet no mesoderm in
this region, the fold at first contains only ectoderm and endoderm. Presently, however, the mesoderm extends into this vicinity, and here, as
elsewhere, is split into the extra-embryonic extensions of the somatic
and splanchnic layers
with the extra-embryonic
coelomic space between
them; both these layers
then become involved in
the fold. The splanchnic
layer together with the
endoderm, however, is
soon withdrawn to the
surface of the yolk, while
t:,§t;§it:,2;?§;8t:;::‘:t..  ;;:a:::‘; the some layer  me
the yolk (,yolk-stilk uénbilicxislk in a ChicFlc of extra-embryonic ectoderm
3 -' ' ‘r. . ' . .
fi‘2‘;:,;:,:"zm;:,:;:e:,mi: 9.32:: winch  -r
a Dorsal aorta. c. Coelom. ebcxexocoelom. ig. In- the two permanent layers
testinal groove. la. Lateral folds of amnion. ‘UYJ.
Vitemne vein of the amniotic head fold.
The embryo has now begun to sink somewhat into the surface of the yolk, and as it does so the
amniotic fold gradually grows back over it. This backward growth is
also accompanied by the development of lateral amniotic folds extending posteriorly on either side. By the end of the second day the embryo
has been covered over in this manner almost as far back as the vitelline
arteries (Figs. 176 and 188). The latter figure shows a cross section
through a region where the folds have not yet quite covered the embryo.
Development during the Third Day. —— About the end of the
second day, or the beginning of the third, another fold appears at the
posterior end of the embryo, and grows forward toward the head fold.
This is the amniotic tail folcl, which soon becomes coextensive upon either side with the posterior ends of the lateral amniotic folds. It is similar to the corresponding head fold except that from the first it contains
only ectoderm and somatic mesoderm. Since the anterior portion of the
amnion starts earlier and grows rapidly, the point at which the converg
l
l
~ SECOND DAY: THE COMPLETED AMNION 359
e is quite near the posterior end of the
ing folds finally meet and {us
sting above the Chick previous to the
animal. The oval opening exi
is the amniotic umbilicus.
Fourth Day. —The end of the third,
or beginning of the fourth day, marks the meeting and fusion of the am
niotic folds at the center of the amniotic umbilicus. The embryo has by D
' ‘this time turned upon its left side throughout the greater part of its
closure
Development during the
ith 35 pairs of somites (about
-third somite. From Kellicott
ransverse section of Chick embryo w
hrough the region of the twenty
(Chordate Developm . .
Dorsal aorta. c. Embryonic coelom. ch. Chorion.
Fig. 189. —-T
72 hours), passing t
a. Amnion. ac. Amniotic cavity. ao.
d. Derrnatome. ebc. exocoelom. g. Rudiment of spinal ganglion. m. Mesonephric
tubule. my. Myotome. p. Posterior cardinal vein. 5. Sclerotome. sa. Sero-amniotic
connection. so. Subcardinal vein. so. Somatic mesoderm. sp. Splanchnic mesoderm.
12. Vitelline artery. W. Wolflian duct.
ds do not turn with it, the closure occurs
not above itsback, but above its right side. It also follows from this,
that the fold of the left side covers the hack of the embryo as well as a
part of the right side. The amnion may now be said to be complete.
_ THE COMPLETED AMNION AND RELATED PARTS
The Amnion and Amniotic Cavity. ——- It is obvious that the amniotic folds, like any other folds, must be composed of two main parts,
ther at the crest of the fold. It is
each part being continuous with the 0
also obvious that one of these parts, i.e., the inner or lower one, lies
everywhere next to the embryo. When fusion occurs, therefore, this inner
length, and inasmuch as the fol
360 THE CHICK .
part will become continuous, completely bounding a new cavity which
surrounds the embryo at every point except for a restricted region on its
ventral side (see below under somatic umbilicus). This continuous inner membrane is the amnion, and the cavity thus formed is the amniotic
cavity. Moreover, inasmuch as the folds involve both ectoderm and
mesoderm, the inner membrane or amnion must likewise consist of ectoderm and mesoderm, the former lining the amniotic cavity and the latter‘ '
forming a coat outside the lining (Figs. 189 and 190).
The Chorion.— At the fusion of the folds the outer part, like the
inner, necessarily becomes continuous. Likewise, it too consists of both
ectoderm and mesoderm, but in this case, the ectoderm will lie outside
and the mesoderm inside, i.e., toward the amnion. The outer membrane
thus formed is called the chorion, serosa or false amnion. Between it and
the inner membrane or true amnion, there is naturally the same space
which separated the inner and outer parts of the amniotic folds, i.e., the
extra-embryonic coelom or exocoelom. This relationship will be made
clear by reference to Figure 190. it may be mentioned incidentally in
this connection that this exocoelomic space eventually becomes filled by
an important sac-like organ (allantois) whose origin and structure will
be described below.
The Sero-Amniotic Connection.———It has been implied that the
extra-embryonic coelom, with whatever may occupy it, everywhere separates the amniotic membrane from the chorionic membrane. This is
true except at one point. At the point of final fusion of the amniotic
folds, i.e., the amniotic umbilicus, the coelomic space is interrupted by
a small area of mesoderm which persists, and serves to unite the above
membranes. It is called the sero-amniotic connection. (Figs. 189 and
190).
The Amniotic F luid.——Shortly after the completion of the amniotic cavity, fluid begins to accumulate within it. Thus the embryo is
soon practically surrounded by a liquid cushion which protects it from
pressure by its membranes and rigid shell. This is the amniotic
Presently, about the fifth day, muscle fibers develop in the mesoderm
of the amnion and begin to send waves of contraction over it. This
causes a gentle rocking of the embryo, and is apparently instrumental in
preventing its adhesion to the various embryonic membranes. It may
also help to obviate the stagnation of blood in the vessels, a condition
which might tend to occur on account of the pressure from the growing
organs.
SECOND DAY: THE COMPLETED AMNION 361
All. Am. char.
5. am. 7
   
Figs. 190, 191, 192.——Diagrams of the" relations of
the extra~embryonic membranes in the Chick. Figures
and description from Lillie (Development of the
Chick). The ectoderm and endoderm are represented
by plain lines; the mesoderm by a cross-hatched line
or band. The yolk-sac is represented by broken parallel lines. In Fig. 190 the allantois is represented as a
sac. In Figs. 191 and 192, where it is supposed Ito be
seen in section. its cavity is represented by unbroken
parallel lines. The stalk of the allantois is exaggerated
in all the diagrams to bring out its connection with
the embryo.
Fig. 190. —Fourth day of incubation. The embryo is
surrounded by the amnion which arises from the somatic umbilicus, Umb., in front and behind: the seroamniotic connection, S.am., is represented above the
tail of the embryo; it consists at this time of a fusion
of the ectoderm of the amnion and chorion. The allantois, AIL, is represented as a sac, the stalk of which
enters the umbilicus behind the yolk-stalk; the allantois lies in the extra-embryonic body-cavity (exocoelom) , and its mesodermal layer is fused with the
corresponding layer "of the chorion above the embryo.
The septa of the yolk-sac, Y.S.S., are represented at
an early stage. The splitting of the mesoderm has progressed beyond the equator of the yolk-sac, and the
undivided portion is slightly thickened to form the
beginning of the connective-tissue ring that'surrounds
the yolk-sac umbilicus. The ectoderm and endoderm
meet in the zone of junction, beyond which the ectoderm is continued a short distance. The vitelline membrane, V.M., is ruptured, but still covers the yolk in
the neighborhood of the yolk-sac umbilicus. The albumen is not represented in this figure. (For complete
explanation of lettering see Fig. 192.)
362 THE CHICK
THE SOMATIC UMBILICUS, THE YOLK—STALK, AND THE
YOLK——SAC
Though they are not a part of the amnion, it seems best to include in
connection with its description an account of these structures which, to
some extent, develop with it. 4
The Somatic Umbilicus. ——During the formation of the amnion,
the gradual separation of the embryo from the yolk has been progressing. This has been accomplished by the steady in-pushing of the ventral
portions of the head, tail, and lateral folds (limiting sulci} beneath the
body of the growing Chick. The result is that by the time the amnion
is completed, these folds have approached one another quite closely,
though without coming into contact. In this manner they give rise to a
short, thick, hollow stalk which connects the embryo with the yolk-sac
and its extra-embryonic membranes. The outermost wall of this stalk is
continuous with that of the amnion, and is, therefore, composed of ectoderm and somatic mesoderm: for this reason, this outer wall is referred
to as the somatic umbilicus (Fig. 190).
The Yolk-Stalk. —— Within this wall and surrounding the inner wall
of the stalk, is a space continuous externally with the extra-embryonic
coelom and internally with the coelom of the embryo itself. Finally, the
inner wall of the stalk consists of splanchnic mesoderm and endoderm.
It is known as the yolk-stalk, but is really merely an inner tube of the
somatic umbilicus separated from it by coelomic space.
The Yolk-Sac.——The wall of the yolk-stalk is coextensive within
the embryo with the wall of the gut, and externally with the layer of
endoderm and the splanchnic mesoderm which overlies the yolk. This
layer is continually growing out around the yolk, and at its outermost
border, i.e., the region of the zone of junction, the endodermal portion
of it becomes continuous with the chorion which overlies it. Thus by
means of the extension of these layers the yolk is gradually enclosed in
a covering, whose inner layer of splanchnic mesoderm and endoderm
constitutes the yolk-sac, attached to the embryo by means of the yolkstalk. Upon the ninth day of incubation this sac has become virtually
complete, save at a point on the side of the yolk postero-ventral to the
body of the Chick, where an opening remains, known as the yolk-sac
umbilicus. This opening, however, is finally closed about the seventeenth day by a solid mass of tissue. It may be recalled in this connection that the rim of the blastoderm, which has thus overgrown the yolk,
was previously homologized with the lip of a very extended blastopore,
4 .. .a_...m.,,.,,,_.,.j,
SECOND DAY: THE ALLANTOIS 353
the true blastopore (primil".Ve Streak) haVing been separated from the
remainder of the rim during gastrulation. Hence upon this basis it is
possible to consider the uncovered yolk mass as a sort of very large
secondary, or yolk-Lla.stopare, the latter term being really only another
name for the yolk-sac umbilicus. A somewhat similar separate blastepore, it may be noted, also occurs in the development of the Elasmobranchs (i.e., the cartilaginous or non-bony fishes) in which the term
yolk-blastopore is regularly applied to it.
On the basis of this description, it is clear that beyond the boundaries
of the amnion the chorion is really nothing more than the uppermost
layer of the blastoderm. It is to be noted, however, that this upper
layer consisting of ectoderm and somatic mesoderrn is soon separated
from the lower layer composed of splanchnic mesoderm and endoderm
by the extra-embryonic coelom. Furthermore, this space presently becomes occupied by another extra-embryonic organ (allantois) , to be described below. Finally it must also be mentioned that early in its development, the lower layer, just indicated, ie, the real yolk-sac layer,
consisting of endoderm and splanchnic mesoderm, becomes covered internally with deep folds, the yolk-sac septa, which gradually press downward into the yolk. These septa in common with the remainder of the
yolk-sac endoderm in the area vasculosa, contain glandular and absorbing cells which digest the yolk in situ before passing it into the
blood vessels. Thus though a slight lumen exists in the yolk-stalk connecting the inside of the yolk-sac with the enteric canal, no yolk appears to pass into the embryo through this lumen. Abnorrnally high or
low temperatures during incubation, e.g., 39.5° C and 3-15° C, appear
to slow up the process of absorption of both yolk and albumen (Romanofl', ’43) .
THE ALLANTOIS
Another extremely important extra-embryonic organ possessed in
some degree by all Amniota is the allantois, and it will be found convenient to consider its entire history also at this time.
Its Early Development. ——The allantois starts in the form of an
out-pushing from the ventral wall of the hind-gut (Fig. 193). This is
scarcely visible before the beginning of the third day, and was, therefore, not referred to in the foregoing description of the alimentary tract.
This out-pushing naturally involves the endoderm and the mesodermal
ventral mesentery which occurs in this region. Thus the sac which is '
presently formed possesses an inner endodermal and an outer mesoder364 THE CHICK
mal layer. By the fourth day the allantois has pushed out through the
coelomic space between the somatic umbilicus and the yolk-stalk, and
is beginning to spread out in the extra-embryonic coelom (Fig. 190}.
The narrow neck of the organ which then connects the outer sac-like
Fig. 191.—Ninth day of incubation. The yolk-sac um
, bilicus has become much narrowed; it is surrounded by
the mesodermal connective-tissue ring, C.T.R., and by the
free edges of the ectoderm and endoderm. The vitelline
membrarie still covers the yolk-sac umbilicus and is
folded into the albumen. The allantois has expanded
around the amnion and yolk-sac and its outer wall is
fused with the chorion. It has pushed a fold of the
chorion over the sero-amniotic connection, into which the
mesoderm has penetrated, and thus forms the upper fold
of the albumen-sac. The lower fold of the albumen-sac
is likewise formed by a duplication of the chorion and
allantois; it must be understood that lateral folds are
forming also. so that the albumen is being surrounded
from all sides. The stalk or neck of the allantois is exaggerated so as to show its connection with the embryo;
it is supposed to pass over the amnion, and not. of course,
through the cavity of the latter. (For explanation of lettering see Fig. 192.)
portion with the gut is known as the allantoic stalk or neck. Along this
stalk pass the two allantoic arteries (later only one), and the single allantoic vein, ‘to end in abundant ramifications over the surface of the
sac. The allantois now grows rapidly, and within a couple of days has
entirely covered the amnion, occupying the space between that organ
and the chorion. Presently the amniotic and chorionic mesoderm fuse,
forming the chorio-allantoic membrane (Figs. 191 and 192). In this
;
3
i
SECOND DAY: THE ALLANTOIS T 365
manner, the above ramifications of the blood vessels are brought very
near to the shell, through which an exchange of gases is possible. Thus
the allantois serves as an organ of respiration for the Chick during embryonic life. Its cavity also acts as a receptacle for the waste products of
gm. 3. Am.
   
All. 5. En:
5-"W AILC.
Chor. »(
Am.
Fig. 192.—Twelfth day of incubation. The conditions
are more advanced than those represented in Fig. 191.
The albumen-sac is closing; its connection with the cavity of the amnion by way of the sero-amniotic connection
will be obvious. The inner wall of the allantois has fused
extensively with the amnion. The umbilicus of the yolksac is much reduced, and some yolk protrudes into the
albumen (sac of the yolk-sac umbilicus, transitory structure soon drawn into the yoll-:-sac proper).
Alb. Albumen. Alb.S. Albumen-sac. .411. Allantois.
AIL]. Inner wall of allantois. /1ll.C. Allantoic cavity.
AZLS. Allantoic stalk or neck. All. + Am. Fusion of allantois and amnion. Am. Amnion. Am.C. Amniotic cavity. Chor. Chorion. C.T.R. Connective--ti.-rsue ring. Eat.
Ectoderm. E.E.B.C. Exococlom (extra-embryonic bodycavity). Ent. Endoderm. Mes. Mesoderm. S.-Am. Sero-amniotic connection. S.Y.S.U. Sac of the yolk-sac umbilicus. Umb. Umbilicus. (somatic). V./ll. Vitelline
membrane. Y.S. Yolk-sac. Y.S.S. Septa of yolk-sac.
metabolism, which are conveyed thither through the allantoic stalk from
the region of the cloaca. lt is thus to be noted that this organ is homologous not only in method of origin, but also partly in “function with the
urinary bladder of the Frog. The latter, however, of course never extends outside of the coelomic cavity, and though it may or may not
be endodermal, the allantois is certainly so.
366 THE CHICK
The Later Development of the Allantois and the Formation
of the A1bumen-Sac. — Meanwhile the albumen is becoming concentrated on the side of the egg next to the yolk-sac urnbilicus, and by the
ninth or tenth day has become very much condensed. Concurrently
the real yolk-sac layer, together with the chorion, has grown around the
yolk so that the edges of the over-growth have more than kept in con
tactfiwith the receding albumen. They have in fact thrust themselves in
between it and the yolk, so that the albumen is bounded upon its inner
side by a layer of chorion. At the same time, save postero-dorsally in
the region of the sero-amniotic connection, the allantois has been following this overgrowth of .the yolk-sac layer and chorion; it lies between
these two layers in the exocoelom, and its walls are fused respectively
with the chorionic layer and that of the yolk-sac. Thus as the latter layers push in between the yolk and the albumen to close the yolk-sac umbilicus, they are accompanied, except postero-dorsally, by the allantois.
Ventro-laterally a fold of the chorion presently pushes its way around
the outside of the albumen between it and the shell membrane. Here too,
moreover, between the two layers of the chorionic fold there follows an
outer fold of the allantois. Meanwhile in the postero-dorsal region, as already suggested, the expansion of this organ is obstructed by the seroamniotic connection. At this point, therefore, it pushes up over this connection, carrying the chorion before ‘it. Thus this dorsal fold, consisting
of a layer of chorion and allantoic wall, comes down between the albumen and shell membrane to meet the similarly constituted ventrolateral folds already described. Hence, at ten days the albumen at the
yolk-sac umbilicus is surrounded by a double layer of fused chorionic
and allantoic tissue, the albumen-sac. There is just one region in the
wall of the sac, however, where all of these layers are not present. This ,
is a small area on its internal dorsal side where the allantois could not
extend because of the sero-amniotic connection. There, therefore, the wall
consists only of chorion, and at one point of the connection itself (Figs.
191,192). A perforation appears in this connection, and on the twelfth ‘
day some albumen enters the amniotic cavity. The remainder of the
albumen is absorbed, and the albumen-sac together with the yolk-sac is
drawn within the embryo just previous to hatching. According to Randles A
and Romanoff, ’50, a periodic turning of the egg is necessary if all these
events are to be accomplished normally at the times indicated. Hatching
is apparently aided by the contraction of the muscular walls of the allantois and by the muscles of the somatic umbilicus (see also Fig. 193).
l
i
i
l
l
“"
SECOND DAY: SUMMARY . 367
SUMMARY OF THE CONDITION AT THE END OF THE
SECOND DAY OF INCUBATION '
I. GENERAL APPEARANCE
The cranial flexure has been initiated, and has brought the fore-brain
to a point where it almost touches the heart, and the mid-brain faces anteriorly. The cervical flexure is also evident in the region of hind-brain
and trunk. In correlation with these flexures lateral rotation has started
so that the embryo lies on its side as far back as the 13th somite.
II. THE SOMITES
There are approximately 27 somites, in which the myotomes and
cutis plates have begun to differentiate, together with the mesenchymatous rudiment of the selerotome.
III. THE FORE-GUT
In the fore-gut the stomodaeum is formed, and in connection with it
Rathke’s pocket, a part of the future hypophysis, is beginning to appear.
Four pairs of visceral pouches and five pairs of arches have begun to develop, and the first pair of pouches have acquired openings to the exterior. The -rudiments of the thyroid, the respiratory system, and the liver
are also present.
IV. THE MID—GUT
This is but slightly developed, although the lateral folds are beginning to mark it off from the extra-embryonic archenteron.
V. THE HIND—CUT
The hind-gut has begun to form and its posterior end has fused with
the ectoderm to form the anal plate or cloacal membrane. In connection
with it there has also arisen the ventral mesentery.
VI. THE CIRCULATORY SYSTEM
The Heart.—-A bent tubular heart has been developed, lined by
endothelium and covered with a myocardium. The regions of the atria,
the ventricles, and the bulbus and trztncus arteriosus are indicated, and
pulsation has been initiated.
368 THE CHICK
The Arteries. —The dorsal aortae are in evidence. Also the ventral
aorta has appeared and become incorporated into the truncus. The first
pair of aortic arches are in process of formation, and the second and
third aortic arches are completed. The vitelline arteries have appeared.
The Veins.——The anterior and posterior cardinals, the sinus venosus, the (luctus venosus, and the ducts of Cuvier have been developed.
In connection with the latter the septa known as the lateral mesocardia
Fig. 193.——Median sagittal section through posterior end of four-day chick. From Kellicott (Chordate Development). After Gasser (Maurer).
al. Allantois. am. Amnion (tail-fold). c. Cloaca.
rn. Cloacal membrane. 11. Notochord. r. Rectum. s.
Spinal cord. y. Wall of yolk-sac (endoderm and
splanchnic mesoderml.
have also been formed. Outside the embryo the anterior vitelline veins
have arisen, and with them the rudiments of the lateral vitelline veins.
The sinus terminalis has become complete.
VII. THE NERVOUS SYSTEM
The Brain and the Cranial Ganglia.-—As indicated under external appearance the cranial and cervical flexures have become well
marked. The fore-brain, mid-brain and hind-brain are now clearly indicated, and within the first main division certain parts are apparent, as
follows: The outgrowth of the optic stalks is well advanced, and there
may also be evident the rudiments of the optic chiasma, the optic recess,‘
the cerebral lzernispheres, the in fundibulum, and some other minor struc‘tunes. The roof of the mid-brain is becoming prominently arched.
SECOND DAY: SUMMARY 369
The cranial ganglionic rudiments of the V, VII and Vlll, and IX and
X nerves are visible, and the latter pair are beginning to separate.
The Spinal Cord and Ganglia. —— The spinal cord has become
thick-walled laterally, and has developed ependymal and germinal cells.
The neural crests are segmenting to form the spinal ganglia.
VIII. THE ORGANS OF SPECIALSENSE
The optic vesicles have become invaginated to form the optic cups,
and the external ectoderm opposite each cup has invaginated in the
process of forming a lens. In connection with the ear, the auditory portion of the ectoderm has become invaginated to form the auditory sac.
IX. THE URINOGENITAL SYSTEM
Only the embryonic parts of the excretory portion of this system appear during the second day. These are the pronephros, including the
Wolflian duct, and the rudiments of the mesonephros. These rudiments
consist of concentrations of nephrogenous tissue, some of which are beginning to become vesicular in the formation of the mesonephric tubules
and the Malpighian bodies.
X. THE AMNION
This extra-embryonic organ begins its development on the second day
with the appearance of the amniotic head fold, the amniotic lateral
folds, and sometimes an indication of the amniotic tail fold.
The complete development of the amnion, the chorion, the allantois,
and the yolk-sac is described in this chapter.
TI
HE CHICK: DEVELOPMENT DURING THE THIRD
DAY OF INCUBATION
GENERAL APPEARANCE
FLEXURES AND TORSION
THE embryo has of course increased somewhat in size, but the
most obvious changes concern the flexures. The cranial flexure is somewhat more marked, while the cervical flexure has greatly increased, so
that the region of the hind-brain, rather than the mid-brain is now the
most anterior part of the embryo. By the close of this day also a new
curvature has become evident at the posterior end. It involves mainly
the tail, and is called the caudal flexure. Between this flexure and the
cervical flexure the back of the embryo is temporarily somewhat bent
in a ventral direction, i.e., opposite to the other curvatures. This is because of the broad attachment to the yolk which still extends throughout
the middle region and tends to draw this part of the embryo ventrad
(Fig. 200). Accompanying these increases in flexure the lateral rotation has progressed posteriorly until by the end of the day the embryo
is on its side about as far back as the twenty-first somite.
LIMB 'RUDS
The limb buds become clearly visible by the end of the third day,
and appear as broad swellings on either side of the embryo. The anterior buds extend from about the fifteenth to the twentieth somite, and
the- posterior buds from about the twenty-seventh to the thirty-third
soniite.
THE SOMITES
During the third day the number of pairs of somites increases to about
36. The newer posterior somites when first formed are in the same condition as were those which are now anterior, and are destined to go
THIRD DAY: THE FORE—GUT 371
through the same process of development. Meanwhile, the more advanced anterior members of the series do not greatly change except for
further modifications along the lines already indicated on the second
day. These modifications are as follows:
Each myotome or muscle plate continues to grow down along the inside ot its respective cutis plate, until in the most mature somites it
reaches the ventral end of the cutis plate and fuses with it. In this manner a complete double layer of cells arises. In the inner layer or muscle
plate thus formed, the cells or rnyoblasts presently begin to become
spindle-shaped, reaching from the anterior to the posterior walls of each
myotome. These are mostly rudiments of dorsal voluntary muscles.
Somewhat later on the third day the outer or cutis plates of somites
which have reached this stage begin to break up into mesenchyme, which
wanders outward toward the ectodermal wall. There it eventually gives
rise to the dermis of the dorsal region, that of the lateral and ventral
parts being derived from the adjacent somatopleurc (Murray, ’28) .
The sclerotomal mesenchyme continues to collect about the notochord
and the sides of the nerve cord.
THE ALIMENTARY TRACT
THE FORE—GUT
The Oral Cavity.———During the third day, the oral plate breaks
through, placing the stomodaeum in direct communication with the
pharynx (Fig. 204-). The region in which the digestive tract opens to
the exterior anteriorly is thus partly stomodaeal and partly pharyngeal.
It is called the oral cavity.
The Hypophysis or Pituitary Body.——It will be recalled that
at 24 hours a hollow diverticulum called Rathke’s pocket was extending
forward from the roof of the stomodaeum toward the floor of the dien_cephalo_n in the vicinity of the infundibulum. At about the 30-somite
stage it has nearly reached the latter organ (Fig. 204), and shortly its
end begins to broaden out and become branched. Finally, near the end
of the incubation period, these branches have become a mass of tubular
tissue well supplied with blood vessels. This glandular mass then loses
all connection with the oral epithelium from which it arose, and be«
comesrfirmly attached to the infundibulum. In this manner the original
Rathl-:e’s pocket comes to constitute the anterior part of the hypophysis
or pituitary body, while the infundibulum becomes the posterior part
372 THE CHICK
and stalk of that organ. Experimental work has shown that the out.
growth of Rathl<e’s pocket is originally induced by the presence of the
infundibulum, and that both structures influence one another in the normal development of the completed organ (Hillemann, ’4-3). It may be
recalled that this same relationship is true in the Frog, except that there
the homologue of Rathke’s pocket is merely a strand of cells.
 
 
   
v.C.d.1 v,P...2, v.C.d.2
;-"v_’.;,~1_‘ M‘ __« v.P.3
:_prfo.G.
buss. ‘
     
     
 
 
tar. - tr. Gr. _
Ls  ,
Fig. 194.—Reconstruction of the fore-gut of a Chick of 72 hours. From
Lillie (Development of the Chick). After Kastschenko.
Hyp. Rathke’s pocket, rudiment of anterior hypophysis. Iar.-tr.Cr. Laryngotracheal groove._ Lg. Lung. Md.a. Mandibular arch. Oes. Oesopliagus.
pr’o.G. Preoral gut. Stom. Stomach. Th. Thyroid. v.C.d. 1, 2. Dorsal division of the first and second visceral clefts. v,C.i:.2. Ventral division of the
second visceral cleft. 1.2.1’. 1,2,3,4-. First, second, third, and fourth visceral
pouches.
The Visceral Pouches and Arches.
The P0uch.es.———lt will be remembered thatduring the second day
four pairs of visceral pouches had appeared; the first three had reached
the ectoderm, and each member of the first pair had acquired a cleft
opening ‘(O the outside. During the third day the first pair of pouches
retain their openings, while each member of the second pair develops
a short dorsal and a long ventral cleft, corresponding to the points of
fusion between ectoderm and endoderm described in the preceding chapter. The members of the fourth pair of pouches now acquire connections
with the ectoderm at their dorsal ends, but never develop any cleits
(Fig. 194).
The Arches. —-—— The visceral arches undergo no special change on the
third day, except the development in some of them of the aortic blood
vessels (arches) which will be described below.
The Thyroid. ~—~— During the third day, the rudiment of the thyroid
which was last described as a slight depression in the floor of the pharTHIRD DAY: THE FORE—GUT 373
ynx, continues to evaginate. By means of this process, the end of the
third day finds the above depression transformed into a wide-mouthed
sac. Figure 195 shows in cross section this and other structures indicated above.
The Laryngotracheal Groove and Lung Prirnordia.——At the
end of the second day a shallow longitudinal groove with a pair of
Fig. 195.—Frontal section through the pharynx of a 35 somite embryo. From
Lillie (Development of the Chick).
a.a. 1, 2, 3, 4. First, second, third, fourth aortic arches. Hyp. Rathkc’s pocket,
ru<iimr:nt of anterior hypophysis. J. J ugular vein. lar.-tr. Gr. Laryngotracheal groove
lpost branchial pharynx). or. Oral cavity. Ph. Pharynx. v./1. 1, 2, 3. First, second,
third visceral arches. L'.G. 1, First visceral cleft. v.F. 2, 3. Second and third visceral
furrows. v.P. 2, 3, 4. Second, third, fourth visceral pouches. III. Third cranial nerve.
postero-lateral expansions had appeared in the floor of the pharynx
just caudal to the visceral pouches, indicating the beginning of the
respiratory system. This groove now becomes much narrower and
deeper, and is called the laryngotracheal groove. Also its postero-lateral
expansions develop into tubelil-re outgrowths "which, as previously indicated, are then ordinarily termed the lung prirnordia. Strictly speaking, however, they really represent, not only the beginnings of the lungs,
but also of the bronchi, i.e., the entire respiratory system.
The Esophagus and the Stomach.—— By the end of thethird day
the esophagus is represented by an abrupt narrowing of the fore-gut
immediately posterior to the pharynx. The narrowed portion leads into
a slightly dilated region just anterior to the liver rudiment, and this di
lation is the beginning of the stomach, i.e., the proventriculus and gizzard (see the fifth day).
374 -THE CHICK
The Liver. —— At the end of the second day the liver was represented
by two anterior-ly directed diverticula from the region of the anterior
intestinal portal; the more anterior of these had extended far enough
forward to overlie slightly the point of union of» the vitelline veins.
During the third day, these diverticula grow somewhat further forward,
the anterior member of the pair along the left dorsal side of the ductus
venosus, and the posterior member along its right ventral side. Both
Fig. 195.--Rec-onstructions of the liver diverticula of the Chick.
From Lillie (Development of the Chick). After Hammar.
A. On the third day of incubation; from the left side; the diverticnlar arise from the anterior intestinal portal.
B. Beginning of the fourth day; from the left side.
a.z'.p. Anterior intestinal portal. D.V. Indicates position of ductus
vcnosus. g.b. Gall bladder. l.d.d'.(cr.). Dorsal or cranial liver diverticulum. l.d.v.(caud.). Ventral or caudal liver diverticulum. pad.
Dorsal pancreas. X. Marks the depression in the floor of the duodenum irom which the common bile duct is formed.
now also branch profusely, the branches spreading around the ductus
venosus and anastomosing freely with one another. At the same time
capillaries from the ductus venosiis begin to develop among the interstices of these anastomosing branches; this is the beginning of the main
body of the liver.
The Bile Ducts. ———- In the meantime, the intestinal portal has, of
course, moved backward beyond the point of origin of the diverticula.
This lengthens the gut and leaves these diverticula attached to its ventral side at their ‘points of origin. The parts of the diverticula between
the region of their anastomosis and the points of attachment to the gut
are at the nature of short tubes, the rudiments of the future bile ducts.
Presently the floor of the gut comprising the region where these ducts
enter it becomes depressed, and then drawn out so as to form a common
. , ,......l......>-yep "
THIRD DAY: THE HIND—GUT 375
duct into which the two original ducts empty. This common duct is
called the ductus choledochus, and is a temporary structure (Fig. 196).
The Gall Bladder. -—- While the above processes have been going on,
the gall bladder has arisen as a posterior evagination from the posterior
the gall bladder is drawn out to form the cystic bile duct.
All of these hepatic structures it should be noted are covered by the
splanchnic mesoderm of the ventral mesentery within which they have
developed. This rnesentery, here termed the gastro-hepatic ligament,
serves permanently to attach the whole mass to the gut and stomach.
The Panct'eas.——This organ first appears on the third day as a
thickening on the dorsal wall of the intestine within the dorsal mesontery about opposite the posterior liver diverticulum. The rudiment thus
indicated gives rise to only about a third of the entire organ whose further development will be described as it occurs (Fig. 196) .
THE MID—GUT
There is no great change in the mid-gut region during the third day
except that it becomes more clearly marked oil as the lateral folds continue to press in toward one another.
THE HIND—GUT
The Postanal Gut.——-It will be recalled that at the close of the
second day the ectoderm had taken so slight a part in the tail fold that
the anal plate retained a dorsal position. On the third day, however,
the fold becomes more marked, and soon takes on the character of a
posterior outgrowth, which is at first anterior to the anal plate. This
outgrowth is the tail bud. As its development progresses it becomes first
postero-dorsal, and then by turning downward postero-ventral, to the
anal plate, which itself becomes ventral instead of dorsal (Figs. 197.,
198). Also as a result of this process there is drawn out into the bud
an extension of the hind-gut, constituting a temporary structure known
as the postanal gut (Fig. 197).
The Allantois. -——The most important structure to appear in connection with the hind-gut during early embryonic life is the allantois.
The rudiment of this organ is usually indicated at about the beginning
of the third day. The method of its development and its final structure
have been described above (Figs. 190, 193) . In connection with the diagrams presented in Figure 198, however, a further word about its early
origin should be said. These diagrams represent the behavior of this re376 THE CHICK
gion as described in the text, and according to Gruenwald (°4«1). It must
he added, however, that in spite of the fact that there is apparent agreement regarding the movements which are taking place, Gruenwald puts
a somewhat different interpretation on them than do certain other an
s.A. Am. Am.cav. Ect. N'ch. n.1.
Fig. 197.-Sagittal section through the tail of an embryo of about 35 somites.
From Lillie (Development of the Clzic/:3.
All. Allantois. Am. Amnion. Am.cav. Amniotic cavity. An.pl. Anal plate. A0. Dorsal aorta. Bl.v. Blood-vessels in wall of allantois. c.C. Central canal of spinal cord.
Cl. Cloaca. Ect. Ectoderm. Ectam. Ectoderm of amnion. E.E.B.C. Exocoelom.
Mesrzm. Mesoderm of amnion. N’c}L. Notochord. n..T. Nerve cord. p’a.C. Post-anal
gut. p.i.p. Posterior intestinal portal. s.A. Segmental arteries, between the somites.
Spl’pl. Splanchnopleure and yolk-sac entoderm. T.B. Tail bud.
thors, e.g., Lillie and the present writer. Gruenwald, following an old interpretation presented by Duval in his atlas, chooses to regard the original “ hind-gut 7 as already “ allantois.” As can be seen from the
figures, it is true that a considerable portion of the original hind-gut is
eventually included in the allantoic outgrowth. It has also been shown
that the elimination of this region results in more or less complete elimination of this organ (Zwilling, ’46) . Nevertheless, it seems to the writer
confusing to identify this gut in its primary condition with the allantois,
THIRD DAY: THE HEART 377
involving as it= certainly does at that time the anal plate. It seems preferable to say that the allantois grows out from the part of this hind-gut
which, by the processes shown, eventually comes to lie anterior to the
anal plate.
an I plate
 
 
tall bud
anal plate
 
A
region of allantoic origln B
beginning of post anal gut
 
 
/Post anal gut
posterior Intestinal, portal——-“
can bud
beglnnlng of allantols
Fig. 198.——Diagrams representing changes in the tail and hind-gut region of the Chick during the third day. up to the 30 somite stage. After
Gruenwald with slight modifications. The successive stages are indicated
in the order of the letters.
THE CIRCULATORY SYSTEM
THE HEART
There are no very marked changes in the form of the heart during
the third day, though the atrium becomes slightly more prominent, and
the hendings and constrictions already described‘ are somewhat emphasized (Fig. 199). Internally toward the end of the (lay sections reveal
the appearance of a slight ingrowth from the atrial wall just to the left
of the sinus venosus. It is the beginning of the interatrial septum (Quiring, ’33) . In the ventricular region the myocardium is becoming thick378 THE CHICK
enecl and spongy, but in the bulbus arteriosus, on the other hand, endothelial thickening has occurred, while the myocardium remains thin
(Fig. 201).
THE EMBRYONIC BLOOD VESSELS
The Arteries. .
The Dorsal Aortae.—During the third day these vessels continue
their development by beginning to form posterior to the point at which
the vitelline arteries leave the body.
These latter arteries thus become
lateral branches of the dorsal
aortae, instead of their continuations, while the further posterior
growth of these aortae brings them
eventually to the extremity of the
tail bud. Meanwhile anteriorly
they have become fused, so that by
the end of the third day a single
aorta extends from just back of
the aortic arches almost to the origin of the vitelline arteries. Finally
during the fifth and sixth days the
fusion of these vessels progresses
Fig. 199.——Heart of 21 Chick embryo
of 72 hours, dissected out and drawn
from the dorsal surface. From Lillie
(Development of the Chick). _ _ , _
Aur.l. Left atrium. Aur.r. Right Into the tall region 3150, resulting
Elirggin-afii“-Igglgtiirgrfirlgfgéf in the formation of asinglé caudal
Ductus venosus. s.V. Sinus venosus. artery. It will not be necessary,
Tina. Truncus arteriosus. V.r. Right however, to trace these processes
limb of ventricle. , , _
of growth and fusion in detail.
The Aortic Arches.———During the third day each original carotid
loop plus the anterior part of each original ventral aorta disappears. At
the same time the part of each original ventral aorta which occupied
the ventral four-fifths of each mandibular arch becomes directly connected with its respective dorsal aorta through the upper fifth of each
of these arches (Fig. 200). In this way the actual first aortic arches are
completed.‘ However, before the end of the day the dorso-ventral con»
nections of these vessels in the mandibular arches have been broken,
1 This statement is based on figures from both Duval and Lillie. It should be
pointed out, however, that Lillie does not actually say that such a direct dorsal
connection occurs, and the writer has not been able. to verify the point at first
hand. If such a connection is established it is certainly for a very brief time, and
confirmation would require the study of closely graded embryos.
THIRD DAY: EMBRYONIC BLOOD VESSELS 379
‘~ :,'_/ L‘ h_' _— 3 I
s.2. V
i
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.7, /
7 {Ma .,
EXfl8E5':7Ial.fi.§‘£1V£'..:‘.1.‘
:§!"'!*"!’:.’.’.£%‘§‘,*.'£."‘e“‘\ \‘.
Fig. 200.—Chick embryo with adjacent portion of area vasculosa, with 35 pairs
of somites (about 72 hours). Dorsal view. From Lillie (Development of the Chick).
ma. 1, 2, 3, 4. First to fourth aortic arches. Am. Amnion. Ar. Branches of vitelline
arteries. Atr. Atrium (Auricle) . A.V. Vitelline artery. B./1. Bulbus arteriosns. cerv.
Fl. Cervical flexure. cr.F l. Cranial flexure. D.C. Ductus Cuvicri. D.V. Ductus venosus.
Ep. Epiphysis. Gn.V. Ganglion of V cranial nerve. Iszh. Isthmus. Jug. External jugular vein. Md. Mandibular arch. M.M. Maxillo-mandibular branch of V cranial
nerve. Myel. Myelencephalon. olf.P. Olfactory pit. Ophth. Ophthalmic branch of
V cranial nerve. 0t. Otocyst. 5.2, 5.10, 5.20, etc. Second, tenth, twentieth, etc., somites. V. Branches of the vitelline veins. V.c.p. Posterior cardinal vein. V.umb.
Umbilical vein. VJ’. Vitelline vein. V.V.p. Posterior vein. W.B. Wing-bud.
380 THE CHICK
and thus the first aortic arches vanish after a very brief existence. The
dorsal aortae in this region do not disappear, however, but extend anteriorly as the internal carotids. Ventrally the stump of each first aortic
arch persists, and presently produces an anteriorly growing twig which
becomes the primary external carotid. (See fifth day for final development.) Meanwhile a fourth aortic arch arises in each of the fourth visceral arches.
Chor. P_ c_ Lens p. Ch.
pl. gr. Am.
1
Fig. 201.—~Transverse section, passing tlirough the eyes and heart, of an embryo
with about 35 pairs of somites (about 72 hours). Compare with F ig. 200. From
Lillie (Development of the C/lick’) . V
Am. Amnion. A0. Dorsal aorta. Atr. Atrium. B.A. Bulbus artcriosus. cI'1..Fis. Choroid fissure. Chor. Chorion. D.C. Ductus Cuvieri. Dienc. Diencephalon. Lg. Rudiment of lung branches. P.C. Pericardial cavity. p.Ch. Posterior (vitreous) chamber.
pl.gr. Pleural groove. V.c. Posterior cardinal vein. Y.S. Yolk-sac.
The Pulmonary Arteries. — During the third ‘day, these arteries appear as rudiments within the walls of the lungs.
The Veins.
The Cardinals and Jugulars.——During the third day, the anterior
cardinals continue to branch considerably in the brain region and may
now be known as the internal jugulars. At the same time a vessel from
the floor of the pharynx joins each anterior cardinal (internal jugular)
just at its point of union with the duct of Cuvier. These new veins are
the external jugulars (Fig. 200). Late on the third day also a new pair
of cardinals begins to develop. They arise from a series of anastomosing
vessels on the ventral side of the mesonephros just lateral to the dorsal
aorta, and are known as the subcardinals. They are scarcely apparent
as definite vessels before the fourth day.
,
a
i
.
THIRD DAY: EMBRYONIC BLOOD VESSELS 381
The Vitelline Veins. —- Before leaving the body of the embryo, these
veins become united by a short transverse vessel which passes over the
intestine just posterior to the dorsal pancreatic rudiment. In this manner,
the intestine is surrounded by
a venous rinv. The anterior
ventral part of this ring is
formed by the posterior end
of the ductus venosus. The
lateral parts consist of the
portions of the vitelline veins
lying between the ductus venosus and the transverse vessel, and the posterior dorsal
part is constituted of the transverse vessel itself (Fig. 211,
A, B; see Chapter 12). Meanwhile, as indicated in the account of the liver, the portion
of -the ductus venosus which
lies within that organ is beginning to give of? capillaries
among the branches of the
liver diverticula.
The Untbilical Veins. —Early on the third day, a vein
develops in the body wall on
each side of the embryo, and
opens anteriorly into the respective duct of Cuvier. These
are the beginnings of the umbilical veins, although at this
Fig. 202.———Part of a transverse section
through the lateral mesocardia of a Chick
with 35 pairs of somites (about 72 hours).
From Kellicott
After Lillie.
a. Atrium. arm. Accessory mesentery. am.
Amnion. ac. Dorsal aorta. be. Bulbus arteriosus. ch. Chorion. cv. Posterior cardinal vein.
dC. Ductus Cuvieri. dm. Dorsal mesentery. 1.
Liver. lm. Lateral mesocardium. pc. Pericardial cavity. pe. Pulmoenteric recess. pg.
Pleural groove. 5. Stomach. sv. Sinus venosus.
um. Ventral mesentery.
(Chordate Development).
time they have no connection with the allantois (Fig. 203). Until such a
connection has been established the blood from this organ is conducted
to the lateral vitelline veins as follows: A transitory vessel, the subintestinal vein, develops upon the dorsal surface of the allantois, from
whence it proceeds up onto the ventral side of the gut, along which it
passes to the posterior intestinal portal. Here it divides into two parts
'which pass auteriorly around either side of the yolk-stalk to open into
the vitellines as these vessels run from the yolk-sac ‘along the margins of
the anterior intestinal portal to the ductus venosus.
382 THE CHICK
THE EXTRA—-EMBRYONIC BLOOD VESSELS
The Arteries.—The vitelline arteries reach further out into the
area vasculosa than during the second day, terminating near its border
in a network of capillaries which empty into the sinus terminalis.
Fig. 203.—Injected Chick embryo of the third day,
showing the arrangement of the cardinal veins and
the formation of the umbilical vein from capillary
networks. From Evans.
A.C. V. Anterior cardinal vein. P.C.V. Posterior cardinal vein. U.V. Umbilical vein.
The Veins.——Posterior to the point where the anterior vitelline
veins have fused, the right vein becomes greatly reduced. During this
period, also, the lateral vitelline veins passing backward and outward
along the margins of the anterior intestinal portal continue to form
from the vascular network lying close to either side of the embryo. In
this manner, they presently reach the region where the vitelline arteries
turn rather directly outward into the area vasculosa, and at this point
they also begin to pass outward just dorsal to the arteries. These veins
5
i
3
.
e
s
I
THIRD DAY: THE MESENCEPHALON 333
never extend all the we)’ t° the sinus terminalis, but branch widely in
the more central part of the vascular area. They receive blood from the
terminalis, however, through several intermediate veins (venous trunks),
which cross the outer network of arterial capillaries to reach them. Before the end of the third day, one other new extra-embryonic vessel
starts to appear, the posterior vitelline vein. At this time it is scarcely
more than a mass of capillaries, but very shortly begins to become distinct. It runs forward from the posterior side of the sinus terminalis,
and empties into the left lateral vitelline vein near its base (Fig. 182).
THE NERVOUS SYSTEM
THE FLEXURES
These have already been discussed under the description oi external
changes.
THE FORE—BRAIN OR PROSENCEPHALON
The Telencephalon. ——The indentation which marks the velum
transversum becomes much more prominent, while the rudiments of the
cerebral hemispheres grow in size and their walls increase in thickness.
In about the center of the lamina terminalis, a thickening appears called
the torus transversus. ltcorresponds to the similarly named structure in
the Frog, and as in that case it represents the rudiment of the future
anterior commissure.
The Di'encephalon.—The more anterior (ventral) portion of the
diencephalon is now sometimes distinguished as the parencepkalon, and
the posterior (dorsal) portion as the synencephalon (Fig. 204.-). Between them is a slight constriction, while the parencephalon is approximately boundcd below by the marked indentation of the velum transversum. Thus the roof of the parencephalic. region constitutes a
relatively raised area from which the epiphysis begins to develop at the
close of the day as a small out-pushing. Upon the floor of the diencephalon, the optic recess, the region of the optic chiasma, and the infundibulum all become more pronounced than they were at the end of
the second day.
THE MESENCEPHALON
The roof of the mid-brain grows rapidly and becomes prominently
arched, while its walls increase uniformly in thickness. This arching of
384 THE CHICK
the mid-brain causes the boundary between it and the roof of the diencephalon to appear gradually more constricted. Likewise posteriorly at
the connection between mid- and hind-brain, a slight constriction in the
roof and lateral walls, indicated during the second day, also becomes
very pronounced. This latter constricted region is henceforth known as
the isthmus.
Fig. 204. —— Optical longitudinal section of the head of an embryo of 30s. The heart
is represented entire. From Lillie (Development of the Chick).
Atr. Atrium. B.a. Bulbus arteriosus. D.v. Ductus venosus. Isth..Isthmus. Lg.
Laryngotracheal groove. Oes. Oesophagus. or.pI. Oral plate, which has begun to
rupture. Parenc. Parcncephalon. Ph. Pharynx. Stain. Stomach. Synenc. Synenceph
alon. Th. Thyroid. S.v. Sinus venosus. Ven.R. Right ventricle. Other abbreviations
as before.
THE RHOMBENCEPHALON
The Metencephalon. ——After the isthmus has become established
the thickening roof of the metencephalon consists largely of the wall
forming the posterior side of the constriction. By the end of the day, the
lateral walls of the metencephalon have also begun to thicken.
The Myelencephalon. —— The roof of the myelencephalon remains
thin, while its ventro-lateral walls have started to thicken somewhat.
The Spinal Cord. —— At the end of the second day, the wallsof the
spinal cord were seen to consist chiefly of ependymal supporting cells
and germinal cells. During the third day, the latter continue to multiply, and theirdescendants migrate out somewhat from their position
THIRD DAY: THE RHOMBENCEPHALON 385
near the central canal. In their new location, they presently become
transformed either into neuroblasts, i.e., primitive nerve cells, or into
primordial glia cells. The nerve cells even at this time have begun to
send out the axones and dendrites typical of the adult neurones. The
central parts of these neurones together 'with glia cells eventually come
to constitute the gray matter of the cord, while the axones form its
white matter.
Fig. 205. —Transverse section through the spinal cord
and ganglion of a Chick about the end of the third
day; prepared by the method of Golgi. From Lillie
(Development of the Chick). After Ramon y Cajal.
c. Cones of growth at the ends of growing nerve
fibers. Nbl. 1 and 2. Neuroblasts of the lateral wall.
Nbl. 3. Neuroblasts of the spinal ganglion. Nbl. 4.
Neuroblasts of the ventral horn (motor neurohlasts).
As regards the final condition of the cord, the following may be said:
Internally, the central canal is obliterated, save for a small ventral portion lined by the inner ciliated ends of the ependymai cells. Surrounding
this and filling the central part of the cord is the gray matter with
dorso-lateral and ventro-lateral extensions or horns reaching out into
the white substance. Externally, there develops along both the dorsal
and ventral sides a median longitudinal fissure. These fissures are
' formed mainly as a result of the enlargement of the lateral regions
through the accumulation of the nerve fibers within them.
The Spinal Nerves. -—The spinal nerves are sometimes described
as constituting parts of two systems, (1) the somatic, and (2) part of the
parasympathetic and the sympathetic; both systems start to develop on
the third day. We shall consider the somatic system. first.
I. The Somatic System. — From bipolar nerve cells within each
spinal ganglion one bundle of fibers (dorsal root) grows into the spinal
386 THE CHICK
cord, and another outward in a ventro-lateral direction. Together these
constitute the aflerent or sensory nerve fibers. At the same time from
the ventro-lateral side of the nerve cord beneath each spinal ganglion,
fibers (ventral root) are growing out from nerve cells located within
the cord. These are eflerent or motor fibers which mingle with those of
the respective outgrowing afferent bundle just at the point where the
latter leave their ganglion. The mixed fibers thus form the common
trunk of a somatic spinal nerve. This trunk then divides again into a
dorsal and ventral part, each part containing fibers of both the above
types. The condition thus indicated is approximately the stage reached
in the development of the somatic nerves at the end of the third day or
early on the fourth (Fig. 205; common trunk not shown).
Inasmuch as it will not be profitable in a work of this scope to follow
further the detailed development of the somatic spinal nerves from clay
to day, their future arrangement will be summed up at this time, as follows: The fibers of the divided trunks increase in number and at the
same time grow outward. Hence, they almost immediately come into
contact with the muscular and dermal plates, which are the rudiments
of the future voluntary musculature and dermis of the Chick. Thus nervous connections are early established with these elements, and as the
latter develop, the nerves (-motor and sensory) develop with them.
It should be noted that some of this musculature just indicated is destined for the limbs, and hence certain groups of the spinal nerves will
constitute the brachial and the sciatic plexuses. In this connection certain experimental results are of interest. Thus it has been shown that
when limb buds are transplanted to abnormal locations as described
above, spinal nerves nearby, which would normally have nothing to do
with limbs, are apparently “ attracted to them,” even forming a characteristic plexus before entering them (Hamburger, ’39). (However, see
conclusions of Detwiler and Piatt on this matter in the section on the
Frog). Hamburger (’39, ’44, ’49), Bueker (’45) and others have also
shown that the number of motor neurons in the cord may be respectively
decreased or increased by the extirpation of an adjacent limb bud or the
implantation of an extra one. Hamburger also showed that the variation
in number of motor neurons was apparently not caused by a difference
in the total number of cells, but rather by the differentiation of more or
less of this particular type of cell as compared with other types. These
results show the effect of developing limb buds on nerves. Lastly, however, Hunt (’32) and Eastlick (’4-3) have demonstrated that in transw_@afl >.
THIRD DAY: THE RHOMBENCEPHALON 387
planted limbs which for any reason fail to be innervated few muscle
fibers develop, and those that do, degenerate after about ten days. In
conclusion it thus appears that there are reciprocal influences between
a growing limb bud and its musculature on the one hand, and the devel.
opment of neurons and their fibers on the other.
11. The Sympathetic and Sacral Parasympathetic Systems. — As in the
Frog there has been much disagreement concerning certain details of the
origin of parts of these systems. For some time all postganglionic neurons at least were alleged to arise from neuroblasts in the dorsal root
ganglia, i.e., originally from the neural crests. Later Jones, ’37, ’39, ’4I
asserted that cells within the neural tube were the exclusive source for
these systems. Further experimental study by Hammond, ’49 and Yntema and Hammond, ’54 ’55 seem now to have resolved the problem as
follows: It appears that all postganglionic ne_urons and their fibers are
derived from the neural crest. All preganglionic fibers, both sacral
parasympathetic and thoraco-lumbar sympathetic arise from special aggregations of motor neurons within the spinal cord. The sheath cells of
all the fibers are from the crest and tube (Brizzee, ’49), and possibly
some mesoderm.
At the end of the third day or early on the fourth the postganglionic
cells derived from the crest collect just above and to either side of the
dorsal aorta. Here they send out fibers anteriorly and posteriorly, forming a pair of delicate longitudinal cords running from the cervical
region to the tail, with thickenings (ganglia) opposite each somatic
ganglion. These are the primary sympathetic and sacral parasympathetic
cords and ganglia, and each of these ganglia is connected with a somatic
ganglion by a strand of fibers, the primary rami communicantes. Lastly
there are a few cells in the dorsal mesentery, probably from the crest,
and destined to form Remak’s ganglion (Chap. 12, Fig. 216).
The Cranial Ganglia and Nerves.—-The ganglia of the V, VII,
VIII, IX and X nerves have already been described as appearing on the
second day. During the third day, the V ganglion shifts its position of
attachment to the brain somewhat, and its characteristic YM shape becomes more marked. The VII and VIII ganglionic mass also shifts to
a more dorsal position. Otherwise the cranial ganglia show no marked
alterations at this time (Fig. 200).
The Mixed Character of Certain Cranial Nerves.——In the Chick, as
in the Frog, it is possible to distinguish the V, VII, IX and X nerves as
mixed, i.e., as containing both sensory and motor elements. In this respect they are of course not different from the spinal nerves, except as
388 THE CHICK
regards the point at which the two types of fibers become mingled. Thus
in the region of the cord, the ventral or motor fibers of any nerve join
the dorsal or sensory fibers of that nerve slightly peripheral to the dorsal ganglion. In the mixed cranial nerves, on the other hand, the two
types of fibers issue from the brain very close together and mingle before entering the ganglion of the respective nerve. It may be further
noted that though the ganglion of the VIII nerve is very closely associated at this time with that of the VII, -its fibers are wholly sensory.
The III or Oculo-Motor Nerve. ——-Besides the mixed or wholly sensory nerves in the Chick, there are also, as in the Frog, certain cranial
nerves which are purely motor and without any connection with the
cranial ganglia. They take their origin from neuroblasts within the
brain itself, just as spinal motor fibers arise from neuroblasts within
the spinal cord. The III or oculo-motor nerve arises in this manner from
the median line of the ventral side of the mid-brain, at about sixty
hours. Its history will be traced a few steps further in connection with
the IV and VI nerves which arise on subsequent days.
THE ORGANS OF SPEClAL SENSE
THE EYE
The Optic Cup. -—— There are two main changes connected with the
optic cup during the third day. The first change is the rapid increase in
its size. Thus at the end of the second day the lens rudiment practically
filled the cavity of the cup, and came in contact with its inner wall. At
the end of seventy-two hours, on the other hand, the lens is entirely
separated from the wall of the cup, and simply rests within its rim.
The second change is the thickening of the inner wall, from whose
neuroblasts axones start to grow at the 30-somite stage (courtesy
Rogers, K. T.) . The optic stalk is still ventral at the point of attachment
to the cup, the region surrounding this point being called the fundus
(Fig. 201).
The Lens. ——The lens becomes detached from the superficial ectoderm during the third day, and forms a hollow ball, whose walls are
at flrst of almost uniform thickness. Presently, however, the cells of the
inner wall (i.e., the one next to the optic cup) begin to lengthen, in a
direction at right angles to this wall, so that the latter is thereby thickened. By the end of the day this thickening has progressed to a considerable extent, the elongated cells which cause it being destined to
form the lens fibers, which constitute the core of the lens.
THIRD DAY: THE EAR 339
THE EAR
At the end of the second day, the auditory pit had been transformed
into the auditory sac, whose mouth was still partly open to the exterior.
' By virtue of the method of the closure of the pit, described in the previ
ous chapter, the major part of the sac lies below the level of its external
Fig. 206.———Two stages in the development of the auditory organ of the Chick. From Kellicott (Chmrclare Development. A. Hemisected model of left auditory sac posterior
view, just before the separation from the head ectoderm,
at about 72 hours. After Krause. B. Median view of a
model of the left membranous labyrinth of an embryo of
7 days and 17 hours. After Riithig and Brugsch.
a. Anterior vertical semicircular canal. aa. Ampulla of
anterior vertical semicircular canal. up. Ampulla of posterior vertical semicircular canal. d. Ductus endolymphaticus. e. Superficial ectoderm of head. l. Lagena (cochlea). p. Rudiment of posterior vertical semicircular canal.
s. Rudiment of saccule. u. Utricle. 9:. Connection between
auditory. sac and superficial ectoderm.
orifice. The connection of this orifice with the dorsal portion of the sac
is then drawn out into a narrow tube, while the dorsal part of the sac
itself is at the same time slightly constricted away from the major ventral part. The former, or dorsal portion, is the rudiment of the endalymphatic duct, which presently ‘grows upward somewhat so that its
roof is slightly dorsal to the level at which the tube leading from it
opens to the exterior (Fig. 206, A).
THE OLFACTORY ORGANS
Early on the third day a small circular spot of ectoderm on each
ventrodateral side of the head somewhat in front of the eye becomes
thickened, in consequence of a lengthening of its cells. These patches
390 THE CHICK
Fig. 207.—Tlze development of the mesonephros. A.B. Transverse sections through the mesonephric tubules of the Duck embryo with 4-5 pairs of somites. From Kellicott (Chordate Development). After Schreiner. C. Transverse section through the middle
of the mesonephros of a Chick of 96 hours. From Lillie (Development of the Clzickt.
A0. Dorsal aorta. B. Rudiment of Bowman’s capsule. c. Conducting part of a primary tubule. coel. Coelom. Cal.T. Collecting tubule. cl. Dorsal outgrowth of the Wolfiian duct to form a collecting
tubule (see fourth day). Glam. Glomerulus. gcrm.Ep. Germinal
epithelium. M’s't. Mesentery. n.t. Nephrogenous tissue. rc. Rudiment of conducting portion of primary tubule. T. 1, 2, 3. Primary,
secondary, and tertiary mesonephric tubules. V.c.p. Posterior cardinal vein. W.D. Wolflian duct. '
then begin to invaginate, and thus form the olfactory pits (Fig. 200).
The thickened epithelium which lines them is the olfactory epithelium,
and is said to consist of two types of cells, simple epithelial cells and
germinal cells. The latter type later give rise to neuroblasts which eventually produce the sensory cells of the olfactory epithelium, while they
in turn give rise to axones which constitute the olfactory nerve. (See
next chapter.)
THIRD DAY: SUMMARY 391
THE URINOGENITAL SYSTEM
During the third day, the pronephros degenerates, while the mesonephros continues to develop, and soon becomes the primary excretory
organ during embryonic life in a manner about to be indicated. Neither
the metanephros nor the reproductive system appears during the third
day.
As regards the changes in the mesonephric region, it will be recalled
that at the end of the second day the Wolflian or mesonephric portion
of the pronephric duct was just beginning to acquire a lumen. Its backward-growmg end, however, was still solid, and had not yet reached
the cloaca. On the third day, this cellular rod connects with the cloaca,
and by the end of the day a lumen has formed throughout its length.
Concerning the mesonephros proper, at 4-8 hours the rudiments of the
mesonephric tubules were forming in the neighborhood of the twentieth
somite or segment, i.e., in the most anterior region of the future organ.
At that time, these rudiments, of which there were two or more to the
somite, consisted merely of spherical condensations of the nephrotome,
which were beginning to become vesicular. Now at the end of seventytwo hours, however, the vesicles opposite the most anterior mesonephric
somites are giving rise to small, hollow evaginafions in the direction of
the Wolfhan duct (Fig. 207, A). There is one evagination to each vesicle, and it is the part of the vesicle which is destined to form the actual
mesonephric tubule. Indeed, just anterior to the twentieth somite or
mesonephric region proper, some of the out-pushings have already become tubules and are connected through conducting portions with the
Wolflian duct (Fig. 207, B). In this region also Malpighian bodies have
appeared in connection with some of the tubules. These most anterior
tubules and glomeruli, however, never become functional.
SUMMARY OF THE CONDITION AT THE END OF THE
THIRD DAY OF INCUBATION
I. GENERAL APPEARANCE
The cranial and cervical flexures have increased, especially the latter. A small caudal flexure has appeared, and the region in between has
developed a slight ventral curvature. The lateral’ rotation has progressed
so that the embryo is on its side as far back as the twenty-first somite.
The four limb buds are clearly visible.
392 THE CHICK
II. THE SOMITES
The number of pairs of somites has increased to thirty-six and in the
more anterior pairs dermatomes and Inyotomes are completely developed. Sclerotomal tissue is still collecting about the notochord and the
sides of the nerve cord.
III. THE ALIMENTARY TRACT
The Fore-gut.-——The oral plate has broken through to complete
the oral cavity, and Rathke’s pocket reaches nearly to the infundibulum.
Subsequent development of these parts to form the pituitary is described
in this chapter. The second pair of visceral pouches has acquired clefts,
and the fourth pair has fused with the ectoderm. The thyroid depression
has become a sac. The depression indicating the respiratory system has
deepened in the laryngotracheal groove, and the rudiments of the
lungs have appeared. The esophagus and stomach are beginning to be
defined. Finally, the liver diverticula have grown forward and anastomosed about the posterior part of the ductus venosus; the rudiment
of the gall bladder is visible, and the dorsal portion of the pancreas has
appeared.
The Mid-gut. —— It has become more clearly defined.
The Hind-gut.~—The anal plate has been carried around to the
ventral side by the growth of the tail bud, and at the same time the
postanal gut has been formed. The rudiment of the allantois has appeared.
IV. THE CIRCULATORY SYSTEM
The Hea.rt.——There are no external changes aside from an emphasis of curvatures and constrictions already present. In the ventricular region myocardial thickening has occurred, and in the bulbus
arteriosus the same is true of the endothelium. The interatrial septum
has started to form.
Embryonic Arteries. —— Fusion of the aortae has progressed. The
first pair of aortic arches has been completed and then disappeared.
The dorsal aortae extend anteriorly as the internal carotids, while the
stumps of the first arches produce the external carotials. The fourth
pair of arches has developed, and the rudiments of the pulmonary
arteries have arisen in the lungs. '
Embryonic Veins. —— The anterior cardinals have branched considerably in the brain region and are now known as the internal jugulars
THIRD DAY: SUMMARY 393
which receive the external jugulars just at the union of the former with
the ducts of Cuvier. The ductus venosus is beginning to develop capillaries among the branching liver diverticula. A new vessel passes over
the intestine in the neighborhood of the pancreas and unites the vitelline veins to form a ring about the alimentary tract. A longitudinal vein
has developed in each body wall; they are the umbilical veins, though
at this time neither has acquired a connection with the allantois. The
rudiments of the subcardinal veins may be visible on the "ventral side
of the mesonephros. The transitory subintestinal vein is present.
Extra-embryonic Arteries.——The vitelline arteries have pushed
out into the area vasculosa until their branches nearly reach the sinus
terminalis.
Extra—embryonic Veins.~¥The right anterior oitelline vein has
almost disappeared; the posterior and intermediate vitelline veins have
started to arise, and the lateral vitelline veins have developed further.
V. THE NERVOUS SYSTEM
The Flexures and the Brain. —— As noted under external appear- '
ance the cranial and cervical flexures are both increased. The cerebral
hemispheres have grown somewhat, and the epiphysis has started to
develop. The optic chiasma, the optic recess. and the infundibulum
have all become more clearly marked. The roof of the mid-brain. is
more prominently arched and the isthmus has appeared. There has also
been thickening and thinning of the brain walls at various points.
The Spinal Cord and Spinal Nerves. ——The germinal cells have
changed their position and have begun to develop into neurones and
glia cells. The sensory and motor nerve fibers issue respectively from
the spinal ganglia and the ventral portion of the cord, the two types
uniting to form the common trunks of the somatic spinal nerves. The
primary sympathetic trunks, ganglia and communicating rami have appeared. The completion of the somatic portion of the spinal nervous
system is described in this chapter.
The Cranial Ganglia and Nerves.—-The ganglia have shifted
their position slightly, and the third or oculo-motor nerves have appeared.
VI. ORGANS OF SPECIAL SENSE
The Eye. —— The optic cup has increased in size and its inner wall
has thickened. The lens has become detached from the ectoderm, and
its inner wall is also thickening. 394 THE CHICK
The Eat. -——The rudiment of the endolymphatic duct has appeared
on the dorsal portion of the auditory sac.
The Olfactory 0rgans.~—The olfactory pits have been formed,
with walls consisting of epithelial and germinal cells. ’
VII. THE URINOGENITAL SYSTEM
The proneplzros has begun to degenerate, while the mesonephros has
started to develop tubules and glomeruli in its most anterior portion.
The Wolflian. duct has reached the cloaca and acquired a lumen throughout its length. °
VIII. THE AMNION AND ALLANTOIS
The folds of the amnion have approached one another above the posterior portion of the embryo and formed the amniotic umbilicus. The
allcmtois, by about the middle of the day, has the appearance of a
slight out-pushing from the hind-gut, and by the close of the day has
extended’ well into the somatic umbilicus.
12
HE CHICK: DEVELOPMENT DURING THE FOURTH
DAY OF INCUBATION
GENERAL APPEARANCE
FLEXURES AND TORSION
T H E cranial flexure remains about as on the previous day, but
the cervical flexure has increased so in degree and extent as to bring
the whole head further posterior. Also it brings the region of the dieti
1:. cephalon around so that it and the anterior part of the optic vesicles
face almost directly caudad. At the same time the mid-region of the
cervical flexure is now the most anterior part of the embryo. From the
anterior to the posterior limb buds the longitudinal axis has in most
cases lost its ventral curvature, and has become virtually straight. Caudad to the posterior limb bud the caudal flexure is more marked so that
the tip of the tail is curled around beneath the body. The lateral torsion
now extends throughout the whole embryo so that it lies entirely on its
side.
THE LIMB BUDS
All the limb buds have increased in prominence.
THE SOMITES
THE COMPLETION OF THEIR FORMATION
By the end of the fourth day the number of somites has reached 42,
and subsequent to this time ten more are added posteriorly. These last
ten, however, later disappear, together with the four most anterior ones
(head somites), which become fused with the skull. Thus at 96 hours
the Chick possesses all the somites which take any part in the development of the adult Bird. The development of the myotomal and derma
l tomal elements progresses posteriorly in the manner already described.
~ I». V
caud. Sci.
int's. F.
int'v. F.
caud. Sci.
X int's. F.
' ‘ Gn.
Derm.
ceph.ScL
snt{v§ 5.
My.
‘caud{ Sci.
4‘ im's. F.
perm. V
ceph. Sci.
My.
int'v. F, .
caud. Sci.
int’s.- F.
Derm.
«ceph. Sci.
. My_.
int'v. F.
caud. Sci.
int's. F.
ceph. sec.
Ep. M
Fig. 208. ——Frontal section through the base of the tail of a Chick embryo of 96 hours. The anterior end of the section (above in the figure)
is at a higher plane than the posterior end. From Lillie (Development
of the Chiclt).
caud.Scl. Caudal division of the sclerotome. ceph.Scl. Cephalic division of the sclerotome. Derm. Dermatome. Ep. Epidermis. Gn. Ganglion.
int’s.F. lntersomitic fissure. int’v.F. Intervertebral fissure. My. Myotome.
N’ch. Notochord. N.T. Neural tube. per’ch.Sh. Petichordal sheath. s.A.
Segmental artery.
THE CHICK
FOURTH DAY: THE SCLEROTOMES 397
THE ULTIMATE FATE OF MYOTOMES AND DERMATOMES
Although the ultimate disposition of these elements of the somites is
not accomplished until some time later, it is not desirable to follow
their development longer by one-day periods. Regarding the dermatomes, or cutis plates, it has already been stated that their substance
gradually moves out beneath the ectoderm, and ultimately forms the
dermis in the dorsal regions, the dermis in the more ventral parts being
derived from the underlying -somatopleure. Likewise Straus and Rawles,
’53 have now shown by carbon marking that the myotomes also are the
source of only about the upper one third of the voluntary body muscles
plus parts of three in the abdomen, the rest being somatopleural in origin.
Head musculature and involuntary muscles develop from mesenchyme.
THE SCLEROTOMES
During the third and fourth days the mesenchyme of the sclerotomes
comes to occupy all spaces about the notochord and between the latter
and the myotomes. Indeed, immediately around the notochord itself it
forms a thin continuous layer, the perichordal sheath. Further peripherally, however, a concentration of the mesenchyme in the cephalic and
caudal portion of each sclerotome, as well as a slight division between
these portions, has long made these parts distinguishable as such. Upon
the fourth day, moreover, it begins to appear that upon either side of I
the notochord the cephalic half of each sclerotome is beginning to become fused with the caudal half of the one anterior to it, thereby establishing a new segmental arrangement (Fig. 208). From the method of
their formation, it follows that the segments thus arising do not coincide with the myotomes; instead, they alternate with them just as they
did in the Frog. In this manner, blocks of mesenchyme are being
marked out on either side of the notochord; these are the rudiments of
the right and left halves of the future vertebrae. Lastly, from the cephalic and caudal portion of each sclerotome, mesenchymatous tissue
has new extended well upward around the sides of the nerve cord. This
forms the rudiments of the neural arches, the cephalic arch of one
sclerotome later fusing with the caudal. of the next to form single arches
corresponding to the vertebrae. The reason for the development of the
alternative arrangement between vertebrae and myotomes, i.e., muscles,
should be quite evident. In order to bend the back or neck it is apparent
that each set of muscles must be attached at each of its ends to a different vertebra.
398 THE CHICK
THE ALIMENTARY TRACT
THE REGION OF THE FORE—GUT
The Tongue. — The tongue appears on the fourth day as two papilliform outgrowths from the floor of the pharynx, one in front of and
one behind the thyroid. These two rudiments then grow forward and
fuse with one another. Eventually the structure thus constituted unites
with a pair of lateral folds to form the tongue of the adult.
The Visceral Pouches and Arches.
The Pouches.——During the fourth day, the third pair of pouches
acquire dorsal and ventral clefts like those of the second, while the
clefts of the latter pouches and of the first (hyomandibulars) become
closed. The second pouches then gradually disappear, whereas the dorsal portions of the first pair extend dorso-posteriorly toward the respective otocysts; here each eventually forms a part of the tubo-tympanic
cavity (see fifth day).
The Arches.—The five pairs of arches reach their maximum development as such during the fourth day, and certain changes in their
blood vessels take place; these changes will be described below.
The Thyroid.——The thyroid sac at this time completely separates
from the floor of the pharynx. Subsequently it becomes divided into
two massive lobes which move backward and take up'a position at the
junction of the subclavian and the common carotid arteries. The effect
of the pituitary upon the later development of this gland has been determined experimentally as follows:
Transplants have been made of thyroid glands from twelve-day old
Chicks to the chorio-allantoic membranes of Chicks with and without
pituitaries. It was found that only in Chicks possessing the pituitary
does either a transplanted thyroid‘ or that of the host develop beyond
the twelve-day stage (Martindale, ’4l).
The Respiratory Tract.——It will be recalled that at the end of
the third day, the posterior part of the pharynx had deepened and narrowed to form the laryngotracheal groove, with the lung primordia at
its posterior extremity. During the fourth day, the posterior portion of
this groove, including the lung diverticula, separates from the ventral
part of the alimentary tract. The anterior portion of the new tube thus
formed is the larynx which continues to open into the pharynx through
a slit-like aperture, the glottis. The remainder of the tube is the trachea,
FOURTH DAY: THE REGION OF THE FORE—_GUT 399
which divides into the lung primordia, really only the primary bronchi,
at its posterior end. This is the condition of the respiratory apparatus
at the end of 96 hours.
The Esophagus, the Stomach, and the Duodenum. —— At the end
of the third day, the fore-gut region posterior to the pharynx consisted of an elongated tube——the esophagus, a slight dilation——the
stomach, and finally another elongated region to which were attached
the rudiments of the liver and pancreas. This last section of the foregut may from now on be termed the duodenum. During the fourth day
the elongation of these parts continues, and also a certain curvature
becomes evident. This latter process extends from the posterior region
of the esophagus to the end of the duodenum, and the direction of the
bending is such that the convex side of the curve is toward the left.
The Liver.——It will be recalled that at the end of the third day
the main body of this organ had formed an anastornosing network
about the ductus venosus, and that it extended somewhat further forward on the left side than on the right. During the fourth day, this network increases, together with its interstitial blood vessels (Fig. 196, B).
As this enlargement proceeds, it will be found that the larger part of
the organ comes to lie more and more upon the right side of the body,
in the hollow made by the bend of the stomach.
The Pancreas. -——At the close of the third day, a thickening in the
dorsal wall of the intestine opposite the posterior liver diverticulum
was noted as the first rudiment of the pancreas. Upon the fourth day
this thickening becomes a solid outgrowth, somewhat hollowed at its
base. By the end of the day, two similar ventral rudiments may also
be visible as antero-lateral outgrowths from the common bile duct (the
ductus choledochus) . The subsequent union of these three elements will
be described in the following chapter.
The Spleen.--Although this organ is not really a part of the
digestive tract at all, it is convenient to describe its development at this
point. During the fourth day a proliferation of cells occurs in the
peritoneum at the base of the dorsal rnesentery just above" the dorsal
pancreatic element. These cells become mingled with the surrounding
mesenchymal tissue, thus forming the main substance of the spleen. Subsequent development results in the formation of a considerable mass,
filled with sinuses which communicate directly with the splenic veins.
Cells from the spleen are buddedyoff into these spaces and pass into the
circulation, where they apparently become transformed into blood corpuscles.
4.09 V THE CHICK
THE REGION OF THE MID—GUT
For purposes of definition, the fore-gut region may be said to terminate at the end of the duodenum, and this point is marked approximately by the opening of the bile duct. The mid~gut, therefore, is the
portion of the alimentary tract extending from the opening of this duct
to the point at which the gut contained in the tail fold begins. It is
difficult to define the latter point exactly at this time, except to say that
since the tail fold never becomes very deep, it is relatively near the
posterior end of the embryo, a short distance in front of the origin of
the allantois. This boundary between the mid- and hind-gut is marked
later by the intestinal caeca (see Chapter 13).
During the third and fourth days the folding-in process has been
going on rapidly in the region of the mid-gut, and due to this, and to
the growth of the entire body, the somatic umbilicus is so relatively constricted as to be called the umbilical stalk. Within it, as already noted,
are the allantoic stalk and the yolk-stalk. The former has always been
small, and the latter has necessarily shared in the constriction of the urnbilical walls. The result of these processes is obviously a mid-gut closed
in at every point save the relatively narrow opening into the yolk-stalk;
it is also a gut which still remains virtually straight. The section of
alimentary tract which has thus been defined is destined to become the
small intestine of the adult bird.
In concluding the discussion of this topic it is well for the student
to realize that there are two aspects to the umbilical constrictions just
indicated. There is, on the one hand, the absolute narrowing of the
umbilical opening. There is also in addition to this the immense growth
of the remainder of the embryo. The girth of the umbilicus is thus a
relative as well as an absolute matter, and the apparent reduction in its
size is due as much or more to the increase in size of the embryo as to
its own constriction.
THE REGION OF THE HIND—GUT
The remainder of the digestive tract posterior to the small intestine
is, by the above definition, the hind-gut, and constitutes the large intestine or rectum. This opens into a terminal chamber, the cloaca. There
is little to be said about the development of the rectum at this time,
since it remains short, uncoiled, and without appendages.
The cloaca at 96 hours consists of a chamber into whose anterodorsal wall there opens, as indicated, the rectum. Just back of the rectal
FOURTH DAY: THE HEART 401
orifice, the cloacal cavity also receives the Wolfiian ducts. Antero-ventrally below the rectal opening is the aperture of the allantois, while just
behind this on the ventral side of the chamber is the original anal plate,
or cloacal membrane (Fig. 193). It consists, as will be recalled, of a
fused plate of endoderm and ectoderm, and during embryonic life separates the cavity of the cloaca from the exterior. Posterior to these
apertures and the cloacal membrane, the cloacal chamber shows a
marked lateral compression.
THE CIRCULATORY SYSTEM
THE HEART
In order to understand the development of the heart during the
fourth and subsequent days, it will be necessary for the reader to refer
to the description of that organ at the end of the second day. Assuming
that this description is clearly in mind, we may then continue the account of the development on the fourth day, as follows:
Changes in the Proportion of the Parts. --The entire loop has
gradually been expanding so that its parts have tended to approach
one another. This has also resulted in a relative shortening of the two
ascending limbs, i.e., the posterior limb comprising the atrium and part
of the ventricle, and the anterior limb comprising another part of the
ventricle and the bulbus arteriosus. At the same time so great has been
the expansion of the transverse portion of the loop connecting these
two limbs that the limbs as such have almost disappeared. What remains of the posterior one is marked by what amounts to a constriction
just below the developing atrium. This apparent constriction, however,
is brought about not so much by an actual contracting of this region
as by the relative expansion of the parts above and below it. Since the‘
part above forms the atrium, and the part below is a portion of the
ventricle, the constriction between constitutes the atria-ventricular canal.
Changes in the Relative Position of the Parts.——At the same
time that these changes in shape and proportion have been occurring,
changes in the relative positions of the parts are also progressing. Of
these there are three principal ones which may be indicated thus:
(1) The bulbus arteriosus is swinging toward the median line beneath
the atrium (Fig. 209, D). (2) The ventricular region is moving backward behind the atrium and also somewhat toward the median line, the
region of the future apex pointing posteriorly. (3) To some degree as
402
THE CHICK
Fi . 209. —- The development of the heart of the Chick. From Kellicott
(C ordaze Development). A, F, after Hochstetter. B—E, after Greil.
A—E, ventral views of the heart. A. of a 40-hour embryo; B. of an embryo of 2.1 mm. head-length; C. of an embryo of 3.0 mm. head-length;
D. of an embryo of 5.0 mm. head-length; E. of an embryo of 6.5 mm.
head-length. F. Frontal section through the heart of an embryo of 9 mm.
head-length.
a. Atrium. b. Bulbus. d. Roots of dorsal aorta. e. Median endothelial
cushion (i.e., the cushion septum}. i. Interventricular groove. la. Left
atrium. le. Lateral endothelial cushion. to. Left ventricle. om. Vitelline
veins. p. Left pulmonary artery. ra. Right atrium. rv. Right ventricle. 5.
Intfrrventricular septum. sa. Interatrial septum. t. Roots of aortic arches.
entnc e.
E
1
»
FOURTH DAY: EMBRYONIC BLOOD VESSELS 403
a part of the latter movement, the posterior portion of the atrium into
which the sinus venosus opens is rotating forward. In this manner, it is
brought just over and then anterior to the atrio-ventricular canal, the
latter remaining at a comparatively fixed point between the ventricular
and atrial regions. Though not completed during the fourth day, these
movements are well under way at this time. Their progress, moreover,
is suflicient to show that their tendency is to place the parts of the heart
more nearly in their adult positions; i.e., the atrium anterior and dorsal,
and the ventricle posterior and ventral.
Interior Changes Involving the Growth of Septa.——Whi1e the
above external alterations in the form of the heart have been going on,
further internal changes are occurring as follows: (1) the interatrial
septum which started to form on the third day becomes more clearly
evident as, a sickle shaped membrane extending postero-ventrally from
the curved antero-dorsal wall, the back of the sickle being attached to
the wall. Eventually of course this septum, augmented by certain other
elements, completely divides the atrium into right and left chambers
(the atria). (2) At the apex of the ventricle, the interventricular septum
arises, and grows forward. Now since the ventricular apex has be</ome
posterior to both the atrio-ventricular canal and the bulbus arteriosus,
it is possible for the forward extension of this septum to meet them
both. This, it eventually does ( see Chapter 13). (3) At the same time
these septa are developing, a third one is beginning to arise within the
atrio-ventricular canal; it starts as two endothelial thickenings, one in
the floor, and the other in the roof of this canal. These are destined to
grow towards one another until they unite in the center of the atrioventricular aperture, thus dividing it into right and left parts. When
completed, this partition isknown as the cushion sept‘-tun (Fig. 209, F ) .
EMBRYONIC BLOOD VESSELS
The Arteries.
The Aortic Arches.-—— It will be recalled that during the third day,
the first pair of aortic arches disappeared, leaving the anterior extensions of the dorsal aortae as the internal carotids. In a similar manner,
extensions from the bases of the first arches continue anteriorly as the
external carotids. Upon the fourth day, the second aortic arches are
likewise obliterated, and the two pairs of oarotids continue posteriorly
to the dorsal and ventral ends of the third pair of arches. At the same
time two new pairs of aortic blood vessels develop in the vestigial fifth
visceral arches behind the fourth and last pair of visceral pouches.
£3
5
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404 THE CHICK
These are the fifth and sixth aortic arches (Fig. 210, A). The fifth pair
is small and quite transitory, being actually attached both dorsally and
ventrally to the anterior sides of the sixth pair. Shortly after the sixth
arches have thus arisen a small branch develops from about the middle
of each and connects with the rudiments of the pulmonary arteries
growing out from the lungs. In this manner the pulmonary arterial sysstem is completed, though throughout embryonic life the branches just
indicated remain small.
LEFT SIDE RIGHT SIDE
4} day 8 day
ductus Batallt subclavian artery
   
 
internal carotid
 
dorsal aorta
artery '  ‘ __:  carotid artery
h ‘ ' ';. 3 3rd aortic arch
5th BOFUC arches I ' j: 4th (systemic)
vitelline artery mm‘ arch
I ’d pulmonary artery th aortic arch
externa caroti
ITIGTY runcus arterlosus
A B
cruncus urtertosus '
Fig. 210.—Aortic arches of the Chick. Left side from a 45-day injected embryo.
Modified from Lillie, after Locy. Right side reconstructed from saggital sections of
an 8-day embryo. Modified from Lillie.
From this description, it is clear that only the ventral portions of the
sixth arches take part in the formation of the pulmonary arteries. The
dorsal portion of each arch, on the other hand, is known as the duct of
Botallo or ductus arteriosus, which, as will be noted below, atrophies
at the time of hatchinv.
The Subclavian Arteries. ——- As noted under the description of external features the primordia of the anterior and posterior limb buds appear by the end of the day as broad swellings on the sides of the body.
Correlated with this we find that on the fourth day the eighteenth segmental artery on each side gives rise to a branch which extends out toward the respective bud. It is the primary subclavian artery. From it,
at the point where it enters the limb, a branch also extends anteriorly
toward the third aortic arch. This is destined to form the permanent
subclavian (see fifth day).
The Sciatic Arteries.-—-Posteriorly, a pair of segmental arteries also
enlarge and grow out toward the hind limb buds. These vessels become
Fig. 211.——Diagrarns illustrating the formation of the
omphalomesenteric and umbilical veins, in the Chick, ventral view. From Kellicott (Chordate Development). Alter
Hochstetter. A. At about 58 hours. B. At about 65 hours
the veins are joined dorsal to the gut by a short transverse
vessel. C. At about 75 hours the anterior intestinal portal
has moved posteriorly somewhat so that the transverse vessel appears to be more anterior. At the same time, the left
side of the loop, which its development created. has disappeared. D. At 80 hours a second loop has been formed by
the fusion of the vitellineveins beneath the gut. E. At
about one hundred hours the right side of this new loop
has also disappeared. F. At about 130 hours, just before
the disappearance of the main portion of the ductus venosus within the liver. This figure is obviously on a much
smaller scale than E. A
c. Vena cava posterior (inferior). dC. Ductus Cuvieri.
dv. Ductus venosus. g. Gut. hl. Left hepatic vein. hr. Right
hepatic vein. 2. Liver. a. Omphalomesenteric or vitelline
vein (the posterior continuation of the ductus venmsus).
p. Anterior intestinal portal. pa. Rudiment of pancreas. ul.
Left umbilical vein. ur. Right umbilical vein. 1;. Vilelline
vein. I, II. Primary and secondary venous rings around
the gut.
406 THE CHICK
the sciatic arteries, and as the legs develop they grow with and supply
them.
The Umbilical Arteries. — During the fourth day, each sciatic artery
gives off at its base a branch which extends into the allantois. These are
the umbilical or allantoic arteries. Later {eighth day), the right member of this pair starts to disappear, while the left becomes a very important embryonic vessel, furnishing blood to the allantois. Indeed, so
large does it become that the left sciatic seems for a time to be merely
a branch from it.
The Renal Arteries and Those of the Conads. -—-Numerous branches
from the dorsal aorta supply the mesonephros at this time, and later on
a few of these persist as the renal arteries. Branches from the aorta also
supply the reproductive organs as these develop.
The Veins. _
The Vitelline Veirzs. —- It will be recalled that at the close of the third
day, the vitelline veins within the embryo had been united by a transverse vessel dorsal to the intestine, so that the latter was surrounded by
a venous ring. Between this time and the close of the fourth day, ‘inither changes have taken place in this region, as follows: Very shortly
after the transverse vessel has been formed the left side of the above
ring disappears (Fig: 211, C ). Later, as the anterior intestinal portal
moves backward, the vitelline veins between the poltal and the transverse vessel fuse with one another beneath the intestine. In this manner, a venous ring is again formed around the posterior extremity of
the fore-gut, and in this case the right side presently begins to grow
smaller. Anterior to the vitelline veins the ductus venosus continues to
receive capillaries from the surrounding liver (Flo: 211, D).
The Cardinal Veins. -— The anterior cardinals, as indicated -in the
previous chapter, have, by this time, reached a stage when they may be
known as jugulars, while the posterior cardinals continue as previously
described. The subcardinals which started to form on the third day become distinct vessels and presently acquire several direct connections
with the posterior cardinals lying on the dorso-lateral sides of the
mesonephros (Fig. 212).
The Inferior or Posterior Vena Cam. -~— This important vessel of the
adult Bird begins to develop at this time out of some of the capillaries
in the dorsal part of the liver on the right side. Slightly further back
it is also augmented by venous islands in a fold (the caval fold) of one
of the liver mesenteries. These capillaries and venous islands soon fuse
Fig. 212. -— Reconstruction of the venous system of a Chick, 90 hours, ventral view.
From Lillie (Development of the Chick). After Miller.
A.o.m. Omphalomesenteric. (vitelline) artery. a.sc.s. Left sciatic artery. A.u.s.
Left umbilical artery. 6. Vessels enclosed within ventral side of mesonephros c.
One of the direct connections of subcardinal with posterior cardinal. V.c.p.d.,s.
Right and left posterior cardinal veins. V.c.i. Venn cava inferior. V ..sc.d.,s. Right
and left subcardinal veins.
407
408 THE CHICK
together so as to form a definite vein which empties anteriorly into the
ductus venosus (Fig. 211, E), and posteriorly establishes a connection
with the right subcardinal (Fig. 212). Its subsequent development will
be described in the following chapter.
The Umbilical Veins. ——— Upon the fourth day, the veins of the lateral
body wall acquire connections with efferent vessels which have developed in the allantois, and at the same time, the right vein begins to disappear, along with the transitory subintestinal vein. The left vein on the
other hand persists, but presently loses its anterior outlet into the ductus
Cuvieri. At the same time, however, it develops new connections with
the anterior half of the ductus venosus (Fig. 211, D, E). Through these,
therefore, blood from the allantois flows quite directly into the latter
vessel, without taking any extensive part in the hepatic portal circulation. Later, these connections with the ductus venosus/fuse into one,
which thus constitutes _the anterior extremity of the single umbilical vein
(Fig. 211, F). Eventually this vein acquires a median position in the
embryo instead of its original lateral one. Subsequent to hatching, its
proximal portion persists as a vein of the ventral body wall.
The Pulmonary V eins.—-These vessels also develop at about this
period in connection with the rudiments of the lungs, and presently become connected with the heart in the region of its left atrium.
EXTRA-—EMBRYONIC BLOOD VESSELS
The Arteries.——-During the fourth day the proximal portions of
the vitelline arteries become fused with one another so as to leave the
dorsal aorta as a single vessel. This fusion, however, occurs for only a
relatively short distance, and never passes beyond the end of the umbilical stalk. From that point, the two main vessels continue to run out
laterally, branching as they go, and terminatingin a network of capillaries just inside the sinus terminalis. Subsequent development does
not fundamentally alter the arterial plan except that as the septa of the
splanchnopleure develop in the yolk-sac, the arterial capillaries come
to occupy the deeper portions of these septa.
The Veins.-—By the end of this "day the right anterior vitelline
vein has disappeared, while the left anterior vitelline vein and the
posterior vein, are well developed. The lateral vitelline veins have also
become larger and more definite at the point where they extend outward in company with the arteries. Furtherout in the area vasculosa,
they continue to branch extensively, the branches connecting with the
intermediate veins as already noted. By this time, however, these conFOURTH DAY: THE PROSENCEPHALON 409
motions are so pronounced that the intermediate vessels appear merely
as the finer endings of the lateral vitellines, uniting these veins with the
sinus terminalis (Fig. 182). Subsequent to the tenth day, the anterior
and posterior vitelline veins are gradually eliminated, the lateral veins
persisting as the main efferent vessels of the yolk-sac. After the tenth
day, the sinus terminalis is no longer distinct, becoming obliterated by
a mass of capillaries. These capillaries and the vessels with which they
are connected, forming the area vasculosa, then continue to spread over
the yolk in company with the yolk»sac mesoderm. Thus, like the latter,
they come at last virtually to surround it.
THE NERVOUS SYSTEM
This system, like the others, continues to develop through embryonic life. The differences observed in it between the fourth and fifth
days, however, are not, in most respects, very great. Therefore, since it
is not proposed to carry a detailed chronological description of any of
the organs beyond the fifth day, we shall conclude the account of the
nervous system in the present chapter.
THE FLEXURES
The cranial and cervical flexures of the brain and nervous system
have already been noted in the account of external changes through the
fourth day. As has been indicated in the general discussion of this matter, only one of the flexures just named, i.e., the cranial, is permanent,
the cervical gradually straightening out until it is entirely gone. Also,
though to a smaller extent than in the Frog, even the cranial flexure is
partly obscured in the adult brain by the development of the cerebral
hemispheres and other parts. There is now to be noted a third flexure,
which though barely visible on the fourth day, later becomes quite
marked. It, like the cranial, is permanent and also like the cranial is
never entirely obscured. This is the pontine flexure which consists of a
ventral bulge in the thickened floor of the myelencephalon (Fig. 214) .
THE PROSENCEPHALON
The Te1encepha1on.——-The cerebral hemispheres continue to increase in size during the fourth day, and their lateral walls in particular, are thickening to form the corpora striata, The other features already noted as characteristic of this portion of the brain have also
increased in prominence. As regards subsequent development the cere410 THE CHICK
bral hemispheres ultimately become one of the most noticeable portions
of the brain, their backward growth causing them to overlap, and to
conceal partially the large optic lobes. Their surface, however, never
attains the complicated convolutions so characteristic of the Mammal.
Anteriorly, beginning about the eighth day, small portions of these hem
Fig. 213.—-Optical longitudinal section of the head of an embryo of 395. From
Lillie (Development of the Chick).
Atr. Atrium. 8.1:. Bulbus arteriosus. D.v. Ductus venosus. Lg. Laryngotracheal
groove. Es. Oesophagus. or.pl. Oral plate, which has now ruptured; Parenc. Parencephalon. Plz. Pharynx. Slam. Stomach. Synenc. Synencephalon. Th. Thyroid. 5.12.
Sinus vcnosus. V en. Ventricle. Other abbreviations as before.
ispheres become partially constricted away from the main posterior
parts to form the olfactory lobes.
Concerning other parts of the telencephalon, as already indicated, the
anterior commissure appears in the midst of the torus transversus. On
the fifth day, also, an evagination develops at the antero-dorsal boundary of the lamina terminalis just between it and the velum transversum;
it is the paraphysis. This structure virtually marks the boundary between the telencephalon and diencephalon, Lillie placing it in the former. and some anatomists in the latter. Above this body occurs the inward bend of the wall which constitutes the velum transversum, whose
more dorsal half at least, according to most authorities, lies definitely
in the diencephalon.
FOURTH DAY: THE PROSENCEPHALON 411
i e
- Hyp. pant. Ft.
Com. ant. Rec. op.‘
<
' Fig. 214.—~Dissection of the brain of an 8-day Chick. From Lillie { Development
of the Chick). The arrows shown in the figure lie near the dorsal and ventral
boundaries of the foramen of Monro.
ch.Pl. Choroid plexus (anterior). Com.ant. Anterior commissurc Com.I’ost. Posterior commissure. C.str. Corpus striatum. Ep. Epiphysis. H. Hemisphere. Hyp.
Hypophysis (anterior stomodaeal, part). L.t. Lamina terminalis. Myel. Myelencephalon. olf. Olfactory nerve. ap.N. Optic chiasma. op.L. Optic lobe. Par. Paraphysis.
Ptzren. Parencephalon. pl.enc.v. Plica encephali ventralis. pon.t.Fl. Pontine flexure.
Recap. Recessus opticus. S.Inf. Saccus infundihuli. Tel.med. Telencephalon medium. Th. Thalamus. T.tr. Torus transversus. Tr. Commissura trochlearis.
hThe lines a-a, b-b, c-c, d-d, e-e, f-f, represent the planes of sections not figured in
t is text.
The wall of this portion of the fore-brain, therefore, gives rise to the
anterior comrnissure and the cerebral hemispheres. Its cavity forms the
anterior part of the third ventricle into which the lateral ventricles of
the hemispheres open through the foramina of Monro.
The Diencepha1on.~——The anterior part of the roof in this region
of the brain, as noted, apparently consists of the dorsal half of the
velum transversum which later becomes folded to form the anterior
choroid plexus. Eventually this plexus develops anterior branches extending forward into the lateral ventricles of the cerebral hemispheres.
Posterior to the plexus the epiphysis shows no great change on the
fourth day. Later, however, it grows out into a long narrow tube, whose
412 THE CHICK
end is dilated and possessed of numerous buds, the epiphysial or pineal
gland. Just posterior to this organ at the boundary between the fore- and
mid-brains, the posterior commissure eventually develops within the
broad constriction which has marked this point from the first.
During the fourth day no striking development occurs in the lateral
Fig. 215.——Median sagittal section through the brain of the Chick of 12_to 13
days. From Kupiier (He-rtwig's Handbuch; etc.).
c. Cerebellum. ca. Anterior commissure. cd. Notochord. ch. Habenular commissure. ci. Infundibular commissure. ck. Central canal of spinal cord. cp. Posterior
commissure. cpa. Anterior pallial commissure. cs. Spinal commissure. cu. Cavum
cerebelli. cw. Optic chiasma. dr. Epiphysial (Pineal) gland. dt. Decussation of the
trochlear (IV) nerve. e. Epiphysis. ex. Paraphysis. hm. Cerebral hemisphere. hy.
Hypophysis (anterior part). 1'. Infundibulum. le. Ependymal lamina of the roof of
the fourth ventricle. lo. Olfactory lobe. 1p. Posterior lobe of cerebral hemisphere.
M. Mesencephalon. opt. Optic chiasma. pch. Choroid plexus third ventricle. pl.
Choroid plexus of fourth ventricle. re. Epiphysial recess. ro. Optic recess. 5. Saccus
infundibuli. si. Posterior intracephalic furrow. tp. _Tuberculum posterius. lpi.
Tuberculum mammillare. tr. Torus transversus. wz. Velum medullare anterius. vi.
Median ventricle of telencephalon. up. Velum medullare posterius.
or ventral region of the diencephalon. Subsequently, however, the former region becomes greatly thickened to form the thalami. On the ventral side, the fate of the infundibulum has already been described (see
discussion of fore-gut, third day) while the optic chiasma comes to comprise a thick bundle of fibers from the optic nerves.
The floor of this posterior division of the fore-brain thus gives rise to
the optic stalks, the optic chiasma and the infundibulum, while the optic thalami develop within the lateral walls. The roof forms the anterior
choroid plexus and the epiphysis; the cavity constitutes the posterior
part of the third ventricle. C
FOURTH DAY: SPINAL CORD AND NERVES 413
THE MESENCEPHALON
There is nothing in particular to be said concerning the development
of this region on the fourth day. Later we find that the growth and thickening of the dorso—lateral parts of the mid-brain greatly exceed that of
a narrow dorso-median strip, thus producing the two large optic lobes,
which the median strip separates_ from one another by a fissure. Ventro.
laterally, the sides and floor of the mid-brain also become thickened,
constituting the crura cerebri. This thickening finally results in narrowing the central canal to form the aqueduct of Sylvius or iter, which con.
nects the cavities of the third and fourth ventricles.
THE RHOMBENCEPHALON
The Metencephalon. ~ The thickening which was noted in the roof
of this region on the third day continues to increase, resulting finally in
the production of a large median lobe, and two small lateral lobes
united with it. The body thus formed extends backward somewhat so
that it partially overhangs the myelencephalon. It is the cerebellum.
About the ninth day, transverse fissures appear on the surface of this organ, which deepen as development proceeds. The ventro-lateral walls of
the metencephal on, which have also been thickening, come eventually to
form the pans Varolii.
The Myelencephalon. —— It has already been stated that the roof
of this region of the brain remains thin; it eventually forms the choroid
plexus of the fourth ventricle. The ventral and ventro—latcral walls, however, showed signs of thickening on the third day. This tendency increases, and these walls finally constitute the medulla oblongata.
THE SPINAL CORD AND SPINAL NERVES
The description of the development. of the cord and of the somatic spinal nerves was completed in Chapter 11. The completion of the sympathetic. and parasympathetic systems, i.e., the autonomic, will now be noted.
The Sympathetic and Sacral Parasympathetic Systems.———It
will be recalled that at the end of the third day the primary sympathetic
and sacral parasympathetic systems had just been established. They consisted of two slender cords and their ganglia lying just dorso-lateral to
the dorsal aorta, and extending from the region of the vagus ganglion to
the tail. On the fourth and fifth days neuroblasts migrate from each
ganglion of the primary systems to positions above the primary cords
just median to where each somatic trunk divides (Fig. 216) . Each such
414 THE CHICK
aggregation of neuroblasts, or ganglion, forms neurons which again send
axones anteriorly and posteriorly to form the paravertebral or permanent sympathetic and sacral parasympathetic cords. For a time both
primary and secondary cords exist to some degree, but eventually the
primary cord is mostly eliminated. It
is generally thought that neuroblasts
from the ganglia ‘of the permanent
cords also migrate to the mesentery
and viscera to form the visceral plexuses, but, save for the sacral ganglia,
Yntema, ’55 denies this, and claims
that in the Chick at least, all these
visceral plexus neuroblasts are from
the vagus crest (see below). Though
unorthodox this view is supported by
extensive investigations.
It should be emphasized at this
point that all the neurons so far de
Fig. Z16.-—Diagram of the chief
elements of the sympathetic nervous system of the Chick, in trans
verse section: From Kellicott
(Chordate Development). After
His, Jr.
a. Dorsal aorta. op. Aortic plexus. J. Dorsal (afiercnt) root of spinal nerve. g. Spinal ganglion. i.
Intestine. m. Me-sentery. n. Notochord. R. Remak’s ganglion. s.
Splanchnic plexus. sg Sympathetic
elements in intestinal wall. 1!.
Mesonephric tubules. v. Ventral
(efferent) root of spinal nerve. I.
Primary sympatheticcord. 11. Secondary sympathetic cord. The rami
communicantes are only partially
scribed as originating from the neural
crests, constitute only the postganglionic elements of the systems under discussion. The preganglionic neurons on
the other hand are all derived from
neuroblasts in the neural tube. These
cells at first occupy the ventro-lateral
parts of the tube along with the somatic motor neurons. From here the
sympathetic and sacral parasympathetic neuroblasts separate from the somatic neuroblasts, and migrate dor
5l‘°‘””° sally taking up positions on either
nuclei of Term’. From these, cell fibers
grow out through the ventral somatic nerve roots to the points‘ where
these roots join their respective dorsal roots. The preganglionic sympathetic and sacral parasympathetic fibers then leave the somatic roots
and through short connections, the secondary or permanent rami commurzicantes, enter the ganglia of the permanent sympathetic and sacral
parasympathetic cords. Either in these ganglia (sympathetic) or in the
ganglia of the visceral plexuses (parasympathetic) they synapse with
the postganglionic fibers of these plexuses. '
side of the neural canal in the
FOURTH DAY: SPINAL com) AND NERVES 415
It should now be noted that all the nerves and fibers of the autonomic
system, i.e., the sympathetic and sacral parasympathetic already dis.
cussed, and the cranial parasympathetic described below, are strictly
motor. Nevertheless there are sensory fibers which convey sensations
from the viscera. These arise from neurons in the cranial and spinal
ganglia where all sensory neurons outside the nose, eye, and ear are
located. They leave the dorsal roots through the rami communicantes
and accompany the motor fibers of the autonomic system to the viscera,
though not part of that system.
The Cranial Ganglia, Mixed Nerves, and Cranial Parasyrnpathetics. Trigeminal Ganglion and Nerve.—-—It has been stated that
this ganglion has the form of an inverted Y. On the fourth day fibers
from the anterodorsal branch, i.e., the ophthalmic, pass anteriorly along
the dorsomedian wall of the optic vesicle. Eventually these ophthalmic
fibers, mostly sensory, reach the face and beak. The other branch of the
Y extends toward the angle of the mouth, where it also divides, one part,
the mandibular, is a mixed nerve, and supplies the lower jaw. The other
all sensory branch, the maxillary, supplies the upper jaw. As usual all
sensory fibers arise from neurons in the ganglion, while the motor fibers
are from neurons in the brain.
T he Acustico-facialis Ganglion and Nerves.-—As indicated above,
the ganglion which gives rise to the VII and VIII nerves is at first in a
single mass. During the fourth day, however, the antero-ventral portion
separates from the remainder, and gives rise to a nerve which extends
chiefly along the hyoid arch, though possessing also a small branch to
the mandibular. This is the rudiment of the future VII or facial nerve
with :1 motor component from the medulla. The remainder of the ganglion gives rise to the VIII or auditory nerve which is purely sensory,
and which communicates with the inner ear as described below.
The Glossopharyngeal Ganglion and Nerve. —-The origin of the IX
cranial ganglion was noted in the account of the second day, where it
was indicated as lying above the third arch. The IX nerve appears on
the fourth day and extends into this arch. Later another branch enters
the second arch, and together they eventually supply the tongue and
pharyngeal region. ,
The V agus and Cranial Parasympathetic S'ystem..——Neuroblasts in
the crest and an adjacent placode above the third branchial pouch, together with neuroblasts within the brain, produce the vagus complex as
follows: Upon‘ the fourth day the crest part of the X ganglion separates
from the placodal portion, and eventually produces the ganglion jagu~
416 THE CHICK
lare, the placodal part producing the ganglion nodosum. The exact
origin of all the neural elements of the X nerve complex in the Chick is
still uncertain, but the situation seems to be thus: Neuroblasts of the
ganglion jugulare produce the somatic sensory neurons, the somatic
motor neurons arising from within the medulla. The crest produces all
postganglionic neurons of the cranial parasympathetic system (Yntema
and Hammond, ’55) except possibly those of the ciliary ganglion, said
by Levi-Montalcini and Amprino, ’47, to be derived from mesenchyme;
the preganglionic neurons of this system arise within the medulla. From
the ganglion nodosum nerves pass into the fourth and fifth neural arches
and posteriorly to the heart, lungs, stomach, and intestine, while the
ganglion moves back into the thorax. Eventually a part of the nodosum
is detached as the ganglion cervical primum.
THE CRANIAL MOTOR NERVES
The Mo’cor—ocu1ar or III Nerve. — The early development of this
cranial motor nerve has already been described. During the fourth day,
it passes down beneath the optic stalk, and there enters a ganglion. This
receives a connection from the ophthalmic branch of the V nerve, and
is known as the ciliary ganglion. The III nerve ends by innervating the
superior, inferior, and internal rectus, and the inferior oblique muscles
of the eye when these develop.
The IV or Trochlearis Nerve.———This motor nerve does not appear until the fifth or sixth day, but will be described at this point. It is
peculiar as a motor nerve, in that it arises from the dorsal side of the
brain, at the bottom of the isthmus. It has no connection with any ganglion, and ultimately innervates the superior oblique eye muscles.
The VI or Abducent Nerve. —— This is a perfectly typical motor
nerve, appearing toward the end of the fourth day. It has no ganglion,
and arises from the ventral side of the medulla median to the point of
origin of the fifth nerve. It innervates the external rectus muscle of
the eye.
The XI or Spinal Accessory Nerve. ——There is no data on the
development of this nerve in the Chick (Lillie).
The XII or Hypoglossus Nerve. — This nerve develops during the
fourth day from two pairs of ventral roots on the medulla at the level
of the third and fourth somites. There are no ganglia, and the roots are
evidently serially homologous with the ventral roots of the spinal nerves.
The nerve to which they give rise eventually innervates the floor of the
pharynx. '
FOURTH DAY: THE EYE 417
THE ORGANS OF SPECIAL SENSE
THE EYE
At the end of the third day the inner wall of the optic cup had thickened, and the whole cup was in the process of enlarging. The lens, meanwhile, had separated from the external ectoderm, and the side of the
lens toward the cup had also begun to thicken. The further development
of the eye may be described as follows:
Parts Connected with the Optic Cup.—-During the fourth day,
pigment begins to appear in the wall of the optic cup nearest the brain,
i.e., its outer wall. At the same time, there is developing upon the innermost surface of the inner wall, the internal limiting membrane. Beneath
this membrane, but still toward the inner side of the inner wall, as noted
on the second day, neuroblasts near the fundus have sent out axones.
These have passed over the retinal elements just beneath the limiting
membrane, and have reached the optic stalk through the proximal part
of the choroid fissure. Here they proceed among cells of the ventral wall
of the stalk, and late on the fourth or early on the fifth day, reach the
brain and form the optic chiasma. Later many more-fibers grow through
the ventral part of the optic stalk, causing it to swell so that the original internal cavity is obliterated. It may then be termed the II or
optic nerve. In this connection it may further be noted that during the
fifth and sixth days the processes of growth occur in such a manner as
to alter the relative position of the point of attachment of the optic stalk
to the cup. The result is that at the completion of these processes the
point in question is no longer at the ventral edge of the cup, but approximately at its center, opposite to the lens.
Subsequent to the fourth day, other changes are also occurring in the
walls of the optic cup. As the various cell layers of the retina are formed
in the inner wall, this wall shows difierentiation into two zones. The
central and larger of these, which includes the fundus, is called the
retinal zone, i.e., the retina proper, and it is only within this zone that
the above retinal elements are developed. The remainder of the inner
wall consists merely of a band around the rim of the cup, and is known
as the lenticular zone. The line of separation between the two is known
as the cm serrata (Fig. 217). Within the retinal zone, the outer wall
forms the pigmented layer of the retina,'but never completely fuses
with it. In the lenticular zone, on the contrary, fusion between inner and
outer walls is complete, pigment penetrates them both, and both remain
418 THE CHICK
ant. ch. ‘ '
corn.
iris
Fig. 217. ——Frontal section of the eye of an eight-day Chick. From Lillie (Development of the Chick).
ant.ch. Anterior chamber of the eye. ch. Choroid coat. cil. Ciliary processes.
Corn. Cornea. l.e.l. Lower eyelid. n.m. Nictitating membrane. olf. Olfactory sac.
op.n.- Optic nerve. as. Ora serrate. p. Pigment layer of the optic cup. post.ch.. Posterior (vitreous) chamher oi the eye. ret. Retina. scl. Sclerotlc coat. scl.C. Sclerotic
cartilage. u.e.l. Upper eyelid‘
relatively thin. From this zone, in connection with certain mesenchymal
elements, are differentiated the iris and the ciliary processes. While these
parts are forming, the cavity of the optic cup is being filled with a gelatinous matrix containing fibers. Both elements are probably derived
from certain cells of the retinal and lenticular zones, and together are
known as the vitreous humor. Certain of the fibers of the humor are con
»
l
8
i
FOURTH DAY: THE EYE 419
nected with the ciliary processes, and help to support the lens. Finally,
the outside of the cup is gradually covered by two layers of mesenchymal origin. The inner is the choroid coat, and the outer the sclerotic
coat, the latter being partly cartilaginous.
The Pecten. -—This body is_also developed in connection with the
optic cup and choroid fissure, but is entirely peculiar to the Birds. It
Fig. 218. — Diagrammatic reconstruction of the pecten of the eye
of (Chick embryo of 71: days’ incubation. From Lillie (Development of the Chick). After Bernd.
Ch.fis.l. Lip of the choroid fissure. Ch./iss. Choroid fissure. Mes.
Mesenchyme. Mes.b. Upper edge of the rpesenchymal ridge covered by the lips of the choroid fissure. Mes.K. Thickening of the
edge of the mesenchyrnal ridge. op.C. Optic cup. 0.St. Optic stalk.
P. Pecten. P.B. Base of the pecten.
The arrow indicates the direction of growth of the lips of the
choroid fissure over the mesenchymal ridge. The line d shows the
plane of the section reproduced in Fig. 219.
has seemed well, therefore, to emphasize it by a separate description.
It arises during the fourth day in the form of a blood vessel embedded
in mesenchyme. This mesenchymal mass is in the shape of a ridge which
enters the cavity of the cup through the choroid fissure near its proximal
end. The distal end of the fissure between this mesenchyme and the rim
of the cup has, meanwhile, been closed. On subsequent days, the mesenchymal ridge pushes up into the cavity, while at the same time it is
being gradually covered over by the in-turning and up-growth of the
edges of the choroid fissure on either side of it. This covering soon becomes more prominent than the relatively thin ridge of mcsenchyme
420 THE CHICK
   
which it has overgrown, and presently (eighth day) the two parts he.
come indistinguishable. Though remaining constricted at its base, the
ridge of fused tissues inside the cavity of the cup continues to grow
somewhat, and later becomes folded, assuming the appearance of a fan,
though in most Birds it is more comb-like, and hence is named the
pecten. It is very vascular and probably helps to nourish the retina. The E
opening in the choroid fissure between pecten and optic stalk provides ‘
Fig. 219.-S£-ction in the plane of (1. of Fig. 218.
to show the histological structure. From Lillie (Development of the Chick). After Bernd. Bl.v. Blood
vessel in mesenchymal ridge. il. Retinal layer of op~
tic cup. Other abbreviations as in Fig. 218.
the exit for the optic nerve fibers from the retina. A few of these fibers
runrdirectly to this point, but the majority come to the base of the pecten,
and run along its sides to the place of exit (Figs. 218 and 219).
The Lens. — At the end of the third day, the inner wall of the lens
vesicle had thickened considerably by virtue of the lengthening of its
cells. This process continues for several days until the cavity of the vesicle is entirely obliterated. Moreover, inasmuch as the lengthening of
the central cells is greater than that of those at the periphery, the inner
surface of the lens becomes distinctly convex (Fio. 217). These lengthened cells of the inner wall form the core of the future lens, while the
cells of the outer layer toward the ectoderm form a simple flat epithelium. The lens now grows, largely by the production of cells at its equa- ‘V
tor where the inner and outer walls meet. These cells become fiber-like
. x
FOURTH DAY: THE EAR 4.21
and wrap themselves around the original elements which form the core,
thus increasing the size of the lens by the addition of concentric layers
of cells.
The Cornea, the Anterior Chamber, and the Lids. ——The
cornea at first consists merely of the external ectoderm opposite the
lens. On the fourth day, however, this layer is augmented internally by
a thin non-cellular layer of mesenchymal origin. On the fifth day, this
thickens slightly, and begins to be covered on the side toward the lens
by a third layer formed of mesenchymal cells. Later, the middle layer
becomes cellular by the migration into it of cells from the mesenchyme,
while the third and innermost layer forms a typical epithelium. The latter finally becomes continuous at its edges with the cells of the sclerotic
coat. The cornea thus constituted arches outward slightly, and thus a
chamber is formed between its inner layer and the front of the lens. This
is the anterior chamber, and it becomes filled with the aqueous humor.
The lids begin to develop about the seventh day as folds of the integument surrounding the cornea (Fig. 217).
THE EAR
The Internal Ear.—-At the end of the third day, the otocyst, or
future internal ear, was in the form of a sac. The uppermost portion of
the sac had been slightly constricted away from the lower major portion, and had started to grow upward somewhat as the rudiment of the
endolymphatic duct. This upper portion, furthermore, still retained its
narrow tubular connection with the exterior (Fig. 206). There is, in
these parts, no marked change characteristic of the fourth day. Upon the
fifth day, however, the connection of the endolymphatic duct with the
exterior is entirely lost. Moreover, the opening of the duct’ into the sac
is being gradually shifted ventrally along the median side of the latter.
At the same time, the dorsal part of the duct is continuing to grow upward, and expanding to form the means endolymphaticus. Eventually,
this becomes embedded in mesenchyme above the hind-brain.
While these events are taking place in connection with the formation
of the endolymphatic duct the remaining major portion of the otocyst is
developing further, as follows: Upon the early part of the fifth day,
there arises from its dorsal half a vertically elongated, hollow out-pushing in the direction of the ectoderm. Then a horizontal out-pushing appears just beneath the first, and therefore at about the equator of the
otocyst. Presently a vertical split develops in the ventral part of the
vertical out-pushing and soon extends dorsally, thus dividing it into an
..q,,,,,,.,/,,.,,a...,..~.,«._.. was 1—,.,~.«— <,..,..«
422 THE CHICK
anterior and a posterior ridge. The anterior, posterior, and horizontal
ridges which have thus arisen are the rudiments of the respective semicircular canals. These canals eventually develop by a gradual constricting away of the hollow ridges, so that they become separated from the
   
Fig. 220.——Model of the auditory labyrinth of the
the right side of a Chick embryo of 8 days and 17
hours; external view. From Lillie (Development of
the Chick). After Riithig and Brugsch.
A.zz. Ampulla of the anterior vertical semicircular
canal. A.l. Ampulla of the lateral horizontal semicircular canal. A.p. Ampulla of the posterior vertical
semicircular canal. C.a. Anterior vertical semicircular canal. C.l. Lateral horizontal semicircular canal.
C.p. Posterior vertical semicircular canal. D.c. Ductus cochlearis. D.e. Endolymphatic duct. La. Lagena.
Sa.c. Endolymphatic sac. U. Utriculus (utricle).
otocyst everywhere except at their ends. During this process a dilation
occurs" on each canal to form its ampulla. The remainder of the dorsal
portion of the otocyst into which the canals open is the utricle.
Meanwhile, most of the ventral part of the otocyst has grown downward and also turned backward and toward the median line of the head.
_,Itsi end forms the lagena, and the portion connecting this with the utricle ‘is the ductus cochlearis or cochlear duct. The sacculus arises about
the seventh day as a pouch on the median side of the uppermost portion
FOURTH DAY: ORGANS or SPECIAL SENSE 423
of the ventral part of the otocyst, i.e., just above the point where the latter receives the ductus cochlearis (Fig. 206, B).
The parts of the inner ear thus "far described constitute the membranous labyrinth (Fig. 220). The walls of this labyrinth are composed
of epithelium, and its cavity is soon filled with the endolymphatic fluid.
Except for small areas within the ampullae and at certain other points,
the above epithelium becomes flat. At these points, however, elongated
sensory cells end in hairs which project into the fluid, and among these
cells grow the endings of nerve fibers (axones) coming from the VIII
cranial ganglion.
On ‘the sixth day, the mesenchyme which immediately surrounds the
developing labyrinth begins to form a membrane (membrana propria)
in close contact with it. At the same time the more peripheral mesenchyme is forming a cartilaginous case, separated slightly from the labyrinth and its membrane, but following all its contours. The space between the two is called the perilymphatic space. It is bridged by tissue
which carries the nerves and blood vessels, and is filled by the perilymphatic fluid derived from loose mesenchyme tissue left within the
space. The cartilaginous case later becomes ossified, and is known as
the bony labyrinth. In it, on the side toward the middle ear, are two
small openings, the fenestra ovalis, and the fenestra rotunda.
The Middle Ear, or Tubo-tympanic Cavity. — As was stated in
connection with the alimentary tract, the first visceral clefts are closed
during the fourth day, and the ventral portion of the pouch of each disappears. The dorsal portion, however, grows up toward the respective
otocyst, and during the fifth and sixth days comes between it and the external epithelium. Each pouch then starts to enlarge, and the space
within it is the dorso-lateral portion of one of the two tuba-tympanic
cavities. Meanwhile," beginning on the fourth day, the ventro-median
portion of each cavity is developed, as follows. In the antero-dorsal region of the pharynx, a horizontal shelf has grown backward, so as to
produce a dorsal chamber virtually separate from the space beneath.
Laterally, the part of each tubo-tympanic cavity already developed
opens into this newly constituted dorso-median chamber. Then, as
growth proceeds, an increasing portion of this chamber becomes drawn
out into the respective cavities. Thus eventually the larger part of each
middle ear space is really developed in this manner, rather than directly from the original “ gill” pouch. When these processes axrgeorri-1 .’~.
plete the median part of the dorso-median chamber still $hains‘“as‘* *'
such. while its lateral parts constitute the Eustachian tubes e({ have a \‘f‘_
V S Q Alhlnbcd ) O
‘ O
4- K
“'r\""'/I lg‘
424 THE CHICK
common opening into the mouth by a single median slit-like aperture in
the horizontal shelf. With regard to the cavities themselves two other
points remain to be noted. First as in the case of the Frog, each tubotympanic cavity contains a bone, the columella. Its development can
best be described, however, in connection with the tympanum. Secondly
there is the peculiar relation which exists between the tubo-tympanic
cavities and certain of the other bones of the Bird’s skull. These bones
like bones in other parts of the Bird skeleton to be described later contain spaces which give lightness to the body. The case of the head bones
is noteworthy at this point, however, because in some of them the spaces
are formed and filled by outgrowths from the tubo-tympanic cavities
(Bremer, ’40). v
The External Auditory Meatus and the Tympanum. —-It will
be recalled that the temporary external opening of the first visceral
pouch occurs only at its dorsal end. Ventrally, however, there is a fusion
with the ectoderm which causes the latter to form a vertically elongated
pit. When the dorsal perforation closes, that point also is marked by a
pit. These pits presently disappear, and on the sixth day the point between them becomes marked by a new depression, the beginning of the
external auditory meatus. It gradually deepens until, except for a thin
layer of mesenchyme, the external ectoderm is in contact with the endoderm of the tympanic cavity. These thin layers of ectoderm, mesenchyme, and endoderm which thus separate the middle ear from the outside, constitute the tympanum or ear drum.
To the inside of the tympanum of an adult Bird is attached one end
of the columella. The other end is in contact with a membrane covering
the fenestra ovalis of the bony labyrinth, i.e., the bony case which finally surrounds the membranous labyrinth. The columella is, therefore,
like a bridge stretching across the tympanic cavity from the tympanum
to the inner ear. It is chiefly developed from mesenchyme which lies in
the dorsal wall of the enlarged tubo-tympanic portion of the gill pouch.
This mesenchymal rudiment, it may be noted, is thought to be derived
from the dorsal end of the second or hyoid arch. However that may be,
as the cavity increases in size, it extends upward on each side of the
above mesenchyrne until it has surrounded it except at its inner and
outer ends. Then as this mesenchyme becomes cartilaginous and finally
ossifies, it forms a bone (the columella), occupying the position already
described‘. Lastly, it should be added that the inner end of this bone in
contact with the membrane of the fenestra ovalis seems to arise, at least
in some_Birds, from an element (the stapes) which, though at first disFOURTH DAY: ORGANS or SPECIAL SENSE 425
Fig. 221.—Sagittal section through the head of a Chick embryo of 5 days, showing the floor of fore~brain, olfactory pit, and developing olfactory nerve between.
From Lillie (Development of the Chick). After Disse.
a. Unipolar neuroblasts near the olfactory epithelium. b. Bipolar cell in the olfactory nerve. c. Unipolar cell near the brain. F.B. Floor of fore-brain. N'bl. Neuro
blast in the olfactory epithelium. olf.Ep. Olfactory epithelium. alf.N. Olfactory
nerve. olf.P. Cavity of olfactory pit.
tinct, eventually fuses with the columella. This stapedial element in the
Bird would thus apparently correspond to the opercular element in the
ear of the Frog.‘
The Olfactory Organs.———lt will be recalled that, at the close of
the third day, the olfactory epithelium consisted of two types of cells:
1 Some writers recognize a third element, the stylohyal, which enters into the
formation of the columella of Birds. It must be stated. however. that the exact
origins, as well as the homologies of the bones of the middle ear in the various
groups of Vertebrates are not yet completely known.
426 THE CHICK
simple epithelial cells and germinal cells. It had also become depressed
to form the olfactory pits. During the fourth day this process of depres.
sion continues to a considerable extent, and thus the specialized olfactory epithelium lying at the bottom of the pits is carried in some dis.
tance from the surface. The epithelium forming the sides of the pits, on
the other hand, is unmodified and similar to that outside. The position
of the pits has also shifted somewhat with the growing of the head, so
that their months now lie just on the antero-lateral border of the oral
cavity.
At the same time that these processes are taking place, the germinal
cells referred to are transformed into neuroblasts, and the latter in turn
into typical neurones. On the external side, these neurones send short
processes to the surface of the olfactory epithelium. On the other side,
they produce axones which extend in toward the brain, the region of
whose future olfactory lobes they do not enter, however, until about the
sixth day. Along the course of these axones are a few bipolar neurones
and also numerous epithelial cells, the latter serving as supporting and
sheath cells for the fibers. Both types are said to migrate from the olfactory epithelium, to their final position during the growth of the axones. The axones, together with the other cells just indicated, constitute
the I cranial nerve (Fig. 221). _
On the fifth and succeeding days, the nasal cavities continue to
deepen somewhat, and become greatly modified in shape. This is partly
the result of the appearance of certain folds in the nasal wall; these
folds are the rudiments of the three nasal turbinals, only two of which
are finally covered by epithelium of the olfactory type.
While the internal development of the olfactory organ is thus progressing, certain external changes are also going on in connection with
the apertures. However, since these changes have more to do with the
development of the face than with that of the olfactory organs proper,
they will be discussed under the heading of general external changes in
Chapter 13.
THE URINOGENITAL SYSTEM
THE EXCRETORY SYSTEM
The Mesonephros.——At the end of the third dayithe pronephros
had virtually disappeared, while the typical mesonephros was beginning
to develop, posterior to the twentieth somite. During the fourth day, the
FOURTH DAY: THE REPRODUCTIVE SYSTEM 427
primary me5°“ePh1"iC tubules are developed from the most ventral vesicles thro11gh°ut_the greater part of the mesonephric region. The remaining vesicles which occur in every mesonephric segment are, moreover,
each giVi1'1g rise '50 a tubule. Thus besides the primary tubules, there are
formed eventually secondary and tertiary tubules and sometimes even
more, all of a similar nature, developing from the nephrotomal mass opposite each somite. As suggested in the previous chapter, the primary
tubules thus formed soon connect directly, through a non-secretory or
conducting portion, with the Wolflian duct. The others as they develop
empty into outgrowths from that duct, which receive the name of collecting tubules (Fig. 207).
At the time that these tubules are developing, the remaining portion
of each vesicle is forming a Malpighian body or corpuscle consisting of
a glomerulus and its capsule. These Malpighian corpuscles are similar
in essential respects to those found in the Frog, and need not be described further. Though its development is still incomplete, the mesonephros apparently starts to function as a kidney at this time (Boyden,
’24). In this connection it is of interest to note that in the Bird a few of
the more cephalic rnesonephric tubules also establish rudimentary nephrostomal relations with the coelom in the manner characteristic of all
these tubules in the Frog.
The Metanephros.—The rudiment of the ureter and collecting
tubules of the metanephros, or permanent kidney of the Chick appears
at the end of the fourth day as a diverticulum from the mesonephric
duct. It arises from the dorsal side of this duct just at the point where
the latter bends to enter the cloaca. During the fourth day, also, the
nephrotomal tissue, just posterior to the thirtieth somite or end of the
mesonephros, begins to degenerate for a short distance (see Chapter
13, Fig. 240). Thus anterior to this point, the mesonephros, and any
undifferentiated nephrogenous tissue overlying it, become entirely cut
off from the nephrotomal tissue posterior to this region. The latter tissue thus cut oi? accompanies the forward growth of the ureter and its
collecting tubules, and is destined to form the secreting portion of the
entire metanephros (see Chapter 13, Fig. 240).
THE REPRODUCTIVE SYSTEM
The Gonads.-—-The rudiments of the two gonads appear on the
fourth day as thickenings of the peritoneal epithelium on each side of
the dorsal mesentery, between it and the respective mesonephros. These
thickenings occur just posterior to the origin of the vitelline arteries
428 THE CHICK
and extend for seven or eight somites, i.e., through the posterior half or
third of the mesonephric region. Presently primordial germ cells appear
in this epithelial tissue, near to which they have been transported from
the anterior part of the germ wall, where they are said to be distinguishable as early as the primitive streak stage. According to the remarkable
observations of Swift (714) and Goldsmith (’28) they are conveyed to
their new location by the blood stream. No sex differentiation is apparent at this time.
The Gonoducts.——The future male gonoducts or vasa cleferentia
are the mesonephric ducts whose development has already been described.
‘The oviducts or Mzillerian ducts begin their development at this time
in both sexes in the form of two ridges, the tubal ridges. Each ridge is a
strip of thickened peritoneum which appears on the fourth day. It lies
on the dorso-lateral face of each mesonephros next to the body wall and
near to the Woliiian duct. It is first found at about the level of the twen
tieth somite; from this point it differentiates posteriorly (see Chapter
13, Fig. 246)
THE ADRENALS
These bodies, though not really a part of the renal system, are closely
connected with it, and their development may, therefore, best be described at this point.
As in the Frog, the adrenal organs are composed partly of cells de-,
rived from the peritoneum, and partly of cells from the sympathetic
nervous system. The former element, known as the cortical substance,
arises from the coelomic epithelium slightly anterior to the germinal
region, and proliferations of this substance presently penetrate the
mesenchyme between the Wolfiian body and the dorsal aorta. The element derived from the sympathetic nervous system (mainly the primary
sympathetic system) is known as the medullary substance, which comes
into contact with the cortical material by the end of the fourth day
(Willier, ’30).
SUMMARY OF THE CONDITION AT THE END OF THE
FOURTH DAY OF INCUBATION
I. GENERAL APPEARANCE
The cervical flexure has increased so that its mid-region is anterior
and the diencephalon faces posteriorly. The caudal flexure has also in
a..;..ys.u.e_.»....m.=»..-,..a»
FOURTH DAY: SUMMARY 429
creased, and the embryo between it and the end of the cervical flexure is
virtually straight. The entire embryo is on its side, and the limb buds
have increased in prominence.
II. THE SOMITES
The number of pairs of somites has increased to forty-two, including
all those which take part in the formation of adult structures, while the
myotomal, dermatomal, and sclerotonzal elements have been developed
in each pair. The last named element forms a nearly complete sheath
about the nerve cord and notochord, and shows slight indications of the
vertebral segments. The account of the further developnfint of the my
otomal and dermatornal elements is completed in this chapter.
III. THE ALIMENTARY TRACT
The Fore-gut. ——The rudiments of the tongue have appeared. The
first and second visceral clefts have closed, and the third opened; the
visceral arches reach their maximum development as such. The thyroid
has completely separated from the floor of the pharynx. Subsequent development of the tongue and thyroid are indicated in this chapter.
The posterior end of the laryngotracheal groove and the lung rudiments have separated from the alimentary tract.
The esophagus, the stomach, and the duodenum have increased in
length, and the two latter parts of the tract have developed a curve to
the left. The liver has increased in size and come to lie somewhat in the
curve of the stomach. The dorsal pancreatic rudiment has become a
solid outgrowth and a pair of ventral pancreatic rudiments have arisen
from the ductus choledochus. The spleen (not really a part of the alimentary tract) has started to develop.
The Mid-gut. —-—The mid-gut or region of the small intestine is now
a virtually straight tube open to the yolk only by the relatively con’
stricted aperture of the yolk-stalls.
The Hind—gut.—The anterior portion of the hind-gut constitutes
the rectum, while its terminal portion becomes the cloaca. The latter is
still separated from the exterior by the cloacal membrane, and its posterior part is laterally compressed.
IV. THE CIRCULATORY SYSTEM
The Heart. — The ventricular region, especially the transverse portion, has expanded and moved posteriorly. The bulbus arteriosus has
swung toward the median line, and the atrium has rotated forward. The
interventricular, the interatrial, and the cushion septa are developing.
430 THE CHICK
The Embryonic Arteries.———The second aortic arches have dis.
appeared, and the fifth and sixth pairs have developed. From the latter
have arisen the roots of the pulmonary arteries which grow out and connect with the rudiments coming from the lungs. The primary subclavian,
the rudiment of the permanent subclavian and the sciatic arteries have
appeared, while the last named have given rise to the umbilical or allantoic arteries. The history of the sciatic and allantoic vessels is concluded in this chapter.
The Embryonic Veins.——The ring about the alimentary tract,
which is formed in connection with the vitelline veins, has been broken
by the disappearance of its left half. A fusion of the above vessels has
occurred beneath the fore-gut, forming a second ring. The capillaries of
the ductus venosus among the branches of the liver diverticula are becoming more numerous. Posteriorly, on the ventral side of the mesonephros, the rudiments of the subcardinals have become distinct vessels
and have acquired direct connections with the posterior cardinals. The
inferior vena cava has begun to form in the liver and caval fold, and
posteriorly has connected with the right subcardinal. The longitudinal
vein in the right body wall is disappearing, along with the transitory
subirztestina.-l vein, and the left, having acquired a connection with the
allantoic vessels, has become the functional umbilical vein. The account of its development is completed. The pulmonary veins appear in
connection with the developing lungs.
The Extra-Embryonic Arteries. —The vitelline arteries have
‘ fused with one another for a short distance as they leave the aorta. Their
branches in the area vasculosa continue to develop in company with the
growth of that region, but are without features requiring further note.
The Extra-Embryonic Veins. -—The right anterior vitelline vein
has disappeared, but the left anterior, posterior, and lateral veins are
well developed. Subsequent development of the extra-embryonic veins is
included in this chapter.
V. THE NERVOUS SYSTEM
The Brain.——The cranial and cervical flexures have increased
slightly; the porztine fiexure may be in evidence. The cerebral hemispheres have increased in size, and their lateral walls are thicker. The
optic lobes are also becoming steadily more prominent. There are no
other marked changes evident at this time.
The Spinal Cord and Spinal Nerves.—-There is no special development on the fourth day.
FOURTH DAY: SUMMARY 431
The Cranial Ganglia and Mixed Nerves.—From the V nerve
ganglion a branch (ophthalmic) has extended toward the future beak
and another (mandibular) toward the angle of the mouth. The VII
nerve ganglion has become separated from the VIII, and has given rise
to the hyoial and mandibular branches. The IX ganglion has sent a nerve
into the third arch. The X ganglion has divided into the ganglion jugulare and ganglion nodosum, and the latter is giving rise to the vagus
nerve.
The Cranial Motor Nerves. —~ The III nerve has entered the ciliary
ganglion, and the VI nerve has just appeared. The XII nerve has also
begun to develop.
VI. THE ORGANS OF SPECIAL SENSE
The Eye. -— Pigment is presented in the outer wall of the optic cup.
On the inner wall the internal limiting membrane is developing and beneath this in the region of the fundus, axones of the retinal neuroblasts
are growing into the optic stalk. The choroid fissure has partly closed,
and its proximal end is filled with the ingrowing pecten. The inner wall
of the lens is continuing to thicken. The middle layer of the cornea has
begun to develop. '
The Ear. -—There is no characteristic change directly connected
with the ear at this time. Within the pharynx, however, the formation of
the tu-bo—tympanic cavities has begun.
The Olfactory Organs. —The depression of the pits has greatly
increased, and their openings now lie on the border of the oral cavity.
The olfactory epithelium is giving rise to the elements of the I nerve.
Besides describing the events of the fourth day, this chapter also in
cludes an account of the subsequent development of the nervous system
and the organs of special sense.
VII. THE URINOGENITAL SYSTEM
The Excretory System. -— Primary tubules have developed
throughout most of the mesonephros, while secondary and tertiary tubules are arising. Collecting tubules are springing from the Wolilian
duct to connect with the two latter types. The Malpighian bodies are
beginning to appear in the functional portion of the organ which starts
to act as a kidney at this time. Rudiments of the metanephros are evident as a divcrticulum from the posterior end of each mesonephric duct.
The nephrotomal tissue just behind the mesonephros is beginning to
degenerate.
432 THE CHICK
The Genital System. —— The Gonads are represented by thickenings
of the peritoneal epithelium on either side of the dorsal mesentery, and
contain primordial germ cells. The oviducts are present in both sexes
in the form of the tubal ridges.
VIII. THE ADRENALS
The cortical substance of the adrenal bodies appears on the peritoneal
wall near the mesonephros, and material from the primary sympathetic
nervous system which is to form the medullary substance comes in contact with it.
IX. THE AMNION AND ALLANTOIS
The amnion is completed upon the fourth day, while the allantois has
pushed out somewhat further into the extra~embryonic coelom.
13
HE CHICK: DEVELOPMENT DURING THE FIFTH
AND SUBSEQUENT DAYS
THE EXTERNAL APPEARANCE
GENERAL
DURING the fifth day, the cervical flexure reaches its maximum
curvature and from then on becomes less and less marked, while the protuberance caused hy the mid-brain also attains its greatest relative prominence at this time. The third and last visceral cleft closes during the
fifth day, and the future neck is slightly indicated; the first three visceral arches, however, are still somewhat in evidence in this region. The
limb buds which were merely rounded swellings on the fourth day are
beginning to give evidence of joints.
By the seventh day the second and third arches are no longer visible
externally, the heart has moved backward so that the neck is clearly
defined, and the external auditory meatus has appeared, as indicated in
the previous chapter. The limbs are distinctly jointed, and by the
eighth day, the fore limbs begin to appear winglike. Upon the eighth
day feather germs are also visible, the tail is relatively much shorter,
and the position of the abdominal viscera is quite clearly marked by an
external protrusion. From this time on, the embryo gradually assumes a
typical bircllike form, one of the most striking changes being the relative increase in the size of the body as compared with that of the head
due to mitosis and rearrangeinent of cells (Gaertner, ’49} (Fig. 222).
THE FACE
In connection with the development of the nose and mouth, the face
undergoes so great a change between the fourth and eighth days, that it
seems best to treat the subject separately.
At four days the openings of the olfactory pits are separated by a
median projection overhanging the mouth. It is the naso-frontal process.
Dorso-laterally each pit is further bounded by the lateral nasal process
lying between the pit and the antero-dorsal part of the eye. Just below
each lateral process there is also another slight out-pushing adjacent to
434 THE CHICK
the antero-ventral side of the eye, termed the maxillary process (Fi«_
223). During the fifth day the lateral nasal process of either side hecomes more closely united with the maxillary process heneath it. the
two being separated only by the shallow lac/Lrymal groove. At the same
time an extension of these
united processes crosses each
nasal pit and fuses with the
frontal process, thus dividing the pits into antacdorsal and postero-verma?
halves. Thereafter as detet.
opment proceeds the f0I:‘:‘;t:’.‘
are carried forward as: ti-to
external hares while the Hiter are drawn back ~-.{t:~.'I;;
the mouth as the z';.::.r»~,;.-1,."
nares (Fig. ‘724«). It is _
evident that the midriic: ;.zs;~:‘—
tion of the upper jaw  to
be derived from the nascfrontal process, and the lateral parts chiefly from the
maxillary process. The lower
jaw is molded upon the ventral and main part of the
mandibular arches (see he
Fig. 222.-—Embryo of 7 days’ and 7 hours‘ low)‘ By Virtue of tiles"
incubation 3:5. From Lillie (Development of changes the eighth day finds
the Chick). After Keihel and Abraham.
 
the nares and rudimentary
beak quite clearly defined, the latter being developed by the co1‘niiication of epidermal cells about the margins of the jaws. Further growth
of these parts, accompanied by a relative diminution in the size of the
eye and the development of the eyelids, brings the face to the condition
found’ at the time of hatchinrr.
FEATHERS
In a preceding paragraph feather germs were mentioned, and because of the peculiarly characteristic nature of these structures in the
whole class of Birds, it seems desirable to indicate very briefly the essentials of their development.
«.... ..»..:
FIFTH DAY: FEATHERS
Feathers, like hair, which
we shall consider briefly in
connection with the Mammal, are epidermal structures. That is to say, the
feather consists of hardened
tightly pacl-zed epidermal
(er.-toclermal) cells, not of
secretion by cells. Initially
:1 point on the skin where the
feather is to appear develops
a slight depression, in the
midst of which rises. 51 small
tipgroxrtli or papilla. The
apex ml the papillzi at iirst is
at about the level of the rim
of the surrounding depression, or sli_s;l1t'i§.* shove it. It
consists of 21 C3iT.' of dermis
(inesoilerm) covered by the
Fig. 22-t.—~Head of an embryo of about 5 days
from the oral surface. (N.L. 8 mm.) From Lillie
(Development of the Chielc).
ch.F. Choroid fissure. E.L. Eye-lid (nictitating;
membrane’). cx.mzr. External nares. l.Gr. Lachrymal groove. Other abbreviations as in Fig. 223.
435
Fig. 223. -—-Head of an emhryo of 4 days’ incubation. from the oral surface (‘N.L. 6 mm.).
From Lillie iflarclopnzent of the Chick).
E12. Epipliysis. H0111. Cerebral hemisphere.
Hy. llyoid arch. I.nas.pr. Lateral nasal process. Id. i\l:tmlihulur arch. flfx. Maxillary
procx.-ss. nas.fr. Nusu-f-rontal process. Olf. Olfactory pit. Or. Oral cavity. Ph. Pharynx.
1).A.3. Third visceral arch.
   
usual Malpighian layer of
the epidermis, and a thin
layer of stratified and
cornified epithelium cells,
the corrzeum. In other
words it possesses the
same cell layers which
constitute the
other regions.
Very shortly this papilla grows outward so
that it protrudes definitely above the Ievel of
the rim of the depression,
at which stage it is known
as a feather germ. Within
this germ the vascular
dermal core is now known
as the feather pulp. At
skin in
436 THE CHICK
the same time the Malpighian layer of the epidermis at the distal end ‘of
the germ forms folds whose cells are modified to make the barbs. More
proximally the folds arise from a nonfolded part of the Malpighian layer
whose cells produce a single axis, the quill. The latter structure pushes
upward and soon throws off its sheath of coreum, emerging as a down
feather, i.e., a short quill with many short, soft barbs. At the base of
the down feather the dermis produces the pulp of the permanent feather,
while the Malpighian layer here forms two main folds opposite each
other, the rachis, other lesser folds again producing the barbs. It is interesting to note that transplantation experiments by Cairns, ’54, have
shown that the underlying dermis determines the special type of epidermal structure which will be formed, i.e., wing feather, leg feather,
claw, or scale. '
THE SKELETON
As in the case of the Frog, only a brief description of the development
of the skeletal system will be given. For a more extended study, the
reader is referred to LilIie’s Development of the Chick, and the books of
reference cited therein.
THE VERTEBRAE, THE RIBS, AND THE STERNUM
At the end of the fourth day the cephalic portion of each sclerotome
was beginning to fuse with the caudal portion of the one anterior to it
to form the rudiment of the right or left half of a vertebra. The occurrence of these vertebral rudiments thus necessarily alternated with the
myotomes. An extension of mesenchyme had also grown up on either
‘side of the nerve cord above both the cephalic and the caudal divisions
of every sclerotome, forming in each case the respective posterior and
anterior rudiment of a future neural arch. This reversed cephalic and
caudal relationship between the original sclerotome on the one hand,
and the future vertebrae and their arches on the other, is of course a
corollary to the alternative arrangement between the vertebrae and myotomes just indicated.
Upon the fifth day, the fusion of the cephalic portion of each sclerotome with the caudal portion of the next anterior to it is completed. The
sclerotomes upon one side of the notochord also have become fused
above and beneath it with the corresponding sclerotomes upon the other.
Furthermore, as a result of concentration, all of the sclerotomal tissue
is beginning to become membranous, and ire; the region of each future
vertebra certain portions of this membrane appear especially condensed.
FIFTH DAY: VERTEBRAE, RIBS, HSTERNUM 437
One such condensation surrounds the notochord as a ring, constituting
the rudiment of a vertebral cenzrzmz. Another occurs in each of the upgrowing primordia of the neural arches, and still another arises in the
membranous mesenchyme extending outward between the myotomes on
either side of the notochord. Each of the latter extensions represents a
transverse or costal process.
During the sixth to the eighth days these eostal processes develop
Iurther, and in the thoracic region give rise to the membranous primordia of the dorsal two thirds of the upper parts of the true ribs, i.e.,
Fig. 225. —The right side of four bisected vertebrae of the trunk
of an 8-day Chick. From Lillie (Development of the Chick). After
Schauinsland.
caud.v.A. Caudal division of vertebral arch. ceph.v.A. Cephalic
division of vertebral arch. N’ch. Notochord.
those movably articulated to the vertebrae. The cervical costal processes
which are not movably articulated are often called cervical ribs.‘ The
first true rib primordlia are those of the fifteenth vertebra, which are followed by six other pairs. The third to the seventh pair of these ribs
possess ventral parts which develop from separate centers, and like the
ventral one third of the dorsal parts come from lateral plate mesoderm,
not sclerotome (Straus and Rawls, ’53). The third to the sixth of these
parts later fuse to the sternum. Further ventrally, the sternum itself develops from bilateral membranous plates also arising within the lateral
plate mesoderm. Presently the membrane of the neural arch primordia
unites above the nerve cord, and their normal development seems to be
. conditioned by both nerve cord and notochord (Waterson, ’54-) . Carti
lagepformation now starts in all of the regions indicated, and in the last
five pairs of ribs the dorsal and ventral part of each has its own center of
chondrification. The sternum or breast bone of the chick, including
1 Since there is no clear cut distinction between cervical and thoracic vertebrae
in the Bird, the writer is arbitrarily defining as thoracic all vetebrae with freely
articulating or true ribs.
438 THE CHICK
heel likewise has two cartilage forming centers, one in each of the lateral membranous plates; these, however, soon fuse. Following chondrification the cartilage is in turn replaced by actual bone; during this procedure the remains of the notochord are completely eliminated. Such
ossification is well advanced by the sixteenth day.
Subsequent to this time several of the thoracic and lumbar vertebrae
become rather firmly united with one another, and these in turn are
fused to the coalesced vertebrae of the sacral region. To this mass there
is also added posteriorly a number of the caudal vertebrae, so that a
considerable portion of the spinal column is virtually inflexible, a condition peculiar to the Birds. Lastly, the extreme terminal vertebrae are
likewise fused into a single piece termed the pygostyle.
C THE APPENDICULAR SKELETON
The Fore-limb. ——- On the fourth day a concentrated mesenchymal
mass—probably of sclerotomal origin appears in the base of each forelimb bud, and on the fifth day there grow out from this membranous
mass four processes. One, the primordium of the limb bones, grows out
into the lengthening wing bud; a second, the scapula, grows backward
and dorsally above the ribs; a third, the coracoid, grows down posteriorly toward the region of the sternum; and a fourth, the clavicle, grows
in front of the coracoid toward the median line. The last three elements
represent the rudiments of the pectoral girdle. Centers of chondrification occur’ in the membranous primordia of the scapula and coracoid
on the sixth day, followed later by ossification. The clavicle, on the
other hand, ossifies directly from membrane, about the eighth day. Like
the coracoid and scapula, all the bones of the fore-limb pass through
both a membranous and cartilaginous stage previous to ossification. It is
interesting to note that in the wrist there are 13 membranous elements
which as a result of fusions produce only two definitive carpals. Likewise in the hand five digits are represented in the membrane, but the
first and fifth soon disappear.
The Hind—1irnb. —-Like the fore-limb, the parts of the pelvic girdle
and hind-limb bones arise about the fifth day as four processes from a
common mass of mesenchyme in the region of each hind-limb bud. The
membranous process representing the limb bones grows out into the
bud; another process, the ilium, which is elongated in an anterior posterior direction, grows dorsally; a third, the pubis, grows anteroventrally, and a fourth, the ischium, grows postero-ventrally. By the
FIFTH DAY: APPENDICULAR 439
eighth day, the distal ends of the pubis and ischium have both rotated
posteriorly so that they are parallel with one another, and with the
ilium. Chondrification and ossification follow the membranous stage,
and the limb develops in a manner fundamentally similar to that of
the fore-limb. There are three tarsal elements and five digits present in
cartilage, but the rudiment of the fifth digit soon disappears. Later the
two proximal tarsals fuse with the tibia, and the distal one with the
three long metatarsals; subsequent to ossification the latter become
united, thus forming with
the distal tarsal element
the single tarso-nzetatan
sus.
As regards the details
of ossification in the long
bones of the Chick, we I , _ ., endochondng
find that the situation dif- , _ -  ‘‘ b°"°
remains of :‘ - — ‘ ‘
_ diaphysial
that 111 the Frog, and ca.-mag:
from what we shall. later
see in the Mammal. As
noted the membranous
stage is as usual followed
by cartilage, and as in the Fig 226__The head of a long bane (femur, in
Frog in the region of the the Chiizk. From Lillie, aftc; Br:11chet.1Thed.sifilua
_ - _  -- lion wit I respect to the epip ysia rarti age i ers
Shaft or d1al’h)“”i"'” ll“: from that in the Ft‘H,[_‘. but the -nizuation in the
cartilage is overlaid by «Iiapi; Z: i» :-in:El'ir to the extent that, save at the
uncjs, horn is little or no bone except that produced by the p:,'rin$t+"um.
 
 
   
fers somewhat, both from
marrow
perlosteum cavity
 
periosteal bone. In this
case, however, the cartilage is presently destroyed, and partly replaced by true endochondral
bone, though of a cancellcus character. Throughout the shaft this cancellous endochondral bone is then likewise removed to be replaced to a
considerable extent by marrow. Thus in respect to having most of each
long bone ultimately of periosteal and membranous origin the Bird approaches, but does not quite equal the condition in the Frog. There is
in the Chick some endo«::honrlral ossification of a permanent nature in
these bones which comes about because of their method of longitudinal
growth which takes place as follows:
The epiphyses or ends in the Chick bones, unlike those in the Frog,
only remain cartilaginous during the increase in length of the diaphysis.
440 THE CHICK
This increase occurs through ossification of the cartilaginous ends on
their diaphyseal sides, with simultaneous addition of more cartilage distally (Fig. 226). Finally as growth is completed the cartilage of the
epiphyses is entirely replaced by cancellous bone. In this manner it
happens that a little spongy bone at the ends of the diaphysis, and all
of that in the completely ossified epiphyses is of endochondral origin.
In concluding this topic it should be noted that among the long bones
of the Bird the humerus is peculiar in one respect. In this bone there is
relatively little marrow, the extensive cavity therein being largely occupied, as will presently be noted, by one of the lung outgrowths called
air sacs. (See below.)
THE SKULL
The Primordial Craniu~m.——The primordial or cartilaginous cranium of the Chick is first indicated by concentrations of mesenchyme
during the fourth and fifth days. Then, during the sixth, seventh, and
eighth days, these mesenchymal concentrations develop into the following fused elements of cartilage. Along either side of and encasing the
anterior end of the notochord, appear the parachordal plates. In the
Chick these elements develop from the first as a single piece, and are often known, therefore, as the basilar plate. Anterior to it are developed
simultaneously upon either side another pair of plates — the trabeculae.
Posteriorly, these are continuous with the parachordals, with which they
form an angle corresponding to the cranial flexure, while anteriorly,
their ends meet and fuse with one another. This fusion then extends
somewhat, so that eventually the central space is closed, except for a
small opening containing the pituitary body. Thus the trabeculae and
parachordals together form the entire cartilaginous floor of the skull.
At the same time that these plates are forming, cartilage also develops around the auditory sacs and the olfactory organs, forming respectively the auditory and olfactory capsules. These are in direct continuity eventually with the plates. From the postero-dorsal part of each
auditory capsule, processes now grow toward one another and fuse
above the hind-brain. Thus is constituted the only portion of the roof of
the cranium which is preformed in cartilage. Posterior to each auditory
capsule, a dorso-lateral plate of cartilage develops, while anterior to
and in contact with the capsule, a transverse partition arises between it
and the orbit. This partition extends medially somewhat, so as partially
to bound the brain cavity in front. Anterior to the cranial cavity, midFIFTH DAY: THE SKULL 441
way between the two orbits, and between the nasal capsules, a continuous longitudinal partition appears and fuses ventrally with the trabeculae. It is the interorbital and internasal septum.
The remaining part of the skull which is preformed in cartilage is
known as the visceral skeleton or cartilaginous splanc/mocralziunz, and
arises from the first three pairs of visceral arches. During the fifth day,
these arches are chiefly membranous. and the antero-ventral or distal
ends of the first mandibular pair have fused with one another in the
middle line. Subsequent to the fifth day, the ventral or main parts of
each mandibular arch become chondrified, and are known as Mec-kel’s
cartilages; they form the core of each side of the lower jaw. From the
proximal (i.e., hinder and upper) end of each of these arches, there develops a tri-radiate piece of cartilage, the palate-quadrate, which eventually ossifies as a separate bone. It is termed simply the quadrate, and
constitutes the articulation between the lower and upper jaws. The second (hyoid) and third visceral pairs of arches later‘ form the hyoid apparatus, consisting respectively of the paired lesser and greater cornuae
and the two median copulae. Moreover, the upper ends of the second
arches are thought to give rise to parts of the colurnellae, as noted in
the account of the ear (Chapter 12) .
Altogether, the final bones of the Bird’s skull which have been preformed in cartilage are the following: the basi-occipital, exoccipztals,
and supra-occipitals about the foramen magnum; the proiitic, epiotic,
and opisthotic about each auditory capsule; the basisp/zenoid, orbitasphenoids. alisphenoids, and interorbital and internasal septum about
the eyes and nasal capsules; the quadrate, and Meckel’s cartilages in
connection with the lower jaw; and the hyozd apparatus in the region
of the throat.
The Membrane Bones. ——These are bones which are not preformed
in cartilage, but ossify directly from the condensed mesenchymc or
membrane. They constitute a good share of the Bird’s skull, and begin
to develop about the ninth day. The bones thus formed are as follows:
the parietals, jrontals, and squamosals, forming together the main part
of the cranium proper; the lachrymals, nasals, and premaxillae, form
ing the face and part of the upper jaw; the maxillae, jugals, quadratojugals, pterygoids, palatines, parasphenoids, and vomer, forming the
rest of the upper jaw and the base of the cranium; and the angulars,
supra-angulars, operculars, and dentals, forming the coveringbones for
the lower jaw.
442 THE CHICK
THE ALIMENTARY TRACT
THE FORE—GUT REGION
The development of the mouth proper has already been suliiciently
described in connection with the discussions of the alimentary tract and
the middle ear in Chapter 12, and of the skull in the preceding paragraph. We shall proceed, therefore, to an account of the further development of the remainder of this tract and its appendages.
Fig. 227.-—— Derivatives of the visceral pouches and associated organs, in the Chick.
From Lillie (Development of the Chick). After Verdun (Maurer). Combined from
frontal sections. A. In embryo of 7 days. B. In embryo of 8 days.
Ep. 3,Ep.4-. Epithelial vestiges derived from ventral portions of the third ancl=fourth
visceral pouches. J. Jugular vein. p’br.,p’br.(V). Postbranchial bodies derived from
fifth visceral pouch. Ph. Pharynx. T h.3.,TH.4. Thymus bodies derived from dorsal
portions of the third and fourth visceral pouches. T’r. Thyroid body. 111, IV. Remains of third visceral cleft and position of fourth which never becomes a real cleft.
The Visceral Pouches and Arches.
The Pouc-hes.——At the end of the fourth day, the first and second
visceral clefts had closed, and the third had opened; during the fifth
day, this latter cleft also closes, whereas the fourth pouch, it will be recalled, has never developed an outer opening. About the seventh or
eighth day, the third and fourth pouches sever their connections with the
pharynx, and thus remain as patches of epithelium in the mesenchyme
of the neck, adjacent to the jugular vein. The dorsal portion of the epi
thelium from the third pouch then fuses with the dorsal portion from
the fourth to form a thymus body on each side of the throat of the
FIFTH DAY: THE FORE—GUT REGION 443
young Chick. Though thus apparently endoderrnal, I-lamrnond, ‘S4, states
that the clefts rather than the pouches may be the source of the thymus
and hence that it is ectodermal. Epithelial vestiges of the third and fourth
pouches are gener:.il.ly thought to produce the para:/iyroirls, while each
fourth pouch also produces a posterior outpushing sometimes regarded
as a vestigial fifth pouch. These separate from the pouches, and the left
one becomes the pose‘-bram:/zial body, somewhat like a small parathyroid.
while the right one clegenerates (Fifi. 227). Dudley, 7112, thinks these outpushings may be rudimentary sixth pouches, the filth having {used with
the fourth.
The Arches. —-The fate of the first three pairs of Visceral arches has
already been suiiiciently described above in ('0I1IlB('li()]] with the visceral
chondrocranium. The fourth pair of arches never develop beyond a
inesenchymal state and eventually disappear. The lifth pair are vestigial
and even more transitory.
The Respiratory Tract. —— At the end of the fourth day, the respiratory tract consisted of the glottis, the larynx, the trachea, and a pair of
posterior outgrowths from the latter. the rudiments of the bronchi and
lungs. All these parts, having arisen from the fore-gut, are necessarily
lined by encloderm. Upon the fifth day, however, the mesenchyme about
them begins to condense to form true mesoderm. through which the
lung rudiments continue to grow posteriorl_v as a pair of tubes. Upon
the sixth day, these tubes begin to branch, and thus it appears that the
original rudiments really represented the lining of only the two main
or primary bronchi. Their branches then constitute the linings of the
secondary bronchi’, and the intercommunicating terticlry or parnbronclzi,
together with the finer ramifications from the latter known as air capillaries. This network of air capillaries, it is to he noted. takes the place
of the blind terminal sacs or alveoli found in the Marnmals. Thus there
are no pockets of residual air in the lungs of the Bird, but continuous
passages which make possible a. complete circulation. The mesoderm
indicated above eventually gives rise in the region of the larynx and
trachea to the cartilages and muscles of these organs. Further hack it
surrounds the endodermal lining of the various bronchi and air capillaries, and ultimately forms the connective tissue substance of the lung.
Through this tissue the blood vessels later rarnify among the tubes
and capillaries.
In the case of the Bird, besides these tubes and respiratory capillaries,.there are also connectgd with the lungs the various air sacs. These
arise, with one exception, as outgrowths from the secondary bronchi,
the exceptional case being the abdominal sacs which originate directly
l
i
r
3
444 THE CHICK
from the posterior ends of the primary bronchi. The rudiments of the
abdominal and cervical sacs are said by some to be distinguishable as
early as the fifth day, while the others appear somewhat later (Fio.
228). In the course of development these peculiar sacs which have thus
originated, gradually push their way to their respective positions among
M°5»'"9‘-‘"' -1  '  ‘ -—  -Lat.moi.
Rec.Br.-:=.'  ‘  .  . ___Me5.mOi_
 
Rec. Br.
Abd. Sc.--""“'
Fig. 228. —A. Lateral view of the left lung of a 9-day embryo, showing branches
of the bronchi within it. B. Ventral View of the lungs and air-sacs of a 12day embryo, with internal branches of the bronchi not shown. After Locy and
Larsell.
Abd. Sc. Abdominal air-sac. A. Int. Sc. Anterior Intermediate air-sac. Br. Extrapulmonary bronchus. Cerv. Sc. Cervical air-sac. Ect. 1. An ectobronchus. Ent. 1.
An entobronchus. Lat. I, 2, 3. Laterobronchi. The ecto. ento. and laterobronchi are
all classed as secondary bronchi in the text description. Lat. moi., Mes. mai. Lateral and mesial moieties of interclavicular air-sac. 0e. Oesophagus. Par. Parabronchi. P. Int. Sc. Posterior Intermediate air-sac. Rec. Br. Recurrent bronchi.
the viscera. Here they come to occupy considerable space, while a branch
of the interclavicular sac extends eventually even into the upper bone
(humerus) of each wing.“ Besides being connected with the respiratory
passages by the bronchi from which they arose, each sac, with the exception of the cervicals, also develops secondary connections with the
parabronchi. In the adult these connections always convey air from the
3 In the latter case the bone is said to undergo a kind of dissolution to make
way for the ingrowing sac, and the dissolution is thought to depend on parathy-K
mid activity which in turn is due to oestrogens derived from the yolk-sac
* (Bremer, ’40) .
FIFTH DAY: THE FORE—GUT REGION 44.-5
sacs to the lungs, and are, therefore, termed recurrent bronchi. The cervical sacs, though possessing no recurrent bronchi, are indirectly connected with branches of the most anterior pair of secondary bronchi,
and these branches probably act as recurrents. The functions of the sacs
are apparently to lighten the Bird’s body, to help maintain air currents and, in the case of the abdominal sacs, to cool the testes.
Fig. 229.—Partially dissected viscera of the Chick, from the right
side. From Kellicott (Chordate Development). After Duval. A. Of a
6-day Chick, enlarged slightly less than six times. B. of a 13-day
Chick, enlarged two and one half times, showing the elongated intestine and its extension into the umbilical stalk. _
zz. Right atrium. al. Allantois. as. Abdominal air-sac. b. Bulbus
arteriosus. c. Caecal processes. zl. Loop of duodenum. dj. Duodenaljejunal flexure (a relatively fixed point during the elongation of the
intestine). f. Fore-limb bud (cut through}. g. Gizzard. go. Gonacl. h.
Hind-limb bud (cut through _). i. Loops of small intestine. l. Liver. lg.
Lung: ll. Left lobe of liver. lv. Left ventricle. M. Rudiment of Mullerian duct (tubal ridge). p. Pancreas. r. Rectum. rl. Right lobe of
liver. To. Right ventricle. s. Yoll-:-stalk. U. Umbilical stalk. W. Wolffian body or mesonephros.
Finally, in connection with the development of the respiratory system, it is to be noted that about the fifth day, the glottis begins to close.
Both larynx and glottis later become entirely shut, but subsequent to
the eleventh day, the opening is gradually re-established.
The Esophagus, the Stomach, and the Duodenum.-—At the
end of the fourth day, the esophagus was a straight tube, while the region of the stomach and duodenum was indicated by a slight curvature
to the left. The esophagus does not alter much on the fifth day, except
to continue to elongate. The stomach, however, is becoming distinguished from the duodenum by its greater dilation. Also, at the extreme left of the gastric duodenal curve, a slight pouch is forming. This
446 THE CHICK
marks the end of the gastric region. Later this pouch enlarges to form
the muscular gizzard. while the part between it and the esophagus develops the gastric glands and comprises the proventriculus. The crop is
evident by the eighth day as a dilation of the esophagus at the base
of the neck. Anterior to the crop at that time, the lumen of the esophagus is temporarily closed. ‘
The duodenum is not very clearly defined on the fifth day, but shortly
afterward it begins to develop as a loop in the tract just beyond the
gizzard. From the gizzard, the proximal limb of the loop descends a
short distance, and then bends upward to form the ascending branch.
Ultimately the pancreas comes to lie in between the limbs of this loop.
The end of the ascending branch marks the termination of the original
fore-gut region and the beginning of" the small intestine (Fig. 229).
The Liver.~——On the fifth and subsequent days, as on the fourth
day, development of the liver consists chiefly in further growth in size.
This is accomplished as already indicated by continuous branching and
anastomosing of the original diverticula together with the accompanying blood capillaries. These diverticular branches are at first solid, but
on the fifth day many of them have acquired a lumen, and this process
continues as growth proceeds. '
As regards the bile ducts, it is to be noted that on the sixth day the
common duct disappears, and the two bile ducts which emptied into it
again empty directly into the duodenum.
The Pancreas.——The pancreas at four days, it will be recalled,
consisted of three separate outgrowths: a dorsal one from the wall of
the duodenum opposite the common bile duct, and the beginnings of
two ventral ones from the duct itself. During the fifth day all three
diverticula continue to grow and branch (Fig. 230). On the sixth day,
the right ventral pancreatic mass becomes united with the dorsal, whose
duct shifts ventrally on to the left side of the duodenum. As noted
above, the common bile duct disappears at this time, and thus the two
ventral pancreatic ducts come to open directly into the intestine. Later,
the left pancreas becomes fused with the other two, and there remains
a single glandular mass lying in the mesentery within the loop of the
duodenum. lts three ducts continue to remain separate, however, and
they open into the distal limb of the duodenal loop near the bile ducts.
THE MID-GUT REGION
It has been indicated that the mid-gut or rudimentary small intestine’
begins at the end of the duodenum. At the close of the fourth day, it
n
F IFTH. DAY: THE MID—GUT REGION 44?
was noted that it extended from this point as a virtually straight tube
across the region of the umbilicus to the beginning of the tail fold and
hind-gut. In zibout the middle, it gave off the yolk-stalk. During the
fifth day a very slight downward bend (the duodeno-jejunal flexure)
Fig. 230.—-Reconstruction of gizzard; duodenum,
and liepato-pancreatic ducts of a Chiclc embryo of
124 hours. From Lillie (_Development of {he Clzz'c'/.15.
Alter Broulia.
D.clI. Duclus (:l10le(lOCllllS. D,r‘y. Ductus cystivus.
DJ1.cy. Duutus hepato-cysticus. 11.11.11. Do1'.~:ul or hepato-enteriv duct. Du. Duodenum. G.bl. Gall bladder.
Ciz. Gizzard. }’u.rI. Dorsal pancreas. Pa.2'.u’. Right
ventral pancreas. Pa.-v.5. Left \‘entr:il pancreas.
appears just at the point where the duodenum ends and the mid-gut
begins. From this bend, the latter extends postero-ventrally for about
half its length; at this point, as noted, it connects with the yolk-stalk. lt
then ascends again to its termination, which is now marked by a small
bilateral swelling, the rudiment of the intestinal caeca. The entire midgut region thus indicated is still quite short, and its dip down into the
umbilical stalk very slight. t
On the sixth day, however, the ventral dip of the small intestine
reaches well down into the above stalk, thus forming in the intestine
448 THE CHICK
as a whole a second distinct loop (Fig. 229, A). The latter soon becomes much more pronounced than the duodenal loop, and during later
development acquires numerous convolutions (Fig. 229, B). These
convolutions lie within the umbilical stalk until about the eighteenth
clay and are then drawn into the body. They are soon followed. by the
EVIL ._. W. D.
An. PL 
Fig. 231.—Chick embryo of 11 days, sagittal section through
the region of the cloaca. Reconstructed from several sections.
(After Minot.) From Lillie (Development of the Chick). Anterior end toward the reader’s left.
All’. Ascending limb of the allantois. Al ". Descending limb of
the allantois. An. Anal invagination, or proctodaeum. An.pl.
Anal plate or cloacal membrane. Art. Umbilical artery. B.F.
Bursa Fabricii. b.f. Duct of the bursa. Clo. Cloaca, i.e., the urodaeal portion. Eb. Ectoderm. Ent. Entoderm of the rectum. Ly.
Nodules of crowded cells, probably primordia of lymphoid structures in the wall of the large intestine. W.D. Wolfiian duct.
remains of the yolk-sac. The intestinal caeca which were barely indicated on the fifth day ultimately grow out into two fingerlike processes.
THE HIND—GUT REGION
On the fifth day, as on the fourth, there is no particular change in
the rectum. On the seventh and eighth days, however, its cavity becomes
occluded. Later, the lumen is restored except for a small plug separating it from the cloaca, and just anterior to this plug a slight dilation
develops. This dilation is the coprodaeum. The plug persists until about
the time of hatching.
The chief change in the cloaca during the fifth day is the fusion of
FIFTH DAY: THE HIND—-GUT REGION 449
the laterally compressed walls of the posterior part. During subsequent
development, a cavity is re-established in the postero-dorsal part of this
closed portion; it constitutes the bursa F abricii of the adult. This is a
sac which remains separate from the original cloaca, but which opens
into another cavity, communicating directly with the exterior. This
mesonephric duct
mecanephrlc duct
 
 
coprodaeu m
allantoic stalk
urodaeum
proctodaeum
Fig. 232.—A diagram of a sagittal section of the posterior end
of an approximately eleven-day embryo to indicate better the relations of the parts partially shown in Fig. 231. The metanephric
duct opening separately into the urodaeum (a condition attained
on the sixth day) is shown, though for some reason it does not
appear in Fig. 231. The anal plate separating urodaeum from
proctodaeum is shown in the diagram, but is unlabelled.
latter cavity is the proctodaeum, and has arisen by an outpushing of
the ectodermal _walls around the edges of the anal plate or cloacal membrane (Figs. 231, 232; compare Fig. 193, Chapter 10). At hatching the
latter disappears and thus the proctodaeum is finally placed in communication with the original embryonic cloaca minus the posterior portion of the latter which went to form the bursa Fabricii. At. about the
same time the plug which closes the rectum disappears. Thus, theadult
cloaca consists of three parts, the coprodaeum, a part of the original
cloacal chamber now called the urodaeum, and the proctodaeum. The
latter opens to the outside through the anus.
450 ‘THE CHICK
THE CIIRCULATORY svsrsm
THE HEART
During the fourth day a series of changes in the position of the various parts of the heart in relation to each other were indicated. During
the fifth day these changes progress rapidly, and upon the sixth day are
virtually completed.
Besides these movements, there were also noticed on the fourth day
the beginnings of certain partitions within the heart. These were the
interatrial, the interventricular, and the cushion septa. During the fifth
and part of the sixth days, all these are practically completed. This
process involves, first, the meeting of the two parts of the cushion septum so as entirely to divide the atrio-ventricular canal into right and
left channels. The interatrial septuin then unites with the cushion septum
on the antero-dorsal side of the latter, while the ventricular septum joins
it postero-ventrally. These fusions, though described separately, occur
more or less simultaneously (Fig. 209, F
In connection with these processes there remain to be added certain
details as follows: As the division of the originally single atrium into
two atria occurs communication between them is preserved by the concomitant development of perforations in the newly formed septum.
These perforations correspond functionally to the foramen ovale in the
heart of the Mammal, and their physiological significance is described
below. It must also be noted that the interatrial septum as thus far described is augmented in the adult Bird by the addition of another part
as follows: Upon the seventh day the proximal portions of the left precava and the pulmonary vein start to be incorporated into the atria,
and as this occurs the tissue between them is added to the septum. This
new part is called the pars cauo-pulmonalis (Quirring, ’33). Lastly,
there is also a small ventricular foramen whose final closure will be
described presently in connection with the development of . the aortic
division of the bulbus.
This completes the description of the septa within the heart proper.
Upon the fifth day, however, another septum develops within the truncus arteriosus. It appears first at the anterior end of this vessel in such
a position as to separate the orifice leading to the sixth aortic arches and
hence to the pulmonary arteries, from that which leads to the third and
fourth "aortic arches. This partition then grows backward through the
FIFTH DAY: THE HEART 451
distal portion of the bulbus, and on the sixth and seventh days it connects with a septum which has formed within the proximal portion of
that vessel. Thus a continuous somewhat spirally twisted partition has
been produced extending through the truncns .-md hullaus clear In to the
interventricular septum of the heart. It is to he noterl that the entire bul
bus, though now ventral,
still lies somewhat to the ca;,¢om_ l\
right of this latter septum. ;'
Nevertheless, the fusion of p 6 xi 3
the hulbus septum and inter- Au 4
ventricular septum is eiiectetl ' "  ,4»
in such a way that in cmmec- ‘pp  .1?‘ S_ GL5.
     
tion with subsequent changes
in the cushion septum the
aortic division (i.e., the division from the third and
fourth arches) of the bulbus
comes to open through the
foramen in the ventricular
septum directly into the left
ventricle. The pulmonary division, on the other hand,
continues to open into the
right ventricle (Fig. 233).
Subsequent to the fifth day
also, certain other changes
are Completed as f°u°w5'The Fig. 233.——The heart and aortic arches of a
semilunar valves develop in Chick embryo the latter part of the sixth day.
both the aortic and pulmo- Efogeacgézfifitiggiefrgfiiyllie (Development
nary divisions of the bulbus, Au. Atria. Car.com. Common carotid ar
and the parts of that vessel Zi§’$ia§°’;'§§'fé?‘ §'i'2'?’ls.“"i’h?f§°"€ii.-‘fulfil:
proximal to these valves are fourth (systemic) and sixth (pulmonary)
incorporated into the ventri- acme arches’
cles. The two divisions of the bulbus and truncus arteriosus distal to
this point are gradually separated so as to form distinct vessels, i.e., the
proximal portions of the aortic and pulmonary arteries. As noted in a
previous chapter, the sinus venosus becomes a part of the right atrium
into which empty all the systemic veins, and finally both atria acquire
small auricular appendages or auricles.
L2
452 THE CHICK
THE EMBRYONIC BLOOD VESSELS
The Arteries.
The Aortic Arches.——At the end of the fourth day, the pairs of
aortic arches which remained fully developed were the third, fourth,
and sixth. The third pair, it will be recalled, ran upward from the ventral aorta. and continued anteriorly as the internal carotids, while pos
teriorly the dorsal end of each of these arches was still connected with
the dorsal end of each fourth arch. From the base of each of the third
LEFT SIDE RIGHT SIDE
6th day 6th day
   
Internal carotid
 
 
 
 
 
i 3 r
aortlcv3rches{-‘i-th : ‘ systemic arch ’  4 h} “me h
gh < . '  r are es
‘ ' ' 6th
external carotid — ' ‘ .
external carotid
common carotid
. \tfUn€US 3l’t¢l‘lOSl.|S
Fig, 234.——Reconstruction of the aortic arches of a 6-day Chick embryo from a.
series of sagittal sections. Modified from Lillie. '
arches, on the other hand, another vessel ran forward as an external
carotid.
Upon the fifth day three further changes are initiated as follows.
First, on each side, the portion of each dorsal aorta between the third
and fourth arches begins to disappear. Secondly, the fourth arch on the
left side diminishes in size (Fig. 234-). Thirdly, there occurs anteriorly
an anastomosis between the internal and external carotids, while the
portion of the latter between this point and the base of the third arch
(primary external carotid in Fig. 235) begins to atrophy.
By the eighth day the changes thus begun have been completed. so
that the condition then obtaining is as follows: First as regards the systemic and pulmonary arches, it is to be noted that on the left side,
the entire fourth arch together with the dorsal aorta between the third
and the sixth arch has vanished. On the right side the dorsal connection between the third and fourth arches is gone, but the fourth arch
itself is well developed It persists as the main systemic arch of the
FIFTH DAY: EMBRYONIC BLOOD VESSELS 4153
adult (Fig. 210, B). It is to be noted that the Bird differs from the
Mammal in that in the latter, it is the left arch which remains. The
immediate cause of this interesting difference between Bird and Mammal according to Bremer (’28) is as follows: In the first place in the
Bird the torsion of the heart tube is somewhat greater than in the Mammal. Secondly this is cor
related Witll 3 greater . Eiigggim
backward movement of ‘°'°“d
the heart in the  in intemolcorotid '
connection with the great- _ 7
er length of the neck. This comm“ €°'°*‘d' . ‘°"""‘°" ‘°'°"d
last ‘feature results in
lengthening the aortic ves- NGHT
 
 
 
 
" internal carotid
LEFT
sels and in involving them
in the increased torsion of §f,;;‘,‘;'v‘f§,', _ §’:§,:{2C;°."..'
the cardiac tube. Thus the
left fourth arch is drawn dums \ [ mm;
into a disadvantageous Borulh ‘ W Soto"!
position on the ventral §?'r'§'.‘,‘,’"°'Y ‘ . °"“ °"°'Y
side of the truncus, while p,i,m,y V primary
the right assumes a dorsal ‘“""°"‘°" ‘”b°'°"'°"
position with a much systemic artery \ 1  p
more direct connection ‘3'*\5¢9'“°"‘°‘°'*°'Y '3'“ “"9'“""°‘”'"'V
with the dorsal aorta Fig. 235.—-Diagram ca; tgi ionic archvés fand
- connecting vesse s in t e ic as viewe rozn
(F1g' 236)‘ In the Mam‘ the ventral side. The vessels in outline indicate
mal on the other hand, the situation existing at one time or_ another in
not only is this not true the embryo. Those shown in black indicate the
1
permanent arrangement. “
but according to Congdon
and Wang (’26) the blood as it comes from the truncus on the right is
necessarily directed toward the left. Hence the left arch receives the
larger stream and so becomes the dominant vessel.
All parts of the sixth arches continue to be well developed on both
sides throughout embryonic life. At the time of hatching, however, the
upper portion of each vessel between the origin of the pulmonary
arteries and the dorsal aorta (i.e., the duct of Botallo or ductus arteriosus, indicated above) becomes atrophied and remains only as an occasional vestige in the adult.‘ In the second place with respect to the carotids it appears that since the atrophy of each external carotid between
i the base of the respective third arch and the point of its anastomosie
3 In the Mammal a remnant of the left duct of Botallo always persists.
454 THE CHICK
with the internal carotid has been completed, each external and internal
vessel now takes its origin and continues anteriorly from this point of
fusion. Posterior to this point certain remaining parts constitute on
either side a newly named vessel, the common carotid. Each common
carotid consists of what was previously the postero-dorsal portion of
the respective internal carotid, the respective third arch, and the part
dorsal aorta
aortlc arch
Fig. 236.——Diagrammatic ventral view of the truncus and the
third and fourth aortic arches in A, the Mammal, and B, the
Chick. After Bremer. Note that in the Chick the fourth arches
are involved in the twist of the truncus, thus bringing the right
fourth arch dorsal, and hence nearer to the dorsal aorta. The
left fourth arch on the other hand is brought ventrad, and hence
further from the dorsal aorta, thus leading to its elimination in
this form.
of each ventral aorta proximal to the base of this arch and the point of
union with the systemic vessel (Fig. 235). It is to be noted in this connection that the point of anastomosis between each external and internal carotid is not shown in Fig. 234. Hence each vessel there indicated
as an internal carotid eventually becomes part of a_ common carotid.
Finally, it must be remembered that while these changes are occurring,
the head of the Bird is being separated from the body by the development of the neck. This process results in the backward movement of the
heart and all its arches, so that by the time they have reached the stage
indicated on the eighth day, they lie entirely within the thorax. The
carotids, on the other hand, are elongated into vessels which pass forward into the head.
The Physiological Significance of the Embryological Structure of the
Heart and Aortic /1rches.—Before considering the remainder of the
FIFTH DAY: EMBRYONIC BLOOD VESSELS 455
blood vessels, it seems well to digress at this time in order to point out
the physiological significance of the heart and its arches as they have
just been described.
The heart, as has been seen, becomes virtually four chambered. It
fails to become entirely so during embryonic life, however, because of
the persistence of the foramina in the interatrial septum. This fact, as
well as the existence of the dorsal portions of the sixth arches, i.e., the
ducts of Botallo, is correlated with the embryonic method of aerating
the blood. This becomes clear upon a consideration of what this method
involves, as follows:
It is obvious that previous to the hatching of a Bird or birth of a
Mammal the lungs cannot act. Instead the allantois of the Bird, or as
will later be explained, the partially homologous placenta of the Mammal, performs the function of blood aeration. There now remains to be
described the relationship which the interatrial foramina and the ducts
of Botallo bear to the distribution of the different classes of blood. The
fully aerated blood from the allantois, the nutrient laden blood from
the yolk-sac, and a relatively small amount of strictly venous blood
from the posterior part of the body become mixed in the ductus venosus,
and from thence are poured together into the right auricle. At the same
time that this occurs the right auricle is also receiving blood through
the ducts of Cuvier or anterior venae cavae (see below). This blood is
returning from the head, and hence, save perhaps in the very early
stages, is relatively depleted of oxygen and nutriment. Up to this point
there is no question about the facts. From here on, however, there have
been two distinct theories as to the fate of the two classes of blood just
indicated. Both have been developed as a result of observations and
experiments upon Mammals, but probably apply equally well in their
essential points to Birds. 1
The first theory was somewhat obscurely outlined by Harvey in connection with his original discussion of the circulation of the blood in
1628. It can be very briefly stated as follows: It holds simply that the
two types of blood are completely mixed as they enter the right atrium,
and hence that there. is no separation of aerated and unaerated blood in
the embryo. This has been accounted for on the ground that the organism is sufliciently small and inactive and the circulation sufliciently swift
so that such separation is unnecessary. The second theory was developed
in 1798 by Sabatier, and may be described thus:
It is supposed that the structure of the right atrium is such that the
blood entering it from the posterior part of the body through the ductus
456 _ THE CHICK
venosus (aerated blood) is turned away from the right ventricle and
guided through the aperture or apertures in the interatrial septum into
the left atrium. From here it passes into the left ventricle, and thence
through the aortic division of the bulbus and truncus arteriosus into
the third and fourth aortic arches. The third arches, as has been seen,
convey this blood newly oxygenated and full of nutriment straight to the
head; the rest passes through the fourth arches (later only one, the
right or left) and backward along the dorsal aorta. On its way, however,
it becomes mixed with the depleted blood which has returned from the
head; this occurs as follows: It was noted above that this blood from
the head also passes into the right atrium. According to the present theory, however, its direction of entrance, together with the structure of the
cavity, is such that it is diverted from the openings into the left atrium,
and emptied directly into the right ventricle. From here it passes out
through the pulmonary division of the bulbus and truncus arteriosus,
and thence a slight part of it flows through the small pulmonary arteries into the rudimentary lungs. The larger part, however, continues
through the dorsal portions of the sixth arches, i.e., the ducts (later
only one duct) of Botallo, into the dorsal aortae; here, as indicated
above, it inevitably mixes with the aerated blood from the fourth arches
(later arch). Some of this mixture then supplies the body posterior to
the head. The larger share of it, however, eventually reaches again the
walls of either the allantois or the yolk-sac, where it receives respectively oxygen or food material, and is returned‘ to the heart in the manner already noted. Thus the posterior part of the body should get blood
poorer in oxygen and nutriment, at least during later stages when the
above arrangement would be in operation (Fig. 236X). Hence some
think there may be a relation between this and the faster growth of the
anterior end, if indeed that end is still growing faster at this time.
However, despite the theoretical considerations in favor of this second theory, all evidence until recently has supported the earlier view.
Thus to begin with, in the human embryonic heart near term at least, it
was shown anatomically that the interatrial aperture is not large enough
to pass all of the blood delivered by the postcaval vein. Hence it would
appear that some mixture of blood from the anterior and posterior veins
must occur in the right atrium. Then Pohlman, in 1909, apparently settled
the matter experimentally by injecting cornstarch into the vessels leading
from -the "placenta of the Pig embryo into the right atrium. He then
withdrew equal amounts of blood from each ventricle and found them to
contain equal numbers of grains. This type of experiment with certain
FIFTH DAY: EMBRYONIC BLOOD VESSELS 457
5 ‘nternal carotid artery
’ xternal carotid artery
internal carotid artery
external carotid artery
 
 
 
 
 
 
 
 
     
           
common carotid artery common carotid artery
~ subclavion artery (3rd arch)
subclavian artery (3rd arch)
main aortic (4th arch)
duct of Botollo (arteriosus
pulmonary artery (6th arch‘
anterior caval vein
ulrnonory artery (6th arch)
uct of Botallo (arteriosusl
- ulmonory veins
trunc us arteriasus
RIGHT LEFT
posterior vena cav
entrances of _
fight at,-.;um anterior cavol veins
entrances of
pulmonary veins
eft atrium
atria-ventricular
(mirrol) valves
hepatic vein
liver
 
entrance of
posterior vena cov
epotic portal vein
atrio-ventricular vaive
‘ oeliac artery ‘
‘ esenteric vein
right ventricle
 
ductus venosu
 
posterior vena covo "
 
       
 
eft ventricle
As. . .
.-,3! posterior mesenteric artery
.«
mbilical (ollantoic) vein
‘ caudal artery
A
umbilical (allantoic) arteries
Fig. 236X.—— Diagrams to illustrate the circulation in the Chick embryo according
to Lillie, and indicating at least a partial separation of aerated from unacrated
blood. Solid arrows represent aerated blood and broken arrows unaerated blood, the
relative amounts of each type being suggested by the size and heaviness of the
respective arrows. A, the complete circulation. B; the heart alone. Note the numerous small foramina in the interatrial septum as compared with the one larger
foramen ovale in the Mammal. The right atrio-ventricular valve is also different
from either of the mammalian valves (Fig. 336). With the substitution of the
placenta for the allantois, essentially the same type of circulation with the separation of the two classes of blood has been alternately denied and claimed in the case
of the Mammal ever since Harvey. For a complete discussion of this controversy
see the text. It seems now to be settled as shown.
Because this is an embryonic stage the sixth arches are connected with the
aorta. Being ‘a Bird the right sixth unites with the aortic extension of the right
fourth arch through the right ductus Botalli. In the Mammal it would be the left.
At hatching both ducts of Bot-allo will close and later atrophy (Coughlin and
Walker, ’53) .
458 THE CHICK
refinements was repeated by Kellogg on both the Pig and the Dog in
1923, and later by others with similar results. Therefore, it was reasonably concluded by both investigators that there had been a thorough
mixture of the two classes of blood in the right atrium. And so the question seemed to be finally answered.
Regardless of all this seemingly overwhelming evidence in favor of
the theory of mixture, however, many embryologists were still intrigued
by the hypothetical desirability of a separation if it could only be
proven. Consequently they have once more returned to the attack with
both similar and improved techniques, and with most interesting results. In the first place Windle and Becker (’40) using the Cat and
Guinea Pig, injected india ink instead of cornstarch. This probably did
not reduce the velocity of flow as did cornstarch, thus providing more
normal conditions, and their results supported the separation theory
of Sabatier. Next, in 194-1, Barclay, Barcroft, Barron, Franklin, and
Prichard performed the most ingenious experiment yet devised. They injected inert material, opaque to X-rays, into the blood stream of living
Sheep fetuses. Then by means of X-ray moving pictures they showed
that there is a fairly complete separation of the anterior and posterior
streams in the right atrium. This brilliant experiment, especially if confirmed, would seem to be conclusive. Finally, Whitehead (’42) has made
a model of an embryo Cat heart in neoprene by the reconstruction
method. With it he has demonstrated that the key to the separation of the
streams entering its right atrium is the pressure at which each stream enters. He, moreover, believes that the pressures with which the blood
streams do enter the actual Cat heart are such as to separate them. Thus
the matter rests at the date this book is written, and we are back once
more to the purely hypothetical conclusions of 1798.
However this may be in the embryo, it is of course certain that in the
adult Bird or Mammal the completely aerated blood from the lungs
(arterial blood) is normally entirely separated in the heart and arterial
circulation from the venous blood. To achieve this at, or shortly after,
the hatching of the Bird or the birth of the Mammal, all that is necessary' is the closure of the interatrial openings, or opening, in the septum and the occlusion of the ducts of Botallo (one duct in the Mammal).
Considering the matter of the septum first, it will be recalled that by
the end of the sixth day in the Chick this structure was closed except
for the existence of numerous foramina. During the embryonic life of
the Bird these foramina are kept open according to current theory in the
FIFTH DAY: EMBRYONIC BLOOD VESSELS 459
following manner: The pressure on the septum from the side of the
right atrium greatly exceeds that from the left side because of the relatively small amount of blood being returned to the left atrium from the
non-functioning lungs. Hence the septum tends to belly out to the left,
and to remain in a stretched condition with the foramina wide open. In
the Bird, as indicated below, the lungs start functioning to some degree
two or three days before hatching takes place. Hence the vessels of
these developing organs receive more and more blood, and the pressure
on the two sides of the septum is gradually equalized. This causes it to
straighten out, the stretch is taken out of it, and as a consequence its
wall thickens and the foramina are functionally closed. Later the tissue
about the former openings presumably becomes entirely fused. The
mechanism in the Mammal is somewhat different, but is supposed also
to depend on an equalization of pressure in the two atria, and a functional closure of the single interatrial opening. The details of the process in this class will be discussed further in connection with the Pig.‘
The closure of the duct of Botallo (arteriosus), at least in the Mammal where it has been most studied, is apparently brought about by the
contraction of muscle fibers within its walls. This has been rather cleverly demonstrated in the Guinea Pig by Kennedy and Clark (111) . Under anesthesia living, almost full term, fetuses were removed from the
uterus while leaving the umbilical cords attached. The fetuses themselves were then opened. so that the heart could be observed. When such
a fetus was in the air it would breathe. and the duct of Botallo could
be seen to close. When it was immersed in normal saline the embryonic
respiratory situation was restored, and the duct of Botallo would
promptly reopen. This could be repeated several times. Thus the closure
would appear to be a result of the stimulus of breathing. Within a
month or so after normal birth. however, the walls of the duct have
grown together, and the structure is reduced to a cord.
In conclusion of this topic it may be noted that in man either a defect
in the interatrial septum or a persistently patent duct of Botallo are
among the causes of infantile cyanosis, “blue babies.” Where a patent
4 The sudden functioning of the lungs as a factor in increasing the blood flow
from them to the heart in the case of the Mammal has been questioned for the
Cat and Guinea Pig by Abel and Windle (’39l. These authors claim that there is
already a good deal of circulation here at term, and that subsequent increase is
gradual. A similar situation is also claimed for other Mammals, including Man
(Patten, ’46). As noted the condition in the Bird is such that in that case gradual
initiation of lung function, and hence of change in the course of the blood, must
always occur.
460 THE CHICK
duct is the primary defect, it may be remedied by tying oii this vessel.
A failure in septal closure, however, is more diflicult to cope with. Yet
now even this may be greatly helped by a clever operation which involves rerouting part of the aortic blood to the lungs.
The Subclavian Arteries. -—-The primary subclavian arteries arise as
outgrowths from the. eighteenth segmental arteries. On the fifth day,
however, an anteriorly growing branch of each primary artery connects
with the respective third aortic arch, which as indicated eventually becomes a part of the common carotid (Bakst and Chaise, ’28; Figs. 233
and Q35) . These new branches then develop, while the original connections with the dorsal aorta through the segmental arteries become atrophied. Thus the permanent subclavians eventuallyarise from the carotids in the Bird. These arteries, of course, supply the wings, and in so
doing, develop various branches. It will not be advisable, however, to
follow them further in detail.
The Remaining Arteries. —— The only other major arteries whose development has not already been indicated in the account of the fourth
day, are the coeliac, the anterior mesenteric and the posterior mesenteric. The coeliac arises from the anterior part of the dorsal aorta, and
supplies the stomach, gizzard and part of the intestine. The anterior
mesenteric originates as an outgrowth from the single vitelline artery
close to the place where the latter leaves the aorta, and supplies the intestine. Lastly the posterior mesenteric develops from the aorta slightly
caudal to the kidneys, and supplies the rectum and cloaca. These three
arteries appear during the fifth and subsequent days (Fig. 237).
The Veins.
The Vitelline Veins. --- At the end of the fourth day, a second venous
ring had been formed about the intestine by a fusion of the vitelline
veins for a short distance beneath it. This second ring was beginning to
be destroyed by the disappearance of its right side, and during the fifth
day, this side is completely obliterated. From a review of the previous
development of this region, it will be evident that the condition of the
vitelline veins at this point has now become as follows. The two veins
unite just in front of the anterior intestinal portal, and ventral to the intestine, to form a single trunk, which is really a posterior continuation
of the ductus venosus. This trunk runs forward beneath the intestine for
a short distance, and then curves upward and to the ‘left. It next turns
sharply to the right and crosses over the intestine dorsally; finally it
bends immediately downward and again runs anteriorly to pass into the
FIFTH DAY: EMBRYONIC BLOOD VESSELS 461
liver (Fig 211, E). During subsequent stages as the anterior intestinal
portal continues to move backward, it is closely followed by the fusion
of the vitelline vessels. Indeed before very long this fusion passes beyond the region of the intestinal portal, and thus the single ductus venosus, or vitelline trunk, comes to extend a considerable distance into the
umbilicus before dividing into its two branches.
Fig. 237. —Diagrammatic lateral view of the chief embryonic blood vessels of the Chick, during the sixth day. From Kellicott (Chordate Development). After Lillie.
a. Atrium. al. Allantoic stalk. ao. Dorsal aorta. c. Coeliac artery. ca.
Caudal artery. cl. Cloaca. cv. Caudal vein. da. Ductus arteriosus. dv. Ducurs venosus. ec. External carotid artery. e]. External jugular vein. i. Intestine. ic. Internal carotid artery. ij. Internal jugular vein. 1. Liver. m.
Mesone‘phros. ma. Mesenteric artery. mv. Mesenreric vein. p. Pulmonary
artery. ,pc. Posterior cardinal vein. pv. Pulmonary vein. 5. Sciatic artery.
31:. Sulfclavian artery. scv. Subclavian vein. st. Yolk-stalk. sv. Subcardinal
vein. ul. Left umbilical artery. ur. Right umbilical artery. 1112. Left umbilical vein. 1;. Ventricle. va. Vitelline artery. vca. Anterior vena cava (anterior cardinal vein). vp. Posterior vena cava. vv. Vitelline vein. y. Yolksac. 3, 4, 6. Third, fourth, and sixth aortic arches.
The Hepatic Portal System. -—— It will be recalled that within the liver
the ductus venosus receives numerous capillaries. These capillaries increase during the fifth day, while at the same time the main channel of
the vein within the liver begins to disappear. This is brought about
through the gradual occlusion of this channel by means of strands of
the hepatic substance which grow into and across it. On the fifth day
also, a vessel starts to develop in the dorsal mesentery of the gut; it is
the mesenteric vein, and~presently acquires a connection with the vitelline trunk at about the region of the pancreas. By the seventh day the
462 THE CHICK
occlusion of the main part of the cluctus venosus within the hepatic sub
stance has been completed. From now on, therefore, the blood enters the
liver by the remaining posterior half of this vein, is distributed through
the hepatic capillaries, and is finally collected again to enter the now
separate anterior half of the same vessel through two main branches.
When development has reached this stage the posterior half of the ductus
venosus may be termed the hepatic portal vein, which receives the mesenteric vessel as its chief tributary. The two branches entering the anterior half of the ductus vencsus, upon the other hand, constitute the
Izepagic veins (Fig. 211, F).
Upon the fifth and immediately subsequent days the blood which enters the liver circulation is largely from the yolk-sac. Before long, however, the mesenteric vein has begun to send out branches which develop
simultaneously with the various digestive organs and spleen. Thus these
organs send an ever-increasing supply of blood through the hepatic portal rein to the liver. When the yolk-sac finally disappears they become
the sole source of the blood which passes through the hepatic capillaries.
The complete system of circulation which is developed in this manner is
then called the hepatic portal system.
The Fate of the Cardinals and Development of the Caval and Renal
Veins. -— On the fourth day, the subcardinals lying ventral to the mesonephros have direct connections with the posterior cardinals lying dorsolateral to it. Upon the fifth day, however, these connections are severed
and new ones established through capillaries within the mesonephric
-substance. At the same time, the subcardinals fuse with one another near
their anterior ends, and the connection of the right one with the posterior end of the vena cava inferior (established on the fourth day) becomes larger (Fig. 238). Thus a part of the blood in the posterior
cardinals now passes through the mesonephros and by way of the subcardinals and vena cava inferior to the heart. In other words, there is in
the embryo of the Bird a typical renal portal circulation. On the fifth
day also, or late upon the fourth, the subclavian veins begin to develop
in connection with the fore-limb buds. They arise as branches of the
posterior cardinal veins, a short distance behind the junction of the latter with the Cuvierian ducts.
Upon the sixth day, the section of each posterior cardinal between the
entrance of the respective subclavian vein and the anterior end of the
mesonephros disappears, thus forcing all the blood from the posterior
part of the body to traverse the renal portal channels. In this manner
also that portion of each posterior cardinal anterior to the entrance of
wag-ea
2;:;.5.~,a....~,..»s,~.._,.~ .. ,.
FIFTH DAY: EMBRYONIC BLOOD VESSELS 463
c. /V. sc. d. V. sc. s.
Fig. 238.--Reconstruction of the venous system of a
Chick of 5 days. Ventral view. From Lillie (Development
of the Chick). After Miller.
a. i\-Iesonephric veins. A0. Aorta. A.o.m. Omphalomesem
teric artery. A.u.s. Left umbilical artery. A.sc.s. Left sciatic artery. V.c.p.d.s. Right and left posterior cardinal
veins. v.c.i. Vena cava inferior. V..sc.d.,s. Right and left
subcardinal veins.
the subciavian becomes simply the proximal part of the latter vessel.
From this time on, the ducts of Cuvier, which now receive the jugulars
(anterior cardinals) and subclavians, may be termed the anterior or
superior caval veins. At about this stage also, the anterior portion of the
ductus venosus, which receives the two hepatic veins and the posterior
vena cava (vena cava inferior), may be said to have become merely the
anterior end of the latter vessel. Thus the posterior caval vein, like the
464 THE CHICK
two anterior cavals, now opens directly into the right atrium (Fig. 237).
While the above changes are occurring subsequent to the fifth day,
there are a pair of new veins arising in connection with the metanephros
Fig. 239. -o- Reconstruction of the venous system of‘ a sparrow embryo, corresponding to a chick of about 14- days. From Lillie (Development of the Chick). After
Miller.
V .c.i.H. Intra-hepatic part of the vena cava inferior. V.c.i.SC. Part of the venecava inferior derived from the suhcardinal vein. V.v.g. Genital veins. V.i.e.d.,s.
Right and left vena iliaca externa. V.i.i. Vena iliaca interna, (or V.c.p.s. Posterior
part of the left cardinal). V.i.l.d.,s. Right and left vena intervertehralis lumhalis.
V.r.m.d.,s. Right and left great renal veins.
or permanent kidney. These are the renal veins which presently take
blood from the permanent kidney to the anterior fused portion of the
subcardinals (now really the posterior part of the posterior vena cava) .
Just anterior to the kidney these renal veins also later establish direct
connections with the ‘posterior cardinals. Thus a new channel is formed
for the blood from the posterior part of the body via the cardinals and
the anterior portion of the new renal veins to the posterior vena cava
(Fig. 239). At the same time that this is occurring, the mesonephros toFIFTH DAY: SEPARATIO1'_V' OF BODY CAVITIES 465
gather with the renal portal system is disappearing. While the latter exists, however, it is essentially similar to the permanent system of the
same name in the Frog and other more primitive Vertebrates, thus affording an excellent example of recapitulation. It remains to note that
the hinder portions of the posterior cardinal veins persist in the adult
Bird as the iliac veins, receiving branches from the hind-limbs. Also in
subsequent stages, branches from the cardinals fuse with one another
medially at the posterior end of the body and give rise to the caudal
vein.
THE BODY CAVITIES
From previous discussion, it will be recalled that the space surrounding the heart has been designated as the pericardial cavity. Up to this
time, however, there has been no mention made of any separation of this
cavity from the peritoneal or general body cavity behind it. It now remains to describe how this separation is effected, together with the simultaneous closing 03 of 51 third space, the pleural cavity (see below). It
will then be possible in conclusion to show also how the walls of the
pericardial cavity come to form the independent pericardial sac of the
adult bird.
THE SEPARATION OF THE PERICARDIAL, PERITONEAL
AND PLEURAL CAVITIES
The separation of the peritoneal and pericardial cavities is chiefly
brought about by the development of a partition known as the septum
transversum. This so-called septum in turn is composed of three parts,
two of which have already been mentioned. The entire septum then is
made up as follows: First, there is a median mass consisting of the liver
and the sinus and ductus venosus, together with the dorsal and ventral
ligaments which unite the liver to the gut and for a time to the ventral
body wall. Second, there are the lateral mesocardia extending obliquely
in an anterior and lateral direction from the median mass to the body
walls. Above and below the lateral mesocardia, the pericardial cavity
still communicates posteriorly with the peritoneal or general body cavity. About the fifth day, however,'the ventral communication begins to
be closed. This is accomplished by the development of the third part of
the septum transversum, i.e., the lateral closing fold, extending from
the mesocardia to the ventro-lateral body wall. By the eighth day, this
closure is complete. In the meantime, the lungs have been developing in
466 THE CHICK
the portion of the peritoneal space which extends forward above the pericardial cavity. This space may be termed pleural cavity, and at this time
(fifth day) the oblique lateral mesocardia have not yet entirely separated it anteriorly from the pericardial cavity beneath it; posteriorly
also it still communicates with the general body cavity. Presently, however, with the further development of the lateral mesocardia and other
parts, the opening between the pleural and pericardial cavities is closed,
and a closure of that between the pleural and body cavities soon follows
(tenth day). This latter is effected by the pleuro-peritoneal septum,
which arises as an outgrowth from the sides of the esophagus. The median pericardial cavity is thus bounded dorsally largely by the mesocardia, laterally and ventrally by the peritoneum of the body wall, and
posteriorly chiefly by the median mass of the septum transversum.
THE ESTABLISHMENT OF THE DEFINITIVE PERlCAR—
DIUM
Eventually, however, the tissue upon the front of the median mass beu
comes thickened and splits into two sheets. The anterior sheet then becomes the posterior wall of the pericardium, the posterior sheet covers
the face of the liver, and the general body cavity extends between them.
At the same time, the latter cavity is also pushing forward beneath and
at the sides of the present pericardium, and as it does so, it apparently
splits the peritoneum of the body wall into two layers. The outer layer
forms the peritoneum of the general body cavity in this region, and the
inner layer constitutes the ventral and lateral wall of the pericardium
proper. In this manner, the final pericardial wall or definitive pericardium of the adult bird comes to surround the heart as a relatively
independent sac with a portion of the liver extending beneath it.
THE URINOGENITAL SYSTEM
THE EXCRETORY SYSTEM
The Mesonephrcs. — During the fifth day, the increase in the numher of the mesonephric tubules ceases, while the organ becomes more
active as a kidney. For a couple of days subsequent to this, however, the
tubules continue to grow in length, thus greatly increasing the bulk of
the organ. Degeneration begins about the eleventh day, and from then
on, the metanephros aids in performing the excretory functions which it
later entirely takes over.
FIFTH DAY: THE EXCRETORY SYSTEM 467
The Metanephros. ——
At the end of the fourth
day, the diverticulum
(ureterl from the posterior end of the W/olfiian
duct had just appeared,
and the nephrogenous tissue immediately behind
the mesonephros had degenerated. During the fifth
day, the above diverticulum, accompanied by the
nephrogenous tissue posterior to the region of
degeneration, grows forward somewhat, and begins to branch dichotomously ( Fig. 240, representing a slightly later
stage). Its position in this
region is adjacent to the
posterior cardinal vein,
upon the median side of
the latter and above the
Wolllian duct. The accompanying nephrogenous tissue lies. in turn, adjacent
to ‘the median side of the
diverticulum, so that the
latter, i.e., the diverticulum, lies between the vein
and the tissue. The nephrogenous tissue, which
is in immediate contact
with the diverticulum and
its branches, is called the
inner zone. Lastly this inner zone is covered on its
median sidelby a layer of
dense mesenchyme which
Fig. 240.——Profile reconstruction of the Wolffian duct and primordium of the metanephrns of
a Chick embryo of 6 days and 8 hours. From
Lillie (Development of the Chick). After Schrei~
ner.
XXV to XXXUI, thetwemy-fifth to thirty-third
somites. ALN. The neck of the allantois. CI. The
cloaca. Int. The intestine. M’s’n. The mesonephros.
71.7‘. The nephrogenous tissue of the metanephros
included within the dotted lines. W.D. The Wolffian duct. Ur. The ureter.
468 THE CHIGK
differentiates in advance of the growing nephrogenous element and diverticulum. It is called the outer zone (Fig. 241).
During subsequent days, the posterior end of the mesonephric duct
bearing the rnetanephric diverticulum (ureter) is drawn into the cloaca,
and thus the ureter acquires an opening separate from that of the mesonephros (Fig. 24-0}; The other end of the rnetanephric duct, with its
’ inner and outer zones,
meanwhile, grows still
further forward till it
reaches the region of the
mesonephros, and then
continues on dorsal to
that organ, nearly to its
anterior extremity. The
inner zone of this tissue
everywhere gives rise to
the secreting tubules and
glorneruli of the permanent kidney in a manner
very similar to that dethe scribed for the mesoneplr
ros. These tubules then
Fig. 241.—Transverse section through ‘
ureter and metanephrogenous tissue of a live
tziygdghick. From Lillie (Dez1elopmen.t of the Connect with the diCh0tO_
A.umb. Umbilical artery. Coal. Coclom. M’s’t.
Mesentery. n.(..i.z. Inner zone of the nephrogenous tissue. n.!.o.z. Outer zone of the nephrogenous tissue. Ur. UI‘€'l€!‘. V .c.p. Posterior cardinal vein. W13. Wolflian duct.
mous branches of the
metanephric duct, which
thus function as collecting
tubules, while the duct itself becomes the ureter of the adult. Eventually the outer zone helps
to form a connective tissue covering for the entire organ.
THE REPRODUCTIVE SYSTEM
The Gonads in the Male. ——~ During the fourth day, it is impossible
to distinguish sex. Occasionally on the fifth day, but more generally
and definitely on the sixth, the distinction becomes possible by the fact
that in the female the left gonad is slightly larger than the right.
This is apparently due to the fact that the right gonad usually possesses
relatively little cortex, and fewer germ cells. These latter facts according to Witschi (’35) are correlated. The left gonad in the female possesses more cortex because of the female chromosomal complex and the
excess cortex this worker thinks acts as an inductor to attract more germ
LY’).
FIFTH DAY: THE REPRODUCTIVE SYSTEM 469
cells. Be this as it may, in the male, which is to be considered first, there
is virtually no difference between the gonads, and therefore the description of one will suffice for both.
It has been indicated in the introductory discussion of germ cells in
general that the primordial germ cells of the Chick are said to be first
Fig. 242.—Section through the gonad of a Chick, the middle
of the fifth day, showing the sexual cords growing inward from
the germinal epithelium. The connections of many of the cords
with the epithelium have been cut across. From Kellicott (Chordate Derelopnzent). After Semen.
g. Germinal epithelium. m. Epithelium of the mesentery (peritoneum). o. Primordial germ cells. 5. Sexual cords. t. Connective-tissue stroma.
discernible well outside the embryo. Indeed, according to Swift (’l4)
and Goldsmith (’28, ’35), these cells are first found at the primitive
streak stage in the zone of junction lateral to the proanmion. From
here they are carried by the blood stream to the vicinity of the germinal
epithelium, whence by amoeboid movements they enter this epithelium
during the fourth and fifth days.
More recently, so far as the representatives of these cells which actually reach the germinal epithelium are concerned, their initial transfer
by means of the blood stream has been denied (Stanley and Witschi,
’40). These authors admit that primordial germ cells are indeed found
470 THE CHICK
I
l
l
l
l
Fig. 243. —-Cross-section through the genital primordium of Limosa aegocephalzz.
From Lillie (Development of the Chick). After I-ioffxnann, from Felix and Buhler.
The stage is about similar to that of a Chick embryo of 4; days, and shows the rote
cords extending from the Malpighian tubules to the germinal epithelium. The lat
ter appears in the figure as a dark mass on the right ventral side of the nn:soneph
ros next to the mesentery. Three primordial germ cells (light colored) are visible
in it. ;
Germ. Germinal epithelium. Ms.t. Mesentery. S.C. Rete cord. V. Posterior cardi-
nal vein. W.D. Wolflian duct. j
in the blood in early stages, but claim that they are only cast offs, never
destined to enter the gonads. According to them all movement of such
cells really on their way to the germinal epithelium is by passive shifting accompanying growth and rearrangement of parts, and later by
active migration as indicated? Be this as it may, by the fifth day the germinal epithelium with the primordial germ cells in it is being drawn
~" It must be further noted that according to Firket (’20) and others all, or most, ‘
of these so-called primordial germ cells in the Chick, as in the Albino Rat, ulti-
mately degenerate and are replaced by definitive germ cells derived from the germinal epithelium itself. ‘
FIFTH DAY: THE REPRODUCTIVE SYSTEM 471
Fig. 244. —Cross-section through the periphery of the testis of a just
hatched Chick. From Lillie (Development of the Chick). After Semen.
The sexual cords have acquired a lumen, and the walls of the canals
thus formed are lined within by the spermatogonia. Next to the latter
come a layer of supporting or Sertoli cells. The connective tissue
(stroma) lying between the sexual cords (now seminiferous tubules‘!
connects at the periphery of the testis with the special layer of connective tissue (albuginea) which covers the entire organ beneath the thin
outermost layer of coelomic epithelium.
Alb. Albuginea. c.T. Connective tissue of the stroma, or septulae
testis. Ep. Remains of the germinal epithelium now forming the outermost or serous covering of the testis. L Lumen of the sexual cords. pr.o.
Spermatogonia. s.C. Sexual cord, lined by supporting cells and spermatogoma.
somewhat on to the ventro-median surface of the mesonephros. Meanwhile from the capsules of the Malpighian bodies of that organ, strands
of cells begin to grow out through the loose mesenchyme to the germinal
epithelium. These strands are the rete cords, and are destined to form
the vasa eflerentia which help to connect the future tubules of the testis
with the vas deferens (see below). At about this period also the germinal epithelium begins to send processes inward among the mesenchyme
cells and the rete cords. These new strands of tissue of epithelial origin
are the sexual cards, which contain primordial germ cells (Figs. 24-2,
243) . Up to this point the condition of the male gonad is virtually iden472 THE CHICK
tical with that of the female. From now on, however, the former begins
to be differentiated to form the adult testis in the following manner:
The sexual cords become separated from the epithelium, and increase
in number so as to constitute the bulk of the organ (seventh day) , while
the rete cords are pressed to the side nearest the mesonephros. Presently also (eleventh day) the mesenchyme, which has been scanty, begins to increase among the sexual cords, forming the connective tissue
or stroma. Eventually it gives rise further to a layer, the albuginea, lying between these cords and the reduced sheet of epithelium which remains as the outer covering of the gonad. Meanwhile the sexual cords
themselves (twentieth day) begin to acquire a lumen, and are thus
transformed into the seminiferous tubules. The walls of the latter are
composed of supporting cells which are lined internally by the multiplying primordial germ cells. The latter may now be termed spermatogonia, from which arise in turn the sperrnatocytes and sperm (Fig.
244) . It is to be noted in this connection that the spermatogonia, unlike
the oiigonia in the Bird, continue to divide throughout the sexual life of
the individual. The ends of the seminiferous tubules eventually become
connected with the rete cords which, as indicated above, become tlt:
vasa efferentia. These in turn connect with the modified mesonephric tuhules in the anterior or sexual half of that organ, which thus becomes
the epididymis. The posterior and non-sexual portion of the mesonephros which remains becomes a vestige known as the paradidymis.
The Gonads in the Female. —Although differences in sex may be
indicated by the disparity in the size of the gonads as early as the fifth
day, there is little else to distinguish male from female at this time.
The description of the testes up to this point will, therefore, suffice
also for the ovaries. The right and left ovary, however, are different in
the Bird, and this difference appears at an early stage.
ln.the left ovary, following; the sixth day, a secondary set of sexual
cords, the ovigeraus cords, grow inward from the germinal epithelium,
and again carry primordial germ cells. The new cords press the original or
primary cords into the medullary region, and the germinal cells in the latter cords degenerate. In the right’ ovary no such secondary growth occurs,
and under normal conditions the primary cords develop only slightly,
the whole structure remaining rudimentary unless artificially stimulated
by injected male hormone to form a testis. In the left ovary, however,
the secondary or ovigerous cords soon break up into nests, each containing at least one germ, surrounded by remaining epithelial cells which
form its follicle. From this point on, the young egg cell begins to grow,
FIFTH DAY: THE REPRODUCTIVE SYSTEM 473
and it may, therefore, be termed an oiicyte (Fifi. 245) . This growth period is reached earlier by some ova than by others, but the oogonial or
multiplication stage ceases for all about the time of hatching. The anterior portion of the mesonephros, which in the male forms the epicli(ly
Fig. 245. -Cross-section of the ovary of a fledgling of Numenius arouatus 3-4 days old. The germinal epithelium is below. From Lillie (1)0velopment of the Chick). After Hoflmann. Note numerous oiicytes surrounded by a single layer of follicle cells.
s.c. Sexual cords degenerating. Germ. Ep. Gerrninal epithelium pruducing ovigerous cords.
mis, remains as a minute rudiment, the epoophoron. The paradidymis of
the male is sometimes evident in the hen as a still smaller vestige, the
pa/'o6p/Loron.
The Gonoducts in the Male. — It has already been stated that in
the male, the Wolffian ducts become the vasa deferentia or sperm ducts
of the adult. They connect with the testes through the vasa eflerentia and
epididymis. Late in ‘development, they become muscular and somewhat
convoluted, with a dilation at their posterior extremities.
474 I THE CHICK
The Gonoducts in.the Female. —— As has been stated, the oviduct:
begin development on the fourth day as the tubal ridges, one on the lat
eral side of each mesonephros adjacent to the respective Wolfiian duct
During the fifth day, a groove-like invagination develops along the an
mttz
   
Fig. 246.——Trans\ erse section through the metanephros, rnesonephros, gonads and
neighboring parts of an 8-day Chick. From Lillie (Development of the Chick).
A0. Aorta. bl.v. Blood vessels. BJ7. Body-wall. Coel. Coelom. COLT. Collecting
tubule of the mesonephros. col.T.M’t’n. Collecting tubules of the metanephros.
Glam. Glomerulus. Gon.l. Left gonad. Gon..r. Right gonad. M.D. Miillerian duct.
M’s’t. Mesentery. n.t.i.z. Inner zone of nephrogenous tissue (metanephric). n.t.o.z.
Outer zone of the nephrogenous tissue. Symp.Gn. Sympathetic ganglion of the
twenty-first spinal ganglion. V.C. Centrum of vertebra. V.s’c.l. Left subcardinal
vein. W.D. Wolfiian duct.
terior portion of each ridge, and the lips of the groove fuse with one
another to make a tube open at its anterior end. This tube which is quite
short, then grows backward independently between the remaining tissue of the ridge and the Wolfiian duct (Fig. 24.6) .
Subsequent development is as follows: By the eighth day each duct
has reached the cloaca, but does not open into it. At this time, there begins the atrophy of both ducts in the male and of the right duct in the
FIFTH DAY: THE ADRENALS - 475
female, accompanied in both sexes by the disappearance of the remains
of the tubal ridges. The left duct in the female, however, gradually enlarges and dillerentiates the infundibulum and glandular portions charaeteristi(- of the adult. It does not, however, effect. an entrance into the
cloaca until the hen is about six months old (Lillie alter Casserl. It always remains attachetl to the body wall and the rudiments of the meso
'nephros by a ligament or mesentery-like fold.
THE ADRENALS
During the fifth day, the cortical substarree, noted as arising on the
fourth day, increases in amount, and cornea into relation with the Malpighian capsules. On the sixth day it begins to be zirrzmged in definite
cords. \'\'l'tlC‘.ll during subsequent days increase in size and number. while
at the same time innervation of the organ begins. On the eighth day
this mass of cords is becoming penetrated by blood sinuses and by the
medullary material previously l!tLll(‘al€d. Within the latter, “ chromaffine ” cells are being differentiated, and eventually this medullary material also acquires a cord-like arrangement.
HATCHING
lt will be recalled that originally the embryo was orientated with its
long axis transverse to that of the shell. and with the head away from
the observer when the large end of the shell is to the obser\‘er’s left. Between the fifth and ninth days the position of the embryo varies considerably, and changes from time to time due to active contractions of the
amnion. By the tenth day, however, a normal embryo agrain assumes the
original position relative to the shell. But at this stage it is nearer to
the large end of the latter, and lies with its back against the yolk-sac in
‘ stead of either its ventral parts or its side. In this position of course its
legs are pressed against the shell. Next, aided by contractions of the amnion, the ‘yolk-sac is moved first toward the small end of the shell, and
then up over the ventral side of the embryo. This movement is usually
completed by the thirteenth or fourteenth day. During the next three or
four days the yolk-sac moves on over the ventral side of the embryo
until the now partially emptied and flabby sac occupies the large end of
the shell. As this is occurring the embryo by means of vigorous wriggling turns itself so that when the process is completed its tail is at the
small end of the shell, i.e., the long axis of the embryo and shell have
476 THE CHICK
now become parallel. According to the schedule indicated this condition is finally achieved on the seventeenth or eighteenth day.‘ The next
step involves the piercing of the egg membrane by the beak so that
breathing of air from the air chamber can begin. Some respiratory
movements may occur, however, even before this, there being by this
time small amounts of air in other parts of the egg. As respiration starts
the amnion and allantois dry up and become detached, while movements of the abdomen draw the remains of the yolk-sac within the body.
At the same time the necessary circulatory changes are occurring within
the embryo as already described. About the last hour before hatching
on the twenty-first day the Chick starts a vigorous counter clock-wise
rotation within the shell aided by strong thrusting movements of the
legs. Presently as a result of the thrusting of the legs and the stretching
of the neck the shell is broken into two parts and the Chick is hatched.
The foregoing description of later positional changes and hatching is
taken from the detailed account by Kuo (’32). One interesting feature
which is not mentioned by this author, however, is the so-called egg
tooth. This is a sharp cone shaped point of horny material developed on
the dorsal side of the beak, and is said by other writers to function in
chipping the shell. At all events it is a transitory structure lost soon
after hatching.
SUMMARY or THE CONDlT!ON AT THE END or THE
FIFTH DAY or INCUBATION C
I. THE EXTERNAL APPEARANCE
The cervical flexure has reached its rnaicimum development, the third
visceral cleft has closed, and the future neck is slightly indicated. The
limb buds are beginning to appear jointed. The nasal apertures are sep
arated into internal and external nares and the beak and mandible are
just startingto form.
II. THE FEATHERS
A depression develops in the skin. At its bottom a slight outgrowth
arises consisting of_a core of mesoderm, the pulp, with a covering of the
Malpighian layer and a thin outer layer of cornified epithelium. This
outgrowth is the papilla. The papilla emerges above the depression, and '
is known as the feather germ. With further growth and the throwing off
“Waters (’35) says usually not until the nineteenth or twentieth day.
FIFTH DAY: SUMMARY 4??
of the cornified cells the Malpighian layer becomes folded and modified
to form the quill and barbs of a feather. Feather germs appear in the
Chick on about the eighth day.
III. THE SKELETON
The definitive or vertebral segmentation of the mesencliymal slzeatlz,
about the notochord and nerve cord has become more marked, while all
the sclerotomal tissue is becoming membranous. These membranous condensations are especially evident in certain regions, representing parts
of the future vertebrae neural arches and costal processes. Mesenchymal
concentrations representing the limb bones and the parts of the pectoral
and pelvic girdles are also visible. The various parts of the primordial
cartilaginous cranium and visceral skeleton. are discernible at this time
as concentrations of mesenchyme about the head
IV. THE ALIMENTARY TRACT
The Fore-gut Region.———The third visceral cleft closes, the lung
rudiments have grown posteriorly somewhat through a mass of developing mesoderm, and faint indications of the abclomirzal and cervical air
sacs may be present. The glottis is partly closed.
The esophagus has continued to elongate, the stomach is slightly dilated, and a pouch representing the rudiment of the gizzard has appeared in connection with it. The duodenal loop is barely defined. The
liver has continued to branch, and some of the branches have acquired
lumens. The three pancreatic diverticula have also branched somewhat.
The Mid-Gut Region. The end of the duodenum is marked by a
ventral bend, the duodeno-jejurzal flexnre. From here the midgut or
small intestine descends to connect with the yolk-sac, and passes dorsally
again to its posterior end, marked by rudiments of the intestinal caecae.
The Hind—gut Region. —-The hind-gut or rectuniis not materially
altered, but the laterally compressed walls of the posterior part of the
cloaca have become fused.
V. THE CIRCULATORY SYSTEM
The Heart.———The alterations in the relative positions of the parts
are nearly completed, as are also the septa within the heart. The septum
of the truncus arteriosus has formed and that of the ‘bulbus has started
to develop.
The Arteries. —-— The portions of the dorsal aortae between the third
and fourth arches have begun to disappear, and the left fourth arch has
478 » THE CHICK
also diminished in size. The subclavian, arteries have become connected
with the carotids and the anterior mesenteric and coeliac arteries are
developed.
The Veins. —-—The right side of the second venous ring about the intestine has disappeared, so that in this region there is only a single vitelline trunk. Within the liver, the capillaries of the ductus venosus are
continuing to develop, while the main channel is atrophying. The mesenteric vein has started to form.
The subcardinals have lost their original direct connections with the
posterior cardinals, and have developed new ones through capillaries
within each mesonephros. At the same time the subcardinals have ‘fused.
with one another anteriorly, and by means of the previous connection
with the vena cava inferior, have thus established a renal portal system. The subclavian veins have started to develop from the posterior
cardinals.
VI. THE BODY CAVITIES
The ventral communication between the pericardial and peritoneal
cavities has begun to he closed by the development of the lateral closing
folds beneath the lateral mesocarzlia.
VII. THE NERVOUS SYSTEM
In connection with the description of this system in the preceding
chapter, it was noted that there are few important developments occurring in it on the fifth day. The following events, however, may be mentioned as having taken place during this period.,The fourth cranial
nerves have originated, and in connection with the ear the rudiments of
the semicircular canals have appeared. In the eye the mesenchymal
part of the pecten. is increasing, while the lips of the choroid fissure are
beginning to overgrow it.
VIII. THE URINOGENITAL SYSTEM
The Excfetory System. —— The mesonephric tubules have ceased to
increase in number, but are continuing to grow in length as the organ
becomes more active. The metanephric diverticulum, accompanied by its
nephrogenous tissues or inner zone, has grown forward and begun to
branch, while about the latter the outer zone is developing from mesonchyme.
The Genital System.——The primordial germ cells have begun-to
pass into the germinal epithelium and the rete and sexual cords have
9 REFERENCES TO LITERATURE » 479
started to develop. The male and female gonads are similar except for
occasional differences in size between the right and left organs in the
female. In both sexes, the oviducts are present as small tubes growing
toward the cloaca.
IX. THE ADRENALS
The cortical substance of the adrenals increases in amount, and comes
into relation with the Malpighian capsules.
REFERENCES TO LITERATURE
CHAPTERS VIII, IX, X, XI, XII, AND XIII
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———-, Franklin, K. J., and Prichard, M. M. L., “ The Foetal Circulation and Cardiouascular System and the Changes That They Undergo at Birth,” Oxford,
1944-.
Barron, D. H., “Observations on the Early Differentiation of the Motor Neuroblasts in the Spinal Cord of the Chick,” Jour. Comp. Neur., LXXXV, 1946.
Barry, A., “The Intrinsic Pulsation Rates of Fragments of the Embryonic Chick
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Bartelmez, G. W., “The Bilaterality of the Pigeon’s Egg: A study in Egg Organization from the First Growth Period of the Oiicyte to the Beginning of Cleavage. Part I,” Jour. Morph, XXIII, 1912. —“ The.ReIation of the Embryo to the
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Boyden, E. A., “ An Experimental Study of the Development of the Avian Cloaca,
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Bueker, E. D., “The Influence of 3. Growing Limb on the Differentiation of
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480 THE CHECK
During its Passage Through the Isthmus and Uterus of the Hen’s Oviduct,”
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Chen, B. K., “ The Early Development of the Duck’s Egg, with Special Reference
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Cole, R. K., “Histology of the Oviduct of the Fowl in Relation to Variations in
the Condition of the Firm Egg Albumen,” Anat. Rec., LXXI, 1938.
Congdon, E. D. and Wang, H. W., “The Mechanical Processes Concerned in the
F urmation of the Differing Types of Aortic Arches of the Chick and the Pig
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Conrad, R. M. and Phillips, R. E., “The formation of the Chalazae and Inner
Thin White in the Hen’s Egg,” Poultry Science, XVII, 1938.
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-——-, and Warren, D. C., “The Alternate White and Yellow Layers of Yolk in
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Danchakoff, V., “ Uber clas Auftreten der Blutelemente im Hiihnerembryo,” Folio
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Eastlick, H. L., “ Studies on Transplanted Embryonic Limbs of the Chick. I. The
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Zoi.il., XCIII, 194-3.
Edwards, C. L., “ The Physiological Zero and the Index of Development for the
Egg of the Domestic Fowl, Callus Domesticus," Am. ./our. Physiol., VI, 1902.
Evans, H. M., “ On the Development of the Aortae, Cardinal and Umbilical Veins,
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Firket, Jean, “On the Origin of Germ Cells in Higher Vertebrates,” Anat. Rec,
XVIII, 1920.
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Fraps. R. M., Neher, B. H., and Rothchild, I., “ The Imposition of Diurnal Ovula~
tory and temperature Rhythms by Periodic Feeding of Hens Maintained
under Continuous Light,” Endocrinology, XL, 1947.
Gasser, E., Beitriige zur Entwiclcelungsgeschichte der Allanlois, M iillerschen. G¢'1'nge
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Goldsmith, J. B., “The History of the Germ Cells in the Domestic Fowl,” four.
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Greil, A., “Beitrage zur vergleichenden Anatomie und Entwickelungsgeschichte
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Cruenwald, P., “Normal and Abnormal Detachment of Body and Cut from the
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Guyer, M., “ The Spermatogenesis of the Domestic Chicken (Callus domesticus),”
Anat. Anz., XXXIV, 1909.
REFERENCES TO LITERATURE 481
Hamburger, V,, “Morphogenetic and Axial Self-differentiation of Transplanted
Limb Primordia of 2-day Chick Embryos,” Jour. Exp. Zo5l., LXXVII, 1938.—
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on the Proliferation and Differentiation in the Spinal Cord of Chick Embryos,”
Jour. Exp. Zo6l., XCVI, 194-4.
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Hillemann, H. H., “An Experimental Study.of the Development of the Pituitary
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Hunt, E. A., “ The Differentiation of Chick Limb Buds in Chorio-allantoic Grafts,
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Jacobson, W., “The Early Development of the Avian Embryo. I. Endoderm Formation,” Jour. Morph., LXII, 1938.— “ II. Mesoderm Formation and the Distribution of Presumptive Embryonic Material,” Jour. Morph., LXII, 1933.
Jones, D. S., “ The Origin of the Sympathetic Trunks in the Chick Embryo,” Anat.
Rec., LXX, 1931+“ Studies on the Origin of Sheath Cells and Sympathetic
Ganglia in the Chick,” Anat. Rec., LXXIII, 1939.—“ Further Studies on the
Origin of Sympathetic Ganglia in the Chick Embryo,” Anat. Rec., LXXIX,
1941.——“ The Origin of the Vagi and the Parasympathetic Ganglion Cells of
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Kcibel, F., and Abraham, K., Normaltafeln. zur Entwickelungsgeschichte des
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E
i.
I
‘E
E.’
E
_.
482 THE CHICK
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9
REFERENCES T 0 LITERATURE 483
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i””""’
E
APPENDIX TO CHICK BIBLIOGRAPHY
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Hamburger, V. and Hamilton, H. L., “ A series of normal stages in the development
of the chick embryo,” Jour. Morph., LXXXVIII, 1951.
Hammond, W. S., “ Origin of the thymus in the chick embryo,” Jour. Morph., XCV,
1954-.
Levi-Montalcini, R. and Amprino, R., “ Recherches experimentales sur l’origin du
ganglion ciliaire dans l’embryon de poulet,” Arch. de Biol., LVIII, 194-7.
Levi-Montalcini, R., “The origin and development of the visceral system in the
spinal cord of the chick embryo,” Jour. Morph., LXXXVI, 1950.
McKeehan, M. S., “A quantitative study of self differentiation of transplanted lens
primordia in the chick,” Jour. Exp. Zob'l., CXXVI, 1954.
Olsen, M. W. and Fraps, R. M. “ Maturation changes in the hen’s ovum," Jour. Exp.
Zob'l., CXIV, 1950.
Randles, C. A., Jr. and liomanolf, A. L., “ Some physical aspects of the amnion and
allantois of the developing chick embryo,” Jour. Exp. Zo6l., CXVI, 1950.
Straus, W. L., Jr. and Rawles, M. E., “ An experimental study of the origin of the
trunk musculature and ribs in the chick,” Am. Jaur. Anat., XCII, 1953.
— Waterson, R. L., Fowler, I. and Fowler, B. 1., “The role of the neural tube and
notochord in development of the axial skeleton of the chick,” Am. Jour. Anat.,
XCV, 1954-.
Yntema, C. L. and Hammond, W. S., “The origin of intrinsic ganglia of trunk
viscera from vagal neural crest in the chick embryo,” Jour. Comp. Neur., CI,
1954.-~ “ Experiments on the origin and development of the sacral autonomic
nerves in the chick embryo,” four. Exp. Zo6l., CXXIX, 1955.
==PART V THE EARLY DEVELOPMENT OF THE MAMMAL AND ITS EMBRYONIC APPENDAGES==
INTRODUCTION
IN taking up the development of the Mammal in a book of this
type, intended primarily for college undergraduates, the writer faces a
dilemma in the choice of material. For those interested chiefly in Zoology the comparative aspects of early stages in several selected Mammals, suggesting as they do evolutionary trends, are highly significant.
On the other hand for those mainly intent upon the study of medicine
the emphasis of interest is likely to be different. Such students, and
many of their teachers, though willing to admit that the study of early
comparative mammalian development is of some value, feel that for
practical purposes they must begin to concentrate. Hence they prefer to
consider chiefly the embryology, both early and later, of a single form.
Preferably this would be Man, but since that is usually not practical,
the next best thing is to select for study some readily available Mammal whose history is nearly akin to that of Man. That Mammal is generally the Pig. If space allowed, there is of course no reason why both
these lines could not be followed in considerable detail. Unfortunately,
however, in a book already dealing at some length with the Frog and
Chick, space does not permit an extensive treatment of both topics. Consequently the following compromise way of treating the Mammals becomes necessary.
To begin with, it will be found desirable as in previous cases to go
back of the start of the embryo itself, and consider somewhat the reproductive organs of the adults. This will be especially necessary in the
mammalian females because of the special relation of certain of their
organs to the reproductive process and to the developing young.
We shall then proceed with the comparisons of the early embryos of
selected orders of Mammals with special emphasis upon the develop* INTRODUCTION 487
ment and character of their extra-embryonic membranes and structures.
This special emphasis is pertinent because we shall find that these membranes and organs are fundamentally similar to those already familiar
in the Chick, and found in all Sauropsids, i.e., Birds and Reptiles. They
are of present interest because of the manner in which both their origin
and structure has been modified in the different mammalian groups to
serve essentially their old functions. The modifications have resulted
from the different environment in which the embryo and fetus of the
Mammal occurs, and from the very special relations with the mother
which this environment makes necessary. That there should be similarities in these structures as between the Mammals and the Sauropsids is
of course natural in view of the known derivation of the Mammals from
the Reptiles. The modifications in the mammalian orders selected then
help to suggest the lines along which evolution has perhaps moved
within that class.
Having thus compared the early stages of certain representative mammalian fonns, we shall finally concentrate upon one of them, i.e., the
development of the Pig. The Pig, however, is an Ungulate, and the
Ungulates are one of the groups whose earliest stages and extra-embryonic membranes have been chosen for comparative study. In this latter
study, moreover, the Pig will be especially emphasized as an example
of the group. Hence when we come to the detailed consideration of this
animal it will not be necessary to start quite at the beginning. We shall
simply pick up where the comparative account left olf.
Lastly, another device by which we shall endeavor to save space and
time is the following: In the embryology of the Frog and Chick we
have already twice gone over in some detail the development of all the
main vertebrate systems. In the Chick, moreover, the processes in many
cases are, as has already been suggested, very similar indeed to those
found in the Mammal. Hence in the Pig we shall not repeat again in detail the development of each system. Instead we shall outline such development rather briefly, emphasizing only those points in which the
process or structure in this animal significantly difiers from that in the
Chick. Such treatment will of course be accompanied by as many illustrations as possible. This should be sufiicient, and will be so if the student of the Pig has reasonably well in mind the corresponding situations in the Chick. Anyone who does not have the Chick development
clearly in mind will find it necessary to refresh the memory by reference back to the appropriate account in that form.
488 EARLY MAMMALIAN DEVELOPMENT
THE ‘REPRODUCTIVE ORGANS OF THE ADULT
, THE MALE
The Testes and Their Ducts. —— In the adult male Mammal there
are normally two testes. These organs may be retained permanently
within the body of the animal, as in the case of the Elephant; more commonly, however, they pass out of the body during development, and are
contained either in two sacs, or in two chambers of a single one, the
scrotal sac or scrotum. This is the case in the Pig. In some cases, however, as among Rodents, an intermediate condition occurs in which the
testes descend into the scrotum only during intervals of sexual activity.
Each testis consists of the usual seminiferous tubules, embedded in connective tissue and leading by way of vasa eilerentia to the respective
vas deferens.
Accessory Organs.—— ln the Mammal there are, in addition to the
testes and other parts just noted, certain accessory organs connected with
the more distal parts of the genital tract. These are the prostate glands,
Cowper’s glands, and, in some animals (e.g., in the Pig and in Man).
the seminal vesicles. The function of the glands is to furnish a suitable
medium for the existence of the sperm after it leaves the organs of the
male. The vesicles presumably assist both in the secretion of additional
fluids and in storing the combined" sexual products or semen previous
to its ejaculation. Finally, there is in the male Mammal a penis. This
has a single duct, the urethra, which serves to discharge urine, and also
to introduce the semen into the genital tract of the female.
_ THE FEMALE
The Ovary.——In the female Mammal there is a single pair of
ovaries, and, as in the other forms studied, these organs are contained
within the body cavity and suspended from its wall by a mesovarium.
The ovaries are whitish ovoid objects, varying in size in different animals, but always relatively small. Thus in the Human Being, for example, each ovary is about 3-4 cm. long, and from 2-3 cm. wide, and they
are about the same in the Pig. Fundamentally, their internal structure is
similar to that already described in the Bird.
The Genital Tract.
The 0viducts.—As in the Bird, the ovaries are not directly connected with the Miillerian ducts or oviducts. The latter, sometimes
OOGENESIS" 439
known as the Fallopian tubes, are, however, provided as usual with a
typical fimbriated funnel, or infur.-dibulum, which serves to embrace the
ovary when an ovum is discharged. The walls of the oviducts are made
up as follows: On the outside is the serous membrane, next to that a
layer of more or less mingled longitudinal and circular muscles, then
a sheet of vascular connective tissue covered by ciliated epithelium. the
connective tissue with its epithelium being known as the mucous layer.
From each infundibulum the respective duct proceeds to join the one
from the opposite side. Between the infundibulum and the point of
junction, however, there is usually more or less bending, and in many
cases the duct actually starts anteriorly before curving backward and
medially to unite with its fellow.
The Uterus and Vagina. —— At some point distal to the infundibula ei
ther above or below the region of junction, or in some cases both above
and below, the character of the tract or tracts changes. The muscular
wall becomes thicker as does also the mucous layer which now contains
lymph spaces and many glands. The part or‘ parts of the genital tract
thus characterized are then known as the uterus or uteri, and the thick»
ened mucous layer plus its epithelium are referred to together as the
uterine endometrium. When these changes occur entirely proximal to
the point of union of the tubes so that there are two distinct uteri (Rodents) the condition is-known as uterus duplex. On the other hand when
they occur both above and below the region of union (Carnivores and
Ungulates) the situation is described as uterus bicornis. Finally, when
the uterine character exists only in the fused part of the tract the-condition is called uterus simplex.
_ Beyond the uterus, or uteri, as the case may be, there is a single passage leading to the exterior, known as the vagina. At the external end of
the latter there are certain rudiments homologous with the penis of the
male.
THE DEVELOPMENT OF THE OVUM UP TO 5EGMENTA—
TION, AND THE SEXUAL CYCLE
OOGENESIS ,
The O6gonia.——-The embryonic ovary of the Mammal contains the
usual primordial germ cells which, as in the lower Vertebrates, have
probably migrated thither from the walls of the gut. At first these cells
lie chiefly in the outer epithelium or cortex of the ovary. According to
490 EARLY MAMMALIAN DEVELOPMENT
Fig. 247. —— Section through part of the ovary of a Dog. From Kellicott (Chordate
Development). After Waldeyer.
a. “ Germinal epithelium.” b. Ovigerous cords. c. Small ovarian follicles. :1. Older
ovarian follicle. e. Ovum surrounded and attached to wall of follicle by cells of
discus proligerus (cumulus oiiphorus), including those of the future corona radiata.
f. Second ovum in follicle with e. (Only rarely are two ova thus found in a single
follicle.) g. Outer layer of follicular capsule. h. Inner layer of follicular capsule. i.
Membrana granulosa. k. Collapsed, degenerating follicle. L Blood-vessels. In. Sections through tubes of the parovarium. y. Involuted portion of superficial epithelium. z. Transition to peritoneal epithelium.
most accounts this cortical epithelium thickens and then produces out-l
growths which push into the deeper mesenchyme. These outgrowths are
the ovigerous cords similar to those described in the Chick, hut in this
instance often called the cords of Pfliiger.1 As in the Bird, they contain
both the female germ cells, or oiigonia, and numerous epithelial cells as
1 Also according to some recent studies by Gruenwald ('42) the development
of the cords is somewhat more involved than this, and varies to some extent in different Mammals. The end result, however, is essentially as indicated.
OOGENESIS 491
well. In the Mammal, however, the two types of cells are not easily distinguishable from one another, and it is quite possible that some germ
cells may arise in situ. from indiilerent cells of Pfliiger. During this period multiplication of all the cells goes on rapidly.
At some time before the birth of the animal in which the ovary is
contained the multiplication of the oiigonia is said to cease. As has been
previously noted, however, this assertion is now seriously questioned,
some workers (E. Allen, ’23, G. I. Hargitt, ’30, and others) maintaining
that in certain cases at least the ova derived from the primordial germ
cells all, or nearly all, disappear. These are then said to be replaced by
new oiigonia arising from the peritoneal (germinal?) epithelium at intervals during the sexual life of the individual. In any event the cells are
eventually arranged in nests or groups, each of which contains a single
oogonium, the remaining epithelial cells in the group being destined to
form the fo1licle.'The young ovum now enters upon the growth period
as an oiicyte.
The Oocyte and the Graafian Follicle. —At about this time, the,
epithelial cells referred to begin to become arranged about the young
ovum to form the highly characteristic mammalian or Graafian follicle.
At first they constitute a thin flat layer only one cell thick, but soon
multiply so as to form a mass of cells about the growing oiicyte. In one
side of this mass there then appears a space, the follicular cavity, which
gradually enlarges and extends around the sides of the oiicyte. These
extensions, however, never quite meet. Thus the oiicyte, still closely surrounded by several layers of cells, is suspended within the follicular
cavity, which becomes filled by a fluid, the liquor folliculi. Meantime,
the outside of the entire follicle has become covered by a capsule (follicular capsule or theca) , formed externally of connective tissue (theca
externa) and internally of cells, blood vessels, and nerves (theca
interna).
The various layers and parts of the entire Graafian follicle may now
be named, as follows: Beginning on the outside there is the follicular
capsule (theca) with its inner and outer layer. Just within this, and
bounding the follicular cavity, there are a few layers of the follicular
cells forming the basement membrane, or membrana granulosa. Upon
the side of the ovum where the cavity has not extended, a neck of cells
reaches from this membrane to those cells which immediately surround
the oiicyte. Thus the latter is attached to the inner wall of the follicle
by this neck, which, together with the more peripheral of the cells immediately surrounding the ovum, is termed the discus proligerus or
492 EARLY MAMMALIAN DEVELOPMENT
cumulus oophorus. Those of the immediately surrounding cells which
have remained closest about the egg are now gradually elongated at
right angles to the surface of the latter. Many of these cells remain attached to this surface for a time following ovulation when they become
known as the corona radiata (Figs. 24-7, 248) . This brings us to the actual egg and its membrane.
Fig. 248.—F'ully grown Human oiicyte just removed from the
ovary. Outside the oiicyte are the clear zona pellucida and the follicular epithelium (_ corona radiate) . The perivitelline space in this
instance is not apparent. The central part of the oiicyte contains
deutoplasmic bodies and the excentric nucleus (germinal vesicle).
Superficially there is a well-marked exoplasm, or cortical layer.
From Waldeyer (Hertwig‘s Handbuch, etc.).
THE MATURE OVUM AND OVULATIONI
The Mature Ovurn. — The mature ovum in all placental Mam~
mals 2 is relatively minute, though naturally varying in size in different
animals. Thus that of the Mouse measures about .075 mm. in diameter,
'-’ It will suflice to state at this point that the term placental Mammal includes
the vast majority of the group. Its exact significance will be fully described in the
section on the yolk-sac, allantois and placenta (see below).
THE FEMALE SEXUAL CYCLE 493 A
that of the Dog about 0.14 mm., that of Man 0.135 mm., and that of the
Whale 0.14 mm. (Hartman, ’29, ’30) . The reason for this minute size is
the fact that mammalian eggs are virtually without yolk (alecithal).
They consist of a central region of opaque endoplasm surrounded by a
thin layer of exoplasm, and within the former is a relatively large nudens (germinal vesicle), somewhat excentrically placed.
The ovum apparently does not possess any true vitelline membrane.
It is surrounded, however, by a thick transparent substance which is
presumably chorionic, i.e., is secreted by the cells of the follicle. This
layer, though clear, frequently appears to be perforated by minute canals through which processes of the follicular cells reach the egg to
nourish it. It is, therefore, known either as the zona pellucida or the
zona radiata. There is usually a slight space between this zone and the
iprotoplasm of the egg, and though there may be no vitelline membrane
this space is known as the perivitelline space (Fig. 24-8) .
Ovuiation.——-As a Graafian follicle and its ovum matures, it is
gradually brought to the surface of the ovary. At the same time one side
of the follicle becomes thin in connection with the formation of a cicatrix, as in the Chick. As complete maturity is reached, the discus proligerus is broken and the ovum floats freely in the liquor folliculi. In
most animals rupture of the follicle then occurs spontaneously, and its
contents is received by the infundibulum of the oviduct. In a few forms,
e.g., the Rabbit and Cat, the breaking of the ripe follicle does not usually occur spontaneously, but only following copulation with the male
(coitus). The liberation of an ovum may or may not take place in both
ovaries at once, and there may or may not be more than one follicle
ready for discharge in the same ovary at approximately the same time.
These variations, moreover, may occur normally in the same species of
animal. In Mammals which ordinarily produce a litter of young, however, the discharge of several ova at once is of course the usual thing.
THE SEXUAL CYCLE IN THE FEMALE
lt is well known that like many other animals, Mammals are capable
of breeding only during certain periods or seasons. Among this group,
moreover, these periods are far more marked in the female than in the
male. In the former sex they are also very definitely related to the process of ovulation so that it seems desirable to discuss the subject at this
point. In all placental Mammals which have been carefully studied, it is
known that during sexual life the walls of the uterus suffer a series of
periodic changes, interrupted only by pregnancy. The placentals, more494 EARLY MAMMALIAN DEVELOPMENT
over, may be divided into two main groups with respect to these uterine
changes, i.e., the Primates and the non-Primates.
The N on—Primate Cycle. —— Among this group the stages involved
are fundamentally similar, and these stages are well represented in the
Pig‘, whose embryology will later be considered. We shall begin therefore by a description of the sexual cycle in the female of this animal. In
the sow each sexual or oestrus cycle, as it is called, occupies twentyone days and in the absence of pregnancy, the cycles are continuous
throughout the year. As regards the behavior of the animal, the activity
of the ovary, and the condition of the uterine endometrium, the periods
or phases of a cycle are characterized as-follows:
I. The Dioestrum.-—-During this period lasting about two and one
half weeks the sow occupies herself with eating and sleeping, and shows
no interest in the opposite sex. A study of her ovaries, however, shows
that within this interval an important event takes place. The empty follicles which remain from the immediately preceding ovulation become
filled with a specialized type of fatty cell. In some cases (Man) these
cells are yellow in color, which has caused each body so formed to be
known as a corpus luteum. In the Pig, however, these bodies are pinkish. They quickly develop to a maximum extent, and persist in this condition for about the first thirteen to fourteen days of the period, at
which time they begin to regress. Correlated with the time of development and persistence of the corpora lutea in the ovary, the uterine mucosa, which was already quite thick at the beginning of this period,
becomes even more hypertrophied, especially the glands. This is a con-_
dition known as pseudopregnancy, because, as we shall see, the state of
the mucosa at this time resembles to a considerable degree its character
during true pregnancy, and due to the stimulus of the same hormone,
progesterone (see below). Finally as the corpora lutea regress the uterine mucosa likewise regresses, and within two or three days has become
relatively thin (F 249, A ). Thus during the last day or so of the dioestrum there is virtually nothing going on in the uterus so that this
brief interval may be thought of as a time of more or less complete
“ rest ” for that organ.
II. The Pro-oestrum.———Following the dioestrum there is a short interval of a day or,so generally known as the pro-oestrum, within which
the behavior of the animal remains about as before. Studies of her ovaries, however, reveal that undeveloped Graafian follicles are starting a
rapid growth, while the uterine mucosa also has again begun to hypertrophy (Fig. 249, A)
THE FEMALE SEXUAL CYCLE 495
III. The Oestrus. —— This period, lasting approximately three days, is
known as the time of “ heat,” and during it the sow becomes extremely
restless and will accept mating at any time. Examination of the ovaries
shows that the Graafian follicles come to maturity at about the middle
 
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Fig. 249.—Diagrams comparing the events of the oestrus cycles of the Pig and
.Dog with those of the ovulatory and non-ovulatory menstrual cycle in Man. The
line vertical rulings in the cycle of the Dog and in those of Man indicate the time
of occurrence and the approximate relative degree of bleeding in each case. There
is no normal obvious bleeding in the Pig. The rise and fall of the curved lines
indicates the relative degree of hypertrophy or degeneration of the tissues or
bodies designated.
of this period, and at that point ovulation occurs. The corpora lutea, already referred to, immediately start development which, in the absence
of pregnancy, continues into the succeeding dioestruxp as already described. The hypertrophy of the mucosa, well under way at the end of
the pro-oestrum, also continues on through oestrus and into the succeeding dioestrum, during most of which periods it remains at a high level
as indicated (Fig. 249, A) .
496 EARLY MAMMALIAN DEVELOPMENT
Variations in the Non-Primate _Cyc1e.—-The non-Primate cycle
as thus described for the Pig may be considered typical for the nonPrimate group of animals so far as its fundamental aspects are concerned. As already suggested, however, there are numerous variations in
detail, some of the more striking of which will now be noted. Probably
the most outstanding is that which occurs in animals like the Dog and
Cat. In these animals there are only two or three oestrus periods a year,
with a long inactive interval, known as an anoestrum between each period of “ heat.” In such cases the corpora lutea, and the uterine hypertrophy in the absence of pregnancy, only persist for a relatively short
time, the uterine mucosa being comparatively thin during most of the
long anoestrum. Breeding of course can only occur during the oestrus
periods which are hence referred to as the breeding seasons. The Dog
and Cow are further peculiar in that at the end of the pro-oestrum the
blood vessels of the hypertrophied mucosa are so gorged that some superficial bleeding occurs. This quirk‘ led to much discussion and misapprehension of the relations between the non-Primate and Primate cycles as we shall presently see. Another peculiarity of a few animals such
as the Cat and also the Rabbit, as already noted, is the fact that ovulation in these forms is not spontaneous during oestrus, even though the
mature ova are present. It only occurs at this time if copulation, or some
form of stimulation which simulates copulation, takes place. Otherwise
the ripe follicles simply degenerate, no corpora lutea are formed, and
hence no pseudopregnancy occurs (see below).
Not only do animals vary as between those with a succession of relatively short dioestrus cycles like the Pig, and those with long anoestrus
intervals like the Dog (Fig. 249, B), but in the latter type some forms
have several short dioestrus cycles between each anoestrum. That is
they have a breeding season perhaps once a year like some sheep, and
during that season they come into “ heat ” several times. Animals with
only one oestrus period at a breeding season are said to be monoestrus,
while those with several at each season, or with continuous short cycles,
are polyoeszrus. Lastly the length of the dioestrus cycles varies greatly
among different anmials. Thus, while it is twenty-one days in the Pig, it
is only five days in the Rat and Mouse, and fifteen in the Guinea-Pig. It
should be emphasized also that these are average times. There is commonly some variation in cycle length even in the same individual, depending upon temperature, food and other unknown conditions.
The Primate Cyc1e.——-In discussing this group it should at once
he pointed out that the peculiarities about to be described do not actuTHE FEMALE SEXUAL CYCLE ‘ 497
any apply to all Primates, e.g., to Lemurs and to the New World Monkeys. They do, however, apply to the Anthropoid Apes, the Old World
Monkeys and to Man. ‘The most complete studies have been made on
Man and Rhesus, an Old World Monkey, and we shall therefore consider the situation particularly as it applies to these forms, and first especially as it applies to Man.
The Menstrual Cycle. — The peculiar characteristic of the sex cycle
as it occurs in the Human female is the inclusion within it of the phenomenon of menstruation, from which the whole cycle takes its name.
The nature of this phenomenon, and its relation to the parts of the nonPrimate cycle, in so far as it can at present be related to them, is as
follows:
Keeping the Pig in mind as presenting a typical example of the situation in the non-Primates, we find that the first but least important
difierence between that animal and Man is in the length of the entire
cycle. Thus in the Pig, as just noted, it is about twenty-one days, while
in both Women and the Rhesus monkey it is normally twenty-eight days,
with numerous more or less minor variations. Proceeding next to a com
parison of the periods within the cycle, and starting with the one in Man
presumably homologous with the dioestrum in the lower animals, we
find conditions at that stage in the Human subject about the same as in
the sow. That is to say there is no sexual urge at this time, the ovary
contains a corpus luteum, and at the beginning the uterine mucosa is
hypertrophied. This phase, comparable with the first and major (pseudopregnant) part of the dioestrum, lasts for about two weeks. At the end
of this time, as in the lower forms, the corpus luteum disappears, and
accompanying this the uterine epithelium regresses. In this instance,
however, this regression instead of being relatively quiet and uneventful,
is a rather violent affair involving a serious breakdown of the endometrium, both mucosa and epithelium. This is accompanied by a sloughing of? of cells and considerable bleeding, and it is this process which
comprises menstruation. Following this as in the Pig, comes a “ rest ”
interval, in this instance, however, lasting four to five days and involving repair of the preceding damage, though the mucosa remains relatively thin. Menstruation plus this interval would therefore correspond
to the end of the dioestrum in the Pig, except that in that animal the
process of regression is much less violent. Hence the menstrual features
are lacking, and no “ repair ” is required during the “ rest ” interval.
The next period should be that of the pro-oestrum, and apparently
‘something essentially similar to this in the lower animals exists in
P 498 EARLY MAMMALIAN DEVELOPMENT
Man. As in the former case it apparently involves no accentuation of
sex interest, the ovary contains a maturing Graafian follicle, and the
uterine mucosa begins again to hyper-trophy. This lasts five to six days.
Following the “pro-oestrum” the next period should be that of
oestrus, but this is another respect in which the Primate cycle difiers
from that of the non-Primates. There is no oestrus. This means that
there is no time in the cycle of greatly heightened sexual activity. Ovulation, which "should occur sometime during oestrus, occurs at the end of
what we are calling the “ pro-oestrum,” though the use of this and other
"terms relating to the oestrus cycle is obviously questionable in a cycle
in which there is no oestrus. This is why the Primate cycle is commonly
referred to as the menstrual cycle in correlation with its most outstanding characteristic. Following ovulation a corpus luteum of course exists, and in the absence of pregnancy a new “ dioestrum ” begins, culminating in another menstruation and “ rest” interval (Fig. 249, C).
From this account it will be evident that ovulation occurs about midway between menstruations, i.e., from the twelfth to the sixteenth day
following the beginning of the last menstrual period (Corner, "43)
From this it is clear that menstrual bleeding has nothing whatever to
do, either in relative time of occurrence, or in character, with the minor
‘bleeding of the pro-oestrum in an animal like the Dog, a phenomenon
with which it was once confused. In this connection it should he noted
that a slight pro-oestral bleeding also. occurs in the Rhesus Monkey and
occasionally in Women, in which cases -it is known as intermenstrual
bleeding or Hartman’s sign, i.e., a sign of imminent ovulation.
To summarize a comparison of the two cycles, then, we may say this:
In both there is what amounts to a “ dioestrum” during which sexual
activity is not evident. The ovary contains a corpus luteum during the
first part of this period, and during this part the uterine mucosa is hypertrophied. Near the end in both cases the mucosa regresses, but in
the Primate cycle the regression is much more thoroughgoing, and is
termed menstruation,_ Finally a short quiescent interval ensues which in
E the Primates is occupied with uterinerepair. In both cycles a “proM 5 oestrum ” follows the “ dioestrum ” involving no change in sex activity,
but the growth of a new Graafian follicle and renewed uterine hyperI trophy. In the norn-Primate cycle this is followed by oestrus or “ heat ”
1 in the midst of which ovulation occurs. In the Primate cycle ovulation
occurs at the end of what we have called, for the sake of comparison,
the “ pro-oestrum,” and there is no oestrus. Instead the “ dioestrum”
immediately follows, and the cycle is complete.
i
I
THE FEMALE SEXUAL CYCLE 499
Having thus described the oestrus and the menstrual cycles there remain the problems of their causes and functions. Much work has been
done in this connection over a long period, but it is only within recent
years that the pieces of the puzzle have begun to fall into some semblance of order. As will presently appear, however, there are even yet
some pieces which are missing.
Causes qf the Oestrus and Menstrual Cyc1es.——It is already
evident that certain events in both the oestrus and menstrual cycles are
closely correlated. Thus we have seen that when a follicle is developing
in the ovary the uterine mucosa in either cycle is undergoing its prooestral hypertrophy. As the corpora lutea form it undergoes still further
hypertrophy, and when these latter bodies start to disappear this mucosa
regresses, either with or without extensive breakdown. Why is this? The
answer is found in the fact that the developing follicle produces a hormone called oestrone (theelin) which causes the initial pro~oestral hypertrophy. It also of course causes the behavioral phenomenon of
“ heat” in most “ lower ” animals?‘ As the corpora lutea form following ovulation they also produce orie or more hormones, including some
oestrone. The most prominent of these, however, is called progesterone,
and this causes the still further uterine hypertrophy of the first part of
the dioestrum. Both these hormones are sterols, have been obtained in
pure crystalline form, and their action repeatedly demonstrated experimentally. The withdrawal of the progesterone as the corpora lutea begin
to disappear would then explain both the dioestral regression and the
menstrual breakdown of the mucosa previously built up. The follicular
and luteal hormones produced in the proper order and then withdrawn
would therefore seem to account satisfactorily and completely for both
types of cycle. This would be true were it not for one curious fact. It
was discovered (Corner, ’23) that Rhesus monkeys, and probably more
rarely Women, experience menstruation without ovulation, and hence in
the absence of corpora lutea. The monkeys, it should be noted, have a
breeding season (the winter months), and it is at the beginning and end
of this season that these so-called anovulatory cycles occur. Women of
course have no such season, and in them cycles of this character have
been thought to occur most commonly in girls beginning to menstruate.
It is now known, however, that such anovulatory cycles, Otllu ». xse apparently normal, occur in a certain percentage of women during their
active sexual life. Indeed it has been proven that such women may only
3 Just what parts of the follicle are responsible for this hormone is not alto»
gather certain, but probably either the theca interns. or the granulosa or both.
500 EARLY MAMMALIAN DEVELOPMENT
actually ovulate two or three times a year in spite of seemingly normal
menstrual periods, causing serious interference with lertility. In any
event such cycles obviously upset theforegoing neat explanation of the
entire phenomenon. Much work has been done in an effort to solve this
problem, but no completely satisfactory answer has yet been arrived at.
It is known for instance that in castrate animals an apparently normal
cycle can be produced by the injection and sudden withdrawal, after a '
suitable interval, of oestrone alone. Yet in non-castrate animals extra
doses of oestrone will not prevent the uterine breakdown. A little progestrone, however, will do so. Hence the latter substance seems clearly
to have some important part in the cycles of normal ovulating animals,
probably in the manner already described.
With these facts in mind two possible explanations of the anovulatory
cycle may be briefly noted. One, considered by many the most probable,
is that a certain amount of oestrone is necessary, first to build up, and
then to maintain, the uterine endometrium in a state of preovulatory hypertrophy. This hypertrophy is of course not quite like that produced
by progesterone, but is nevertheless considerable. The necessary oestrone for this is furnished by the partially developed follicle, which instead of going on to ovulate, persists for a time, periodically regresses,
and is replaced by another. The regression of course produces a temporary lack of oestrone, and an anovulatory endometrial breakdown very
similar to menstruation occurs (Fig. 249, D). The second possibility,
suggested by Hisaw, is that the partially developed Graafian follicle produces not only oestrone, but a little progesterone as well. Then if, in the
anovulatory cycle, the production of the progesterone for some reason,
such as the-regression of the follicle, declines, this may be enough to produce menstruation even in the absence of ovulation and the ensuing
corpus luteum. There is a little suggestive evidence for this, but it is
diflicult to prove. So much for this part of the oestral cycle and menstrual mechanism.‘ '
4 It may be added that these hormones also have several other significant’ effects
not directly pertinent to the present discussion. Thus oestrone not only starts the
hypertrophy of the mucosa in each cycle, but is necessary to bring the infantile
uterus to a stage of development where progesterone can act on it. Also it controls
the growth of the muscles of the pregnant uterus, first stimulating, and then checking, and causes corfiification of the vagina of the Guinea Pig, thus revealing its
presence in this animal. Lastly it stimulates development of the breasts to a condition where they can be acted on by the pituitary hormone, prolactin, but at the
' same time prevents milk flow until birth. Progesterone in addition to its elfect on
the uterine mucosa has a decidedly quieting action on the normal rhythmic contractions of the uterine muscles, and is said by some to cause relaxation of the pelvic,
THE FEMALE SEXUAL CYCLE‘ 501
There still remains the question as to what sets ofl" these cycles, i.e.,
what starts the follicles to developing, and what stops them. The answer
to this appears to be found in that gland-of-all-work, the pituitary. The
anterior lobe of this gland is known to produce, among other things, a
follicle stimulating hormone (F.S.H.) which causes Craafian follicles
to begin their growth. What then seems to happen is that when the
growing follicle achieves a certain output of oestrone this acts in turn
to suppress secretion by the pituitary. (There is some experimental evidence for this.) The follicle then ovulates, and its extensive oestrone
production ceases, thus allowing the pituitary secretion to rise again,
and so the cycle repeats itself.
Here again, however, a problem arises which has not been entirely
satisfactorily answered. The scheme just presented works well enough
for animals like the Pig or Man with continuous cycles, but what of
those with an anoestrum? What causes the cycles to stop? We do not
know. It has been suggested that during the anoestrum in such animals
as the Dog or Cat the secretion of the pituitary and the ovarian follicle,
exactly balance each other so that nothing happens. Perhaps so, but
< there is no proof of it. Also if this is true, what produces an unbalance.
and starts off a new cycle?
Functions of the Female Cycle. —- Thus far the oestrus and menstrual cycles have been considered without reference to the possible occurrence of pregnancy. As might be suspected, however, each cycle is in
fact an invitation to, and a preparation for, this important event. In
cases where oestrus occurs the behavior of the female is such as to permit and encourage mating at this time, and it is of course at just this
point also that a ripe egg is released into the oviduct ready to be fertilized. In the menstrual cycle the same thing is true, except that here
I there appears to be no special sexual urge at the time of ovulation. Fol’ lowing this event in either case the egg is subject to fertilization in the
upper end of the oviduct. If this occurs the egg becomes what amounts
to a blastula in a manner to be described below, and after 3-4 days finds
its way into the uterus. Here meanwhile the climax in the hypertrophy
of the uterine mucosa is coming about. It now appears that this hypertrophy is just what is needed to insure the firm attachment of the developing egg to the uterine wall by a process known as implantation. This
i
A
1
»
ligaments of the Guinea Pig. Hisaw, however, has claimed a separate luteal hor) mane, relaxm, to be responsible for this. In some cases progesterone also acts as an.
acciassory in aiding the oestrogens to prepare the breasts for final stimulation by
pro actin.
-n.7<ma.t:..a
502 EARLY MAMMALIAN DEVELOPMENT
process varies considerably in different animals, and will be discussed
at some length later on. The point to be noted at the moment is that apparently the hypertrophy of the mucosa is a necessary preparation for it.
As has been noted, if fertilization and implantation fail to occur, the
hypertrophy regresses and a new cycle is initiated, with, as M.- 3. Gilbert
so cleverly suggests in her book, Biography of the Unborn, “ hope for
better luck next time.” On the other hand, if implantation does occur,
the hypertrophy persists and in fact increases. Because of the similarity
of this hypertrophy to that of the dioestrum, the latter, as previously
noted, is frequently termed pseudopregnancy. This persistence of the
hypertrophy when it is needed, and its disappearance when it is not
needed leads to some further questions to which we have at present only
partial answers. Some of these questions and the tentative answers are
as follows: .
What for instance makes the hypertrophy of the mucosa persist in
pregnancy and not at other times? In this connection it is of interest to
find that in many animals the corpora lutea also persist throughout
pregnancy instead of disappearing as in the non pregnant cycle. Is
there a causal connection here? It would appear that in those cases
where both corpora lutea and mucosal hypertrophy persist together
there is. Thus in the Rat and the Cow removal of the corpora lutea of
pregnancy causes regression of the mucosa and abortion, though in
other cases, like that of Man, this is not true. The answer as to what
makes the hypertrophied uterine mucosa continue in the former animals
then seems to be fairly clear. It will be recalled that one of the chief
hormones of the corpus luteum is progesterone. This hormone, however,
was so named because of the very fact that it maintains an ‘hypertrophied
condition of the mucosa not only during most of the dioestrum, but especially during pregnancy. Thus the corpora lutea apparently rather obviously persist during pregnancy in these cases in order to secrete the
progesterone which maintains this condition. There is also, as noted, evidence that the corpora lutea produce some oestrone, or something
closely akin to it. This and the progesterone appear to assist in causing
the hypertrophy of the muscles of the uterus as well as that of the mu~
cosa during pregnancy.
The next question is, how do the corpora lutea know, so to speak,
when to persist and when not to? The answer to this appears to be that
the organ which attaches the embryo to the uterine wall, termed the
placenta, itself secretes several hormones, one of which is luteinizing, i.e.,
helps to keep the corpus luteum developed. There is also a pituitary horTHE FEMALE SEXUAL CYCLE 503
nlone which has a luteinizing efi'ect, but this is apparently not the one
chiefly involved during pregnancy. _As just suggested the placenta pro
" duces other hormones, i.e., oestrogens (oestrone like hormones), and also
quite definitely progesterone. This source of these substances, it is now
generally agreed, soon becomes the main one in cases like Man where
the corpus luteum functions for only about the first four months of pregnancy, being operatively removable after the first few weeks without
harm.
Also, in Man at least, certain other gonad stimulating hormones, similar in action to the F.S.H. of the pituitary, are produced by the placenta.
They are called Prolan A and B, and are used in the Aschheim-Zondek or
Friedman tests for pregnancy. Thus so much of these hormones is produced under this condition, even within the first month, that they are
excreted in the urine. Advantage is taken of this fact to make a test for
their presence, and hence for pregnancy, by injecting a specified amount
of the suspected urine into a female rabbit (Friedman test). If the hormones are present they will cause the animal to ovulate within ten
hours.5 The particular tissue of the placenta from which these various
sterol substances appear to be derived in Man and Monkeys is a special
material called trophoblast to be described below (Wislocki and Bennett, ’43; Baker, Hook and Severinghaus, ’4-4) .
Finally, in this connection, what if any function has menstruation as
such? It would indeed be comforting to be able to assign it one, but to
date no adequate explanation for this excessive breakdown of the uterine
endometrium exists. It seems to be merely an overenthusiastic expression in some Primates of the regression following luteal hypertrophy
and withdrawal which occurs in a more restrained manner in other more
humble Mammals.
Parturition. —This is a process which might naturally be considered at the conclusion of development rather than here. However, possible dependence upon the hormonal substances which we have been discussing makes this an appropriate point to mention the factors which
may be involved. As a matter of fact there is not a great deal to say, because comparatively little is really known as to just what factors are
actually concerned in this phenomenon. It may be that among others a
- reduction of progesterone, which quiets uterine contraction, and an in
crease in oestrogens, which are known to stimulate it: play a part. This,
5 Another peculiar effect of these hormones is to cause the release ‘at sperm
from the testes of the Frog when so-called pregnancy urine is injected into a lymph
sac of one of these animals. This fact furnishes another pregnancy test which
promises to be of value ( Miller and Wiltberger, ’48).
504 EARLY MAMMALIAN DEVELOPMENT
however, is only a guess, and according to Corner many other elements
such as the balance of still other hormones, the rate of blood flow
through the placenta, the state of nutrition in the fetus, and probably
various other conditions are concerned. Indeed some have claimed that
the mere size and weight of its tenant finally irritates the uterus into
initiating the contractions of labor. Some evidence for this latter notion
is perhaps furnished by certain cases in the Cat studied by Markee and
Hinsey (’35) . In an abnormal situation in this animal one horn of the
uterus contained embryos differing considerably in age from those in
the other, a condition known as superfetation. In this case the born with
the older fetuses delivered itself thirteen days ahead of the other, the
normal full term in this animal being from sixty-three to sixty-five days.
This would thus seem to indicate that the conditions responsible for delivery are not entirely hormonal, and hence general, but are at least
partly quite local. These investigators also showed that thickness of
endometrium and muscle depends on the number and weight of fetuses
present in the horn in question. This again emphasizes the effect of local
factors on conditions which may "affect delivery. In concluding this
topic it is pertinent to note the normal term of gestation in the animal
we are about to consider in some detail, i.e., the Pig. As usual this period varies slightly with breed and other factors, the range being from
112-115 days, or just under four months (Asdell, ’46) .
THE SEXUAL CYCLE IN THE MALE
As regards the male among Mammals, it is found that here also there
is a tendency toward cycles of sexual activity. This phenomenon, however, is not so common as among the females, or among the males of
lower forms. In thosespecies of Mammals in which the male does experience special periods of heightened sexual desire, however, these normally coincide with the breeding season of the female, and are known
as the rutting periods. At such times the males may develop very special
secondary sexual characters, such as the antlers of the buck deer, as
well as great irritability and desire for combat with other males. On the
other hand, the males of many Mammals have no such special periods
of sex activity. Instead, they are apparently able to breed at any time,
even though the females of their kind will only receive them at certain
seasons.
With this understanding concerning the nature of the sexual cycle and
its relation to ovulation and sexual activity, we are now prepared to
return to the history of the ovum.’
MATURATION AND F ERTILIZATION 505
MATURATION AND FERTILIZATION
Although in Mammals the first maturation division often occurs before ovulation and fertilization, the second, with apparently only a few
exceptions (e.g., the Mole, Rabbit, and probably Man) occurs after
Fig. 250.——Reconstruction of four sections through the fertilized
ovum of the Cat. From Longley (combined from two figures). No
zona pellucida is visible in these sections. The corona radiata is
disintegrating.
s. Remains of second polar spindle. I. First polar body. II. Second polar hody. o”. Sperm nucleus. 9 . Egg nucleus.
ward. Hence it has seemed best to mention both divisions in connection
with the latter phenomenon.
The First Maturation Division.—At some time during the
growth of the oocyte, the preliminary stages of maturation are completed without any peculiarity of note. The first polar, spindle is then
formed, and usually a short time before ovulation the first polar body
is given off. In the latter connection the only feature to be noted as pe
‘culiar to Mammalsis the fact that this polar body is normally relatively
large, i_.e., often as much as one fourth the diameter of the ovum itself,
506 EARLY MAMMALIAN DEVELOPMENT
and in abnormal cases sometimes equal to .the latter. The fate of these
exceptionally large bodies is not known. After the extrusion of the first
polar body, the spindle for the second is formed and moves into position for division. The completion of the process may then take place in
the ovary (e.g., in the Mole and Rabbit) or it may be inhibited while
ovulation and fertilization occur.
Fertilization. —— Sperm introduced into the vagina of the Mammal
rapidly make their way into the uterus and up the oviducts. A few hours
0
Fig. 251. — Cleavage of the ovum of the Rabbit. From Kellioott (Chordate Development). After Assheton. A. Two-cell stage, 24- hours after coitus, showing the two
polar bodies separated. B. Four-cell stage, 25% hours after coitus. C. Eight-cell
stage.
a. Albumenous layer derived from the wall of the oviduct. z. Zona radiata.
or even less suiiices for them to reach the upper ends of these ducts
where the actual process of fertilization usually takes place.
Considerable work has been done on the rate and method of progress
of the sperm up the oviducts of different animals. Thus Parker (’31)
showed that in the Rabbit the sperm are transported up, both by contractions of the tube and by cilia, despite the fact that the latter beat in
an abovaxian direction. By contractions the tube is divided into small
compartments, and as soon as sperm get into the first of these they are
spread throughout it by ciliary currents which move down the walls and
up the middle of the compartment. Then the location of the contractions
shifts, and new compartments are formed. Sperm do of course swim, but
as just suggested, this auto-motility is not the only, or even the main
factor, involved in getting them to the upper end of the oviduct. In the
Sheep, Schott (’4l) found the sperm to reach the upper ends of the ducts
in about twenty minutes, and to travel at the rate of 4- cm. (40 mm.) per
minute. He does not, however, state that they swim at that rate. Phillips
and Andrews (’37) claim an average swimming speed in vitro of only
4.83 mm. per minute over a distance equal to the length of the ewe’:
MATURATION AND FERTILI7ATION 507
genital tract, though they do much better at first. In the ewe, however,
they travel, according to these authors, by swimming or otherwise, at a
rate of at least 12.4 mm. per minute. In the Rat, Blandau and Money
(’44.} say that in twenty-six out of thirty cases sperm reached the infundibulum in forty-five minutes. They do not say just how, but Rossman (’37) suggests a peristaltic activity of the uterus as responsible for
mnvement through that region. In this connection Asdell (’46) also
notes that contractions of the uterus probably aid in the transport of
the sperm, but gives the “ average” time required to reach the infundibulum “in all animals studied ” as about four hours. This, it will be
noted, is considerably longer than any of the times indicated above, and
he does not say what animals were involved. This author further states
that none of the first few sperm to reach an egg fertilize it, but they do
secrete an enzyme, hyaluronidase, which disperses the cells of the corona radiata, thus making the egg accessible to one of the sperm which
follow. He states that about one million sperm at an insemination are
necessary to insure fertilization by the one sperm required per egg
This is obviously only a rough estimate, since the kinds of animals, and
the numbers of eggs are not given.
Most recently some interesting data have been acquired concerning
these matters in relation to Man. These data were presented at the Washington meeting of the American Society of Zoologists (’-48) by Dr.
E. J. Farris under the title, “ Motile Spermatozoa as an Index of Fertility in Man,” and the results are ‘quoted with the author’s permission. According to this investigator Human sperm swim in vitro at the rate of
3 mm. per minute, a rate not so different for one of those claimed for
the Sheep. This author admits, however, that other factors, such as those
indicated above, are also active in the movement of the sperm in the fe«’
male genital tract, and claims that actually they reach the ovum at the
upper end in about an hour. This is much better than the “ average time
in all animals studied ” given by Asdell. Farris also notes that at least
130 million motile sperm per c.c. of semen, and preferably more, are
necessary to insure fertilization. ‘
Aside from such studies there are others indicating the time which
sperm retain their fertilizing capacity. In the Rat, Soderwall and Blandau (’41) say it is at the most fourteen hours, and that it falls off considerably after ten hours. In the Guinea Pig, on the other hand, Soderwall and Young (’4«O) place the maximum time at twenty-two hours,
while in Man, Farris places it at twelve hours, even though the sperm
may remain motile much longer than this. An extreme survivaltime is
508 EARLY MAMMALIAN DEVELOPMENT
found in the Bat where insemination occurs in the fall, and the sperm
apparently survive and retain fertilizing capacity in the hibernating females all winter (Wimsatt, ’44) . '
The functional survival of the egg previous to fertilization has also
been studied, ‘though not so extensively as in the case of the sperm. It is
said, however, to be able to retain its fertilizability for ten hours in the
future inner cell mass
   
 
«. _‘ ' A
future trophoblast
Inner cell mass
Fig. 252.—Semi-diagrammatic sections through stages of
early cleavage, blastula tblastocystt and early gastrula of the
Pig. After Heuser and Streeter. A. Early cleavage. B, C and D
formation of biastocyst with inner cell mass. E. S:art of epihlast and hypohlast differentiation (gastrnlation), probably by
delamination. or possibly some infiltration. of cells from the
inner cell mass. Trophoblast, often first called subzonal layer.
Rat (Blandau and Jordan, ’41), and for twenty hours in the Guinea Pig
(Blandau and Young, ’39) .
From these data it will be evident that even though ovulation may
not occur so that an egg is present at the moment sperm reach the upper end of the oviduct there is still good opportunity for fertilization to
occur there over a reasonable period. When a viable sperm does reach
an egg it malies its way through any remaining cells of the corona radiata and through the zona pellucida which still cover it. Usually only
one actually enters the egg, presumably due to mechanisms similar to
those previously described. In many cases, only the head and middle
piece of the sperm enter, but in others (Mouse), the entire spermatozoon is taken in; when this does occur, however, the tail soon degenerates. The head of the sperm next forms the sperm nucleus (male pronucleus) in the usual manner.
SEGMENTATION 509
The Second Maturation Division. — If this has not already been
completed its completion occurs following the entrance of the sperm
and while the nucleus of the latter is forming; it results in a second
polar body, usually smaller than the first. This division is soon followed
by the union of the sperm and egg nuclei, and the process of fertilization
is complete (Fig. 250).
SEGMENTATION, GASTRULATION, AMNION FORMATION, AND THE PRIMITIVE STREAK
SEGMENTATI ON
The Type of Cleavage. — Segmentation in the placental Mammals
is total, as might be expected from the virtual absence of yolk. The arrangement and behavior of the cells, however, is quite different from
that observed in the first yolkless form which was studied, i.e., Amphioxus. The reason for this is apparently due to the fact that the egg of a
Mammal is almost certainly only secondarily without yolk. The evidence
for this assumption will become more and more obvious in the course of
this chapter, but a couple of the more striking proofs may be indicated
here. Thus as will appear, the embryos of the primitive non-placental
Mammals known as Monotremes possess both yolk-sac and yolk, while
all the placental Mammals retain the sac, though it is empty. Secondly,
there are the origin of the embryo from what amounts to a blastoderm.
the method of gastrulation, and other features all characteristics of
large-yolked forms. We may now proceed to the actual method of segmentation. _
The Blastocyst.——Cleavage, though total, is irregular from the
start (Fig. 251) . The result is the formation of a spherical mass of cells
known as the morula in which the cells are of two types. On the outside
they are at first cubical, but soon assume the form of a flattened epithelium, which being covered temporarily by the zona radiata is called
the subzonal layer, later the trophoblast. The cells on the inside, on the
other hand, are spherical and are called the inner cell mass. Presently,
vacuoles appear on one side of this mass, beneath it and the subzonal
layer. These run together and increase until more than half of the
morula is occupied by a fluid-filled cavity. On the other side, the inner
mass hangs from the wall like a suspended drop (Fig. 252). The morula
has now become a ‘blastodermic vesicle or bldstocyst, which corresponds
in a general way to the blastula of lower forms. Hence the cavity may
510 EARLY MAMMALIAN DEVELOPMENT
be termed the blastocoel or subgerminal cavity, while the fluid within it
occupies the place of the yolk. Finally, as subsequent development
shows, the inner cell mass lying above the fluid virtually plays the part
of a blastoderm (Fig. 253).
Cleavage occurs while the ovum is passing down the oviduct, and in
some instances it may even have reached the blastocyst condition by "the
time it arrives in the uterus. The time required for this passage varies
‘ much in different animals, but is
ordinarily considerable, e.g., about
four days in the Rabbit, and eight
or ten days in the Dog. The movement down the duct is apparently
accomplished mainly by peristaltic
action, though in the Rabbit,
Parker claims that the cilia heating in-an abovarian direction are
involved.
Within the uterus the cleaving
egg, or morula, soon becomes a
blastocyst, if it is not already one,
and this begins to enlarge through
Fig. 253.—-Section through the fully
formed blastodermic vesicle of the Rabbit,’ From Quain’s Anatomy, after Van
Beneden.
f.c.m. Granular cells of the inner cell
mass. troph. Trophoblast. zp. Zona pellucida.
the multiplication and flattening
of the cells of the subzonal layer
(Fig. 253). There is considerable
variation in the size and shape
_ which is reached in this manner.
Thus in the Rabbit, the vesicle after three days in the uterus becomes
ovoidal, measuring about 4.5 x 3.5 mm. In Ungulates, on the other
hand, it becomes very long and tapering, that of a nine day Pig measuring about 8am. in length and .5 mm. in diameter, while in a day or two
more the length has reached about a meter, and the diameter a few millimeters. In all cases, however, the inner cell mass remains very small,
and in instances where the vesicle is elongated, as in the Pig or Sheep,
the mass is attached about midway between its ends (Fig. 254) .
' .< GASTRULQTION
As in the other forms studied, this term is here used to denote the formation of an archenteric cavity, and the setting aside of epiblast and
kypoblast. In most Mammals the latter appears to arise either by a splitting off (delamination) of cells from the ventral side of the inner cell
GASTRULATION 511
Fig. 254.——Photographs of Pig blastocyst by Heuser and
Streeter showing the transition from an oval to an elongated
form. In group A the long axis of the smallest specimen was
approximately 7.5 mm., while in the largest it was about 13.8
mm. In group B the magnification is less so that the smallest
specimen on the extreme left actually measured about 15 mm. in
length, and the greatly elongated specimen at the top of the
group measured about 150 mm.
mass, or by an infiltration of cells from this area. It will be recalled
that both these possibilities are identical with some of those recently suggested as occurring in the origin of the primordial hypoblast of the
Chick. At all events the cells so ‘produced then multiply and spread
around the inside of the vesicle until in many forms they eventually
completely line» it, just ‘as they line the archenteron ‘and yolk-sac of
the Bird. This extension of the hypoblast and later mesoderm around
the inside of the blastocyst is of course essentially epibolic, though the
overgrowth covers only a cavity. The cavity so lined constitutes the
archenteron, while part of it presently becomes the yolk-sac in a man512 EARLY MAMMALIAN DEVELOPMENT
Fig. 255.—Sections through four stages in the early development of
the lnsectivore Tupaia jauanica. From Hubrecht. A. Blastodermic
vesicle completely closed; hypoblast still continuous with the embryonic epiblast. B, C. Embryonic epiblast split and folding out upon the
surface of the vesicle, pushing away the trophoblast cells. D. Embry> oniclepiblast forming a Hat disc on the surface of the blastodermic
‘. vesic e.
E. Inner cells mass, now embryonic knob. ec. Embryonic epiblast. en.
Hypohlast. tr. Trophoblast.
ner to be indicated, despite the absence of yolk. Thus the situation differs from that found in previous forms, and particularly in the Bird, as
follows: In the latter case the original archenteron consisted only of a
shallow space between the hypoblastic roof and the underlying yolk.
i The central region of the roof, later augmented by mesoderm, then
folded off to form the gut, while the borders grew out and around the
yolk to form the sac. In most Mammals, on the other hand, there is of
IMPLANTATION i513
course no yolk at all, so that the cavity of the blastocoel beneath the
hypoblast may all, at first, be called archenteron. Later on the hypoblastic roof of this cavity now accompanied by mesoderm, and hence
termed endoderm, folds of? as in the Bird to form a gut. Meanwhile the
remainder of the cavity may or may not have become completely lined
with endoderm. In the Guinea Pig for example only the roof is ever so
constituted. In any event the part of this cavity not eventually occupied
by the allantois, amnion and extra-embryonic coelom becomes the yolksac, with or without a ventral wall. In many cases, as in the Rabbit,
Cat and Pig, this sac is fairly extensive, especially at first. In others,
like most Primates, it is very insignificant. Certain special details and
peculiarities of. these extra-embryonic structures will be considered later.
Meanwhile it is to be noted that with the origin of the hypoblast the remainder of the inner cell mass together with the original subzonal layer
may now be termed the epiblast. This epiblast is then further divided
into that which composes the inner cell mass proper, now termed the
embryonic knob, and that which composes the subzonal layer, now
termed the trophoblast. It is to be noted that the latter completely encloses, for a time at least, the embryonic knob and the yolk-sac. Hence
though originating differently, it occupies the same position as the chorionic ectoderm of the Chick (Fig. 255, A). In fact, with the mesoderm
which in some cases later comes to line it, this layer constitutes the
clwrion of the Mammal.
It is to be clearly understood that the process of gastrulation which
has just been described is entirely one of delamination or infiltration,
and proliferation; there is apparently no involution, invagination, nor
epiboly, and hence also no concrescence. Consequently, it is not surprising that there is no well marked blastopore, at least in connection
with the actual process of hypoblast formation. Later, as in the Chick, a
primitive streak arises as a thickening in the epiblast, and again as in
the Bird, parts of this streak are interpreted by many as the homologue
of a blastopore. This will be discussed further when the origin of the
primitive streak is described.
IMPLANTATION
By the time the stage described above has been reached, and some
' times somewhat earlier, the blastocyst has become attached to the uter
ine wall. This process is known as implantation, and there are several
methods by which it is brought about. It will be best, however, to postpone their detailed discussion until the description of the placenta is
514 EARLY MAMMALIAN DEVELOPMENT
taken up. Sufiice it to say at this point that it is brought about largely by
the activity of the trophoblast, aided by certain changes in the uterine
wall itself.
THE AMNION
There are two chief methods by which the amnion is formed in the
Mammal: ‘
I. The First Method of Amnion Formation.——-This method
may be defined briefly as the method of amnion formation by folds. The
Fig. 256.-—Formation of the amnion in the Rabbit (Lepus). From Jenkinson
(Vertebrate Embryology). After Assheton.
i.m. Inner cell. mass. Ll. Lower layer (i.e., hypoblast) . e.p. Embryonic plate (i.e..
blastoderxnal epiblast). R. Cells of Rauber. tr. Trophoblast.
first step in this method involves the transformation of the epiblast of
the embryonic knob into a flattened plate overlying the hypoblast, the
two layers being virtually homologous with the similar ones of the avian
blastoderm. This flattening is accomplished, however, by two different
processes. Thus though subsequent development of the amnion itself is
similar, it is convenient upon the basis of the above differences in the
initial stages to describe Method I under two headings, Type (a) and
Type (b). «
Method 1, Type (a) .-—-This type is illustrated by one of the Insectivores, T upaia (Fig. 255) ; in this animal a depression appears in the top
of the embrvonic knob, and extends well down into it. The bottom of the
depression then rises to the surface, and the edges are at the same time
pushed apart. As this occurs the trophoblast cells above are broken and
scattered. Thus the epiblastic plate of the blastoderm so formed comes to
lie directly on the surface of the blastocyst.
THE AMNION 515
Fig. 257.—DiEerentiation of the early Pig blastoderm.
After Heuser and Streeter. A, B and C are from blastocysts
measuring .6 mm. in diameter, and show clear differentiation
of the inner cell mass (chiefly epiblast), and a thin layer of
hypoblast, the whole being covered by a layer of trophoblast.
D measured .8 mm., but does not show the hypoblast. The
trophoblast over the inner cell mass is scattered, only two
cells (cells of Rauber) remaining. '
‘ Method I, Type (b). —-— In this type, of which the Rabbit or the Pig
form equally good examples (Figs. 256, 257), the process‘ is simpler,
for here the knob merely flattens without the occurrence of any previous depression. In such cases after the flattening is completed, scattered
trophoblast cells may remain for a time over the blastoderm, and are
known as the cells of Rauber; these, however, soon disappeux.
Subsequent Stages of Method 1, Types (a) and (b). —— As. suggested
above it will now appear that the later stages of types (a)" and (b) are
virtually alike. Before they are described, however, it should be noted
that during or soon after the above processes, mesoderm has been proliferated between the epiblast and the underlying hypoblast in a man516
EARLY MAMMALIAN DEVELOPMENT
Fig. 258.—Diagrams of the formation of the embryonic membranes and appendages in the Rabbit. From Kellicott (Chardate
Development). After Van Beneden and Julin (partly after Marshall). Sagittal sections. A. At the end of the ninth day, after
coitus. B. Early the tenth day. C. At the end of the tenth day.
Ectoclerm black; endoderm dotted; mesoderm gray.
al. Allantois. as. Allantoic stalk. b. Tail-bud. c. Heart. d. Allantoidean trophoderm (see page 543). e. Endoderm. ex. Exocoelom. f. Fore-gut. h. Hind-gut. m. Mesoderm. N. Central nervous system. p. Pericardial cavity. pa. Proamnion. s. Marginal
sinus (sinus terminalis). t. Trophoblast. ta. Tail fold of amnion.
v. Trophodermal villi. vb. Trophoblastic villi. y. Cavity of yolksac. ys. Yolk-stalk.
THE AMNION 517
ner to be described below. The two first layers may henceforth therefore
be referred to as ectoderm and endoderm. Moreover, there has arisen
within this mesoderm the usual coelomic split, separating it into the
somatic and splanchnic layers. In either type (a) or (b), the amnion is
then formed by folds of ectoderm and somatic mesoderm, which arise
about the rim of the flattened embryonic knob (i.e., the blastodermal
-ectoderm), in essentially the same manner as in the Chick (Fig. 258).
Thus as the amnion is completed by the meeting of the folds at the seroamniotir: connection, the chorion is at the same time re-established above
it. This portion of re-established chorion now consists as usual therefore
not only of an outer layer of ectoderm, but also of an inner layer of
somatic mesoderm. Between the latter and the somatic mesoderm of the
amnion is of course the extra-embryonic coelom.
There are, however, certain minor points of difference to be noted
between the case of the Bird and that of the placental Mammal. In the
first place there is the origin of the chorionic ectoderm. In the Bird this
arises entirely from ectoderm of the extra-embryonic blastoderm which
has grown out over the yolk. In the Mammal, on the other hand, since
the folds arise just at the border between blastodermal ectoderm (embryonic knob) and trophoblast, a large portion of the ectoderm in the
folds, i.e., that of the outer layer, seems to be formed from the latter
substance. Thus_while the lining of the amnion may be chiefly blastedermal, the ectodermal part of the chorion which covers it is apparently
entirely of trophoblast, a tissue which seems to have no real homologue
in the Bird. A second but rather less important diflerence between Bird
and Mammal is the fact that in the latter the tail fold often appears
earlier than the head fold, and is therefore the longer of the two. In the
Pig, on the other hand, head and tail folds are virtually equal, and are
continuous with the lateral folds which arise coincidentally (Fig. 300).
II. The Second Method of Amnion Formation.-—In the second method of amnion formation, the trophoblast above the embryonic
knob is never interrupted, a condition known as entypy. In contrast to
Method I, the amniotic cavity then arises merely as a space within the
embryonic knob or in connection with the knob and the trophoblast
above it. Here again, however, there are variations in the process, so
that it may best be described under the headings, Type “(a) , Type (12) ,
and Type (c). '
Method II, Type (a).——This type is illustrated by the Hedgehog
(Erinaceus, Fig. 259) in which the rudimentary amniotic cavity appears,
not in the knob itself, but as a space between the center of its dorsal side
518 EARLY MAMMALIAN DEVELOPMENT
and the trophoblast. The edges of the knob, however, remain adherent
to the trophoblast, and these edges now turn and grow toward one another between the trophoblast and the cavity. Thus when they meet and
fuse, the epiblastic (future ectodermal) layer of the amnion is completed. Later, the extra-embryonic coelom lined by mesoderm forces its
way in between the trophoblast (now chorionic ectoderm) and the epiblast, now ectoderm, of the amnion, so that in this manner the latter receives its mesodermal covering and the former its mesodermal lining. It
am.c.
C.
Fig. 259. -—Formation of the amnion in the Hedgehog (Erinaceus) . From Jenkinson (Vertebrate Embryology) . After Hubrecht. A. Early. B. Later stage.
am. Amnion. c. Extra-embryonic coelom. ec. Ectoderm. e.k. Embryonic knob.
l. Lacuna. m. Mesoderrn. n. Notochord. tr. Trophoblast. y.s. Yolk-sac.
may be noted that the type of amnion formation thus exemplified by the
Hedgehog is quite similar in many respects to that just described under
Method I, and may, therefore, represent a transitional stage between
Methods I and II. Later, as the embryo develops, the edges of the flat
blastoderm are folded downward in the usual manner, and portions of
the mesodermal layers are of course involved in this process. The layer
lying next to the endoderm is then splanchnic mesoderm, and the one
next to the ectoderm (either trophoblastic or embryonic) is somati;
mesoderm.
Method II, Type (b). — The second type of Method II is typically illustrated in the development of the Guinea Pig (Cavia), in which the
process is as follows: _
Shortly after gastrulation is completed, the embryonic knob becomes
separated from the trophoblast above it, and moves down near the opposite side of the blastocyst.‘’ In so doing, it pushes the central portion
5 In this case and that of the Mouse and Rat the blastocyst, presumably be
cause of its shape, has been termed by some the “egg cylinder,” though it is of
course neither an egg nor a cylinder.
THE AMNION ‘ 519
of the hypoblast layer before it; the edges of this central portion, nevertheless, remain attached to the dorsal trophohlast. This process presently results in the production of a clear space between the knob and
the trophoblast, bounded on its sides by the upstretching hypoblast. A
cavity now develops in the middle of the embryonic knob; this is the
rudiment of the amniotic cavity (Fig. 260, A, B). On the floor of this
cavity, the cells remain columnar, and are homologous with the upper
Fig. 260.—Fo1-mation of the amnion in the Guinea Pig (Cauia).
From Jenkinson (Vertebrate Embryology). After Selenka. A. Early.
B. Later. C. Latest stage.
a.tr. Allantoidesn trophoderm. o.tr. Omphaloidean trophohlast (see
page 543) . l. Lacuna. e.k_. Embryonic knob. am.c. Amniotic cavity. y.s.
Yo1k~sac hypoblast in A and B, endoderm in C.
or epiblastic layer of the embryonic portion of the blastoderm in previous forms. The cells of the roof and sides, on the other hand, soon flatten and form the epiblastic layer of the amnion. The latter now begins
to expand, filling the space above it (Fig. 260, C). In the meantime mesoderm begins to arise between the epiblast of the hlastoderm ‘and the
hypoblast beneath it. Thus the former becomes ectoderm and the latter
endoderrn, while within the mesoderm the coelomic split occurs, producing two layers. These layers then spread out upon either side, the
lower layer extending over the endoderm as the splanchnic mesoderm,
and the upper layer extending up over the ectoderm of the amnion as
the somatic mesoderm. The amnion is now completely formed, and consists, as in previous cases, of an" outer layer of mesoderm and an inner
one of ectoderm. Further development merely involves an increase in
size and a gradual folding in about the embryo to form thenumbilical
stalk.
Fig. 261.—Formation of the amnion in the Mouse (Mus). From Jenkinson. ( Vertebrate Embryology). A.—E. Successiye stages. am. Amnion. am.c. Amniotic cavity. a.tr. Allantoidean nophoderm. c. Extra-embryonic coelom. e.k. Embryonic knob. l. Lacuna. l.l. Lower layer, L6.
hypoblast. m. Mesoderm. m.g. Medullary groove. n. Notochord. a.tr. Omphaloidean trophoblast. py. dy. Proximal or upper, and distal or
lower walls of yolk-sac. tr. Trophoblast. tr.c. Temporary trophoblastic or false amniotic cavity. y.s. Yolk-sac.
THE AMNION 521
In anticipation of the method which is next to be described under
type (c), however, it may finally be added that besides the amniotic
cavity thus formed, there has also arisen a cavity in the dorsal trophoblast from which the knob was separated. This second space is often referred to as the false amniotic cavity, but in the type under discussion it
never has any connection with the true cavity. It presently disappears
and has no further significance.
Method 11 Type (c).—This last type of amnion formation is well
shown in the Mouse (Mus, Fig. 261). In this form the embryonic knob
moves down as in the Guinea Pig, pushing the endoderm before it, but
does not become separated from the trophoblast. Instead, the latter simply thickens, thus filling up the space which would otherwise result. A
cavity now appears in the upper part of the knob, and at once comes into
communication with a cavity in the lower part of the thickened trophoblast, i.e., the false amniotic cavity. The mesoderm next arises between
the hypoblast, now endoderm, and the epiblast, now ectoderm, of the
knob, whence it spreads upward between the endoderrn and the thickened trophoblast. Within this mesoderm the coelomic split next develops
upon either side, and the two coelomic spaces then press toward each
other and finally unite. In this manner the mass of ectoderm and trophoblast, including the cavity, is cut in two in approximately the region
where the ectodermal and trophoblastic elements were in contact. This
process is such as to leave one closed cavity in the trophoblast and
another closed cavity in the embryonic knob, with the extra-embryonic
coelom lined by mesoderm between them. The cavity in the knob is,
of course, the amniotic cavity with its usual layers, while the one in
the trophoblast is the false cavity already referred to. The latter. it will
be noted, is in no wise different from its homologue in type (b), except
that in this case it temporarily communicates with the true cavity. Later,
as in the former case, it disappears.
The Inversion of the Germ Layers. —— Before passing on to a discussion of the relative primitiveness of Methods I and ll, it is worth
while to note a peculiar misconception which arose in the minds of early
students of forms like Cavia and Mus. These are cases, it will be recalled, where the embryonic knob moves far down into the blastocyst.
The obvious result is that the endoderm extends well up on either side,
considerably above the level of the blastoderrn. Hence, if in examining
the blastocyst of such a form, the investigator overlooked the outer layer
of trophoblast, the first layer he would come to would be endoderm. He
would thus get the impression that in some mysterious manner the endo-“rm
522 EARLY MAMMALIAN DEVELOPMENT
derm had gotten on the outside of the blastocyst. This oversight was exactly what occurred, and the phenomenon was, therefore, referred to as
an “inversion of the germ layers.” As a matter of fact, it is now clear
that no such inversion really exists, and hence the phrase is of only historical interest.
 
beginning
primitive streak P”'“m"° 3'9"‘
substance
   
Fig. 262.—Graphic reconstructions of the Pig hlastoderm in the prestreak and early streak stages. After Streeter. A. Pre-streak stage. B.
Early primitive streak, showing beginning mesoblast formation. C and
D. Later stages in primitive streak development with greater extension
of the mesoblast. As in the Chick, the mesoblast can be seen spreading
out from the sides of the streak.
The Relative Primitiveness of Methods I and II. ——There has
been some discussion as to which of these two main methods of amnion
formation is the more primitive among placental Mammals, one view —
that of Hubrecht— being strongly in favor of Method II. The reasons
. for this attitude are based chiefly upon the characteristics of the mam
malian chorion indicated in connection with Method I, and are as follows: In the Bird or Reptile (i.e., the Sauropsids) , there is, as suggested,
no chorion (the layer corresponding in relative position to the mammalian trophoblast) until it is formed by the outer walls of the amniotic
folds. In all the Mammals whose early development is known, on the
~‘ other hand, the blastocyst is entirely enclosed in trophoblast, or chtfiifi
[, onic epiblast, before any amnion has been formed, either by folds or
...~..._.,_..m.. . . .,._.—...._t,...__.......,._t_
THE AMNION 523
otherwise. It is true that in those cases where the process of folding occurs (e.g., in the Rabbit), the original trophoblastic chorion above the
embryo virtually disappears, and the new one in this region is formed
from the outer walls of the folds. Nevertheless, even in these cases there
is no denying that there was a trophoblastic chorion previous to the
C
Fig. 263.--Later primitive streak and mesoblast formation in the Pig. After
Streeter.
folding, and further that most of the (chorionic) portion of the folds is
still really trophohlastic. Hence, as indicated above, it is said that the
original trophoblastic chorion of Mammals cannot be regarded as homologous with the layer of the same name in the Sauropsids. From this
statement it then follows, according to proponents, of this idea, that the
cases of the formation of the mammalian amnion and chorion by folds
could not have been derived from this process in the Reptiles; it must
rather represent a reversion to the reptilian condition, or else a piece of
independent evolution.
524 EARLY MAMMALIAN DEVELOPMENT
Fig. 264.——Surface view of two stages of the Pig blastoderm with parts of the
adjacent blaslocyst. After Streeter. A. Primitive groove stage, length of blastederm about 1 mm. B. Blastoderm showing primitive groove and also beginning
neural groove. length 1.7 mm. Crest of chorio-amniotic fold shows around margin
of blastoderm.
H.n. Hensen’s node (knot). N.gr. Neural groove. P.gr. Primitive groove.
There are, however, many zoologists who do not subscribe to the theory just presented. Instead they regard Method I as the more primitive,
for the following reasons: In the first place it is known that Mammals as
a class sprang from Reptiles, in which group the method of amnion formation is by folds as in the Birds. Furthermore, among those Mammals
which are in other respects most primitive, i.e., the Monotremes and
Marsupials, the formation of the amnion by folds (according to the evidence of those stages which are known in these animals) in all probability prevails. Lastly, as admitted by the opponents of the view now being
presented, the trophoblastic ‘chorion of the Mammal is not really homologous with the true chorion of the Bird; it is rather a secondary developTHE PRIMITIVE S'l‘REAK 525
ment, whose early and complete enclosure of the blastocyst is made possible by the absence of yolk. Consequently, though the trophoblast
usually takes a large part in the formation of the mammalian chorion,
it has not, contrary to the
argument stated in the foregoing paragraph, necessarily anything to do with the
formation of the amnion.
Indeed, as has been seen,
the latter frequently forms
by’ folds in spite of the
presence of the precocious 3"; owpnajsgmue
trophoblastic chorion, and
those cases where it does
not (Method II) are mere
ly another secondary devel
opment" In_ conclusion’ it Fig. 265.——Reconstruction of a surface view of
may be Sald that 01’! the a Pig blastoderm, length 1.56 mm. After Streeter.
Heavy dotted line anterior to Hensen’s node is
whole the lfrgurllents for the notochord. Cross hatched region is mesothe conception Just p1‘6- derm. Darkly lined area posterior to Hensen’s
sented appear to be rather node is remains of primitive streak.
more cogent and reasonable than those opposed to it and it is the one
which is more widely held.
THE PRIMITIVE STREAK AND RELATED STRUCTURES
It will have been noted that during the process of amnion formation
(in Method I, slightly preceding it) there arises in one way or another
from the embryonic knob a flat plate of epiblast. This area of epiblast
together with the hypoblast directly beneath it is the area from which
the embryo proper is now to develop. As has been suggested, in the
Chick it is termed the embryonic blastoderm; in the Mammal it is the
embryonic disc.
The Primitive Streak and Groove. ——The primitive streak arises
along the mid-line of the embryonic disc in what later proves to be the
longitudinal axis of the embryo. The questions as to its source are very
much the same as they were in the case of the Chick, but not so much
experimental work has been done in an eiiort to answer them. The reasons for this are fairly obvious in view of the conditions under which
the Mammalian embryo develops. However, careful study of fixed material has been made by Streeter and others in the case of the Pig, and
 
' Henserrs node
526 EARLY MAMMALIAN DEVELOPMENT
Fig. 266.-—A. Sagittal section through the embryonic shield of the Hedgehog,
showing the transitory blastopore. From Kellicott (Chordate Development). After
Hubrecht. B. Posterior part of a sagittal section through the embryonic disc of
the Mole. C. Diagram of a sagittal section through the embryonic disc of the Mole.
From McMurrich (Development of the Human Body). After Heape.
ant. Amnion. b. or bl. Blastopore. ce. Chorda endoderm. ec. Ectoderm. en. Endoderm. nc. Neurenteric canal. prm. Peristomial mesoderm. ps. Primitive streak.
t. Trophoderm.
the following conclusions seem justified. There first.appears a thickened crescent of epiblast about what proves to be the posterior margin
of the disc (Fig. 262, A). This crescent then assumes the form of an
oval (Fig. 262, B, C ), and this gradually elongatesy into the primitive
streak (Fig. 262, D; Fig. 263). Presently, as in the Bird, a primitive
groove forms along the middle of the streak and at its anterior end there
develops a thickened spot, Hensen’s knot (Figs. 264, 265) . It is to be particularly noted that in this knot there is likewise a pit which in some
Mammals, e.g., the Hedgehog, as in some Birds, temporarily opens into.
the archenteron (Fig. 266). In some others the pit merely pushes into
THE PRIMITIVE STREAK 527
the notochord where it is known as the notochordal canal. In either case
its possible homology with the part of the blastopore which in other
cases forms a neurenteric canal is obvious, even though it disappears before the neural folds arise. Just what is going on during these changes of
shape from a crescent, to a streak with a groove and knot, is not certain.
It seems highly probable, however, that the process is again one of convergence of material toward the mid-line, and perhaps even some concrescence. Also as in the
Chick, there is apparently
rapid proliferation of cells
in this region. The meanings
of the groove and knot are
no more or less clear than
in the case of the Chick, and
whatever their significance
in that form they probably
have the same significance
in the Mammal (see below).
Origin of Mesoderm
and Notochord. —-As in
the Chick, so in the Pig, and
presumably in other Mam
primitive streak
     
ectoderm
mesoderm
x section pig blastoderm
mals, the streak is again the Fig. 267.—-‘Transverse section of one side of v
d a Pig blastoderm similar to one from which
5°urce Of the meso erm’ surface reconstruction in Fig. 262, C, was
which is proliferated from made. After Streeter. Long axis measurement
. . of the blastoderm from which this section was
its sides, and spreads out on
taken was .5 mm.
either hand and posteriorly
(Figs. 267, 268). Indeed as shown in Figure 262, this proliferation actually begins even before the streak primordium has assumed its definitive elongated form. Whether there is later any actual movement of
cells through the streak from the upper surface, i.e., anything like infiltration (involution), as was suggested in the case of the Bird is not
known, but it seems quite possible. If this were true it might help, again
as in the Bird, to account for the development of the groove. Be that as
it may the mesoderm having thus originated as a single sheet, very early
begins to split into the usual somatic and splanchnic layers. This splitting starts in random isolated areas, thus producing small vesicles,
which presently coalesce, to form more extenisve coelomic spaces (Figs.
262, 263). It willebe noted incidentally that the coelom first formed in
this manner actually lies outside the definitely embryonic area, i.e., ap528 EARLY MAMMALIAN DEVELOPMENT
proximately the region comparable to the area pellucida of the Chick.
Hence this first coelornic space is extra-embryonic, but very shortly it
spreads within the embryonic region. Finally the notochord (headprocess) of the Pig arises according to Streeter (’27) as a rod of cells
 
 
 
“-‘9..3'.~‘4‘:-"W
,
Fig. 268.—A. Transverse section through the primitive streak of
the Mole. B. Transverse section through a Human embryo of 1.54
mm. (Graf von Spee’s Embryo Gle.) From Minot (Laboratory
Zfggt-Book of Embryology), after Heape (A), and Graf von Spee
ch. Notochord. ct. Somatic mesoderm of amnion. df. Splanchnic
mesoderm. Ec. or ek. Ectoderm. en. or En. Endoderm. df. Dorsal
furrow. g. Junction of extra-embryonic somatic and splanchnic mesvoderm. me. or mes. Mesoderm. p. Rudiment of embryonic coelom.
p.gr. Primitive groove. Pr. Primitive streak.
proliferated at the primitive knot and pushed anteriorly. This it will be
recalled is identical with one of the theories of notochord origin in the
Chick. According to one of the most recent theories, however (Spratt,
’47) , the notochord in the Bird lengthens by growing posteriorly rather
than anteriorly, as the primitive streak shortens. It is quite probable that
whatever the true process proves to be in that case it will be found to
hold also for the Mammal. However that may be, it should be noted
that there is an interesting difference between the relation of the mesoderm and notochord in the Pig from that observed in the Chick. Thus it
YOLK-SAC, ALLANTOIS, AND PLACENTA 529
will be seen that in the Pig the notochqrd has no mesoderm free area
(proamnion) anterior to it as was true in the Bird (Fig. 265). The only
suggestion of this occurs much earlier in front of the beginning primitive
streak sometime before the notochord has begun to develop (Fig. 262).
The Nature of the Mammalian Primitive St-reak.——From the
above ‘description it is very evident that the parts here indicated are
virtually homologous with the similarly named structures in the Bird.
Consequently if the primitive streak of the latter can be further homologizecl with the remains of an elongated closed hlastopore, it would
appear that this homology holds equally well for the primitive streak
of the Mammal. As previously suggested, however, because of practical
difliculties experimental observations on the behavior of materials dur-.
ing and immediately after the formation of the primitive streak are not
as yet available in this instance as they were in the Chick. The chief
evidence therefore arises from observation of the relations of the streak
to the formation of the notochord and mesoderm already noted, and to
parts of the future embryo. Thus in the latter connection it may be
stated that the anus forms at the posterior end of the streak, and a. very
marked pit, amounting in some cases to a virtual neurenteric canal, at
its anterior end. '
In the case of the preceding topic as in others to follow the student
who does not recall the comparable situation in the Chick is again
urged to refresh his memory on the points in question, since we shall
not repeat identical material. '
THE YOLK-SAC, THE ALLANTOIS, AND THE PLACENTA:
THEIR STRUCTURE AND FUNCTIONS IN THE MAMMAL
Among the Amniotes of which the Chick is a type, i.e., the Birds, the.
chief organs through which the embryo receives its nutriment and
effects respiration have been seen to be respectively the yolk-sac and the
allantois. Among the vast majority of the Amniote group known as
Mammals, however, these organs are very largely, and in many cases
completely, supplanted in these functions by a new structure, typically
associated with the allantois and termed the placenta. The large group
of Mammals among whose members this organ is most fully developed
is therefore known as that of the placental Mammals, a group which
hastalready been frequently referred to. It will presently appear, however, that within this group there are certain types of placentas which
vary from one another, ‘both in their structure, and in the degree to
530 EARLY MAMMALIAN DEVELOPMENT
Fig. 269.—Fetal membranes of A, Monotremata; B, C, D. Marsupials. B. Phalangista, Aepyprymnus, Didelphys, Bettongid; C. Dasyurus; D. Perameles and
Halmaturus. (In Didelphys the proamnion persists as in Dasyrus.) From Jenkinson (Vertebrate Embryology). (A, B, D, after Semon; C, after Hill.)
In this diagram of Mammalian fetal membranes the trophoblast (ectoderm of
mammalian chorion) is stippled, the ectoderm oi the amnion represented by a
continuous line, the endoderm by a broken line, and the mesodertn (somatopleure
and splanchnoplenre) by a thick line swollen at intervals.
all. Allantols. am.c. Amniotic cavity. pr. Proamnion, i.e., portion of amnion without mesoderm. y.s. Yolk-sac. s.t. Sinus terminalis of area vasculosa.
which they have assumed the place and functions of the allantois and
the yolk-sac‘. There exist also two relatively small mammalian groups,
the Monotremes and the Marsupials, whose members possess either no
placenta at all or only a very rudimentary one. Under these circumV or F‘ stances, therefore, it appears most convenient to treat the subject by
i taking up the conditions of the above organs in one group at a time.
The Monotremes and the Marsupials will be considered first, since they
are most primitive, and exhibit a condition most nearly akin to that in
the Reptiles and Birds. After these there will be discussed certain orders of truly placental Mammals which best illustrate the various types
noes-2 ax:
‘ma.
THE MARSUPIALS 531
of allantoic placenta, and perhaps suggest its method of evolution. The
orders to be thus considered are the Ungulazes, the Carnivores, the
Rodents, and the Primates. Finally before passing to a study of the first
group, it may be mentioned incidentally that the discussion of this subject also necessarily involves in each case a more extended reference
to the matter of implantation referred to above.
THE MONOTREMES
These curious mammalian forms comprise the Spiny Ant Eater
(Echidna) , and the Duck Bill (0rnithorhynchus) . They are remarkable
as Mammals in that they lay hard-shelled eggs like Birds. As might
be expected in such a case, the yolk—sac is well developed and illed
with yolk, while the allantois is also prominent. The placenta, on the
other hand, because of its peculiar nature and functions, which its study
will presently reveal, is naturally entirely lacking. In short, in eggs of
this sort the embryonic parts under discussion are in all respects characteristically reptilian or avian (Fig. 269, /1)-.
THE MARSUPIALS
This group comprises the Kangaroos (Macropodidae), the Opossums
(Didelphyidae), the Marsupial Cats (Dasyuridae) and the Bandicoots
(Peramelidae). These animals are all characterized by the fact that
their young are born in a comparatively undeveloped condition. They
then crawl inside of the Marsupial pouch of the mother and become
attached to her teats, where they remain for some time. As might be
expected under such circumstances, the means for obtaining nourishment and aerating the blood previous to birth are very primitive. In
fact, among the various members of the group there occur some very
excellent examples of graded transition from the condition in the Monotremes to that in the real placental Mammals. The Opossum is per»
haps as primitive a form as any in this respect, and will therefore be
considered first.
The Most Rudimentary Type of Placenta. -—ln Didelphys, or
the Opossum (Fig. 269, B), the yolk-sac, as in all the Marsupials, is
well developed though it contains no yolk. Nevertheless, upon its upper
surface there is a clearly defined area vasculosa, bounded by a sinus
terminalis. Since there is no yolk, however, the nutriment which the
above area is to convey into the embryo must be obtained from some
other source; this is accomplished in the following manner: Although
532 EARLY MAMMALIAN DEVELOPMENT
the mesoderm, and consequently the area vasculosa, do not reach to the
opposite side of the yolk-sac, the endoderm on that side comes into contact with the trophoblast of the blastocyst. During implantation this
trophoblast becomes thrown into folds (not shown in the figure) which
fit into depressions in the uterine wall. The latter then secretes a viscid
fluid, the uterine milk, which is absorbed via the trophoblast and endoderm, and finally reaches the embryo, partly at least by way of the area
va.sculosa.- This contact of the embryonic trophoblast and the uterine
tissue may be regarded as a very primitive beginning of what will later
berecognized as a placenta. The allantois is very small in this case, as
in most other Marsupials, and has no contact with the trophoblast. The
exact means by which the embryonic blood is aerated, therefore, is a '
little uncertain. Very possibly, however, it also is accomplished through
the contact of yolk-sac and maternal tissues.
A “ Yolk-Sac Placenta.” —-— Dasyurus is the second form to be considered, because it exemplifies the next step in the development of a
true placenta (Fig. 269, C). The allantois, however, is still small, and
the placenta-like structure which occurs is, therefore, again associated
entirely with the yolk-sac. Furthermore, the trophoblast in contact with
the non-vascular area of the sac once more forms the connection with
the uterine wall. In this instance, however, this implantation is more
thoroughgoing, and there appears for the first time that process ‘of
uterine erosion so noteworthy among some of the higher forms. This
erosion is accomplished by the trophoblast which, after becoming
thickened and syncytial (i.e., trophodermal) in certain regions, eats
into the uterine epithelium and engulfs some of the maternal blood vessels. The blood so obtained passes in between the trophoblast and yolksac, secretions from one or both of which digest it so that it can be absorbed. Presumably also such an arrangement makes possible respiratory
exchange of gases between embryonic and maternal blood. The type of
contact which is here illustrated is so intimate that the area in which it
occurs is sometimes referred to as a yplk-sac placenta.
A Primitive “ Allantoic Placenta.” —-— Finally, the most advanced
condition in this Marsupial series is illustrated in Perameles, where
the following situation occurs (Fig. 269, D) : Here the yolk-sac is again
large, and possesses an area vasculosa which is probably functional
in absorbing some nourishment by way of the trophoblast. In this case,
however, the allantois also is well developed,vand comes into contact
with the mesoderm of the chorion. Implantation then occurs and the
trophoblast in the area of this contact becomes attached to the uterine
—
‘-,
i
THE PLACENT-ALIA 533
wall, whose epithelium in this region is transformed into a vascular
syncytium. The trophoblast finally disappears, and the maternal blood
vessels come into intimate contact with those which have grown out
through the mesoderm of the allantois (Fig. 270). Thus there is established a true allantoic placenta. As will presently appear, however, the
exact relationship of its embryonic and its maternal parts is different
from that described in any of the subsequent types.
4/
 
f. b. v.
:73. mt.
Fig. 270.——Section through the placenta of Perameles. From Jenlcinson (Vertebrate Embryology). After Hill.
all. Allantoic epithelium. m. Mesoderm of allantois together with xnesoderm of
chorion. f.b.v. Fetal blood-vessel. ep.s. Syncytium of uterine epithelium. m.b.v.
Maternal blood-vessels. c.t. Sub-epithelial connective tissue of uterus.
In connection with this, the first real placenta to be noted, there is
one very important fact to be pointed out. Neither in this placenta nor
in those of any other type does the fetal and the maternal blood actually mix. It is always completely separated by one or more membranes.
Through these membranes, however, it is easily possible for an exchange of nutritive and waste materials, as well as gases, to take place.
This-completes the account of the Marsupials, and we are now prepared to pass on to the orders of the genuine placental Mammals. As
has been indicated, the latter are so named because here an allantoic
placenta of one sort or another becomes the usual and chief means of
embryonic nutrition and respiration. In the Marsupials, on the other
hand, such a condition occurs only in the single instance last cited. '
THE PLACENTALIA OR TRUE PLACENTAL MAMMALS
Within this large group, the embryonic appendages whose condition
is being considered are probably in their-most primitive form among
534 EARLY MAMMALIAN DEVELOPMENT
the Ungulates, and this ‘order, therefore, will be treated first with special reference to the Mammal; we have selected for later detailed study,
the Pig.
The Ungulates (the Pig).
The Early Means of Nutrition and the Yolk-Sac. —- Before the blaste
cysts enter the horns of the bicornate uterus, the latter have been prepared for their reception during the pro-oestrum, oestrus and early
Fig. 271.—Diagram of a fetal and maternal cotyledon of the Cow.
From Jenkinson (Vertebrate Embryology).
all. Allantoic epithelium. tr. Trophoblast. 11. Villus. ep. Uterine epithelium continued into crypt. c.w. Wall of crypt. The maternal conneco
live tissue is shaded.
dioestrum periods as explained in cohnection with the oestrus cycle. As
a result of this the uterine walls are thickened, and their glands hypertrophied to produce the secretion (uterine milk) which helps to supply
the embryos with nutriment and is eagerly absorbed by the trophoblast
of the blastocysts. Meanwhile gastrulation has occurred, the endoderm
(hypoblast) has grown around the inside of each blastocyst, and thus
with the advent of mesoderm and the folding off of the gut, an empty
yolk-sac is established in each. It is relatively large, and in the early
stages possesses a well developed area vasculosa. Thus it is able to function actively in passing nutriment from the uterine cavity into the embryo. Later, however, the yolk-sac becomes insignificant, its function
being entirely taken over by the allantois and the placenta, whose development will now be described. '
The Placenta arid the Allantois.—The blastocyst of this group, it
will be remembered, soon becomes greatly elongated, reaching a length
1
1
i
I
I
I
THE PLACENTALIA 535
of as much as a meter. It is not, however, to be understood from this
that it is actually extended to this extent, for if it were it would be
longer than the uterine horn in which it and several of its fellows are
contained. Instead, as the threadlike blastocyst of the Pig grows, it becomes greatly folded, the folds fitting into corresponding folds of the
blastodermic vesicle
amnion ¢mb")'°
   
l‘  I horlonlc crophoblast
diagrammatic x section r
of blastodermlc vesicle _.- '
 
Fig. 272.—-Drawing of a Pig blastodermic vesicle measuring about 350 mm. in
length and 4-0 mm. in diameter, and a diagrammatic :ransverse section of same.
The contained embryo measured about 40 mm. in length. Note the folds which
replace the villi of many Ungulates.
uterine walls. Later when the embryo develops and the blastocyst expands, the latter is very much dilated and shortened, after which the
term blastodermic vesicle is more commonly applied to it. As the vesicles reach their maximum length on about the thirteenth day. their
trophoblast has become relatively adherent to the uterine epithelium,
and implantation is said to have occurred.’ In the case of the Pig the
surface of the endometrium remains folded as does ,the surface of the
7 The implantation time varies in difierent animals, but in most of them it
occurs within a few days, often about seven, after the blastocysts reach the uterus.
In a few cases, however, implantation may be markedly dela'yed. Thus in the Long
Tailed Weasel and the Martin the blastocysts are said to lie dormant in the uterus
for many weeks (Wright, '42).
- 536 - EARLY MAMMALIAN DEVELOPMENT
blastocyst, though not to the extent that it was at its greatest length.
This arrangement of course increases the area of trophoblastic and uterine contact through which the exchange of nutriment and excretory
products can occur. This capacity for exchange is still further augmented
by the fact that in certain spots (areolae) microscopic projections
(villi) push out from the chorion into small spaces between the latter
and the uterine epithelium. These spaces are filled with the uterine secretion referred to above. In some Ungulates such as the Cow, the villi
atrial part posterior ardinti vein
 
 
 
ventricular area .
temporary viteiime and intcstlnai arteries
 
Fig. 273.——A 6.2 mm. Pig embryo (23 somites), injected, showing the circulatory system and beginning allantois. After Sabin.
are larger, and arranged in bunches or cotyledons, while the corresponding areas in the uterine wall with which the cotyledons come into
contact are called caruncles. These latter are permanently located, and
are said to exist as raised areas even in the uterus of the unborn calf.
Thus in these instances the locations of the embryonic cotyledons are
secondary, being determined by the positions of the maternal caruncles.
Meanwhile, to return to the Pig, by the time the embryo has reached
a length of 4-6 mm. the allantois has begun to outstrip the yolk-sac,
and soon comes to occupy the major part of the extra-embryonic space.
It appears first as a rather conspicuous crescent-shaped outgrowth encircling the posterior of the embryo, with its -horns extending anteriorly
(Fig. 273). In this respect it difl'ers considerably from the Chick allantois which it will be recalled is first noted as a roundish bladder pushing anteriorly and upward to the right from beneath the curled tail.
The crescentic allahtoic outgrowth of the Pig rapidly works its way
around the amnion, pushes aside the now useless yolk-sac, and eventuTHE PLACENTALIA 537
ally extends everywhere throughout the extra-embryonic space of the
vesicle except in the extreme ends (Fig. 272). The mesoderm which covers the allantois carries the umbilical blood vessels, and this mesoderm
together with the capillaries of the vessels becomes closely adherent to
the mesoderrn of the chorion into which these capillaries penetrate. In
this manner the fetal vessels come close enough to those of the uterine
mucosa for the necessary exchanges to occur. Thus is constituted the
Ungulate (in this case Pig) placenta, which as will be noted, comprises
almost the whole surface of the blastodermic vesicle.
It is to be especially noted that in the processes just described there
is absolutely no erosion of the uterine epithelium.‘ Instead the chorionic folds simply fit in between those of the endometrium from which
they may be easily stripped away at any time. Indeed during gestation
the endometriumicontinues to secrete nutritive substances between itself
and the chorion. This is absorbed by the latter and taken up by the
embryonic vessels, so that in this case, as in some others, the embryonic nutriment is not all obtained directly from that which is carried
in the maternal blood. A placenta in which the contact. between fetaland maternal tissue is such as indicated is often defined as indeciduate.
This term implies that at the time of parturition, the wall of the uterus
is literally not deciduous. That is, there is no tearing away of maternal
tissue when the fetal part of the placenta separates from that of the
mother.
In concluding this discussion of implantation in the Pig a curious
fact may be noted which apparently applies also to other Mammals
which have two horned uteri and produce litters. Thus it is well known
that the number of eggs ovulated by the two ovaries may be quite unequal as indicated by the corpora lutea present. Yet Corner has demonstrated that the number qf embryos developing in each uterine horn
is practically the same. This can only mean that enough of the embryos
from the side which produced more eggs have migrated to the opposite
side to equalize the numbers in the two horns. How this is brought
about no one knows, but in the case of the Pig it apparently occurs
previous to the elongation of the blastocysts.
The Carnivores.
The Yolk-Sac. —As in the Ungulates, the period of the pro-oestrum
results in the accumulation within the uterine hornsof a nutritive mix
3 According to some authorities there is erosion of the inaternal epithelium in
the Ruminants.
538 EARLY MAMMALIAN DEVELOPMENT
ture somewhat similar to that already described. In some cases, however (e.g., the Cat), it appears to be less abundant than in the Ungulates, and of a more watery consistency. The uterine mucosa is of course
also hypertrophied in the usual way, and everything is ready for the
Fig. 274.-——Fetal membranes and placenta of the Dog. From Jenkinson (Vertebrate Embryology). After Duval. '
all. Allantois. am.c. Amniotic cavity. In. Mesometrium, or sheet of connective
tissue attaching the uterus to the body wall. pl. Zonary placenta. (See text under
description of the placenta of the Carnivores for the definition of this term.) y.s.
Yolk-sac. The fetal mesoderm, connective tissue and blood-vessels are in black.
reception of the blastocyst, which in this instance is oval, never at any
time threadlike. Again the latter begins its development by absorption
of the nutrient fluid. A yolk-sac has meanwhile developed, in_ the usual
Mammalian manner, and apparently it plays about the same part in
this process as was noted in the Ungulates. As in that order, also, this
appendage later becomes relatively insignificant (Fig. 274) .
The Placenta and the Allantois. —— While these events are occurring,
3. change is taking place in the uterine wall. In a band which completely encircles this wall the epithelium disappears. Likewise, in the
THE PLACENTALIA 539
‘7‘;'';-~;-‘'7‘/=‘7‘-—--— 8"’:/’ %g___,.
tr.
rn.b.c.
f. c. t.
f. b. c.
In .b.v.
Fig. 275.—Section through the placenta and uterine
wall of the Cat. From Jenkinson (Vertebrate Embryology).
all. Epithelium of allantois. f.b.v. Large fetal bloodvessels. f.b.c. Fetal capillaries. f.c.t. Fetal connective
tissue. tr. Trophoblast (finely shaded). m.b.c. Maternal
blood capillaries; these are immediately surrounded by
maternal connective tissue (coarsely stippled). m.b.v.
Maternal blood-vessels passing through the maternal
glandular tissue (d). cp. Compacta (necks of glands).
sp. Spongiosa (dilutions of glands).
region of a corresponding band about the equator of the oval blastecysts, the latter begins to adhere to the prepared uterine wall. During
this process of implantation, trophoblastic villi similar to those of
some of the Ungulates begin to develop from the wall of the blastocyst
in the region of its adherence. Because of the obvious band or zone-like
shape of this region, the type of placenta which develops in this order A
is called zonary. The villi of the chorion, which may contain a core of
540 EARLY MAMMALIAN DEVELOPMENT
mesoderm, now push their way directly iillio the mucous tissue of the
uterus. As they do so, they absorb any remaining epithelial debris
which comes in their way. In this manner, they soon.become firmly embedded in the maternal tissue and surrounded by maternal blood vessels. While this is going on, the allantois has grown out, and as in the
Ungulates, soon becomes the chief appendage of the embryo. When the
allantoic mesoderm comes into contact with the chorionic mesoderm in
the zone of implantation, the allantoiccapillaries penetrate the villi,
and the placenta is virtually complete. During subsequent development,
however, it becomes thickened somewhat by growth and branching of
the villi and capillaries, and also of the maternal connective tissue in
which they are embedded. The glands of the latter continue to supply
debris and fat, which is absorbed by the chorionic villi up to the end
of gestation. The main source of embryonic nutrition, however, is presumably material contained in the maternal blood (Fig. 275).
It will be noted that the attachment of the fetal and the maternal
parts of the placenta is much more intimate in this case than it was in
the Ungulates. This has resulted from the disappearance of the uterine
epithelium, which allows the capillaries in the fetal villi to come that
much nearer to those of the mother. Because of this very close attach-.
ment, it also happens that at birth a large portion of the maternal tissue
is torn away with the fetal portion of the placenta. For this reason, this
type of placenta may be regarded as deciduate. Indeed, as will appear
from a study of the remaining groups, the Carnivores are probably the
only animals possessing a placenta of which this is true in any large
degree.
The Rodents.—-As in the forms previously studied, the uterine
epithelium of the horns is in, a hypertrophied condition following the
proioestrum and oestrus, and is thus ready to receive the blastocysts
(“ egg cylinders ”) when they reach the uteri. The method of attachment and of placenta formation which now follows varies somewhat
in different Rodents, although it is fundamentally similar in all of them,
and leads to practically the same results. It will further be found that
in this case, the former process, i.e., attachment or implantation, is
somewhat elaborate, and therefore requires more detailed attention than
has hitherto been necessary. The chief conditions with respect to this
process as well aslto the general character of the yolk-sac, may be illustrated by reference to two forms, the Mouse and the Rabbit. _
Implantation and the Development 0/ the Yolk-Sac. —— In the case of
the Mouse, each elongated uterine horn becomes lined with pits upon its
anti-mesometric side. This is the side opposite its point of attachment to
r_
THE PLACENTALIA 54.1
the coelomic wall, the latter region being termed the mesometric side.
Each of the ovoid blastocysts, of which there are several in the -Mouse,
becomes embedded in one of these pits with the embryonic knob facing
the narrow lumen of the uterus (Fig. 276, B). That this anti-mesometric
Fig. 276.—-Five stages in the formation of the placenta in the Mouse. From Jen»
kinson (Vertebrate Embryology). A. The blastocyst free in the uterus. B. The
blastocyst attached and the placental thickening of the developed allantoidean
trophoblast (trophoderm) (a.t.r.). C. Later stage, after closure of the amniotic cavity (am.c.) and the obliteration of the uterine lumen. D. Placenta becoming established, and reappearance of uterine lumen (l’u-.). E. Elaboration of the placenta.
l()isap)pearance of the distal wall of the yolk-sac and omphaloidean trophoblast
0.tf. .
c. Extra-embryonic coelom. l'u. New uterine lumen on the anti-mesometric si .
lu. Original lumen of the uterus. y.s. Yolk-sac. ;v.st. Yolk-stalk. u.c. Umbilical cor
m.. Mesometrium.
 
 
   
 
 
implantation is not the result of gravity has been clearly demonstraf
in the Rat by Alden (’45). He cut out the middle portion of a uter '‘
horn, leaving blood vessels intact, and replaced it in an inverted « .
tion. Implantation in this section was still on the anti-mesometric,‘;
now dorsal, side. Continuing with the case of the Mouse the furth
tory of a single blastocyst will suflice. ’ ..
As soon as the embedding has occurred, the trophoblast imm N
starts to erode the epithelium of the pit, and to devour the debris '
542 EARLY MAMMALIAN DEVELOPMENT
results. Meantime the blastocyst enlarges sufliciently so that the side containing the embryonic knob crosses the uterine lumen and comes in contact with the opposite wall (Fig. 276, B, C). In this way, each blastocyst
obtains attachment at every point, and completely obliterates the cavity
of the‘ ‘uterus where it is situated. At every place where contact is thus
97- am.
Fig. 277.--Fetal membranes and placenta of the Rabbit. From Jenlrinson (Vertebrate Embryology). After Duval and Van Beneden.
pr.am. Proamnion. Other letters as in Fig. 276.
established, i.e., on the bottom and sides of the original pit, and also
upon the uterine wall opposite to it, erosion of the uterine epithelium
is carried on. The placenta, which will presently he described, is established on the mesometric side of the uterus at the second point of
contact, and therefore next to the embryo. Then, owing to the intimate
relation of trophoblast and allantois in this region, the thickened trophohlast (trophoderm) on this side of the blastocyst is called allantoideon.
On the opposite side, i.e., at the original bottom of the pit, the uterine
lumen is later again established. Here for a while epithelium once more
develops, and covers both the wall of the uterus and the blastocyst (Fig.
F______ _. .. .Hhm_a_
THE PLACENTALIA 543
, 276, D). Inside the latter, the yolk-sac has meanwhile formed, and on its
3 upper surface has acquired an area vasculosa. Its lower wall, on the
other hand, which is in contact with the trophoblast of the blastocyst, finally degenerates. The trophoblast (in this region termed omphaloidgun) and the newly formed epithelium at this point then also vanish,
and thus the interior of the yolk-sac is placed in immediate communication with the re-established uterine cavity (Fig. 276, E) .9
Tufning now to the method of implantation in the Rabbit, it is found
to be somewhat less complicated. Here a pair of folds arise upon the
mesometric side of the uterus, and the blastocysts become attached to
these. Each blastocyst in this case lies between the folds and becomes
i attached by the trophoblast on either side of the embryonic disc. In
3 these regions, the uterine epithelium is eroded, and two placentas are
established which later merge into one (Fig. 277). The opposite side of
the blastocyst forms no intimate contact with the uterine wall and presently disappears. Concurrently the ventral wall of the yolk-sac also disappears, so that again, as in the case of the Mouse, the cavity of the sac
x is directly continuous with that of the uterus (this stage not shown in
the figure).
Having thus described the two chief types of implantation among the
Rodents, we are now in a position to discuss the nature of the placenta
and other means of nutrition common to all this group.
The Placenta and the Allantoi.s.———During the erosion of the uterine
epithelium indicated above, the allantoidean or placental trophoblast
becomes greatly thickened, to form trophoderm. This trophoderrnthen
continues to eat down into the mucous layer of the uterine wall, engulfing, as it does so, maternal blood vessels, together with glycogen from
the glycogen-filled cells (maternal glycogen tissue). There next appear
in the trophoclerm numerous lacunae, and into these is emptied the maternal blood from the vessels whose walls have been destroyed (Fig.
278, A). Meantime an allantois has arisen. In the Rodents, the endodermal portion of this organ containing the cavity is usually small,
although in the Rabbit, which in this as in most other respects is more
primitive, the allantoic cavity attains a considerable size (Fig. 277).
The mesodermal part, however, is always well developed, and soon
reaches the trophoderm of the placental region, bringing with it the umbilical blood vessels (Fig. 278, B). The capillaries of these vessels then
” The assumption has been that in this as in other cases the vascularized wall
of the empty yolk-sac functions in obtaining nutrimc.-nt for the early embnyo. Recent
experiments on the Rat. however, involving the tying 03 of‘ the vitelline vessels.
seem to indicate that such a function is negligible, at least in this animal
(Noer’47).
544_._ EARLY MAMMALIAN DEVELOPMENT
a. m. f. b. v.
 
\\\\
vs‘
\§.\
.\
it
\\\\ ( ‘
.t\\\ r ‘~" ‘L
Fig. 278.—Placentation of the Mouse. Details of the five stages of
Fig. 276. From Jenkinson (Vertebrate Embryology».
A. Strip of a section through the allantoidean trophoblast (trophoderm) and overlying maternal tissues in stage C, Fig. 276.
a.t.r. Allantoidean trophoderm. mu. Muscularis. m.v. Maternal bloodvessel, opening below into I. lacunae of the trophoderm. Lu. Original
lumen of the uterus. m.g.c. Maternal glycogen tissue.
B. Similar strip of the same parts in stage D, Fig. 276.
_ fjmv. Fetal blood-vessel. a.m. Allantoic mesoderm. Other letters as
in .
C. Similar strip of the last stage, Fig. 276.
tr.g.c. Trophodermal glycogen tissue. Other letters as in 3.
Note that ,ultimately this placenta is very largely composed of
trophoderm, which is a non-maternal tissue. Hence, since at parturition the line of separation passes through the placenta (the trophodermal glycogen tissue), little or no maternal tissue is lost, and the
placenta is essentially indeciduate. (See text.)
l
THE PLACENTALIA 545
penetrate the trophoderm so as to come near to the cavities containing
the extravasated maternal blood. This blood is being constantly poured
into the central space of the placental region, and withdrawn at the
periphery through the maternal veins. Gradually, toward the maternal
side, the trophoderm surrounding the lacunae becomes further vacuolated through the secretion of glycogen, thus establishing a trophoder.
mal glycogen tissue (Fig. 278, C). Eventually through the increase of
the latter, the layer of original maternal glycogen tissue is entirely eliminated.” Such is the character of the completed placenta of the Rodents,
which, because of its development upon only one side of the blastocyst,
has the general shape of a disc or button. It is, therefore, termed discoidal, as distinguished from the zonary form found in the Carnivores.
Comparing the placenta in this case with that noted in the Carnivores,
the chief difference will be found to be that, in the completed organ of
the Rodents, maternal tissue plays very little part. The placenta indeed
is principally composed of the fetal trophoderm with its capillaries,
lacunae, and glycogen tissue. This difference seems to be achieved by
the fact that the trophoderm erodes not only the uterine epithelium, but
a large part of the mucosa and its blood vessels as well. Because of this
peculiar structure, it happens at parturition that, aside from the blood
in the lacunae, very little real maternal tissue is lost. This follows from
the fact that the actual line of separation runs through the region of
vacuolated cells which have now lost their glycogen and collapsed, and
this region, as noted, is held to be entirely trophodermal. On account
of this lack of maternal tissue to be torn away, many authorities regard
the term deciduate as a misnomer when applied to placentas of this
type. If the above description be correct, it apparently is a misnomer.
Nevertheless, such placentas are still commonly classified under this
head.
As regards the method of nutrition in this order, it is apparent that,
aside from the glycogen, nutriment is chiefly obtained, so far as the placenta is concerned, from the maternal blood. It will be remembered,
however, that among the Rodents, the yolk-sac is always eventually open
to the uterine cavity. Thus, for instance in the Mouse and the Rabbit,
the lower epithelial wall of this organ was found to disappear com- ‘
pletely, while in the Guinea Pig it is never even formed. This being the
case, the upper wall of the sac may, in’ some cases at least, function
throughout gestation in the absorption of uterine secretions. To the ex
1° The maternal glycogen tissue is said to be more abundant and persistent in
the Rabbit.
546 EARLY MAMMALIAN DEVELOPMENT
Fig. 279.——Diagrams illustrating the formation of the umbilical
cord and the relations of the allantois and yolk-sac in the Human embryo. From McMurric_h (Development of the Human Body). The
heavy black line represents the embryonic ectoderm; the dotted line
marks the line of the transition of the body (embryonic) ectoderm
into that of the amnion. Shaded areas, mesoderm.
Ac. Amniotic cavity. Al. Allantois. Bc. Exocoelom. Bs. Body-stalk.
Ch. Chorion. P. Placenta. Uc. Umbilical cord. V. Chorionic (tropho
dermal) villi. Ys. Yolk-sac.
tent that this is true, therefore, the Rodent yolk-sac, both in its form and
in its activity, differs markedly from the types previously studied within
the strictly placental group.
The Primates.“
The Allantois and the Yolk-Sac. —— In the order of Primates, the nature of the yolk-sac and allantois is somewhat unique, while the latter
11 The characteristics of the embryonic appendages which are ascribed to this
order apply to only'une of the family of Lemurs, i.e., Tarsius. This animal, in
respect to these organs, may be classed with the lower Monkeys. So far as is known,
however, all other Lemurs are similar to the Ungulates as regards the yolk-sac and
allantois, and also even in the possession of a difluse indecidiiate placenta. This
exception must be home in mind with reference to all statements concerning the
Primates as a whole.
THE PLACENTALIA 547
 
 
 
 
Fig. 280.——Diagrams of sagittal sections through the Human blastoderrnic vesicle, showing the formation of the amnion and trophoderm. From Kellicott
(Chardate Development). /1-D, after Keibel and Elze. E. From McMurrich (Development of the Human Body), after Graf von Spec. In all the figures the anterior
end is toward the left, and in all the figures except E the following conventions.
are used: Black, embryonic ectoderm: heavy stipples, trophoblast and trophoderm;
light stipples, endoderm. Ohlique ruling, mesoderm except in A. A. Hypothetical
early stage; oblique ruling represents magma reticulare (see text). 8. Amniotic
cavity and wide exocoelom established; endoderm limited to a small vesicle beneath the embryonic ectoderm. The exocoelom in reality contains scattered mesenchyme cells. C. Blastodermic vesicle enlarged and covered with trophedermal villi,
into which’ the mesoderm is extending. Endodermic vesicle (yolk-sac) very small
(stage of Peter’s ovum). D. Embryonic portion only, of an older vesicle showing
the neurenteric canal, primitive streak (in the plane of the section posterior to
canal), and body-stalk. The mesoderm of the yolk-sac is becoming vascular. E.
%;a;gi)ttal section through a Human embryo of 1.54 mm. (Graf von Spec’: embryo
C
a. Amniotic cavity. at. Allantois. am. Amnion. B. Body-stalk‘ (umbilical cord).
ch. Chorion. e. Exocoelorn. nc. Neurenteric canal. V. Chorionic villi. Y. Yolk-sac.
y 548 EARLY MAMMALIAN DEVELOPMENT
organ is also peculiar in its method of development. An account of these
structures will be given, therefore, before proceeding to the matter of
implantation and placenta formation within this group.
First, as regards the allantois, it will be found that the endodermal
sac is even more limited than it was in the majority of the Rodents. Furthermore, the mesoderm of that organ does not comprise, as in most
   
Trophcblosl‘
.. Ex traembryonic
mesoblusi
Pfimmve Exccoelo "c
cndodmn ,,,,,,,,,,;“,;
Extrcemrycmc mdoderm Uterus
ring,“ 5"," db“ Trophoblufl mesoblast Amnion Gerrfldigk
xh-cembryonic
‘"‘l°d"’"‘ mesobtast
   
E 1 A Primitive
",:.::ni,:::. endodtrm
Fig. 281.—Mid-sagittal sections through four Human blastocysts
(“ ova") and surrounding uterine wall. After Hertig and Rock. A and B
are estimated as 11 days old plus, while C and D are estimated as 12
‘days old plus. B is the Miller “ovum,” while D is the Werner (Stieve)
previous cases, a mere covering for the sac; instead, it forms a thick
stalk, the body-stalk, or umbilical cord, which attaches the embryo to
the chorion or wall of the blastocyst. Into the proximal end of the mesoclermal cord, the hollow endodermal element then projects for only a
short distance (Figs. 279 and 280). This condition is brought about as
follows:
From what is known of the earliest human embryos (7-15 days, see
i ‘ below I} the blastocyst, following cleavage and gastrulation, contains the
§ _ following structures and materials. First there is the blastoderm, con” E ‘ sisting of a layer of ectoderm and endoderm with a small amniotic cav;  ity derived appariantly from a split in the embryonic knob (Method II,
Type b, seeahove): Second, the greater part of the blastocoelic space is
,Ԥ
THE PLACENTALIA 54,9
occupied by a reticulate material, the magma reticulare, which probably
consists of coagulated protein containing fluid. Scattered through this
reticulate substance, and lining parts of the trophoblast, are a few mesoderm cells ':(extraembryonic mesoblast) presumably derived from the
blastoderm L,‘( Fig. 281, A, B). At about the center of the blastocyst in
these human specimens there occurs a particularly definite space
bou_nded laterally and ventrally by an especially clearly defined layer
of the reticulum, termed the exocoelomic membrane or Heu.ser’s mem
Remnant
exocoelamic membrane
Fig. 282,-Mid-sagittal section of a Human blastocyst and
surrounding uterine wall with an estimated age of 15 days, the
Edwards-.lones-Brewer “ovum.” After Hertig and Rock.
brane (Fig. 281). Dorsally this space is lined by the endoderm of the
blastoderm, and it has therefore been interpreted by some as the yolk— '
sac. Others maintain that the true yolk-sac does not appear until slightly
later, about the 13th day. It is difiicult, however, to distinguish the
_ “endoderm” of this later yolk-sac from the exocoelomic membrane
bounding the central “ exocoelomic space ” of the earlier embryos. At
all events in these later stages the magma reticulare has mostly disappeared and the trophoblast is lined by a definite layer of mesoderm.
This also extends around what is now termed the yolk-sac, up over the
amnion, and at what proves to be the posterior end of the embryo, serves
to attach the blastoderm to the trophoblast (Figs. 280, D; 281, D; 282).
This mesodermal attachment later comes to constitute the umbilical
stalk already referred to, and into it there presently grows a small outpushing from one side of the sac where the latter joins the blastoderm.
It is the beginning of the very small allantois (Figs. 279, 280, D, E).
550 EARLY MAMMALIAN DEVELOPMENT
Although at first located somewhat dorsally, the embryonic end of the
stalk soon moves around so as to be attached to the embryo on its ventral side. It retains, however, its original point of attachment to the
chorion since it is here that the placenta is to be formed.” From this
description it is evident that in the Primates, the allantois, or more
strictly in this case, the umbilical cord, does not grow out from the embryo to the trophoblast. It is there from the first.”
As concerns the yolk-sac, it is only necessary to state that it is very
rudimentary, having little or no function. The space which might otherwise be occupied by these appendages, however, is eventually filled in
this order by a very large amnion.“
Implantation and Placenta Formations-—According to previous accounts ovulation occurs following what amounts to a pro-oestral uterine
hypertrophy, and the blastocyst reaches the uterus while the latter is
under the influence of the progesterone of the succeeding corpus luteum.
Here implantation takes place through the erosion of the hypertrophied
endometrium by the newly arrived blastocyst between one or ‘two weeks
following ovulation. This is of course previous to the time of the menstruation which would have occurred had pregnancy not intervened.
As in the case of the Rodents the details of the implantation process
vary somewhat. In this instance, the chief variation occurs. so far as is
known, between two groups, i.e., Tarsius, together with the other lower
Monkeys, and the higher Apes, together with Man.
As regards the first group, i.e., that of Tarsius and the Monkeys, the
description may be brief. The region of implantation may occur on the
dorsal or ventral wall of the uterus, depending upon the form in question, and is not marked by either pits or folds, as in the Rodents. When
" In Tarsius the placenta is’ formed on the opposite side of the blastocyst, and
the stalk shifts its point of attachment to the trophoblast accordingly. V
‘3 In a more recent human specimen. the Martin-Falkiner blastocyst C38),
estimated at seventeen days of age, a somewhat different theory is expressed concerning the development of these structures. These investigators seem to think that
both the yolk-sac and allantois may arise as vesicles developing in the inner cell
mass itself, and that they may later all run together. If this is true it involves a
somewhat novel method of gastrulation, and a peculiar fate for the allantois. Since
there is some question about the normality of this embryo, theories based on it
should await confirmation from the study of more specimens.
“ Though not (iertainly known, it appears that the amnion in the Primates (excepting the Lemurs, in this instance including Tarsius) is formed in a manner
similar to that described under method II, i.e., by the development of a cavity in
the embryonic knob!" The process in this group differs from that described under
types I) or c of the second method, however, in that in this case the embryonic
knob does not move down to the opposite side of the blastocyst.
THE PLACENTALIA 551
the trophoblast of the blastocyst comes into contact with the hypertrophied uterine endometrium it promptly erodes the epithelium. A discoidal placenta which is very similar, if not identical, with that described for the Rodent, then develops at the place in question. Later, a
Fig. 283. —— Development of the fetal membranes in Tarsius. From Jenkinson (Vertebrate Embryology). After I-lubrecht.
a. Blastocyst before Rauber’s cells have disappeared. I). The embryonic knob
(e.k.) is being folded out to the surface; the yolk-sac is complete. c. The embryonic
plate (c.p.) is at the surface, the extra-embryonic coelom (c) is formed. (1. The
tail fold of the amnion is growing forward (t.am.), the allantois (all.) has pcnc-'
trated the mesoderm of the bodystalk, a placental thickening has been developed
at the anti-embryonic pole. e. The amnion is closed and the body-stalk or umbilical
cord (u.c.) is shifting its position, to be attached to the placenta (pl.).
second similarly shaped placenta may form where the blastocyst comes
in contact with the opposite side of the uterus. The umbilical cord, of
course, reaches only one of these, but the two are connected by blood
vessels (Fig. 283, only one placenta in this case). ‘
Considering now the second group, i.e., the higher Apes and Man, it
unfortunately happens that as regards the earliest ‘stages relatively little
is definitely known, chiefly because of the scarcity of material. Some of
552 EARLY MAMMALIAN DEVELOPMENT
the earlier classic cases which have been studied comprise the Miller
blastocyst Streeter (’26) with an estimated age of ll days and a diameter of 0.4 mm., the Bryce-Teacher blastocyst, estimated age 12—14 days,
diameter 0.64 mm., and the Peters blastocyst, estimated age 14-15 days.
diameter 1.1 min.” Somewhat more recently others have been added to
m. b.v. _ d. b. tr.
d. r. ep.
Fig. 284.——Early Human embryo with its membranes. From Jenkinson (Vertebrate Embryology). After Peters. "
am.c. Amniotic cavity. c. Extra-embryonic coelom. d.b. Decidua basalis (serotina). d.r.ep. Uterine epithelium covering the decidua reflexa or capsularis. l. Lacuna in trophoblast (tn). gl. Uterine gland. m.b.v. Maternal blood-vessels opening
here and there into lacunae. cl. Clot marking (probably) the point of entrance of
theblastocyst; here the uterine epithelium is interrupted. y.s. Yolk-sac.
the list, all of about the same or slightly greater estimated age. Thus
there is the Werner (Stieve) blastoeyst at 12 days, and the EdwardJones-Brewer blastocyst (Brewer, ’37) at 15 days with internal dimensions of 1.85 x 1.71 x . 1.01 mm., and the previously mentioned
Martin-Falkner hlastocyst, estimated age 17 days with possible abnormalities. Latest of all, are the Hertig-Rock blastocysts, one of which (not
shown in the figures) is estimated at about 7 days, the youngest yet dis
15 Whether some of these specimens have quite reached the blastocyst stage is
perhaps open to question: but they are certainly not “ ova ” as they have sometimes
been designated. ' '
v
»
__ .... _,.,, A,.,._
THE PLACENTALIA 553
covered (Hertig and Rock, ’4l; Figs. 281, 282). The additional data
from all the clearly normal sources, however, has not substantially modified the conclusions previously held concerning the early stages already
described, and the processes about to be discussed. From information
obtained from these early specimens, and from conditions which are
known to exist later on, implantation and development both in Man and
the higher Apes is thought to be as follows:
The blastocyst usually becomes attached to the dorsal (i.e., posterior)
wall of the uterus in Man, and to the ventral (i.e., anterior) wall in the
Apes; here the trophoblast promptly starts its work of erosion. In this
case, however, the process goes much further than in the instances so far
noted. In fact, it is thought that by this means the blastocyst becomes
completely buried in the mucous layer of the uterus, while the epithelium closes behind it. It thus virtually occupies the position of an
internal parasite within the uterine tissue (Fig. 284). As growth now
proceeds, the blastocyst, covered by a layer of uterine mucosa and some
epithelium, begins to project into the cavity of the uterus. Meanwhile, it
appears that changes are taking place in the trophoblast, or chorion, as
it may be called, quite similar to those which occurred in the Rodent,
i.e., a thickening, and the formation of lacunae. In this case, these processes by which the trophoblast is thus converted into the trophoderm at
first occur on every side of the blastocyst. Presently, however, the trophodermal development becomes much more marked on the inner side,
i.e., that side away from the cavity of the uterus, and it is here that the
permanent discoidal placenta is soon formed.
Throughout the trophoblast or chorion (now trophoderm) but especially on the placental side, the embryonic blood vessels, surrounded by
a sheet of connective tissue (chorionic mesoderm), are working their
way among the lacunae, into some of which they project. These vessels
and their connective tissue are covered with a’ thin trophodermal cell
layer known in human embryology as the cell layer of Langhans. Outside of this, there is an added layer of the trophoderm which is syncytial,
and is apparently derived from the cells of Langhans, the latter being
gradually used up. Thus, where the blood vessels, pushing their trophodermal and mesodermal layers before them, project into the lacunae,
they have something like the appearance of villi, and are often so referred to (Fig. 285). It should be clearly understodd, however, that
these “ villi” are in no sense homologous with the true villi described
in connection with the indeciduate placenta of the Ungulates. They are
not indeed essentially different from the capillaries‘ which push into,
554 EARLY MAMMALIAN DEVELOPMENT
Fig. 235.~— Diagrams illustrating the development of the “villi” in the Human
placenta. From Kellicott (Chonlate Development). A, B. After Peters. C. After
Bryce. A. Chorionic mesodetm just beginning to extend into the villi. B. Mesoderm
invading the villi which are now branched. Layer oi Langhans cells forming beneath the syncytintrophoderm. C. Continued branching of the villi, all now covered
only by the syncytiotrophoderm and the single layer of Langhans cells.
_ b. Decidua basalfs. cb. Capillaries of the decidua basalis. cv. Capillaries of the
villi. e. Endothelium of the maternal capillaries. f. Fibrin deposited at the junction of the trophoderm and decidua basalis. i. lntervillous cavity (i.e., lacuna or
sinus) filled with maternal blood. L. Langhans ‘cells. In. Chorionic mesoderm. s.
Syncytiotrophoderm. t. Trophoderm. 1:. Villi. vf. Fixation villi, i.e., those which extend clear across a sinus.
THE PLACENTALIA 555
Fig. 286. —A. A diagram of an idealized section through the inner portion of the
wall of the non-pregnant uterus a short time previous to the beginning of menstruation. The muscular layer is very thick, and only a small portion of it is shown.
Beyond this layer on the outside of the uterus would come the peritoneal covering
or serous membrane which here as elsewhere is quite thin. B. A diagram of a similar section through the Human placenta at a slightly later stage than that shown
in Fig. 2§S {according to Jenkinson). The trophoderm, it will  mired, has pen.
etrated slightly into the compacta in this stage, so that the_ villi are more firmly
attached. Note that these “ villi ” are quite different in their relation to the niaternal tissue from that observed in the Ungulates, (Compare Fig. 271). No attempt
has been made to distinguish between affereiit and efierent hlood vessels, although
itdis to be understood that both types exist on both the embryonic and maternal
si es.
.bc. Blood capillaries in the mucosa. c.l.L. Cell layer of Langlians, still clearly in
evidence. Chr. Chorion consisting of trophoderm plus extra-embryonic imz.-tvoderm.
co. Compacta. d. Decidua; for explanation of terms see further in text. f.bz-. Fetal
blood vessels. m. Muscular layer of uterus, or muscularis, ,only a small portion of
which is shown. mbv. Maternal blood vessels. n.ugl. Necks of uterine glands in the
compacts. s. Sinus lined by syncytial trophoderm, and filled with maternal blood.
That the syncytial layer and cells of Langhans line the sinuses on the side of the
decidua is questioned by some authors. sp. Spongiosa. str. Syncytial trophoaerm.
tunes. Tgophodelrrlrlial ”(chorionic) rnesoderm. u.ep. Uterine epithelium. u.gl. Uterine g an s. v. i us.
1
556 EARLY MAMMALIAN DEVELOPMENT
and are hence covered by, the trophodermal material in the Mouse or
Rabbit. As regards the lacunae, they are again filled with maternal
blood, and are often termed “ sinuses.” They also are lined by a syncytial layer of the trophoderm augmented to some extent by a layer of the
cells of Langhans, similar to, and continuous with, that which covers the
connective tissue of the fetal
capillaries (J enkinson) .
Outside of the discoidal placental region, the whole blastocyst is growing out so as to
fill the"cavity of the uterus
(Figs. 287 and 288) . Its wall
in this area consists internally
of extra-embryonic mesoderm,
and externally of the trophoderm, the two together as
usual constituting‘ the chorion,
while within this chorionic
trophoderm the “ villi ” and
lacunae are only slightly developed. Lastly, tightly adherent to, and covering this
trophoderm, comes the uterine mucosa and epithelium
which covered the blastocyst
after its embedding in the
Fig. 287.——-Human embryo of the fourth uterine wall‘ A5 growth con’
month in ulero, showing the arrangement of tinugs, this epithelium is even.
the membranes and placenta. From Kellicott
(Chonlate Development). After Strahl. many bound to come in con‘
c. Chorion and amnion. p. Placenta. LL. tact with that which lines the
Umbilical Cord‘ walls of the uterus at other
points. By the time this occurs; however, the uterine epithelium and
mucosa covering the growing blastocyst has become distended and is
disappearing. Thus the trophoderm of this region is brought into direct
relations with the epithelium which elsewhere still remains on the walls
of the uterus, and this epithelium too presently disappears. Concurrent
with the complete filing of the uterus and the disappearance of all its
epithelium the chorionic layer of the blastocyst is everywhere united to .
the sul)-epithelial mucosa of the uterine wall. It is only in the region
THE PLACENTALIA 557
I.-u.
d. v.
Fig. 288.—Diagrammatic section through the pregnant human uterus and embryo at the seventh or eighth week. From Jenltinson (Vertebrate Embryology).
After Balfour, after Longet.
am. Amnion. a.m.c. Amniotic cavity. The latter has enlarged until it occupies
nearly all of the extra-embryonic coelom (c), the amnion being reflected over the
umbilical cord (u..c.) and yolk-sac (y.s.). The yolk-sac, it will be noted, is very
small. d.b. Decidua basalis (serotinal, in connection with which the trophoderm or
chorion, represented everywhere by fine stippling, gives rise to the placenta. Thus
the chorion in this region is the chorion frondosum. d.r. Decidua capsularis (refiexa), consisting of a thin layer of. uterine epithelium and mucosa. It soon disappears, exposing the vacuolated trophoderm (chorion) beneath, which in this region
becomes the chorion laeve. d.v. Decidua vera, whose epithelium also disappears
when the trophoderm beneath the capsularis (chorion laeve) comes in contact with
it. Lu. Lumen of uterus, presently obliterated. o.d. Oviduct whose direction in the
non-pregnant uterus would be nearly horizontal. pl. Placenta; for details see Fig.
of the placenta, however, that the chorion normally continues to be
vascularized and to thicken by the growth of villi.
The placenta, as so far described, consists then essentially of a greatly
thickened layer of trophoderm containing lacunae or sinuses filled with
maternal blood, while into and across these sinuses extend chorionic
processes or “ villi” containing fetal connective tissue and capillaries.
The layer thus indicated is obviously essentially tissue of embryonic
origin, and is sometimes known as the “ placenta proper.” Between it
and the muscular wall of the uterus there still exists a certain amount of
558
EARLY MAMMALIAN ‘DEVELOPMENT
the uterine mucosa, i.e., that part of the mucosa which the trophoderm
has not destroyed. It now remains to state that in some of the higher
Apes and Man (as well as in certain of the lower animals already discussed, e.g., the Cat) this portion of the ‘mucosa is itself differentiated
Fig. 289. —— Reconstruction of a human embryo of 2.6 mm. From Minot
(Laboratory Text-Book of Embryology). After His.
/1. Aortic limb of heart. All. Bodystalk. A0. Dorsal aorta. Au. Umbilical arteries. Car. Posterior cardinal
vein. Jg. Anterior cardinal vein
(internal jugular). Om. 0mphalomesenteric vein. op. Optic vesicle.
or. Otocyst. V It. Right umbilical vein.
This completes the description
into two main layers. The outermost
of these layers adjacent to the muscularis is filled with glands, and is
known as the spongiosa. The second
layer, to which the trophoderm is
firmly adherent, and in which it is in
fact slightly embedded, is occupied
by the straighter smaller portions of
these glands, i.e., their necks, and is
called the compacta (Fig. 286).
Moreover, the compacta and spongiosa not only exist in the region of
the placenta, but likewise at all other
points around the uterine wall.“
Thus, when the non-placental trophoderm of the enlarging blastocyst
eventually comes into contact with
this wall from which the epithelium
soon disappears as indicated in the
preceding paragraph, it becomes here
also adherent to the compacta. During the later stages of pregnancy,
both the compacta and spongiosa
tend to degenerate and to become
stretched and thin. It is then through
the region of either one or both of
these layers that the tissue breaks at
the time of parturition.
of the placenta and the adjacent re
gions in Man and the Apes. It remains, however, to indicate the names
by which the various parts are known in human embryology. To understand the significance of this nomenclature, the student must bear in
mind the older idea that placentas of this type were truly deciduate.
16 The spongiosa and compacts indeed occur not only in the pregnant Primate
uterus, but in the non-pregnant uterus as well, particularly just previous to men
struation.
THE PLACENTALIA V p . 559
That is, it was thought that a large part of the uterine wall was deciduous, i.e., torn away or shed at parturition. Hence those layers of the wall
(i.e., the mucosa) which were supposed so to behave were termed the
decidua. Also in correlation with this idea, most of the placenta and the
covering of the blastocyst was supposed to be formed out of this decidua,
rather than out of trophoderm. With this in mind, the reasons for the
following names are fairly
evident:
That part of the uterine
wall to which the placenta is
attached is known as the decidua serotina, or decidua
basalis (Fig. 288). The portion of uterine mucosa and
epithelium which, during the
earlier development, covers
the blastocyst on the side opposite the placenta, is called
the decidua reflexa or decidua
capsularis. That is, this portion is, as it were, reflected
Fig. 290.—Human embryo of about 23 days
(4.0 mm.). From Minot (Laboratory Text
over the blastocyst, forming 300/t of Embfyolvgfb After His ‘Emb1‘:v'0 0)
. dl. Fore-limb bud. BS. Body-stalk. Op. Op31 cover or capsule for It‘ tic vesicle. pl. Hind-limb bud. IV. Fourth ven
L t1 the 1-emainin art of tricle of brain. 1. Mandibular process. 2. Hythaes uiérine wan witghpwhich oid arch. 3, 4. Third and fourth visceral
arches.
the thin chorion, now lack
ing the overlying decidua reflexa, finally comes in contact, is known as
the decidua vera, and as this contact occurs the decidua Vera disappears down to the compacta. Not only are the parts of the uterus thus
named, but the parts of the chorion are also defined. That part which
forms the placenta and adheres to the decidua serotina is termed the
chorion frondosum. The remainder, at least after its loss of the first
slightly developed “ villi,” is the chorion laeve.
Comparing the means of embryonic nourishment in the Primates with
those in the Rodents, there appears at least one notable difference. In
the Rodents the yolk-sac probably plays at least some part in obtaining
nutriment for the embryo throughout development; in“the Primates (except the Lemurs), on the other hand, this function, as well as that of
respiration, is entirely subser-ved by the placenta. Coming to the actual
structure of this organ itself, there exists a striking similarity between
560 EARLY MAMMALIAN DEVELOPMENT
the two orders. There is also, however, a slight difference here, which is
perhaps worth noting. At the time of parturition in the Rodents scarcely
any maternal tissue, save blood, is lost, and hence the placenta is not at
all deciduate in the strict sense of the word. In the Primates, on the
other hand, there is a certain amount of the compacta and perhaps of
the spongiosa lost at birth, and this is maternal tissue. Hence the Primate placenta, at least to this slight extent, may be said to be truly deciduate. The body—stalk in the two groups is in general similar in lack
ing any extensive endothelial element. As has been noted, however, its
method of formation is different.
15
EVELOPMENT OF THE PIG TO THE TEN MILLIMETER STAGE
I N the preceding comparative discussion of the early stages of various representative groups of Mammals we have carried the history of
the Pig in particular to about the thirteenth day of its development.
This means of course thirteen days from the time of fertilization in the
upper part of the oviduct. During this time, as we have seen, the egg has
reached the uterus, developed into an elongated blastocyst, and the
blastocyst is becoming implanted. The embryo itself is represented by a
blastoderm in which a primitive groove and notochord are evident, and
in which the three primary germ layers have already been diHerentiated as previously described. The nature of the archenteron, and its re
lation to the blastocoel has also been indicated.
Having reached this point, we are now prepared to proceed with a
description of the further development of this animal. In doing so we
are once more faced with the problem of whether to describe the complete development of one system at a time, or to carry all systems along
together as it were, in a series of stages. For fairly obvious reasons it is
not practical in the case of the Mammal to proceed very far by daily
periods. Furthermore, through study of the Frog and Chick we are now
familiar enough with the vertebrate plan of development so that we are
aware in a general way of what other systems are doing while we concentrate our attention upon one. For these reasons a sort of compromise
between the system plan and the stage plan becomes possible. Beginning
at the present point therefore we shall carry each system of the Pig to
completion in two main steps. The first step will take us to the condition which exists at the 10 mm. stage (20-21 days), a condition more
or less comparable with that of a 4-5 day Chick. The second step will
then bring the system in question to completion, or as near to it as it is
necessary to go. As we proceed with these steps, however, it is desirable
from time to time to mention the number of somites present, and also
the approximate length of the embryo. In the latter connection certain
facts concerning the general form of the animal need to be mentioned,
562 THE PIG TO TEN MILLIMETERS
and we shall take those up at this point, together with a few comments
on other external features.
Embryonic Flexions and Rotation. — As in other Vertebrates, so
in the Pig, the very early stages pose no question as to what line constitutes the longitudinal embryonic axis. This is obviously indicated by
the line of the primitive groove and notochord, and presently also by
the line of the fused neural folds, and the contours defined by the folding oil of the embryo. This simple condition persists up to about the
ten somite stage, when the
embryo is approximately
fifteen days old and measures from 3 to 4.5 mm.
in length (Fig. 291).
Shortly after this, how="¢U"3' §"°°V° ever, as in the Bird, vari
 
 
     
 
T‘ eural fold
Cgtedgfi “ ous curvatures begin to
o ammon .
:- r , , develop, and certain flex
; smus rhomboldahs . _ d
pmnmve streak: ures are again recognize .
The cranial and cervical
flexures are the same as
in the Chick, and in addi
Fig. 291. — Surfacfi View of a Pigf ernbrylp fofd7 {ion two others are named
somites (3 mm.), 5 owing c osing o neura 0 s. . . .
Amnion removed. After Keibel. whlch mlght 3150 be de5‘g'
nated in the Bird, but usually are not. These are the dorsal and lumbo-sacral flexures which refer
simply to the successively more posterior parts of the continuous curvature. The caudal flexure mentioned in the account of the Chick also
exists in the Mammal as a continuation of the lumbo-sacral flexure, but
is not generally especially designated (Fig. 294-). It should also be
noted thatlfor a brief interval before the caudal and lumbo-sacral flexures develop there is, as was also true of the Chick, a slight ventral bend
in the m_id—body region due again apparently to the pull of the yolkstalk (Fig. 292). This, however, is quite transitory. As soon as these
curvatures develop the question at once arises as to which of the infinite
number of straight lines which might be drawn through the embryo is
to be designated as its length. In Mammalian embryos, including Man,
there are two such lines which are quite commonly used. One is a line
passing from the most anterior point of the cranial flexure (mid-brain)
posteriorly through the “ rump.” The latter may be defined as a point
at about the middle of the convexity of the lumbo-sacral flexure, i.e.,
3
EXTERNAL FEATURES 563
somewhat posterior to a point dorsal to the origin of the hind-limbs.
This line of measurement is the crown rump axis. The other is a line.
from the posterior side of the cervical flexure, i.e., just over the ear,
anteriorly, and again terminating at the rump posteriorly. Because of
the position of the anterior point above the ear this may be called the
auricular rum p axis. All measurements referred to in this account will
be those of the straight embryo previous to the development of its flexures, and
later those of approximately the crown rump
axis.
In this general connection one further matter pertaining to the curvatures of cut edge
Mammalian embryos may ‘a’:‘3':h";‘:i';n;.
be mentioned, though it i»
has no reference to the E
problem of measurement.
It will be recalled that
when the Chick developed
its various flexures it also _ 1
acquired a lateral rotation .  under chorion
or torsion. In that case this i " ‘
rotation prevented the
burying of the anterior end Fig. 292. -— Surface view of a Pig embryo with
. about 16 somites (4.5 mm.), showing outpush“1 the yolk‘ In the Mam‘ ing of allantois beneath chorion. After Keibel.
cut edge
of yolk sac
 
mal of course there is no
yolk, but it is an interesting fact that the lateral torsion still takes place
to some degree (Figs. 292, 293). It is quite variable, as all vestigial structures and activities are apt to be, and soon vanishes entirely.
Other External Features.—Finally before proceeding to a dis-.
cussion of the specific systems a few further remarks are pertinent with
regard to general external features, aside from the various curvatures.
As will be apparent from Figure 294, four visceral arches and four
“ clefts ” are in evidence, while about the two posterior clefts is a general depression termed the cervical sinus. As sections‘ reveal, however,
these are not true cleft's since they do not normally actually open
through into the corresponding visceral pouches, but- it is convenient to
refer to them as such. Also from the figure itmight at first be supposed"
564 THE PIC TO TEN MILLIMETERS
that there are five-clefts and five arches rather than four. The apparent
first cleft, however, is really the space between the maxillary process
and mandibular arch, and is therefore not counted as a cleft, nor is the
maxillary process an arch. Immediately anterior to the maxillary process is still another depression separating this process from the front
parts of the face (see below). This depression is the lachrymal groove.
At its dorsal end is the eye, and at its ventral end the nasal pit. In this
connection it may be appropriately noted that one of the few rather
striking difierences between the appearance of the head of a 4-5 day
Chick and that of a 10
 
hyomndibuhr def‘. ':- mm. Pig is the much
auditory Pit Jolt ,3; greater size of the eye in
2nd  optic vgfldg the Bird.
"l“°"l‘l°f“ 3rd 0" i Viewing the embryo
somites'  llnmd‘ from the front it will fur.
amnion;
ther be seen (Fig. 295i
_‘ that antero-ventral_t.o,the
; eyes, between them and
‘ the olfactory pits, lie the
naso-lateral processes,
which as in the Bird
bound the pits laterally.
Medially the pits in the Pig are bounded by the naso-medial processes,
structures not indicated in the Bird. A comparison of these forms, however, reveals that these last named processes are really only special differentiations (prominences) of the lateral parts of the naso-frontal
process, which in the Chick is shown bounding the pits on their medial
sides. In the Pig the region between the naso-medial processes, i.e., the
middle of the “ naso-frontal process ” is sometimes termed simply the
frontal process. However, this region is soon (10 mm.) merged with
the naso-medial processes which may then be said to join each other in
the mid-line. The oral cavity of the Pig soon appears therefore as an
opening immediately beneath the fused naso-medial processes. This cavity as usual is bounded ventrally by the mandibular arches, while the
maxillary processes are pushing into it from either. side. The latter are
separated from the naso-lateral processes by-the lacrymal groove.
Finally, among external features of the 10 mm. Pig, are the prominent paddle-like fore and hind limb buds and the numerous well-marked
somites. Both of course are highly reminiscent of the appearance of
these structures in the Chick in a corresponding stage.
 
Fig. 293.-—Surface view of a 3.5 mm. Fig embryo
with chorion removed to show allantois. After
Keibel.
NERVOUS SYSTEM: EARLY DIFFERENTIATION 565
THE NERVOUS SYSTEM
As in the case of the Chick, much of the general form of the early
mammalian embryo, as well as various prominences appearing upon it,
are determined by the developing nervous system. It is therefore convenient to consider this system first.
Illrd viscera! arch
h 'd
Nth visceral arch yo. arch
 
 
 
 
'mandibu|ar arch
cervical sinus
forblimb bud - maxillary process‘
33% 5. :2 6””
7 mm. erribryo
Fig. 294.—Lateml View ‘of a 7 mm. Pig embryo with amnion and
chorion removed.
EARLY DIFFERENTIATION
The System as a Who1e.—The nervous system first appears in
embryos of about 2 mm. as the usual groove in an ectodermal medullary plate immediately anterior to the primitive streak (Fig. 264).
Slightly later definite folds arise upon either side of this groove in essentially the same way as in the Bird (Fig. 291). The location where
the folds most closely approach each other represents the future hindbrain region, while the wide open part immediately anterior to this is
the future fore-brain. The neural tube proper is obviously not yet repre566 THE PIG TO TEN MILLIMETERS
sented, which means that the anterior parts of the system are as usual
the first to form, and as in other cases maintain their advantage in precocity till very late in development. It will be noted that the chief difference between the situation in the Chick and the Pig at this stage is the
wider flare of the folds in the anterior region of the latter. Slightly later,
 
   
 
frontal PTOCCSS
olfactory plt
naso-lateral process _ '
- naso~medlaI process
maxillary process
mandibular arch
hyomandibular clef:
hyoid arch
lllrd viscera! arch 1
from 7 mm. embryo
Fig. 295.——Antero-ventral view of the head of a 7 mm. Pig embryo
showing parts constituting jaws and face.
at about 10 somites, another difference becomes evident in that, as previously stated, the optic vesicles of the Pig are much less prominent than
were those of the Chick at a comparable stage, and this remains true
throughout the earlier periods of development. As will be apparent from
the figures, these vesicles, at their earlier stages, are also somewhat differently shaped from those of the Bird.
DIFFERENTIATION TO TEN MILLIMETERS
The Brain. —— Following this early condition the cranial flexure
makes its appearance (13 somites), and shortly thereafter the cervical
and caudal flexuresiare also under way. Thus by the 25 somite stage the
anterior extremity is almost touching the heart in about the manner of
NERVOUS SYSTEM: TO TEN MILLIMETERS 567
a 48-hour Chick with the mid-brain at approximately the most anterior
point of the embryo. By this time also the various divisions of the brain
are evident, and are the same as those in the Bird, i.e., the prosence phalon, mesencephalon and rhombencephalon. As will presently be noted
these main parts are soon further subdivided, and give rise to the same
structures as enumerated in the previous form. Thus at 10 mm. (Figs.
296, 297) about the same degree of development of the brain exists,
with the same parts in evidence as in a 4-5 day Chick. The proscncephalon is divided into telencephalon and diencephalon, and the former is
giving rise to outgrowths (telencephalic vesicles) which will become the
cerebral hemispheres. The diencephalon, which is separated from the
telencephalon by the same features as characterized the Bird, has, as
before, given rise to the optic vesicles and the infundibulum. The chief
difference between this part of the Pig brain at this time, and that of the
4-5 day Chick, is the lack of an epiphysis in the Pig, in which it does
not appear until considerably later. The mesencephalon is as usual 3.
very prominent region whose protruding anterior side marks the apex
of the cranial flexure. It is, however, not so well developed as that of the
Chick at a corresponding stage. This is correlated with the fact that this
region is the site of the future optic lobes of the Bird, which are more
prominently developed than the partially comparable ‘corpora quadrigemina of the Mammal. A sharp fold, the isthmus, separates the mesencephalon from the following rhombencephalon, and the division of ‘the
latter into metencephalon and myelencephalon is now distinguishable
by the thickened sloping roof which characterizes the former (Fig. 297).
The Neural Tube and Crests. —- Passing posteriorly we find that,
as in the Frog and Chick, the neural tube has been formed by the closing neural folds so that its dorsal and ventral walls are thin and its lateral walls relatively thick. By the 10 mm. stage the cells in these walls
are becoming differentiated into several different types, some of which
have already been mentioned in the case of the Chick. Near the delicate
internal limiting membrane lining the neural canal the original germinal cells have given rise to spongioblasts and the latter to supporting
cells with long fibers running toward the outer periphery of the cord.
Again as in the Bird these supporting elements are called ependymal
cells. The larger part of the cord, however, is occupied at 10 mm. by
the mantle layer, consisting of other germinal cells in process of further
division and differentiation as follows: Some of the germinal cells become spongioblasts which in this layer eventually form other types of
supporting cells known as short and long-rayed astrocytes. The remain568 THE PIG TO TEN MILLIMETERS
tier of the germinal cells in the mantle layer are neuroblasts which later
differentiate into actual nerve cells. Finally outside the ependymal and
mantle layers, beneath a thin outer" limiting membrane, there occurs a
non-nucleated region termed the marginal layer. Because of the lack of
myelencephalon Vlllth and Vllzh (genleulate) ganglia
 
   
 
 
lX“(‘ 8‘"8l:)°" audkor), “Sid, metencephalon
Xth gangllon jugula . '¢ Vth(Gasserian) ganglion
' A lVth nerve
Xlth spina'laccess_ory nerve '5ml°'::;";‘°:;:l°"
Fig.1” Frorlep sgangluo . F3’. 2”
xnth-he.-.,¢.,-00;; - ophthalmic nerve
petrosal ganglion
. maxillar nerve
ganglion nodosum  Y
‘ 'diencephalon
Fig. 302 ' F?s- 302
xth nu,” / Rathke's pocket
esophagus ‘ Seesell‘s pocket
' OptIC cup
Fis_ 3°“ ‘ ' Fig. 301
_mng telencephalon
F‘ 305 W c:narndIib‘ular n rveF;'_ 305
---?{7;--— ° 3 0 Y P yuzel Inelyolldstalk
533.306 Fin» 306
t ch " - _
*a:t":..a 3» 3;;
d°rsa s allantoie stalk
, Fig. 3l0
FIg.3l0
l ncreas .
ventrzamabladder postncloacal gut.
Fig. an “9~ 3"
:a,.:m “-9- 313
l
spinal ganglion C mm
mesonephros
nephrogenous tissue of metancphr
mcsonephricvduct
Fig. 296.——Reconstruction of a 10 mm. Pig embryo, designed to show primarily
the main features of the nervous, digestive, respiratory and excretory systems at
this stage. Drawing made chiefly from a study of sections, with aid from a wax
reconstruction produced under the author’s direction in the Oberlin College Zoological Laboratory. Lines at the sides with figure numbers over them indicate where
the sections represented in these figures pass through the embryo. By laying a
ruler along any pair of lines the structures cut by the respective section may be
seen. *
nuclei, it stains very lightly compared to the darker more central regions. It will further be noted in sections of the 10 mm. Pig that portions of the mantle layer extend ventro—laterally somewhat, causing the
lower sides of the cord to bulge slightly. These extensions are the beginnings of the ventral horns (Fig. 298).
Aside from the cord itself it will be found, as in the case of the Frog
NERVOUS SYSTEM: TO TEN MILLIMETERS 569
and Chick, that as the neural folds come together a hand of cells is
pinched off between the tube and the overlying ectoderm. The cells of
this band soon become concentrated on either side to form the continuops neural crests. The latter are then further concentrated segmentally
pharynx metencephalon
 
 
 
 
   
 
 
 
notochord
mesencephalon
Ra:h$<e's pocket
v Seesell’s pocket
./ optic chiasma
, Optic recess
lamina terminalis
um bi Iical
artery
vltelline vein
posterio
vena cava
mesoneph ros
dorsal root ganglion
Fig. 297.-—Mid-sagittal section of a 10 mm. Fig embryo.
to form the groups of neuroblasts which develop into the spinal ganglia.
By the 10 mm. stage each such ganglion is clearly defined, and has
given rise to the dorsal roots of the spinal nerves which are definitely
connected with the cord. y
The Cranial Nerves. —— In the 10 mm. Pig all the cranial ganglia
-and nerves are represented except the I or oljactorf, and the II or optic,
the optic stalk not yet containing any actual nerve fibers (Fig. 296).
570 THE PIG TO TEN MILLIMETERS
The III or oculomotor nerves can be plainly seen emerging from the
ventral side_ of the mesencephalon, while the IV or trochelar nerves are
just starting from the dorsal side of the fissure (isthmus) between midand hind-brain. The V or trigeminal nerve ganglion of each side appears on the ventro-lateral side of the myelencephalon near its anterior
end. It is united to the brain by a large root, and from it emerges anteriorly the ophthalmic nerve, while more posteriorly and ventrally arise
 
 
 
       
 
external llmltlng membrane
lumen of
neural tube
mantle layer
prlmordlum of
ventral horn
blood vessel
K
I .- 1.1‘
»‘3 ventralnerve root
~:
internal Ilmltlng Vf
membrane
   
   
Fig. 298.———-Transverse section of the center and right side of the nerve
cord and a spinal ganglion of a 10 mm. Pig embryo.
-the maxillary and mandibular nerves. The entire complex lacks the distinct V shape which it had in the Chick due to the large mass of the ganglion proper which obscures the base of the V. More ventral than the V
nerve ganglion, at about the middle of the myelencephalon the VI or
abducens nerve of either side takes its origin, while above it at about
the level of the V ganglion occur the ganglia of the VII and VIII nerves.
These latter ganglia are somewhat dorso-ventrally elongated structures
much less massive than the V. The VII or geniculate ganglion is very
close to the VIII pr acoustic, but is slightly anterior to it, and the
branches of the VII or facial nerve are little developed at this time. The
acoustic or auditory ganglion in turn is in contact with the auditory
vesicle which lies posterior to it, the short branches of the auditory
nerve not being in evidence as yet. There is no single glossopharyngeal
NERVOUS SYSTEM: TO TEN MILLIMETERS 571
ganglion in the Pig. Instead the erve cells which would constitute this
ganglion are divided into two groups, a dorsal and a ventral. The dorsal
group is in close contact with the posterior side of the auditory vesicle,
and is called the superior ganglion of the IX or glosso pharyngeal nerve.
The ventral group occurs both ventral and slightly posterior to the superior ganglion, and is known as the petrosal ganglion of the same nerve.
As in the Chick, the X or vagus ganglion occurring just behind the IX is
also divided into two parts, the ganglion jugulare and the ganglion
lXth
Xth
   
 
}cranlal nerve ganglion
hind-brain (metencephalon)
1.3-‘
branches of anterior cardinal velni
mid—brain (mesencephalon)
Xlth cranial nerve (spinal accessory)
cndolymphatic duct
Fig. 299.—Transverse section through the brain region, including some of the
spinal ganglia, of a 10 mm. Fig embryo. See reconstruction Fig. 296.
nodosum. The former is so closely in contact with the superior ganglion
of the IX at this time as to be scarcely distinguishable as a separate ganglion (Fig. 299). From it there arise two thick strands of nerve fibers.
The more dorsal of these proceeds posteriorly to meet the XI nerve,
along whose posterior part it extends for a way, as the elongated commissural or accessory ganglion. The second strand passes postero-ventrally, and shortly enlarges to form the ganglion nodosum indicated
above. From the latter the vagus nerve containing both afferent and efferent fibers is evident at this stage proceeding toward the viscera. The
fibers of the XI or spinal accessory nerve, already referred to, also pass
antero-dorsally from the nodosum toward the ganglion jugulare along
with those of the X nerve. Before reaching this ganglion, however, these
fibers branch off in a well-defined strand which curves dorsad, and proceeds along the side of the myelencephalon until it ends in F r0riep’s
ganglion. This latter ganglion later disappears, and the nerve is entirely motor. The XII or hypoglossal nerve is also entirely motor, and
J‘:
572 THE PIG TO TEN MILLIMETERS
hence has no ganglion. It arises as a g oup of fibers ventral to the spinal
accessory, and these shortly unite to form a single trunk (Fig. 296).
The Spinal Nerves. — We have already noted the origin of the dorsal root ganglia and the fibers connecting them with the dorsal part of
the spinalicord. These are of course sensory nerves. The ventral root
motor nervefibers originate in the ventro-lateral portions of the mantle
layer of the cord, whence they emerge opposite each dorsal root (Fig.
298). As in the Chick, they then very shortly join the sensory fibers running outward from the dorsal root ganglion, and from near the point of
union three branches arise. The most dorsal branch of each spinal nerve
is a dorsalsomatic ramus, and the middle one a ventral somatic ramus,
both containing mixed sensory and motor fibers just as they did in the
Bird. The third and most ventral branc-h, also as in the Bird, is a ramus
conzmunicans of the sympathetic system, except in the sacral region
whose communicating rami belong to a part of the parasynz pathetic
system. The cell bodies which give rise to the fibers of all these rami lie,
as in previous cases, within the nerve cord, and are known as preganglionic Izeufanes. On the other hand the neurones ( postganglionic) which
constitute the chain ganglia of the sympathetic and parasympathetic systems to which the fibers of the rami run, have as usual migrated thence
from the nerve cord, the dorsal root ganglia, or both. This is also of
course true of the neurones in the various visceral plexuses. In the case
of the Pig, however, it has not been possible to analyze the exact sources
of these postganglionic and visceral neurones as carefully as in the Frog
and Bird. This is because of obvious limitations on experimental procedure. Also there seems to be no data as to whether the permanent system is preceded by a temporary primary one as in the Chick-. Lastly, in
connection with the parasympathetic system referred to above, it may
be noted that the preganglionic neurones of this system not located in the
sacral region, occur in the brain. The parasympathetic and sympathetic
systems together are often referred to as the autonomic system.
One interesting point concerning the spinal nerves which is true of all
the vertebrate embryos with appendages, comes out especially clearly
inrthe 10 mm. Pig. This is the modification in the original strictly segmental arrangement of the spinal nerves. Though this arrangement is
still marked, the fusing of several branches in their respective regions
to form the brachial and sacral plexuses is very evident. Also the caudal
migration of the appendages is indicated by the fact that the branches
which form the respective plexuses arise from regions of the cord considerably anterior to the limbs which they supply. The caudal movement
DIGESTIVE SY STEM: EARLY STAGES 573
of the diaphragm is likewise evidenced by the anterior origin and backward extension of the phrenic nerve it this stage. In later stages this
nerve continues to follow the diaphragm as it moves posteriorly.
The Organs of Special Sense. — As inthe case of the parts of the
nervous system just described, the organs of special sense in the 10
mm. Fig are also developed to about the same extent as those of a 4-5
day Chick. Thus the olfactory pits already noted in the account of the
exterior, are present opposite the prosencephalon. Further back the optic vesicles have formed cups in the usual manner, and each cup is oc} cupied by a hollow sphere of cells destined to become the lens. As in] dicated above, these forerunners of the eye are definitely much smaller
1 relatively than they were in the Bird, but they have formed in the same
; fashion from the same parts. Likewise the auditory vesicles have arisen
on either side of the hind-brain by invagination from the surface ectoderm in a way already familiar. They are about the same shape as those
of a 5-day Chick with the endolympliatic ducts extending dorsalward in
the usual manner. As in previous cases these parts are in close proxim- .
ity to the hyomanclihular pouch which will form the middle ear and
Eustachian tube (Figs. 296, 299, 302).
THE DIGESTIVE SYSTEM
‘ EARLY STAGES
The Primitive Gut and Related Parts. —— We have already noted
, that in the Pig. as in the Chick. the embryo forms from a fiat plate of
cells by a folding off process. Also by the time this occurs the germ lay‘ ers have arisen and the. mesoderm has been more or less completely split
into the somatic and splanchnic sheets. Hence the innermost layers of
the folds which form the gut will consist as usual of the splanchnic mesoderm and the endoderm (splanchnopleure) . As in the Bird, the folding '
off is accompanied by the outgrowth of the distal rim of the fold, especially anteriorly and posteriorly. Thus the fore-gut and hind-gut are
lengthened (Fig. 300). As in the Bird the proximal rim of the fold, on
the other hand, either remains stationary or actually draws together
i somewhat. Insofar as this latter movement involves the splanchnopleure
Q it produces a great relative narrowing of the yolk-stalk or yolk-sac um‘ bilicus (see Chick, Fig. 190), so that the gut cavity is more and more
I
1
sharply separated from the remainder of the extra-embryonic portion of
the archenteron. The folds of the somatopleure of course follow, thus
narrowing also the somatic umbilicus, or as it is called in the Mammal,
the body stalk, or later the umbilical cord.
574 THE PIG TO TEN MILLIMETERS
In connection with this process there are, however, certain differences
to be noted between the Chick an l Pig. In the first place it appears that
the folding off is somewhat more nearly simultaneous anteriorly, laterally and posteriorly in the Pig than it was in the Chick, though even in
the former the head fold is a little precocious. A second difference is perhaps more striking, and has already been referred to. It is the fact that
at a very early stage the mesoderm develops anteriorly as well as lat
amniotic heiad told
,7
"method Mung PI“: amniotic tail fold
' anal plate
   
 
   
A °"3lPlfl€ periczrdtal coelorn Ik mflodflm
yo 5“ endoderm
chcfionk uaphabhn amniotic head fold neural tube amniotic nail fold amnion
 
eczoder  chorionic trophohlasl
,,,,m°n  notochord  . Fla“ cmdum
\ mesoderm
mesoderm ‘_  ‘O \‘
hind-gut
   
yolk sac mesoderm
yolk sac endod:rm/
perlardial coelom
B
Fig. 300. —-— Diagrammatic mid-sagittal sections through early Pig embryos to
show primarily the method of origin of the allantois which is slightly difierent from
that in the Chick. See Fig. 198. Note also the relatively equal growth of the head
and tail amniotic folds as compared with their unequal growth in the Bird.
erally and posteriorly, so that there is no proamnion region which is
free of it. Hence the mesoderm is involved in the head fold of the Pig
from the first, the same as everywhere else. Still a third dilierence between Bird and Mammal has to do with the behavior of the mesoderm
beneath the forming gut. In both organisms it will be noted that as the
lateral folds of the splanchnopleure press toward each other the layers
of endoderm are the first to meet. Wliereupon they fuse and at once close
off to form the completed endodermal tube, save for the opening of the
yolk-stalk. The splanchnic mesodermal layers of the splanchnopleure
meet next and fuse, but do not close off. Instead they remain as a double
sheet, the ventral mesentery, which unites the gutto the ventral body
wall formed by the subsequent fusion of the somatic mesoderm and
ectoderm. In both Bird and Mammal the dorsal part of this mesentery
persists to help support the heart and liver. In the Bird, however, the
most ventral part, i.e., the part which makes contact with the body wall,
DIGESTIVE SYSTEM: EARLY STAGES 575
it may be recalled, almost immediately disappears. In the Mammal, on
the other hand, this part persists much longer. Indeed in the latter, as
we shall see, some of it exists permanently, and we shall have occasion
to return to it later on.
The Yolk—Sac. — While the folding of the splanchnopleure is forming the gut‘ and yolk-stalk, what remains ventrally of the original archenteric space becomes the yolk-sac. The endodermal lining of this sac
mcdullary plate
 
 
splanchnlc _
mesoderm ‘
somatic mesoderm
chorlonlc trophoblast
Fig. 301.——-Transverse section through a Pig blastocyst cutting the blastoderm
and embryo at the level of the second somite. After Streeter, modified to complete
the blastocyst ventrally. The embryo is the same as that reconstructed in Fig. 265.
and measures 1.56 mm. in length.
has of coursebeen completed ventrally by the growth of this layer clear
around the inside of the original blastocoel. The downgrowth of the
mesoderm followed by its split into two layers, however, proceeds more
slowly. Thus there is a time when this split mesoderm is pushing its
way ventrad and medially from both sides, but has not yet met ventrally (Fig. 301). Shortly, however, it does meet, thus everywhere separating the endoderm of the yolk-sac from the trophoblast by a layer
of extra-embryonic splanchnic mesoderm, the extra-embryonic coelom
and a layer of extra-embryonic somatic mesoderm.
The Allantois. — As the above events are taking place (2—4.5
mm.) , it should be noted that at the posterior end of -the embryo a condition exists which at first seems very similar to that which prevailed in
the Bird. Thus as in that case there is the same fold of the splanchnopleure which in the Bird we have called hind-gut, but which some have
576 THE BIG TO TEN MILLIMETERS
chosen to interpret as allantois. So far as the detailed events in this region have been described for the Pig, however, the subsequent differentiation of the actual allantois and the definitive hind-gut appear to dif~
fer somewhat from the history of these parts in the Chick. Thus in the
latter the original fold constituting the primordial hind-gut (by some
labeled allantois) is, according to our previously stated position, only
partly allantoic. This was on the ground that it is not until after the
tail-bud has swung around to the ventral side that a portion of this re
lnrq visceral‘ arch
Xth cranial nerve
end of 4th visceral pouch
 
 
 
mandibular arch
maxillary process
Pl‘3")’"* _/ i « _ -.  , ' i ‘ portion of
' ' ' ' ‘ ' « cerebral hemisphert,
nerve I'O0C
dorsal spinal ,
nerve root ganglia ‘
cervical nerve
 
anterlor cardinal veln
3rd vlsceral clef: hyommdl I "I" dd‘
Fig. 302.—Transverse section through the eye and visceral arch region of a 10
mm. Pig. See reconstruction Figs. 296, 318, 320.
gion gives rise to an anterior outgrowth which is entirely allantoic. In
the Pig, on the other hand, all of the original posterior fold continues
its backward growth to form allantois. Shortly afterward another fold
develops in the dorsal splanchnopleure slightly anterior to the allantoic
outpushing, and grows posteriorly above the latter to form the definitive
hind-gut (Fig. 300). '
FURTHER DEVELOPMENT OF THE GUT
The Stomodaeum. — As in the Chick the fore-gut does not at first
open to the outside. Soon, however, the ectoderm becomes invaginated
to meet the endoderm at a point slightly posterior to the extreme end of
the gut. This invaginated ectoderm is as usual the stomodaeum, and the
double membrane formed by its fusion with the endoderm is the oral
plate. Sometime between the 15 and 25 somite (4.5—-6.5 mm.) stage, this
plate breaks through, and puts the stomodaeal cavity in communication
with the future pharynx. The short portion of gut extending anterior to
the stomodaeum isii temporary structure known as the pré-oral gut, or
FURTHER DEVELOPMENT OF THE GUT 577
in the Mammal as Seesel’s pocket (Figs. 296, 297) .’ The stomodaeum itself later gives rise to the oral region involving the nasal, maxillary and
mandibular processes. At 10 mm., however, the only structure which it
has produced is an anterior outgrowth in the direction of the infundibulum of the brain. This diverticulum, as in the Chick, is Rathke’s pocket,
Fig. 303.——Reconstructions of the developing bronchi of a Pig’s lung at the
stages indicated. After Flint. The arteries and veins, though only labeled in one
figure, are represented in the same manner in each.
and is of course, the primordium of the anterior part of the pituitary.
(See footnote on this topic in the section on the Frog.)
The Pharynx.——This region of the gut is rather shallow dorsaventrally, and at an early stage begins to show the lateral outpocketings
which form the visceral pouches. There are usually four pairs of these
in the Pig, the hyomandibular and three posterior to that pair, though
, the last (fourth) pair aresmall and sometimes entirely lacking (Fig.
302). In a 10 mm. specimen all the pairs destined to appear are well
developed, and have come in contact with the corresponding ectodermal
“ clefts ” (Figs. 294, 296). As already indicated, in the case of the Pig,
it is to be noted that, as in most other Mammals, these regions of con578 THE PIG TO TEN MILLIMETERS
Xth cranial nerve endocardi! cushion
ductus Cuvier valvulae venosae
Fig. 304.—~Transverse section through the heart and trachael region of a 10 mm. Pig. See reconstruction Figs. 296, 318, 320.
esophagus
   
mesonephros
“mg posterior vena cava
subcardinal vein
Fig. 305. ——Transverse section through posterior of heart and the
lung region of a 10 mm. Pig. Umbilical stalk not included in figure. See reconstruction Figs. 296, 318, 320.
FURTHER DEVELOPMENT or THE GUT 579
tact seldom become perforated, so that no real visceral slits are formed.
In occasional instances, however, such perforations do occur even in
Man, as reminiscent anomalies, while in the Cow the second pair regularly develop slits for a brief period (Anderson, ’22).
The Trachea and Bronchi. — Just posterior to the visceral
pouches the pharynx develops a deep ventral groove which, as in the
stomach
fore- limb bud
     
left umbilical vein(ductus venosu§
coelom
ericardial cavity
ventricle
ventral vein of mesonephros
Fig. 306.———Transverse section through the region of the stomach, liver, and posterior tip of heart of a 10 mm. Pig. See reconstruction Figs. 296, 318, 320.
Bird, is the laryngo-tracheal groove. As in that case also it shortly hecomes converted into a separate tube the trachea, which at the 7.5 mm.
stage has already produced a couple of outgrowths at its posterior end.
These of course are the primordia of the main bronchi, though they are
commonly referred to as lung buds. At 10 mm. they in turn are just
starting to give rise to stubby outpushings, the beginnings of the branchial tubes (Figs. 296, 303, 304, 305).
The Esophagus and Stomach. —— Above the trachea the part
which remains after the former structure has been pinched off beneath
it, is the esophagus. Between the 5-10 mm. stages a dilation develops
in the enteric tube at the posterior end of the esophagus just behind
the limb buds. It is the beginning of the stomach (F igs.296, 306).
580 THE  TO TEN MILLIMETERS
The Liver and Related Parts. ——- In the Pig the liver primordium
arises as a single rather wide diverticulum from the ventral side of the
gut immediately caudal to the stomach region (duodenum) at about the
4 mm. stage. In the Bird, it will be recalled, there were two original
hepatic outgrowths. The single outgrowth of the Pig, however, very
shortly gives rise to several anteriorly directed buds which grow out ,
into numerous hepatic ducts. The posterior part of the same outgrowth
becomes extended as the cystic duct while its end enlarges as the gall
Fig. 307.—Reconstruction of the stomach, dorsal
and ventral pancreas and gall bladder of a 10 mm.
Pig, enlarged from Fig. 296.
bladder. The anteriorly growing hepatic ducts and the posterior cystic
duct remain connected with the gut by the original single outgrowth
which becomes extended as the common bile duct or ductus cholcdochus (Figs. 296, 307, 308, 309). All these structures, it should be
noted, do not just lie freely in the coelom, but are, as in the Chicl-:, embedded within the ventral mesentery whose existence in this region has
_ beenipreviously explained. Their development to the pointindicated
occurs between the 5-10 mm. stages.
The Pancreas. -— At about the same time that the liver diverticulum
first appears (4 mm.) a dorsal evagination occurs, in this case within
the‘ dorsal mesentery, and slightly posterior to the liver outgrowth. It is
the dorsal part of the pancreas. At 5 mm. a single ventro-lateral pancreatic rudiment has grown out from the ductus choledochus near the point
of union of the latter with the gut. It may be recalled that in the Chick
there were two of these ventro-lateral. pancreatic primordia from the
common bile duct,'as well as the single dorsal one. At 10 mm. each
single dorsal and ventral pancreatic primordium in the Pig consists of
numerous -budding cords of cells, and the two parts are almost fusing
(Figs. 296, 307, 308, 309).
FURTHER DEVELOPMENT OF THE GUT 581i
The Mid-gut Region.——Immediately posterior to the liver and
pancreatic diverticula the intestine of the Pig, like that of the Chick,
turns ventrad. It proceeds in this direction as far as the origin of the
yolk-stalk, and then passes dorsad again to the region of the rectum. By
the 10 mm. stage the gut in this region has become a rather small tube,
. and its ventral bending has become a very clear cut loop whose sides
are quite closely‘ approximated. At the most ventral point of this loop,
30,53‘ Pancreas ventral vein of mesonephros
Pegterior cardinal v
 
posterior vena ca
8l°m hepatic portal vein
Fig. 308.——Transverse section through the region of the
anterior and of the mesonephros, the bile duct and liver of
a 10 mm. Fig. Umbilical stalk not included in figure. See
reconstruction Figs. 296, 318, 320.
from its rather sharp apex, the yolk-stalk still takes its origin. By this
time, however, this stalk is extremely constricted to form an even
smaller tube than the intestine, and the yolk-sac at its extremity exists
merely as a shriveled vestigial diverticulum within the body-stalk (Figs.
296, 297, 309, 310). In some instances at this time a small enlargement
appears on the posterior ascending limb of the loop. It is the beginning
of the caecum.
The Hind-gut Regi0n.——_,-Continuing posteriorly it has already
been noted that an evagination or fold has arisen in the dorsal wall of
the splanchnopleure of this region just anterior to the allantoic outgrowth to form the hind-gut (Fig. 300) . The crest of this fold is almost
from the first in contact with the ectoderm above it, the fusion constituting the anal plate. Thus this plate is at first dorsal.just as in the Chick.
With the outgrowth of the tail bud the caudal portion of the hind-gut
region is. drawn posteriorly and ventrad. The result is that the anal
582 THE‘ PIG TO TEN LMILLIMETERS
genital ridge dorm Pancreas
posterior mrdinzl vein
 
 
 
I
for
ventral pancreas
ventral vein of mesonephros
Fig. 309.--Transverse section through the region of mesonephros, pancreas and
posterior of liver of a 10 mm. Pig. Only a part of the umbilical stalk included in
the figure. See reconstruction Figs. 296, 318, 320.
vltelline vein
subcardinal veins left umbilical vein
vitclline vein
’ ‘ K gut loop
 
umbilical arteries
ventral vcln ofmesonephros
right umbilical vein
Fig. 310.——Transverse section through the region of mesonephros, gut loop, um
bilical and vitelline argeries and veins, allantoic stalk and ti
p of embryo of a. 10
mm. Pig. See reconstruction Figs. 296, 313, 320.
FURTHER DEVELOPMENT OF THE GUT 583
plate, as in the Bird, is presently swung clear around to the ventral
side. With the further outgrowth of the tail bud a small portion of the
hind-gut is pulled out into this bud a short distance beyond the anal
plate. As in the Chick this extension is the postanal gut, but unlike the
case of the Chick it is entirely a temporary structure with no future
function, and so need not be referred to again. Both it and the anal
plate, it should be noted, are nowcaudal and ventral to the allantoic
stalk. Thus with the shift in these parts the latter no longer extends pos
ventral vein of mesonephros
fused subcardinal veins
7 ’ ‘- \ umbilical veim
 
posterior cardinal vet
 
mesonephros 'w-- i’ - T " ‘ ’
T \. — —* ut umbilical arteries
mesonephric duct
Eig. 311.— Transverse section through the region of rnesonephros, gut, umbilical
veins, allantoic stalk and cloaca of a 10 mm. Pig. See reconstruction Figs. 296, 318,
320.
teriorly, but rather proceeds at first dorsad before curving antero-ventrally into the body-stalk (Figs. 296, 311). Just within the embryo
postero-dorsal to the anal plate, the slightly enlarged end of the gut constitutes the cloaca, and the anal plate may now be termed the cloacal
membrane. This enlarged region of the gut is called the cloaca because
as in the Chick it presently receives not only the gut opening (anus),
but those of the urinogenital ducts and the allantois. The opening of
the anus is furthest postero-dorsal, those of the urinogenital ducts,
slightly more cephalad and ventro-lateral, and that of the allantois more
antero-ventral (Fig. 296). By the time this situation has developed, e.g.,
in a 6 mm. embryo, there has also occurred, according to some, the
usual depression in the ectoderm surrounding the cloacal membrane to
form the proctodaeum. The latter, though, seems not to be much in evidence at 10 mm. Thus we have a condition essentially similar to that in
forms previously studied. From this point onward, however, the situation in the Mammal begins.to diverge from that previously observed.
584 THE PIG TO TEN MILLIMETER_S
The divergences just suggested, though not far advanced in the 10
mm. stage, are definitely underway, as a result chiefly of one process.
Within the cloaca a crescentic sheet of tissue, the urorectal fold, is
growing from the postero-dorsal wall toward the cloacal membrane and
from the lateral walls toward the median line. When completed the result will be to divide the cloacal chamber into two parts. One, the
postero-dorsal into which opens the large intestine, will constitute the
rectum. The other, antero-ventral, part is called the urinogenital sinus,
and constitutes essentially an extension of the neck of the allantois
which now receives the urinogenital ducts (Figs. 311, 337). Although
this change has been initiated in the 10 mm. embryo, the cloacal division is not yet complete, nor is the cloacal membrane yet ruptured as is
the case with the oral plate.
MESODERMAL STRUCTURES
Under the headings of systems, we have thus far considered the nervous system, which of course is exclusively ectodermal, and the digestive
system. The latter because of its lining is often thought of as primarily
endodermal, though of course much of its walls are derived from mesoderm. Now, however, we are about to consider systems which are exclusively mesodermal in origin, e.g., the circulatory system, and the urinegenital system. Before embarking upon our discussion of these definite
systems, however, it is also necessary to make a few further comments
regarding the condition of the mesoderm in general.
The Sornites.-—— We have already discussed the origin of the lateral
plate mesoderm, but there has been no occasion to refer to the somites
except in a general way as criteria of development. It may now be
noted that these structures develop in the Pig in almost exactly the same
manner already made familiar in the Chick. As in that case the first ones
formed turn out to be the most anterior, each new. one being added between the most anterior old one and Hensen’s knot. Not only is the order of their origin similar but their character and method of development is the same. Thus the original ridges of mesoderm adjacent to the
notochord and nerve cord flrst become segmented. Then each segment
(somite) becomes a roundish mass with the cells radiating from its
slightly hollow center. Next the cells adjacent to the notochord and
nerve cord become loosely arranged about these structures as sclerotome. At the same time the cells of the dorsal part of the remaining
outer wall grow ventrad between this wall and the sclerotome. Thus is
formed a new dorso-ventrally elongated double layered structure with
THE CIRCULATORY SYSTEM 585
a space between the layers. The outer layer as before is called dermatome, and the inner wall myotome, the space between them being myocoel. The question of what these layers eventually give rise to, is still
uncertain in the case of the Mammal as it was in the Bird. The inner
layer certainly goes largely to form skeletal muscle, but to what extent
the outer layer or dermatome really forms dermis is not so clear. Probably only part of it so behaves. The sclerotome, however, again unequivocally gives rise to the parts of the vertebrae. By the 10 mm. stage the
parts of the original somites indicated above are no longer evident, except to a slight extent toward the posterior (Fig. 310).
The Intermediate Mesoderm. ——Though this term was not used
in the case of the Frog and Chick its equivalent was present. It is merely
the mesoderm between the somites and each lateral plate, i.e., it is the
part previously designated as nephrotome. The latter term indicated its
fate in the previous cases, and it is the same here. The details of this
will of course be taken up in connection with the urinogenital system.
The Somatic and Splanchnic Mesoderm.——The origin of the
somatic and splanchnic mesoderm, has already been discussed, and
need not be gone into here. However, it is pertinent to note that by the
10 mm. stage the intermediate mesoderm on each side no longer connects the lateral sheet of that side with the disappearing somites, but
throughout much of its length forms a discrete mass, the developing
mesonephros (Figs. 305, 309) . As the latter pushes out into the coelom
it ofqcourse carries a layer of mesoderm before it as its covering of
coelomic epithelium. It thus comes about that on the median side of
each mesonephros this covering passes dorso-medially until the two
sheets of epithelium are separated only by the mesentery of the gut.
With this arrangement the division between somatic and splanchnic
mesoderm might now seem to be somewhat confused. It is customary,
however, to designate only the mesodermal covering of the outer body
wall as somatic. The remainder covering the mesonephros (and later
the metanephros), the mesentery and the viscera is then splanchnic.
THE CIRCULATORY SYSTEM
The Blood Islands. -——- It will be recalled that in the Bird one of the
first manifestations of the beginning of the circulatory system is the _
formation of blood islands in the area vasculosa, which is of course
extra-embryonic. Virtually the same situation obtains in the Pig where
the blood islands also appear on the surface of the empty yolk-sac corresponding to the area vasculosa of the Chick. It will be recalled that
586 THE PIG.TO TEN MILLIMETERS
in the Bird, however, the mesoderm from which they arise in this region
is supposed to have migrated out from the area pellucida. It then forms
blood islands, and these in turn bud 0H mesoderm cells between them
and the ectoderm. No such indirect method seems to occur in the Pig.
The mesoderm is already in this area, and is divided into somatic and
splanchnic layers. The blood islands are then organized out of cells
from the splanchnic layer between it and the endoderm. As before, these
cells become aggregated into clum-ps, and while those around the periphery of each clump become flattened to form blood vessel endotlzelium, the more central ones 'transform into blood corpuscles. It
should be noted also that in the Mammal this activity is not confined to
the mesoderm of the yolk-sac. The allantois, which is somewhat more
precociously developed than in the Bird, likewise produces blood islands in a similar manner. It has recently been demonstrated, moreover,
that in certain Monkeys red blood corpuscles continue to be formed
from the endothelial walls of the blood sinuses of the chorionic villi
during early pregnancy (Wislocki, ’4-3). It is further claimed that in
the Baboon even the amnion produces red blood cells (Noback, ’46).
While early genesis of blood cells occurs in these various extra-embry
. onic locations their later formation is relegated to special organs such
as the mesonephros, liver, spleen and finally the bone marrow. Meanwhile the differentiation of the endothelium of numerous vessels goes
on constantly throughout the embryo. As the circulatory system thus
develops it is quickly supplied with both corpuscles and fluid from the
various blood islands, and later from the other sources just indicated.
Whether these later centers possess their capacity as a result of the migration to them of blood forming mother cells from the original blood
islands is still an open question. Some hold this view, while others maintain that the later centers give rise to their own blood-forming cells from
local mesoderm. Possibly both methods occur. In any event there are of
course many kinds of blood cells produced from the original mother
cells, and their varied diiferentiations make a complicated subject which
we shall not go into. '
The Heart. — One of the first parts of the intra-embryonic circulatory system to develop is the heart, and the method of its early formation
is virtually identical with what we have already described in the Chick.
On either side of tlie pharyngeal region, before this part has been closed
in ventrally, the endothelium of a blood vessel forms between the
splanchnic mesoderm and the endoderm in the manner described above.
As the closure occurs these two blood tubes fuse beneath the pharynx to
THE CIRCULATORY SYSTEM 587
4
t
t
6
dorsal acme
truncus arterloxus somlte posterior cardinal veln
 
   
 
vitelline (omphalomesenteric) veins
amum duct of Cuvier
1 anterior cardinat vein
i truncus arteriosus
_' . - g”
dorsal aortae
 
 
vitelline(omphalomesenteric)veins, arteries
Fig. 312.—A. Partial injection of the vessels of a Pig embryo of 14- somites, 4‘ mm. in length. After Sabin. B. Partial injection of the vessels of a Pig embryo of
1 27 somites, 6 mm. in length. After Sabin.
588 THE PIG TO TEN MILLIMETERS
form the usual single heart tube. The splanchnic mesoderm follows the
endothelium and while the latter constitutes the endocardium, the mesoderm covers it to form the epicardium, and the dorsal and ventral mesocardia. Because of the latter the two coelomic spaces on either side (_in
the Bird called the amnio-cardiac vesicles), as in that case, do not at
first communicate. Presently, however, the ventral mesocardium disappears, and the two parts of the pericardial space are united. The dorsal
mesocardium, as in the Chick, persists somewhat longer. This condition
septum ll
 
l I
J «I 4/
2 «._.,%2«:d? mcerventrlcular
. ,,
trabcculae I? T‘
Ca Ynea e
Fig. 313.—Frontal section through the heart of a 10 mm. Pig.
is reached at about the 4.5-5 mm., or 13 somite stage. (See Chick, Fig.
l 79.)
The next steps in cardiac development in the Pig are again very familiar. The dorsal mesocardium in its middle region disappears, leaving
the double-walled tube free to bend. Then as the latter increases in
length it becomes thrown into the usual curve to the right, and this
shortly becomes a loop whose apex is rotated backward. As in the Chick,
the postero-dorsal part of the loop becomes the atrium, the apex of the
loop and a portion of each limb the ventricle, and the antero-dorsal end
of the more anterior limb the truncus arteriosus. These parts then rotate so that the atrial region becomes antero-dorsal, and the apex of the
ventricle postero-ventral with the truncus running cephalad along the
antero-ventral face of the ventricle. From a comparison of this description and of the figures of the heart of the Frog and Chick at similar
stages the essential Ilikeness will be apparent (Figs. 108, 184-, 312).
By 10 mm. the befidings and shiftings indicated above are complete,
and the heart presents externally almost the adult appearance. Interl
i
!
THE CIRCULATORY SYSTEM 539
nally a crescentic septum, the septum primum (I) has grown from the
antero-dorsal wall of the atrium, and has partially divided it into right
and left chambers. Postero-ventrally, i.e., toward the ventricle, however, the growth is not quite complete, and the very small opening
briefly remaining is all that is left of the originally wide-open orifice
between the atria, the interatrial foramen primum (Figs. 313, 314).
Meanwhile dorso-anteriorly a new opening has developed in the septum
called the interatrial foramen secundum. Also another septum, the sep
 
 
 
 
 
 
P°5t"'°' °°'d'"°' M" anterior cardinal vein
duct of Cuvier
sinus venosus—«\
posterior vena cava ' interatrial foramenll
valvulae venosae - septum i(primum)
hepatic vein interatrial foramenl
bulbo-conus
septum Ii (secundum)
cushion septum
interventricular foramen
interventricular septum
Fig. 314.-—Reconstruction_of the heart of a 7.9 mm. Fig
with the right atrium and right ventricle opened from the
right side. After Morrill.
tum secunclum (II), is sometimes slightly in evidence to the right of the
septum primum (Fig. 313). The further fate of these septa, their openings and their functions will be fully discussed in the section on ‘later
development. Another conspicuous structure apparent within the right
atrium at 10 mm. is a pair of flaps guarding the orifice from the sinus
venosus to this atrium, the valvulae venosae (Fig. 304). Later on one
of these valves forms a minor ridge, the septum spurium, which soon
disappears.
Between the atrium and the ventricular region the heart is somewhat
constricted to form the atria-ventricular canal, and this also has become almost or quite divided by growths proceeding from its dorsal
and ventral walls. When complete these growths, as in the Bird, will
form the so-called cushion septum (Fig. 304). At the same time a third
septum, the interventricular, is growing from the apex of the ventricle
toward the atrio-ventricular canal (Fig. 304). All these septa will
shortly meet to divide the entire organ into completely separated right
and left chambers, save for the existence of one of the interauricular
foramina which persists until birth and even after. Finally the walls of
.._ .......__...._.. _. . . . ..s......,
590 THE PIG TO TEN MILLIMETERS '
 
V: /e-///ne vem
   
 
Dorsal rpm nan! a/I?/1‘ 1'1-'aor/Ic arch
/‘rt /mzry .‘1eaa’re/‘.1
Lell 4 §"aor}/‘c arch Pflmmy head mm Le/7 I '3-’aor//c are/t
V ‘ L7//c vesicle '
 
 
Le/I an):-rior cardinal Van
Luff dorsadaorla
 
Left dorsal not-la.
5egmenlaIar/er/es
 
Fig. 315.—St_ages in the development of the aortic arches and other anterior ar
. 4.4 mm., 10 somites. B. 4.15 mm., 19 somites. C.
3.8 mm., 26 sornites. D. 4.57 mm? 28 somites. E. 4.46 mm., 30 somites. F. 6 mm.,
stages of development
as indicated by the number of somites are not always exactly correlated with the
relative lengths of the embryos. The former is usually the more accurate criterion of
degree of general development in the earlier stages. Hence both items are given.
‘.s .4 4 . A
THE CIRCULATORY SYSTEM 591
   
     
       
 
 
 
 
     
/Ior//c /run/<
.3 ‘Paar!/c arc/7
E3-—i. eff dorsal aar/a
/f/gh/dorsa.’ aorla
 
/"/
4_"'ao.'/Ic arch
_:‘L
 
E »/’u/manar} arc/7 '
‘«\\\\\\'«\\‘\““““.“_‘T'“‘m"” ’ ' M‘
\\“\ .  . ‘. .
uuwlw/" """'“‘
Ex/erna/carom/ar/cry
Cxfernal camlrd arfery
4 " aor//c arch
Pu/mamzryarc/1
Pu/marlaly ar/cry
2/2’ dorsal aarla E
Pulmonary vein Ssgmrnlal ar/cries
 
   
Fig. 316.—Stages in the dgvelopment of the aortic arches" and other anterior
arteries of the Pig. After Heuser. A. 24 somites. B. 4.3 mm., 26 son-mites. C. 6 mm.,
36 somites. D. 8 mm. E. 12 mm. '
m
——
592 THE PIC TO TEN MILLIMETERS
 
   
   
M Tivigemrz-up
/ ,
vary/zeadvein /’
 
   
 
   
(Sr/ema/s arn- “
ht! arlery
florsalrerrmaninf  IA‘  “ I I
z#‘7°’A"5”"}’ ~~\\\\V§m\ ‘.  J5'aarIicarc}2
4"’aur1icarc}i 2;" V  , E  §g,ra‘a’r”J:"'}
/zu/manager:/L 2 [1, ,  \_.Lx_ ' II,’ Exlerna/carol/dar/er}
‘I ' t ‘V  /V LL-//4"~"aar//c an:/z
Fulmonaryarleiy ; -- I
I ~ 7 » tel/pz//monaryam/2
P”/"'°£',',’,-,',"  -  Pulmonary arfery
' '  £2/fa/nrsa/aar/:1
 
 
     
 
 
,. .
‘ I’!-(M 3 Dorsal remnanf /Z"'aar/‘ aid:
§ n\\\7(?\‘\_.€ ‘  0 "
 
Darsal remnanl 9/ . N
Z "7"aorlic arch '
Exlema/cara/- __ V
zdarlery '
4 4’aar//‘tart/2
LII /rnormry
e W are/1
 
5vb:/avian ar/cry 5"“""""" °’/”-V
Aor/a.
   
Fig. 317.—-Stages in the development of the aortic arches and other anterior arteries in the Pi . After Heuser. A. 12 mm. B. 14 mm. C. 17 mm. D. 19.3 mm.
THE CIRCULATORY SYSTEM 593
the ventricles become definitely thickened, and muscular bands, the
trabeculae carneae project into the ventricular lumen.
The T runcus and Aortic Arches. ——.The truncus arteriosus has
already been mentioned as it comes up underneath the pharynx. As in
iind visceral pouch
is: visceral pouch
   
   
 
internal carotid artery
external carotid artery
3rd aortic arch
?
II
.5:
‘<
F‘-s-299
     
   
 
 
F:g.3oz I RF?th|<e's pockeg
pulmonary artery I  9"”:
vertebralarter Ii:
Interatrigfomcn  V
subciaviaingdrtery  ’  fi"3°“
intervcntricular foramen
n,.3o5  “M05 »
. FI'g.3O6 ’
cociiac art:  3 u -in I
macs ' ,;,_ °‘ °' 9‘
E33.-L-'2
53%
d°"3' ‘°"‘ aliantolc stalk
F'»g.3lO '
3.3:‘:
Fig.323 ‘
superior mesenteric artery '
Fig. 318. —-—Reconstruction of a 10 mm. Pig embryo designed to show primarily
the main features of the arterial system at this stage. Drawing made by same
methods as used for Fig. 296. As before the lines at the sides indicate where the
sections denoted by the figure numbers above the lines, pass through the embryo.
the case of the Chick this large vessel does not, contrary to what most
diagrams suggest, really extend any distance cephalad in a horizontal
position before giving off the aortic arches. Instead it extends dorsally
and only ‘slightly cephalad directly into the -midst of the pharyngeal region (Fig. 318). Here it gives rise to the six aortic arches, but again as
in the Bird, not all at one time. The mandibular aortic ‘arch appears first,
then the hyoid, and by the time the other four pairs have developed in
the remaining visceral arches (10 mm.) the first two aortic vessels have _
disappeared (Figs. 315, 316). Also again as in the Chick, the‘ fifth pair
594 THE PIG TO TEN MILLIMETERS
are vestigial, sometimes appearing briefly as loops on -the front sides of
the sixth arches, and sometimes on the posterior sides of the fourth.
With respect to the sixth arches themselves it must be noted that as early
as 7.5 mm. each has given rise to a small posterior outgrowth which
 
 
 
 
 
 
   
   
circle of Wlllls
nternal
left carotid artery
lnterna xmlml
right carotid artery
external
lntersegmennl arterl 3", mm: "ch
_. common carotld artery
3rd , ' l l _ _ brachlo-cephalic artery
aortic arches-[4th I l . . <' I 4th aortic arch
Gil‘! ' ‘ -' l ' duct of Botallo
*:'::.:::'z,:.':r.:::: «me»
pulmonary arteries
trun_eus
arterlosus
left vertebral artery
dorsalaorta  ‘
Internal mammary arcerl
A B
Fi . 319.——Semi-diagrammatic representation of the development of the aortic
arc es and other anterior arteries of the Pig. A. Arteries at the 10 mm. stage. B.
Arteries of a specimen near term.
left subclavlan
artery
reaches the developing lung buds. These outgrowths, together with the
proximal parts of the arches, constitute at the 10 mm. stage the pulmonary arteries (Fig. 316, E). It may be noted that in other Mammals
studied the proximal parts of both the sixth arches continue to form a
part of these arteries. In the Pig, however, as we shall see, only the
proximal part of the left sixth arch persists as a part of the pulmonary
system (Figs. 317, A, B; 319). Anteriorly, the first two pairs of arches
THE CIRCULATORY SYSTEM 595
have disappeared, and each member of the third pair has given rise near
its base to a new vessel. These vessels are the external carotids, and appear at lffmm. as very tenuous strands extending cephalad toward the
ventral part of the head (Fig. 318). Both fourth arches at this time remain well developed.
The Dorsal Aortae. —— At their dorsal ends the arches of each side
are connected anteriorly and posteriorly by the two dorsal aortae. Cephalad these aortae remain separate, and extend into the head as the internal carotids. Posteriorly they also continue separately at first (Fig.
312, B), but at about 6.5 _mm. (17 somites) they become united at ap
proximately the middle of the embryo to form the single dorsal aorta. '
By the 10 mm. stage this fusion has progressed to the tail, and as far
forward as the anterior appendages (Figs. 316, 318).
Other Arteries Anterior to the Heart.——ln the Pig and other
Mammals the internal carotids are not the only dorsal arteries extending into the head. There early arise from the aorta throughout most of
its length small branches between each pair of somites, the inter segmental (or segmental) arteries. These were also noted in the Chick. In
the Pig, however, these arteries soon form antero-posterior anastomoses
in the region extending from the seventh cervical somite into the head,‘
and at the same time lose their connections with the dorsal aorta. As a
result of this process there are established in the neck region anterior to
the seventh cervical intersegrnental arteries, a pair of longitudinal vessels called the vertebral arteries (Fig. 317). These arteries, however, do
not continue separately clear into the head. Beneath the myelencephalon
they fuse into a single median vessel termed the basilar aitery. As re
- gards the seventh cervical intersegmentals, it may be noted that they are
starting to enlarge slightly to take part in the formation of the subclavian arteries, whose development will be described further in the
next stage. The fate of the intersegmentals posterior to the seventh cervical will also be noted at that time. Meantime by the 10 mm. stage the internal carotids have each sent a branch medially to unite with the basilar, thus producing a part of the future circle of Willis about the
hypophysis (Figs. 317, 318, 319).
Arteries Posterior to the Heart.—To complete the history of
the arteries at this stage we find that somewhat caudad from the middle
of the embryo, the two omphalomesenteric or vitelline arteries are
among the first to‘ arise from the dorsal aortae. These arteries connect
the aortae with the vessels formed in the wall of the yolk-sac, and since
the vitellines arise before the dorsal aortae have fused, they are at first
a
596 THE PIG T0 TEN MILLIMETERS
double (Fig. 312, B). Their function of course is to take blood from
the embryo to the yolk-sac, where it receives nutriment absorbed by
this organ from the uterine walls prior to the development of the allantoic placenta. At 10 mm. the aortae in the region of the origin of the
vitelline arteries have fused and with them the arteries, so that a single
 
 
 
 
   
 
 
fig. 299
j H9299
anterior cardinal vein
external jugular vein
Fig-302 F:‘g.302
right duct of Cuvier
valvulae venosae
I-75.304 Fig. 301!
“CW5 V¢"°‘"‘ omphalornescnteric vein
Fig-305 Fig.305
posterior vena cava
Fi .306 Fig. 306
right hepatic vein _
p;s_3o8 Fig. 303
fis.309 .F_li'_§2?.
hepatic portal vein 3"3|'|§°l€ 5'-ilk
I-'Ig.3lO Fi§.3IO
imemml am left umbilical vein
Fig.3" Fig.3l|
5* 3” right ugfiialu zcgl vein
' cloaca
esonephros
ventral vein of mesoncphro:
Fig. 320.——Reconstruction of a 10 mm. Pig embryo designed to show primarily
the main features of the venous system at this stage. Drawing made by the same
methods as used for Figs. 296 and 318. As in these figures the lines at the sides
indicate where the sections denoted by the figure numbers above the lines, pass
‘throughthe embryo.
vitelline artery extends along the mesentery into the body-stalk (Fig.
318). With the disappearance of the yolk-sac this vessel persists within
the body as the anterior mesenteric artery. A short distance anterior to
it the coeliac artery has developed at~10 mm., and extends toward the
stomach region, but the posterior mesenteric artery has not yet appeared.
In addition to the segmental arteries already mentioned the aorta also
gives off numerous small’ branches at the level of the mesbnephros to
the glomeruli and tubules of that organ, the renal arteries. Lastly, so far
as branches from the aorta are concerned, are the umbilical arteries to
2 . THE CIRCULATORY SYSTEM 597
the allantois. These arise quite early before the two aortae have fused
in this region, and even after their fusion at 10 mm. the umbilicals rernain separate. By this stage also each has produced a small branch in
ugh: duct of Cuvier let‘: duct of Cutler
   
 
sinus venosus mt! duct of Cums: Id: due: at Crmer
mad" anterior cardinal vein i
‘ cu-dinalveln ' "mm"
W . ' ——Post.erior ardlnll vctn “"“"" "l"
E P°“¢'l°|'  - poxzerlor
3; liver
‘inhumane: vein left umbllica! van
Ugh! umbmal veln aft umblllul vein
umphalemestntcrlc(virelline) veins
A ‘ omplulomesenreriz (vizemne) um!
i
4
I
5
left‘ due: of Cuvler
 
4 I umbilical vein
- hlombflid vein
Intestinal vein
oenptualomesenxerlc (yi::IIing)vgm
Fig. 321.—Reconstructions of stages in the development of the veins of the liver
and immediate vicinity. A. The veins in a. 5-6 mm. embryo, semi-diagrammatic.
Veins in the liver according to Butler, with the omphalomesenteric (vitelline)
veins extended posteriorly to show their relation to the gut. B. Veins in a 6 mm.
Pig embryo, semi-diagrammatic. Again the vessels within the liver are according
to Butler, with the omphalomesenterics posterior to it added. C. Veins in the liver
of a 10 mm. Pig embryo viewed from the right side (enlarged from Fig. 320). D.
Veins in the liver of a Pig at the same stage as C, but viewed ventrally.
connection with the developing hind limb bud, the external iliac. The
aorta itself continues on as a single vessel into the tail (Fiv. 318).
, The Omphalomesenteric Veins.—As in the Bird, among the
i earliest, if not the earliest, veins to develop in the Pig are the am phalaA mesenteric or vitelline veins. They. arise just as they did in the Chick
coincidentally with the formation of the cardiac tubes which fuse an-_
teriorly to form the heart. Posterior to the region of fusion these tubes
extend caudad and laterally out onto the yolk-sac where _theyi’become
continuous with the capillaries and blood islands which we have noted
593 ' THE PIG TO TEN MILLIMETERS
     
   
. Illllll VEVIOSUI '
   
ventral vein 0! mesonephrot
posterior ardlnnl veln
-right subcardlnal vein
Early stage in any young 5-6 mm. Fig embryo.
mammalian embryo.
sinus venesus
anterior ardlnal vein
duct of Cuvler
subclavian Vein
posterior ardlnal vein
I I
rlghl umbilical vcln e E umbmal "In
hepatic portal vein
3-7 mm. Fig embryo. 12-14 mm. Pig embryo.
Fig. 322.--Diagrams of developing venous system posterior to heart in: A. Any
very young mammal; B, C, E, F, H, I, J, in Pig at stages indicated. D. Transverse
section of C at level shown by arrow.
as originating there. As development proceeds the fusion of the vitellines continues for a very short distance posterior .to the atrial region of
the heart to form a thin walled sac, the sinus venosus (Figs. 312, A; 322,
A, B); At about this time also (3.5—4 mni.) the previously noted interatria  prirrium begins to develop, and in such a way that the sinus
l ._ 'j‘,e*‘6pa1-in\to"“l:Z1e right atrium (Fig. 304).
K.
‘t
2»~
I
posterior vena_u:va
(right mpnudlnafi
owunulllhc — " /\ /1 ’\
30-35 mm. Pig embryo; - Adult Pig.
Fig. 322 cont.—F, H, I, J, as noted ahove. G. Transverse section of F at level
shown by arrow. All stages after Butler. Princeton Embryological Collection.
At this point a difference may be noted between the further development of the vitelline veins in the Chick and that in the'Pig. The two
veins in the Pig do not continue their fusion to form may large part of
the ductus venosus as in the Bird, the major portion of that trunk arising from a different source in a way to be described be '
remain mostly separate,  the liver and pre ' fl t9-fie
600 THE PIG TO TEN MILLIMETERS
stage their middle portions have broken up into a capillary network.
Their anteriorstumps, however, remain as the two hepatic veins, while
their posterior parts persist for a time in the caudal half of the liver as
two distinctuvessels (Fig. 322, A, B). From there these vessels issue to
pass along either side of the gut to the regressing yolk-sac. As the latter
disappears they become simply two veins bringing blood from the intes
tine, and by the 10 mm. stage a further change has occurred, resulting
in the reduction of these two vessels to the one hepatic portal vein. The
method by which this takes place, producing the peculiar spiral course
of this single vessel about the gut, is illustrated in figure 321. It involves essentially the same process as in the Chick, i.e., a fusion of the
vitelline vessels first above the intestine, and then below it, with the
subsequent disappearance of the left and right sides of the loops thus
formed. The chief diiierence between the Chick and the Pig in this connection is that in the latter both sides of the loop are formed before ei
' ther disappears, but as indicated the end result is the same.
The Allantoic (Umbilical) Veins. —— Another pair of veins which
develop very early in the Mammal are the allantoic or umbilical veins.
In the Bird these are somewhat slower in forming, and it will also be
recalled that at first the allantois is drained by a transitory vessel, the
subintestinal vein, which opens anteriorly into the vitellines. This preliminary arrangement does not occur in the Pig. Instead the umbilical
veins develop at once in essentially the same way that they ultimately
do in the Bird. They arise as vessels in the lateral body wall which
open anteriorly directly into the sinus venosus (Fig. 322, A). Posteriorly they extend around the sides of the wall, and thence via the bodystalk onto the neck of theallantois (Fig. 273). This is the situation at
first, but by 10 mm. certain changes have developedas follows:
Anteriorly the two veins no longer empty directly into the sinus venosus. Instead as the liver comes into contact with the body wall, the umbilicals in that wall develop new channels connected with the hepatic
capillaries (6 mm.) (Fig. 322, B). By the 10 mm. stage some of these
capillaries in line with the flow of blood from the two umbilicals have
developed into well marked channels which soon become definite vessels
within the liver. The left one even at this stage is larger than the right,
which soon disappears in this region. Hence the part of the left umbilical within the liver now forms the major part of the ductus -venogms,
the short anterior section which opens into the sinus, being derived from
the very limited fusion of the vitellines indicated above (Figs. 320, 321,
322, C). Thus, as noted, the ductus has for the most part a quite difl'erTHE CIRCULATORY SYSTEM 601
ent origin from the similarly named vessel in the Chick where it arose
entirely from the posterior fusion of the vitelline veins. Caudad to the
liver the two allantoic or umbilical veins continue at this time to exist
as separate vessels as far as the umbilical stalk, but within this stalk
they have become fused into one. Thus there is but one umbilical vein
in the stalk, but two umbilical arteries. Even at this stage, however, the
right umbilical vein within the body wall is becoming smaller.
The Anterior and Posterior Cardinal Veins. —— So far we have
considered venous systems which are both intra- and extra—embryonic.
It now remains to indicate the development of those veins which are entirely within the embryo. Among these the most prominent up to the 10
mm. stage are the various cardinals, whose development very closely
parallels that in the Bird. Thus the anterior cardinals arise anteriorly
on either side of the neck and headregion slightly dorso-lateral to the
aortae, and soon develop a capillary network connecting with the latter
vessels. The posterior cardinals likewise develop in the same relative
position to‘ the aorta posterior to the heart. Dorso-lateral to that organ
the anterior and posterior vessels of each side dip.downward slightly,
and join one another to form the wide, short ducts of Cuvier which
slope ventrally and medially to enter the sinus venosus. A short distance cephalad to the point where the anterior cardinals enter the ducts
each cardinal is joined by a ventral branch coming from the region of
the mandibular arch. It is of course the future external jugular. Very
slightly posterior to, or at its junction with, the respective duct of Cuvier
each posterior cardinal receives the subclavian from the adjacent forelimb bud. This vein, as was the case with the corresponding arteries,
results simply from the enlargement of one of the numerous intersegmental veins which drain into the posterior cardinals (Fig. 321, 304).
The Subcardinals and Posterior Vena Cava. ——Again as in the
Chick, with the development of the mesenephros the original cardinal
circulation is supplemented by certain new vessels which in a 10 mm.
embryo are well established. Indeed by this time the posterior cardinals
have actually begun _to degenerate, and their functions to be taken over
by these new vessels as follows:
Along the ventro-medial border of each mesonephros a plexus of
capillaries is formed (5-6 mm.) , and soon these have anastomosed so as
to constitute continuous vessels running the length ‘of each mesonephros.
These are the subcardinals, and through further mesonephric capillaries
they are soon more or less connected with the posterior cardinals‘ (Fig.
322, B). In fact anteriorly these connections presently become quite
602 THE PIG T0 TEN MILLIMETERS
definite and direct. Now as the mesonephroi grow the suhcardinals are
crowded still nearer the mid-line, and at about the middle antero-posteriorly, ofthe mesonephroi they fuse into a single large sinus (Figs.
311, 322, C, D, E). Into this drain all the surrounding capillaries. This
comes about because, as this sinus is formed, the posterior cardinals at
this level disappear entirely, though they persist for a time both anteriorly and posteriorly. Thus it happens at 10 mm. that among the capillaries draining their blood into the median subcardinal sinus through
the mesonephros are many from the posterior parts of the posterior cardinals (Figs. 320, 322, C, E). At the same time anterior to the subcatdinal sinus, the left subcardinal begins to become smaller, and to lose
its connection with the anterior part of the left posterior cardinal,
though this is still functioning at 10 mm. (Fig. 322, C, E). The right
subcardinal, however, just as in the Bird, becomes more prominent, and
at 10 mm. has affected a connection with still another new vessel. This
vessel has formed from capillaries within the liver mesentery, and also
from some of those within the liver itself. It is the mesenteric and
hepatic part of the posterior vena ctwa, the subcardinal sinus and the
anterior portion of the right subcardinal, being the other parts developed at this time (Figs. 320, 322, C, E). Anteriorly the part of the new
vessel developing in the liver opens into the ductus venosus near its anterior end, where it also receives the two hepatic veins. As the caval
vein grows, the anterior part of the ductus between this vein and_ the
sinus becomes the anterior end of the vein (Fig. 321). The complete
development of its posterior end will be explained in our discussion of
the next stage.
In connection with the description of this vessel up to the present
point, however, there is, already one feature concerned with its posterior
part which is becoming evident, and which merits attention. This
feature is the development of a renal portal system in essentially the
same way that it was formed in the Bird (Fig. 322, E). When fully developed, these systems function more or less like that of the Frog,
though they arise somewhat difierently, there being no subcardinals in
the Frog.‘ It is interesting of course that this system exists in all these
forms, yet in the Bird and Mammal is only temporary. It is perhaps
even more remarkable that it is always the right side (in the Bird and
1 It appears that in the Pig,‘ and very probably the Bird, not so much of the
blood coming from the posterior of the embryo is actually supplied to the mesenpheric tubules as in the Frog. Instead more of it seems to be routed more directly
through the organ, while the tubules, as well as the glomeruli, are supplied more
from arterial sources.
THE EXCRETORY SYSTEM 603
Mammal the right subcardinal (in the Frog the right posterior cardinal)
which enters into the formation of the posterior vena cava. Such facts
can scarcely be entirely coincidental.
One minor feature regarding the cardinals in the 10 mm. Fig which
differs from that in the Chick should be mentioned to avoid confusion.
In the Chick there are no other vessels than those just described. In the
Pig, on the other hand, some of the capillaries along the ventro-latera-I
side of each mesonephros also anastomose to form a small vessel extending antero-posteriorly along this region. It is called the ventral
vein. of the mesonephros, and since it also connects through capillaries
with the respective posterior cardinal, it might be mistaken for a subcardinal. Its smaller size and superficial ventral position, however, distinguishes it and it soon disappears (Figs. 320, 322, C, D, E).
The Pulmonary Veins.-—0ne other important intra-embryonic
venous system which has no relation to the cardinals, but which also
starts to develop at an early. period is the pulmonary. Since the pulmonary arteries have been seen to arise as early as the 7.5 mm. stage,
the development of the veins at about that time might be anticipated,
and they have in fact arisen. There is some question, however, as to just
how these vessels have been formed, e.g., whether as an outgrowth from
the atrium, or as in so many other cases, by an anastomosing of plexuses along their course. In any event they exist at this stage as small
i veins which proceed from each lung bud, and unite in a common trunk
which enters theleft atrium. Later as in the case of the arteries the pulmonary veins also suffer certain alterations which will be noted in due
course.
THE URINOGENITAL SYSTEM
Although these systems are ordinarily considered together because of
the close association of some of their parts both embryologically and
anatomically, it is convenient as previously, to describe their development separately. We shall begin with the excretory systemsince it is the
first to become clearly evident.
E THE EXCRETORY SYSTEM
I The Pronephros. —— In the Pig, as in the Bird, there is a gesture
made toward the development of a pronephros. On-each side its rudimentary tubules arise as usual from the intermediate mesoderm, and occur in the cephalic region from about the sixth to the fourteenth somites.
These vestigial organs are of course without functional significance, but
I
. ‘T
604 THE PIG TO TEN MILLIMETERS
the tubules turn and grow caudad to give rise to the pronephric ducts,
in the way with which we are already familiar. By 10 mm. all parts of
this system, save the ducts, have virtually disappeared. ,
_ The Mesonephros.—The mesonephros arises in the intermediate ‘
mesoderm from about the fourteenth to the thirty-second somite of the I
Pig.‘ As usual it first appears as spherical concentrations in this meso- ' ii
derm, three or four such concentrations being developed opposite each
somite. These form vesicles, and the vesicles produce tubular outgrowths
which become coiled, and open into the old pronephric, now mesonephric, duct. The vesiculariportion of each tubule is invaginated by
the usual knot of capillaries forming a glomerulus, supplied with blood
by branches from the aorta, and draining into tributaries to the subcardinal veins. The invaginated part of the tubule of course constitutes
Bowman’s capsule.
Anteriorly the mesonephric duct is often difficult to distinguish in
cross section from the numerous mesonephric tubules, but more caudally
it can generally be located along the ventral border of the organ. Poste
rior to the mesonephros this duct continues to the cloaca, and by the 6
mm. stage has entered it. By 10 mm. the antero-ventral region, into the
sides of which this entrance was affected, is beginning to be separated
from the postero~dorsal part by the urorectal fold in the manner already described (Fig. 337). Thus the ducts are coming to open into the
part of the cloaca termed the urinogenital sinus which is in the process
of being added to the neck of the bladder (allantois). These arrange—
ments in the cloacal region are the beginnings of changes which will
ultimately bring about fundamental diflerences between conditions in
these parts in the Bird and the Mammal. These dilierences will be discussed in detail later in connection with the development of the external
genitalia. At this time, however, the most striking peculiarity of the
mammalian excretory system lies in the remarkable relative size of the
mesonephroi themselves. Thus in a 10 mm. Pig these organs are far
larger than at any period in the Chick, being in fact much the largest structures in the embryo (Figs. 296, 310, 311).-The functional significance of this difference is not known.
The Metanephros and Ureter. ——As the student is already aware,
the mesonephric kidney in all Amniotes is ultimately replaced by a
third or metanephric kidney. This kidney starts to appear at the 5-6
mm. stage as a very smalldiverticulum growing out from the posterodorsal side of each mesonephric duct just dorsal to the point where
if these ducts enter thelcloaca. By 10 mm. the diverticula still issue from
THE GENITAL SYSTEM 605
the mesonephric ducts rather than the neck of the bladder, but have
grown anteriorly somewhat, and the cephalic portion of each is enlarged slightly. The enlarged portion represents the lining of the future
pelvis of the kidney, and is already surrounded by a concentration
of intermediate nephrogenic mesoderm (Figs. 296, 323). This meso
mesonephros mesonephric duct
subcardinai veins hind-limb bud
Tm
_ \\ posterior cardinal vein
£“v
   
posterior cardinal vein
\_> .vI’
ventral vein of mesonephros ""“b""3 3'"‘°"Y
Fig. 323. —Transverse section through the region of the mesonephros, umbilical
arteries, mesonephric and metanephric ducts and hind limb buds of a 10 mm. Pig.
See reconstruction Figs. 296, 318, 320.
derm is carried forward with the pelvic portion, and later furnishes
the material from which the kidney tubules are made. The remainder
of_ the outgrowth of course becomes the future metanephric duct or
ureter.
THE GENITAL SYSTEM
The Gonads.——These are barely in evidence at the 10 mm. stage.
They may sometimes be detected, however, as very slight thickenings on
the medial sides of the mesonephroi, somewhat anterior to the middle.
16
HE LATER DEVELOPMENT OF THE PIG
HA V I N G completed our descriptions of the Pig embryo as a whole,
and of the various systems at the 10 mm. stage (20—21 days), we are
now prepared to indicate the further development of this animal as far
as it is profitable to carry it. This means in most instances, either to the
adult condition, or to a condition near enough to it so that the ‘steps required to attain the adult state are quite obvious. As in the discussion of
the earlier development we shall begin by a consideration of external
features. E ' '
The F lexures. —— Following the 10 mm. stage the Pig embryo grad
ually straightens to some extent. "This pr'ocess first involves mainly the _
dorsal flexure (15-20 mm., Fig. 324), and later the cervical and lumbesacral flexures. As in other vertebrates the cranial flexure is permanent,
but since it concerns chiefly the brain it also ‘becomes less obvious externally as development proceeds.
External Features Posterior to the Head and Neck Region. —At 15 mm. the boundaries of the somites are still clearly visible, and
the milk ridge has become evident. By 20 mm. the ,somite markings have
pretty much disappeared, while along the lower border of the milk ridge
fiveior six mammary anlagen are present. Ventral to these anlagen in
both these stages the abdomen protrudes greatly, due to the developing
mass of viscera within it. By the 50 mm. stage, however, these have been
drawn up, and the ventral contour is about _that of a well-fed adult.
Throughout all these periods there has been relatively little growth of
the umbilical cord. Its diameter does ultimately increase, however, due
to growth of the contained blood vessels and connective tissue, so that
at term it measures from 8-10 -mm., while the length of the whole animal may be as much as 25-30 cm. The paddle-like appearance of the
feet at 10 mm. has been referred to, and this condition still prevails at
20 mm. By that time, however, the existence of five toes in each foot is
clearly in evidence, and the limb joints are slightly suggested. In the
Pig and other Artiodactyls, as is well known, the first digit (homologue
of the thumb or great toe in Man) soon vanishes entirely. The third and
fourth digits develop evenly to form the cloven hoof, while the second
,;
EXTERNAL FEATURES 607
and fifth digits remain short and more or less vestigial. This condition
is well advanced in an embryo of 40-50 mm.
The Head and Neck Regions. —— Probably the most striking
changes of all in any mammalian embryo are those connected with the
head and neck, especially with relation to the face, and we shall now
indicate these changes in their main outlines.
 
 
   
' hair follicles
ear plnna
hear: area
milk ridge .
nipples
umbilical stalk
genital tubercle
20 mm. embryo
Fig. 324. +A 20 mm. Pig embryo viewed from the right side.
When last described at 10 mm. it will be recalled that there were
four visceral clefts and four arches visible in a side view, the first arch
being the mandibular (Fig. 294). Also apparent were the maxillary
processes and nasal pits. Each pit was bounded laterally by a nasclateral process which was separated from the adjacent maxillary process by'a groove running from oral cavity to eye, the lachrymal groove.
Viewed anteriorly (Fig. 295) the frontal process separated the nasal
pits, and adjacent to each pit this process was thickened to form the
naso-medial processes. Reference to the appropriate figures makes evident the great similarity of these facial anlagen in a 4-5 day Chick and
608 THE LATER DEVELOPMENT OF THE PIG
a 10 mm. Pig. It may now be added that the resemblance between Pig
and Man at comparable stages is even closer. Indeed the latter are so
much alike not only with regard to facial features, but in other respects,
that to a casual observer the differences between a 10 mm. Pig embryo
and a 10 mm. Human embryo would be scarcely noticeable. The
changes which gradually ensue to produce the condition in the head of
the adult Pig will now be indicated.
   
naso-lateral process
naso-lacrymal groove
naso-medlal process
eye
external naris
maxillary process
mandibular arch tdngue
auditory opening
Pl"“3 °f °a" (hyomandibular cleft)
Fig. 325.-—-A view of the face of a 17 mm. Pig embryo from the
antero-ventral side. '
The lower jaw, it may at once be noted, is derived entirely from the
mandibular arches which grow antero-medially until they meet. Posteriorly they form an angle with the maxillary processes which constitute
the larger part of the upper jaw. However, these latter processes do not
meet one another anteriorly, and hence do not form the antero-median
part of this jaw. Instead this part is comprised of the naso-medial processes whose forward extremities grow together. In so doing they crowd
the original median region, i.e., the frontal process, backward (Figs.
295, 325). Thus the naso-medial processes come to form the pre-maxillary part of the upper jaw, and the nasal septum, while the frontal process forms only the nasal bridge. While this fusion between the naso~
EXTERNAL FEATURES 609
medial processes is occurring in tlie mid-line, each of these processes is
also fusing postero-laterally with the respective maxillary process, and
also with the respective naso-lateral process. These fusions serve to
bound the nasal pits antero-laterally, and cut them off from the edge of
the oral cavity, thus producing the external nares (Fig. 325). Posteriorly these pits breakthrough into the oral cavity, and so give rise to."
the temporary internal nares, of which more will be said in connection with the development of the mouth proper. While the bridge of the
nose is formed as noted from the frontal process, its sides (alae) are
constituted by the naso-lateral processes. Also the lachrymal groove separating these processes from the maxillary processes is closed over so
as to form a tube, the lachrymal duct connecting eye and nose.
Further development of the Pig’s face consists largely of the outgrowth of all these parts. Indeed the whole procedure from 10 mm. onward may be roughly pictured thus: It is much as though all the above
processes were approached from the front by invisible fingers which
grasp these processes, squeeze them together, and then draw them out
anteriorly to make the Pig’s snout. Essentially these same changes occur in the development of the human face from the same original parts,
except that, fortunately from our point of view, the “ drawing out ” procedure is not carried to such an extreme. It is of some interest to note
in this connection that a failure in the fusion of the naso-median processes with the respective maxillary processeson one side or both results in the formation of the defect known as “ harelip.” An inspection
of Figure 325 will show why this is true.
On the sides of the head the almond shaped eyes do not possess lids,
even at 20 mm., though the follicles of the coarse bristles constituting
the Pig’s eyebrow _a_re,, clearly visible. Both upper and lower lids appear
very shortly, however, ‘at about 24 mm., as folds of skin. Eventually
these folds meet and fuse so that the eye is completely covered for a
time, and in some animals this condition even persists for a while-after
birth, e.g., in the Cat, in which case the animal is said to be born
“ blind.” As regards the eye itself, it has previously been indicated that
one prominent difference between the Chick and the Mammal is the
fact that in the earlier stages the eyes of all mammalian embryos are
definitely smaller than those of comparable Bird embryos. This is still
true at the stage of the latter corresponding to that of the 20 mm. Pig,
and it may be further remarked that the Pig eye is even smaller relatively than that of many other _Mammals, e.g., Man pr Rat.
610 THE LATER DEVELOPMENT OF THE PIG
THE NERVOUS SYSTEM
In the preceding chapter the development of the nervous system was
carried to the point characteristic of a 10 mm. Pig, and in so doing it
was found convenient to treat it by parts. These involved the brain, the
c°,.p°,., qu,d,..:¢m|m cerebral hemisphere
     
   
cerebellum
olctory bulb
   
 
   
A from 81 mm. embryo
cerebral hemisphere
parietal lobe
Sylvlan fissure
I frontal lobe
sulcus rhlnalis
   
pyriform lob
was hypophysls
spinal cord
 
olfactor tract
7 olfactory bulb
B from 230 mm. embryo
Fig. 326.—Lateral views of two stages in the development of the Pig brain. In B
the corpora quadrigemina are entirely covered by the cerebrum and cerebellum.
neural tube, the cranial nerves, the spinal nerves and the organs of special sense. We shall now proceed with the further development of these
parts so far as seems profitable. ‘
THE BRAIN
The Telencephalon. —' This structure is of course the anterior part
of the prosenoepharlon which is separated from the posterior part (diencephalon) by the same boundaries already familiar in the Chick. As
previously noted it has already started to give rise to its most important
and conspicuous products, the cerebral hemispheres. As in the Chick
THE BRAIN 611
these antero-lateral outgrowths contain cavities, the lateral ventricles,
which communicate with the small remaining space within the telencephalon by the foramina of Monro. This latter space as usual constitutes a small part of the anterior portion of the third oentricle.
It was noted in the discussion of this region in the Chick, that although the cerebral hemispheres are relatively prominent structures in
that form, they never attain the size and complexity characteristic of
the Mammal. In the latter animal their size eventually causes them to
constitute by far the larger part of the brain, and to cover entirely
the mammalian homologues of the Bird’s conspicuous optic lobes. In
addition to their mere size, in the Pig and most other higher Mammals,
their surface area (cortex) is increased by complex foldings, the narrow depressions or fissures between the folds being known as sulci. It
should now he noted that one of the more conspicuous of these sulci
extends horizontally along the ventro—lateral region of each hemisphere,
serving to separate the upper portion, or neopallium, from t.he lower
portion or rhinencephalon. It is therefore called the sulcus rhinalis.
Other sulci within the neopallium serve to divide it into the frontal,
parietal, temporal and occipital lobes or regions, which in turn are still
further subdivided (Fig. 326). The rhinencephalon does not contain
conspicuous sulci, but does give rise at its anterior extremity to the
olfactory lobes or bulbs, while its lateral walls constitute chiefly the
pyriform lobes. Quite evidently the.rhinencephalon is phylogenetically
the older part of the telencephalon, while the neopallium is a recent addition reaching its most conspicuous development in Man.
The Diencepha1on.——This posterior portion of the prosencephalon, whose laterally compressed cavity comprises most of the third ventricle, has already been noted as giving rise to the optic vesicles and infundihulum. The connection of the optic stalks with this part of the
brain is marked as usual by the optic recess which really constitutes the
ventral boundary between telencephalon and diencephalon. Immedi
p ately posterior to this recess and hence definitely in the wall of the di
encephalon, is a thickening which, as in the Bird, is the optic chiasma,
within which eventually the fibers of the optic. nerves cross each other.
Adjacent to the chiasma on the posterior side (i.e., the floor) occurs a
thin region of wall termed the lamina post optica, and immediately beyond that the pouch-like infunklibulum presently makes contact with
Rathke’s pocket growing antero-dorsally from the stomodaeum. As
previously indicated these two latter structures together produce the
adult pituitary or_ hypophysis. The anterior part of this organ, compris612 THE LATER DEVELOPMENT  THE PIG
ing the pars distalis, pars intermedia and pars tuberalis, is derived entirely from Rathke’s pocket, while the posterior part forming the pars
nervosa. and the stalk are derived entirely from the infundibulum.
Upon the anterior side, i.e., the roof of the diencephalon, two structures appear. The more posterior, or really dorsal, is an outpushing
whose lumen later becomes occluded, and which develops into the epiphysis. Anterior to this the rather thin roof of the third ventricle becomes
invaginated, and this invagination divides into two parts which extend
forward into each lateral ventricle. These invaginations or folds are
partially produced and augmented by the development of blood capillaries within their walls, and they thus come to constitute the anterior
‘choroid plexus or plexuses.
The sides of the diencephalon. are eventually thickened to form the
optic thalami, the thalami of each side being connected by a median
fusion of the walls. The transverse band of tissue formed by this fusion
is called the massa intermediu.
The Mesencepha1on.——As previously indicated, the roof of this
region, which in the Bird forms mainly the optic lobes, in the Mammal
gives rise to the corpora quadrigemina. As the name suggests, these consist of four, instead of two, thickened outpushings which, as already
noted, are well covered in the adult by the large cerebral hemispheres.
The more anterior pair are apparently more or less homologous in function with the avian optic lobes, and might be so named, but are not. Instead they are called the superior colliculi. The posterior pair are cen
s ters for auditory reflexes, and hence might be referred to as auditory
lobes, but again their actual names are the inferior colliculi. The sides
and floor of the mesencephalon become greatly thickened by fiber tracts
connecting the anterior andposterior parts of the brain. In the Bird
they were designated as the crura cerebri, though this term is not so
commonly employed ‘in the Mammal. Here these regions are often referred to as peduncles. At all events the growth of these parts com-4
presses the lumen of this region of the brain into a narrow canal connecting the third and fourth ventricles, and universally termed the
aqueduct of Sylvius.
The Rhombencephalon. — It will be recalled that in the Mammal,
as in the Bird, thenposterior part of the brain, i.e., the rhombencephalon,
is early divided into two parts, the anterior metencephalon and the posterior myelencephalon. The former is the shorter region, and indeed consists primarily in itsdorsal aspect of the thickened ‘sloping roof of the
posterior side of the isthmus fold (Fig. 297). As in the Chick this dorTHE NEURAL TUBE 613
' sal region presently undergoes extensive growth to form the cerebellum,
a part of the brain especially concerned with muscular coordination.
The division of this organ into a median lobe, the vermis, and lateral
lobes, which appeared to some extent in the Bird, is still further emphasized in the Mammal, and in addition each lobe develops, extensive foldings (Fig. 326). Ventro-laterally beneath the cerebellum the walls of
the metencephalon are greatly thickened by fiber tracts, partly from
fibers originating in the cerebellum itself, and partly from fibers
passing through these walls to and from anterior parts of the brain.
In this region, as in the mid-brain, the thickenings so caused are
often designated as peduncles. The ventral thickening becomes so pronounced eventually as almost to comprise a sort of reversed flexure. It
is called the pans, and because of the eflect just indicated is sometimes
referred to as the pontine flexure (see the Chick). Beside the thickenings caused by the fiber tracts there is also at deeper levels the development of numerous neurones connected with the cranial nerves which arise
from the sides of this part of the brain. The lumen of the metencephalon remains fairly large, and is considered a part of the fourth ventricle.
Posterior to the metencephalon the myelencephalon becomesa tube
which tapers off into the spinal cord, and is designated as the medulla.
In most respects the medulla resembles the cord except that it is wider,
especially anteriorly, and its extensive roof consists of a thin membrane
into which blood capillaries soon press. This produces a vascular ‘infolding similar to that described in connection with the diencephalon,
and in this case termed the posterior choroid plexus. The broad shallow cavity of this region into which these folds push is also quite extensive, and constitutes the larger part of the fourth ventricle. The ventralateral walls of the medulla are essentially similar to what has already
been described with respect to the walls of the neural tube. They consist
internally of a lining of ependymal cells, a middle mantle layer of neuroblasts which become nerve cells, and an outer marginal layer of iibers. It may be further noted that dissection, or cross sections, show
that a groove runs along either side of the internal wall of this region,
termed the sulcus limitans, dividing it into a dorsal and ventral part.
THE NEURAL TUBE
When last noted at 10 mm. the essential layers and types of cells in
the tube were already beginning to differentiate. Further development
consists mainly in the continued production and difierentiation of these
cells, so that the cord not only becomes larger, but assumes its charac614 THE LATER DEVELOPMENT OF THE PIG
teristic shape. Thus in cross section we find the ependymal cells lining
the now relatively small central canal, and sending their supporting
processes transversely through the substance of the cord. Within the
mantle layer the spongioblasts ultimately all become supporting cells
of other types, while the neuroblasts all finally become transformed into
nerve cells. As a result of growth this layer finally assumes in cross section a somewhat butterfly shape (i.e., with wings extended), constituting
the so-called gray matter of the cord. The dorsal andrventral extensions
(horns) of the “ butterfly wings ” serve to divide the outer marginal
layer into four tracts of relatively white material. These tracts or columns consist of bundles of medulated fibers, the myelin substance in
the fiber sheaves giving the tracts their white appearance. The dorsal
column consists mainly of sensory fibers conducting impulses to the
brain, while the two lateral columns and the ventral column are motor '
paths from the brain to the various spinal nerves.
THE CRANIAL NERVES
The origins of all cranial nerves, save the I and II, have already been
indicated, and there is little more that need be said about them except
to note briefly the parts which they ultimately innervate in the Pig. In
general the relationships of nerves and parts are the same as in the
Chick in so far as comparablestructures exist. Thus the III or oculamotor nerve as usual supplies the inferior oblique, and the superior,
inferior and internal (anterior) rectus muscles of the eye. The IV or
trochlear nerve innervates the superior oblique eye muscle, while the
external (posterior) eye muscle is innervated by the VI or abducens
nerve. Passing to the most anterior of the mixed neigves we find that the
ophthalmic branch of the V or trigeminal nerve comes to supply the
snout, eyeball, and upper eyelid; the maxillary branch supplies the upper lip, jaw, palate, face and lower eyelid; the mandibular branch supplies the tongue, lips, muscles of the jaw, the lower jaw itself, and the
external ear. The VII or facial’ nerve was but slightly developed at 10
mm. As its name suggests, it supplies the face, and is primarily motor,
though the existence upon it of the geniculate ganglion shows that it
contains some sensory fibers. These fibers come eventually to join the
mandibular branch of the V nerve and evidence indicates that they concern the sense of tiiste. The VIII is of course" the auditory nerve, and is
‘entirely sensory, being concerned with both hearing and the sense of
equilibrium. Though at first closely associated with the VII its ganglion
later becomes more distinct, and eventually divides into two parts the
THE SPINAL NERVES 615,
vestibular ganglion and the spiral ganglion. The branch from the former supplies the semicircular canals, is termed the vestibular nerve,
and is concerned with equilibrium. The cochlear nerve from the spiral
ganglion innervates the cochlea, and is concerned with hearing. The IX
or glossopharyngeal nerve fibers are mainly sensory, and come to sup-V
ply the pharynx and tongue. Such motor fibers as there are pass to the
pharynx. The X or vagus nerve develops further as follows: Sensory fibers from the ganglion jugulare come to innervate the external ear,
while sensory fibers from the ganglion nodosum eventually reach the
pharynx, larynx, trachea, esophagus and thoracic and ‘abdominal viscera. Motor fibers of the X nerve innervate the pharynx and larynx,
while other motor fibers connect with the sympathetic ganglia, and supply the visceral musculature. The XI or spinal accessory nerve, as previously noted, loses Froriep’s ganglion (which disappears), and thus
this nerve becomes entirely motor, and its fibers are very closely associated with the motor fibers of the vagus. Many of them also run to sympathetic ganglia, and thence to the viscera. Other motor fibers of this
nerve help to. innervate the pharynx and larynx, while still others originating along the cervical region of the cord proceed to the trapezius
and sterno-cleido-mastoid inuscles. The XII or hypoglossal nerve is the
motor nerve oflthe tongue. The muscles which it innervates originate
further back and migrate anteriorly as the tongue develops, carrying the
branches of the XII nerve along with them. Indeed phylogenetically the
tongue muscles are probably derived from the occipital myotomes, and
the XII nerve was -originally a spinal nerve which has recently become
cranial. '
The origin and development of the I and II cranial nerves will be
taken up in connection with the organs of special sense along with
which they develop.
SPINAL NERVES
The Somatic Nerves. —— As regards the further development of the
somatic spinal nerves, it may be said that their afierent and efferent fibers grow until they come in contact respectively with skin or muscle.
Then as the latter parts develop and move further away the. fibers grow
so as to maintain their contact. The sheaths of these fibers have two.
sources. The neurilemma is formed of cells of ectodemial origin which
accompany the fibers as they grow out. The myelin sheath. on the other
hand is not itself cellular; but is a cell product which accumulates at
numerous points between the neurilemma and the nerve fibers. These
616 V THE LATER DEVELOPMENT OF THE PIG
accumulations then spread until they meet, the meeting points forming
the nodes of Ranvier.
The Autonomic Nerves. ——The origins of the autonomic nervous
system have already been stated, and the fact that it involves both parasympathetic and sympathetic parts. Each part of course has to do with
controlling the involuntary movements of the viscera, and as in the case
of the somatic nerves, when the fibers make contact with the organs
which they are to innervate they grow with them. It is of interest that
the two parts of the system largely overlap with respect to the structures
which they reach, and that they have opposing functions. Thus the symp_athetic fibers reaching the heart from certain postganglionic neurones
carry accelerating nerve impulses. On the other hand, impulses in the
parasympathetic fibers from the brain via the vagus nerve to postganglionic neurones on the organ itself, have a retarding influence.
THE ORGANS OF SPECIAL SENSE
THE OLFACTORY ORGAN AND I NERVE
Following the formation of the olfactory pits, and the establishment
posteriorly of their communications with the oral cavity, the further
development of the olfactory organs proceeds as follows: In the lateral
walls of each nasal chamber folds develop known as conchae or nasaturbinals, these folds being more numerous in many lower animals and
in the human fetus than in the human adult. Meanwhile the epithelium,
at first simple cuboidal, soon becomes more or less stratified columnar
throughout a large part of its extent, with the occurrence of many ciliated and goblet cells. On the more dorsal conchae, and on the median
septum formed by the fusion of the naso-median processes, however, the
original cuboidal epithelium becomes transformed into that of the specifically olfactory type. In these regions no goblet cells are formed, and
the tall columnar cells which develop here lack cilia.‘Also just beneath
the surface certain of the cells turn out _to be neuroblastic. From each
of these a fine bristle-like process projects through the epithelium to
the surface. At the same time from its opposite pole each of these cells
sends an axone to the olfactory bulb or lobe of the brain. The bundle of
axones from each of the two olfactory areas then come to constitute the I
or olfactory nerves’. Eventually the various nasal sinuses, i.e., the'ethmoid, maxillary and frontal are developed by the invasion of the bone
by the non-olfactorys nasal mucosa which gradually excavates the bone
‘substance, and then lines the spaces so formed. The further development
THE AUDITORY ORGAN -617
of the posterior nasal passages and the internal nares will be referred to
in connection with the account of the oral cavity.
THE EYE AND ‘OPTIC NERVE
Except for one feature the development of this important organ is essentially the same in the Mammal as in the Bird, where it was described
in some detail. The vascular pecten, presumably an organ aiding in the
nutrition of the inner parts of the Bird eye, does not exist in the eye of
the Mammal. There are, however, blood vessels of course which supply
the mammalian retina and lens. These are capillaries arising from a
branch of the ophthalmic artery. This branch enters the optic cup along
t the groove on the ventral side of the optic stalk by way of the proximal
part of the choroid fissure. It is atfirst called the Ityaloid artery because it supplies only the developing lens, but later it supplies the retina also, and is then called the central artery of the retina. Shortly after
it appears, axones from the cells of the neuroblasts (future ganglionic)
layer of the retina start growing back along the artery which they soon
come to surround. As the number of these fibers increases they encroach on the tissue of the original stalk. Finally they become medullated and surrounded by a connective tissue sheath, while the old stalk
cells are virtually eliminated. Thus are produced the I or optic nerves.
As is well known, in the case of the mammalian eye the fibers from the
median sides of the two retinas cross in the optic chiasma, while those
from the lateral sides do not.
As suggested the development of the eye proper, aside from the
points noted, is so similar to that of the Chick that no further comment
on it is deemed necessary.
THE AUDITORY ORGAN
The Membranous Labyrinth. -—- In the 10mm. Pig the only indication of the auditory organ was the occurrence of the usual otic vesicle with its upgrowing endolymphatic duct. It now remains to state
that from this vesicle the membranous labyrinth of the inner ear develops essentially as in the Bird, except that 'n the Mammal one feature of
it develops considerably further. Thus it wi 1 be recalled that in the former case the semicircular canals arise from the upper part of the otocyst
‘termed the utricle. Then the lower portion of the otocyst partly con- _
stricts away, and produces an outpocketing called the sacculus. Up to '
this point the situations in the Bird and Mammal are similar. In the
Bird, however, it will be remembered that the larger part of the ventral
618 THE LATER DEVELOPMENT OF THE PIG
 
 
 
posterior
semi cir;ulu
~ canal
endolymphatk duct
endolymphatlt duet “Perm.
semicircular canal _-_ ‘
posterior
 
 
superior semicircular canal
endoiymphatic duct
' posterior}
 
_._..——_cochiur duct
(ductus eociilearls)
A rgan oi‘Corti
'‘ cochlear duct
3
E
n
3
In
S
U‘
E
:
 
 
ienestra rotunda ‘
zochlear duct
Euxuehian tube
. auditory nerve
seal: vestibuli
scaia tympani
 
Fig. 327.--A,'B, C and D, stages in the development of the membranous labyrinth of the Human ear. After Sireeten Although this is the Human ear and not
that of the Pig, the latter is presumably very similar, as are those of all Mammals
so far as known. All views are of the left ear from the left, i.e., lateral, side. A.
The otic vesicle from a 6.6 mm. embryo, showing rudiments of the membranous
semicircular canals» starting to form, also the beginning of the endolymphatic duct.
B. Membranous labyrinth from a 13 mm. embryo. C. Membranous labyrinth from
a 20 mm. embryo. D. Membranous labyrinth from a 30 mm. embryo. E. A semidiagrammatic representation of the middle and inner ear opened from the side.
Modified from various sources. F. A diagrammatic section through one side of the
cochlea, including of course the scala tympani kind vestibuli and the cochlear duct,
showing the organ of Corti.
portion of the otocyst is not involved in the sacculus, but grows out into
a relatively short tube termed the lagena. In the Mammal these same
parts exist, but here'the whole “ lagena ” is called the ductus cochlearis
or cochlear duct, and its connection to the utricle becomes narrowed to a
slender tube, the ductw -reuniens. Furthermore the remainder of the
mammalian ductus cochlearis continue: to grow until it has produced
i
I
i
1
i
fE5¢a.
THE AUDITORY ORGAN 619
an extensive spiral tube on whose floor the cells eventually become re.
arranged and differentiated to form the organ of Corti, and the tectorial
membrane. These last named structures, the most elaborate parts of the
organ of hearing, have no counterpart in the Bird. This, it may be suggested, is a somewhat remarkable fact in view of the auditory stimuli
which some members of this latter group can produce, and hence presumably appreciate. Surely the song of the Nightingale should require a
more complicated organ of reception than the Pigs grunt! Finally it remains to state that, as in the Chick, the whole membranous structure
derived from ectoderm is closely covered by a mesenchymal layer, the
membrana propria (Fig. 327).
The Bony Labyrinth.—Again as in the Bird, there has been developed around the membranous labyrinth and its mesenchymal membrana propria a bony labyrinth, the two labyrinths being separated by
the perilymphatic space. Naturally, however, in this case the bony capsule or labyrinth has also to be more elaborately formed in order to encase the spiral ductus cochlearis. Not only does it also become spiral in
order thus to encase this region, but in doing so it becomes divided into
two channels. One, dorsal to the ductus cochlearis, is the scala vestibuli,
while the other ventral to it is the scala tympani. At the apex of the
spiral, at the end of the ductus cochlearis these channels communicate.
At the other end surrounding the sacculus and the utricle the wall of
the scala vestibuli contains the fenestra ovalis to whose membranous
covering is attached a bone of the middle ear. The wall of the scala
tympani in this region contains the fenestra rotunda also covered by a
membrane.
The Middle Ear. —— Considering next the middle car we find again i
the same parts involved as in the Chick, but once more with a slightly
‘ dilierent outcome in certain respects. The first or hyomandibular pouch.
grows out-until it makes contact with the ventral part of the corresponding visceral furrow. This initial contact, however, does not long continue. The upper part of the pouch enlarges, but at the same time withdraws somewhat from the ectoderm of the furrow, while between them
mesenchyme develops. Presently within this mesenchyme cartilaginous
concentrations arise, representing the developing ear bones or ossicles.
In this case, however, instead of there finally developing only one such
bone, the columella, three of them appear——-—the‘ malleus, incus and‘
stapes ( Fig. .327). At the same time that the cartilaginous anlagen are
becoming ossified to form these bones, the mesenchyme surrounding
them is being absorbed. As this occurs, the upper end of the visceral
620 THE LATER DEVELOPMENT OF THE PIG
pouch once more extends so that it surrounds the developing ossicles,
including a little of the disappearing mesenchyme. It also again almost
reaches the outer ectoderrn, being separated from it only by a thin sheet
of mesenchyme. Thus there is formed the permanent cavity of the
‘ middle ear, or tympanic cavity. The part of the visceral pouch between
this cavity and the pharynx remains, of course, as the Eustachian tube.
It thus also comes about that the tym panic membrane or tympanum consists, as in previous cases, of tissue derived from each of the germ layers, the outer lining being ectodermal, the middle layer mesodermal,
and the inner lining endodermal. On its median side the lining of the
tympanic cavity is in contact with the bony capsule of the inner ear, and
so forms a membrane over each of its two fenestra. To the membrane
covering one of these, the fenestra ovalis, the stapes is attached, while
at the other end of the bony chain the malleus of course is fastened to
the tympanum. Though most of the mesenchyme about the ear bones is
ultimately absorbed, some of it becomes dilferentiated into the small‘
muscles attaching the ossicles to the wall of the tympanic cavity. It is
also interesting to note that in Man this mesenchyme does not entirely
disappear until a few months after birth. This apparently serves to prevent free movement of the ossicles, and thus to protect the ear of the infant from too strong stimulus by loud noises.
Homologies.——Turning now to the possible homologies of the
mammalian ear bones, it will be well to recall the situations which were
described in the Frog and Chick. Thus in the former animal, though
only one bone, the columella, finally existed as a separate entity within
the completed middle ear, there were originally two elements concerned.
For, fused to the inner end of the columella, there was also the operculum, lying within the fenestra ovalis. At its outer end, moreover, the
columella connected with a ring of cartilage around the tympanic membrane called the annulus tympanicus. In the Chick there was again a
columella which fused with an opercular element, in that case called
the stapes, but the -annulus tympanicus was lacking. In these cases it
was suggested that the columella was possibly the homologue of the
hyomandibular element ofthe hyoid arch of the primitive fishes, and
that the annulus tympanicus might be the homologue of the pa1atoquadrate cartilage of such forms. In the Mammal, where there are three
separate ossicles, the question of possible homologies again arises. It
has been suggested that the mammalian stapes corresponds to the columella, and hence ultimately to the hyornandibular, the incus to the
palato-quadrate (primitive upper jaw) and the malleus to Meckel’s carnuxzw-—-.—. .....‘,ws.,..._........._..., C,»
THE ORAL CAVITY 621
tilage (primitive lower jaw). This obviously leaves the opercular element of the Frog and the stapes of the Chick quite out of the picture.
As stated in connection with the Frog, there is good evidence, experimental and otherwise, to support these suggested homologies, and they
are, therefore, quite generally accepted. Thus the intriguing notion that
parts once connected with the coarse work of seizing food have finally
been promoted to the delicate “ white collar ” task of transmitting sound ‘
waves, seems to be well established. It probably affords an example of
functional adaptation correlated with a changing environment.
THE DIGESTIVE AND RESPIRATORY SYSTEMS
The Oral Cavity.-——0riginally the anlage of the oral cavity existed merely as the stomodaeum, a relatively shallow pocket lined with
ectoderm. By the 10 mm. stage, the oral plate which constituted the
stomodaeal union with the fore-gut had broken through, and the roof
of the stomodaeal cavity had given rise to Rathke’s pocket. Subsequent
to 10 mm. the stomodaeum becomes greatly deepened to form the ac~
tual oral cavity, while Rathke’s pocket becomes separated from it, and
as already noted, gives rise to the anterior part of the pituitary. The
deepening of the cavity as just suggested is extensive; so much so in
fact, that eventually we find the tonsils occurring "at about the original
site of the oral plate. This enlargement is brought about chiefly by the _
outgrowth of the mandible, and the various processes giving rise to the
face, nose and upper jaws. The external aspects of this procedure have
already been described, but it remains to indicate some of the details
more especially concerned with the mouth itself. Thus it will be recalled
that the maxillary processes formed the sides of the upper jaw (maxillae) , while the anterior tip was derived from the fused naso-medial processes. This tip is the premaxillaryt region, and from it there grows backward a small median plate constituting the more anterior portion of the
palate, and termed the median palatine process (Fig. 328). By far the
larger part of the permanent roof of the mouth, however, is formed by
the two lateral plates, the lateral palatine processes. These are simply
median extensions of the maxillary processes which soon meet and fuse
in the middle line. The more posterior plate so formed then unites with
the median palatine process and thus together these parts constitute the
complete hard palate. It is now to be recalled that ‘the temporary internal nares open into the oral cavity through its original roof fairly near
the front.,The formation of this new roof beneath the first one, however,
creates a new chamber between the two roofs into which the nares open."
622 THE LATER DEVELOPMENT OF THE PTG
2: mm. D 29 mm.
 
 
median ‘palatine process _
     
   
 
 
 
 
cerebral hemispheres
nasal cartilage
\
.§:_
nasal chamber
 
   
 
 
$3
'5'‘.**?-’*%
..~.\
 
 
 
nasal septum
\\
lateral palatine process '
tongue
nasal cartilage
   
 
nasal septum
fused lateral palatlne process
tongue ,. _
' mandibular cartilage
Fig. 328.——Illustrations to show the development of the roof of the mouth and
the nares of the Pig. A and B. The roofs of the mouths of specimens of the sizes
indicated, the lower jaw having been removed. C and D. Transverse sections of the
snouts of the same specimens at the levels indicated by the lines at each side of A
and of B. E. A transverse” section, made with a microtome, of a snout of a somewhat
older embryo than D at about the same level. This section appears somewhat
smaller than D because it does not show the surrounding parts of the head, and
because it was apparent1y.somewhat compressed laterally in cutting.
THE ORAL CAVITY 623
The further development of the nasal septum to fuse with the new or
1 lower roof then divides this chamber into two lateral parts. In this way
' there is produced essentially a posterior extension of the nasal cavities
 
 
 
 
 
 
tongue
ameloblast layer
5 enamel
- entine
' odontoblast layer
enamel pulp of
enamel organ
dental sac
dental papilla (pulp)
‘ 9--‘iosteum
bone crabesulae of mandible
Fig. 329.—-A. Transverse section through the right side of the
lower jaw and tongue of a Pig embryo somewhat older than the
oldest in Fig. 328, showing the beginning of tooth development. BThe same section shown in A, but at a much lower magnification soas to show the whole jaw, with an indication of the part from which
A was taken. Connection of enamel organ with dental ledge has
gone.
so that the definitive internal nares eventually open well back toward the
throat.
' While this is going on in the roof of the month, the tongue is being
formed in the-floor. As in the Chick it is made up of three thickenings,
a median one called the tuberculum impar, and a pair of lateral ones.
These lateral primordia soon overgrow the median one to form a single
mass which for a time lies between the lateral palatine processes. As
these come together, however, the tongue drops down to its adult posi
tion (Fig. 328).
624 THE LATER DEVELOPMENT OF THE PIG
Finally by the 23-30 mm. stage a thickening of the oral epithelium
(ectoderm) has developed around the border of both jaws. This thickening, termed the labio-clental ledge or lamina, pushes into the underlying mesenchyrne, and presently its inner and outer edges become particularly developed. The outer edge or part is called the labia-gingival
lamina (later a groove), and serves to separate the lip from the inner
part of the originally single thickening (Fig. 329). This inner part is
called the dental ledge or lamina, and within it the teeth eventually develop. Since these latter structures do not occur at all in modern Birds,
and were not mentioned in the Frog where they are not highly evolved,
we shall consider their formation separately along with that peculiarly
mammalian product, hair. _
The Pharynx.—~The pharynx begins at approximately the line
where the oral plate disappears, and thus is the most anterior part of
the alimentary and respiratory tracts to be lined by endoderm. It is also
the part which is flanked laterally by the remains of the visceral arches
posterior to the mandibular, and by the pouches. These arches and
pouches very shortly disappear as such, but as will be apparent, their
remains give rise to Various adult structures as follows:
Thus the second or hyoid pair definitely produce the styloid processes and lesser horns of the hyoid. There is also the possibility, as
noted, that the columella (mammalian stapes) of the car may be derived from it. The third pair of arches give rise to the greater horns of
the hyoid, while from the fourth pair of arches is derived the thyroid
cartilage of the larynx. No distinct fifth arches are ever visible, in the
Pig, but from the region where they should lie come the cricoicl and
arytenoid cartilages. All of these parts are of course involved in the
formation of the larynx, and immediately adjacent structures.
Turning to the products of the visceral pouches we find that, as we
have already noted, the first or hyomandibular pouches take part in the
formation of the Eustachian tubes and tympanic cavities. The second
pair in connection with ingrowths of lymphoidtissue produce the main
or palatine tonsils. The third pair give rise to the main or definitive
thymus bodies (thymus III), which migrate posteriorly until they are
eventually located in the upper part -of the thorax. It is interesting to
note that in the Guinea Pig the thymus bodies are permanently in the
neck instead of the thorax. This is apparently because the third pouches
in this case are so firmly fused to the ectoderm that they cannot be carried backward (Klapper, ’46). In addition to becoming transformed
into thymus tissue this third pouches also produce outgrowthswhich beaun:¢».». %. ._.- -
THE PHARYNX 625
come the chief pair of parathyroids (parathyroid III). These are located in the neck where they are ultimately associated closely with the
posterior parts of the thyroid. With respect to the fourth pair of visceral
pouch derivatives there has been some disagreement. So far as the Pig
is concerned Godwin ( ’4~0) concludes that, as noted, this pair of pouches
are not always present. When they are, he thinks that the remains of the
pouches proper become incipient thymus bodies (thymus IV) which
later disappear. In addition there are produced in this animal two distinct outgrowths either from the pouches if they are present, or if they
llnd visceral pouch
 
 
lst visceral pouch
lllrd visceral pouch Rathke 5 pocket
 
Nth visceral pouch
Fig. 330.—The pharyngeal region of a 10 mm. Pig embryo, showing diagrammatically the regions iroin which the thyroid, thymus and parathyroid bodies either
have been, or will be, derived.
are not, from the region of the pharynx where they would be. One of
these outgrowths is an additional pair of parathyroids (parathyroid
IV), each of which, according to ‘Godwin, soon divides into two parts
which persist. Others, however, have claimed that they disappear. The
other outgrowths are the pair of post-branchial bodies. Each of these
bodies eventually becomes embedded in the thyroid gland. According to
Godwin, however, there is nothing to indicate that they ever become actual thyroid tissue as believed by some (Fig. 330).
The thyroid gland as in other forms arises as an evagination from the
‘ floor of the pharynx between the first and second visceral pouches. It
soon loses its connection with the pharyngeal floor and becomes almost,
though not quite, completely divided into two lobes (Fig. 296). These
lobes then migrate posteriorly somewhat to lie eventually at the base of
the neck. As noted the parathyroids are closely associated with the thyroid, and the ultimo-branchial body becomes imbedded in it, whether a
part of it or not. Though the thyroid becomes separated from its point
of origin this point at the future root of the tongue“ is marked, in Man»
626 THE LATER DEVELOPMENT OF THE PIG
   
 
     
   
 
—-—dorsal mesentery
(rnesogastrium)
line of stomac
h attachment of
“°'““ dorsal mesentery
(mesogutflum) ventral mesentery
“var g3St|’0—hep:ttlc omcntum
n —-—ventral rnesentery
‘,5 B (falciform ligament)
d I stomach dorsal mesentery
0'53 m¢$¢“‘¢"Y meso astrium
(mesogutflum) \’ stomach J ( 3 )
liver
D falclform ligament
eplplol: foreman --——§u
gastric-hepatic omencum 5°33‘ mew“?-=77
(mesogastrlum)
dorsal mesentery
(mesogastrlum) “°"‘3Ch
eplplolc foramen ‘ spiem
falciform ligament
dorsal mcsogasrrlurn
spleen
Fig. 331.———A, .C and E are semi-diagrammatic representations of the developing
stomach and mesenteries of the Pig, as viewed from the ventral side. The dash
lines in C and E represent the part of the mesogastrium on the dorsal side which is‘
covered by the stomach in this view. The liver and ventral mesenteries (gastro
hepatic otnentum and falciform ligament) are not shown in these figures as they
would obscure the stomach. B, D and F are diagrams of transverse sections through
A, C and E viewed from the anterior. G is a diagram of a transverse section of
liver, stomach and colon in Man at a later stage when the stomach and colon have
become transverse to the body. Hence this section is mid-sagittal for the body as a
whole» The great ofnenturn, which does not occur in the Pig, is obviously an extension of the fall}. of the original dorsal mesentery down across the anterior (ventral)
wall of the abdomen. It,is largely this fold which accumulates fat in older persons.
THE ESOPHAGUS 627
at least, by a permanent depression, the foramen caecum. The histological differentiation of the thyroid is fairly simple. The endodermal derivatives become broken up into nests of cells which form the secreting
follicles, surrounded by mesodermal connective tissue ‘and blood capillaries.
One other structure of the pharynx remains to he mentioned, the epiglottis. It arises as a thickening in the floor of the pharynx just posterior
   
 
esophagus
 
 
 
 
 
stomach
yolh stalk future duodenum
future ileum
   
bulbus coli
future colon
I2 mm. 24mm.
stomach
caecum
30mm. i 35 mm.
Fig. 332.— Stages in the development of the intestine of the Pig from
the gut loop stage to that in a 35 mm. embryo. After Linehack.
to the lower ends of the third pair of visceral arches. It grows posteriorly, and eventually overhangs the slit-like opening to the larynx, i.e.,
the glottis.
The Esophagus.—At the back of the pharynx the original gut
canal had become separated at 10 mm. into a dorsal and ventral division, and the latter was starting to become differentiated into the respiratory system. The dorsal part, on the other hand, was already becoming
narrowed to constitute the esophagus. In carrying on the description of
these parts it will be convenient to discuss the digestive portion of the
originally undivided gut separately from its respiratory derivatives. In
so doing we shall consider the former first.
628 THE LATER DEVELOPMENT OF THE PIG
The esophageal part of the digestive tract posterior to the pharynx is,
as previously indicated, already relatively constricted. Its inner endodermal lining becomes differentiated into a smooth non-ciliated epithelial layer, and into mucous glands which extend into the connective
tissue (submucosa) beneath the epithelium. The connective tissue and
muscular coats are of course derived from»-mesoderm. ~
The Stomach and Its Mesenteries. — At 10 mm. the stomach was
represented by an enlargement in the primitive gut posterior to the
esophagus. As elsewhere this part of the gut was attached to the dorsal
body wall by its dorsal mesentery (dorsal mesogastrium). This en
 
descending colon
50mm. 95mm
Fig. 333. ——-A continuation of the development of the Pig intestine shown in Fig.
332 with special reference to the region of the colon. After Lineback. .
larged region is already slightly bent with the convex side dorsal, and
very shortly three things happen to it. (1) The bend increases, (2) the
anterior end shifts to the left, and (3) the whole structure rotates on its
longitudinal axis in a clockwise direction when viewed from the
esophageal end. As these movements take place it is obvious that some
adjustment must be made by the attached mesenteries. What occurs is
that the dorsal mesogastrium is extended to accommodate the bending
and rotation of the stomach. Furthermore, since the line of attachment
of mesentery to stomach does not change as the stomach rotates, ‘this line
necessarily rotates with it. Thus in the new position the line of mesenteric attachment simply follows the curve around the left convex side of
the organ. As these changes occur with respect to the dorsal mesentery,
the ventral mesentery has likewise had to shift its position so that it
now leaves the stomach on the concave side of the latter (Fig. 331).
In connection with these alterations certain further facts need now
to be noted as foliows. We have seen how, as the stomach changes its
THE INTESTINE 629
position, the dorsal and ventral mesenteries change to accommodate it.
In the course of this accommodation it is clear that the dorsal mesentery must increase in extent. It remains to add, however, that this mesontery increases more than would be required by the shift of the stomach.
As a result a fold of the mesentery comes to extend out beyond the
stomach so as to form a sort of wide pocket. This fold and pocket are
called the omental bursa, the spleen later developing within the walls
of the fold. Inspection of Figure 331 will show that an opening from the
general coelom into this more restricted pocket area occurs from one
side. This opening, at first quite wide, becomes much narrowed later on,
and is known as the epiploic foramen. In Man the fold itself also develops further to form still another structure which will be noted in
connection with the development of the intestine.
The Intestine. -—The intestine at 10 mm. consisted anteriorly of a
short region to which the liver and pancreas were attached, the duodenum, followed by a loop whose limbs passed into and out of the umbilical stalk. At the ventral apex of this loop a very narrow tube still
represented the yolk-stalk, while the upper end of the posterior limb
bent around caudally to the rectum (Fig. 332, 12 mm.). The whole
structure was of course supported by a mesentery. By the 24« mm. stage
the anterior limb of the former simple loop has become very markedly
coiled, and it is this region which forms the main part of the small intestine. Upon the posterior limb of the loop a short distance from the
apex, a slight outpocketing or caecum was evident at 10 mm., and
shortly thereafter it becomes a distinct diverticulum (Fig. 332, 24 mm.) .
In Man this caecum gives rise to a finger-like extension, the vermiform
appendix. From the point where the caecum grows out the distal part of
the original posterior loop becomes the large intestine or colon. Eventually this part bends so that the small intestine enters it at a right
angle. Also it too becomes coiled, forming a loop, a condition not found
in Man (Fig. 333). In correlation with all this bending and coiling the
dorsal mesentery of these parts of the intestine also becomes thrown
into somewhat involved configurations which it is not necessary to go
into. It is of interest, however, to note a further development of the
mesentery in the region of the stomach which occurs in the case of Man,
but not in the Pig. It occurs as follows:
The fold of the bursa, as previously described for the Pig continues
subsequently to increase in extent in the human embryo, and to grow
caudad, until eventually it comes into contact with the‘ parts of the colon
occupying a transverse position in Man. When this condition is reached
630 THE LATER DE/"EL -0P"MENT OF THE PIG
the bursal fold fuses with theepaetitczmeal covering of the colon, and
later, after birth, continues ‘to grocsw still further in a caudal direction.
At the same time the two liimlitsctftltie fold beyond the line of fusion
with the colon unite with ome antotlzflier no form a double sheet. This sheet,
the great omenlum, thus co-nstiitutmesa. sort of apron covering the lower
abdominal viscera on their veentrz-3 al( anterior) side between them and
the ventral body wall (Fig. 3311]." This is possible because in this region
the ventral mesentery haslnng‘; sithnce disappeared. Later this part of the
omentum usually becoxrnesastoontgeptt lace for fat, a feature which is frequently all too obvious in caldeerirz:-en .and women.
The Recturn.—At the 1(1) Ir::-.1111. stage the cloaca, into which the
large intestine opens, vtras in garoczess of being divided by the urorectal
fold to form the rectum arid tjhe urimogenital sinus. The cloacal membrane also had not yet rutptumerl I. Tlte completion of these processes,
however, is more readilydescr"ihoo din. connection with the description of
the development of the exte» rnulg<=_=2nitalia. and related parts. It will therefore be deferred until that subojeci-tis discussed.
The Liver and Its Nfesantaerie-s.——We are now prepared to return to the develop‘me1:1‘o‘E th:-istx_)utg:roWth of the duodenum. It will be
recalled that in the Pig th_ere is only one hepatic diverticulum insteadof two. This single outgr owtth 0 (ductus choledochus), moreover, had
produced several anter iorl ytllireoctecl buds, the anlagen of the liver tubules, while the remainso ftlneo outgrowth was extending posteriorly as
the anlage of the cystic duct zantllgall bladder (Fig. 307). This anlage
rapidly elongates to form theieefiuiiive duct while its end. enlarges to
produce a bladder. Me anwhiloethaeamteriorly directed tubules grow out
into the ventral mesentery’ wlr:1ete-ethey soon come into contact with the
Vll"‘lliI16 (oniphalozmesenteric ]v-veins; into which they push. They thus
break these vessels up in to finnr umeerable sinusoidal capillaries which
ramify amongst the liv’ert:uh1..t1les.a.ln this manner the tubules and capil
laries come .to constitute the manin mass of the hepatic substance with
i only a relatively small arnountttcifstupporting connective tissue. Having
completed our description-_ of ties dervelopment of the organ itself it remains to say a few words zregs arrli ing its mesenteries.
It has been repeatécllyslatecdthliat the liver develops within the ventral
mesentery of the Stomach ancfiitnodeanum. It may now be ‘added that the
part of this mesentery which .at ztachnes the hepatic mass to the intestine
and stomach is known as the: lesaser orrrentum, or sometimes the gastrohepatic omentum (gastro—lie1::patio cligarnent in the Chick). Beneath the
liver, i.e., between it amdtzlto ' van-ntral body wall, a small portion of mesLIVER AND ITS MESENTERIES 631
entery also permanently persists in the Mammal, where it is termed the
falciform ligament, connecting liver and body wall. This ligament is
absent in the Bird as previously noted (Figs; 331, 335).
The Pancreas. ——~Even as the liver in the Pig has only one origin
instead of two, so the Pig pancreas has only two origins instead of three.
The two primordia in question were already in evidence at 10 mm. One
consisted of an outgrowth from the dorsal side of the intestine of a mass
   
 
—-ductus choledochus
pancreatic ducts
d uodenai
diverciculum
ventral pancreas
Fig. 334.—-Later development of ‘the dorsal and ventral pancreas. Slightly modified from Thyng.
of cords at a level slightly caudad to the origin of the ductus choledochus. The other arose from the ventro-lateral side of the duct itself
(Fig. 307). The two growing masses soon fuse, and the cords of which
they consist become tubular. These in turn produce numerous buds
which develop into one of two things. Part of the buds remain connected
with the tubules, and form the pancreatic acini which produce digestive
secretions. The remaining buds become segregated, and constitute
among the tubules little aggregations of highly vascularized tissue, the
islets of Langerhans. Although the pancreas in the Pig has two origins
as indicated, the adult organ has only one duct. This is derived from the
dorsal outgrowth, and hence connects directly with the duodenum. The
ventral connection with the ductus choledochus in this case disappears
(Fig. 334). ~ , ‘
It is of interest to note at this point that in the Mammals generally
632 THE LATER DEVELOPMENT OF, THE PIG
this double, rather than triple, origin of the pancreas is the common procedure. Whether one or both primordia are to persist as ducts, however,
and if only one, which one, varies in different animals. Thus in the
Horse and Dog there are two permanent pancreatic ducts. In the Sheep
and Man on the other hand there is only one, and in these cases the ventral one opening into the base of the common bile duct. In the Ox, and
in the Pig (as already indicated), however, the dorsal duct is the persistent one, opening as noted into the duodenum.
Lastly, it should be recalled that as the liver outgrowths occur into
the ventral mesentery, so the pancreatic outgrowths push into the dorsal
mesentery. Furthermore, though they start into this mesentery at the
level of the duodenum, the fused pancreatic elements soon extend anteriorly into that part of the mesenterylsupporting the stomach, i.e., the
rnesogastrium. Then later as this forms the omental bursa we find the
pancreas in the more dorsal limb of the bursal fold, which eventually
becomes adherent to the dorsal wall of the coelom (Fig. 331).
The Respiratory System.——— The cartilages of the larynx have already been noted in connection with the fate of the visceral arches.
Also the initial development of the trachea and bronchial outgrowths
were indicated as present at 10 mm. Following this period the main
bronchial tubes and their branches continue to push out into the coelomic spaces (pleural cavities) beneath the esophagus and above the
heart‘( Fig. 303). The lining of the tubules is columnar or cuboidal, but
at their terminals the tubules produce little sacs, the lung alveoli, and
here the epithelium becomes thin and flat. ,
It must now be pointed out that when these endodermal outgrowths
first occur they do not really lie in the pleural cavities. Rather they lie
in a thick sheet of mesoderm which hangs from the dorsal body wall
like a rnesentery, and which, in addition to the trachea and lung buds,
also contains the esophagus. It is the dorsal part of the mediastinum.
Though within this structure at the start, the branching bronchi, as indicated, soon push out of it into the antero-lateral extensions of the
coelom termed the pleural canals or cavities. As they do so they carry,
reflected over them, a layer of mesoderm. This produces the mesothelium of the visceral pleura, the connective tissue about the alveoli
and bronchi, and the cartilaginous rings of the bronchi.‘ At the roots
of the lungs the mésothelium is of course reflected laterally onto the
1 It has been claimed (Clements, '38) that the endoderrnal epithelium of the
alveoli in the Pig (and probably other Mammals) later disappears entirely, leaving
the blood capillaries covered only by a very thin sheet of connective tissue.
~c,.a_..
DIVISION OF BODY CAVITY COMPLETED 633
outer wall of each pleural canal to form the parietal pleura. Finally it
remains to note that the pleural (coelomic) spaces within which the
lungs lie are not at first separated posteriorly from the rest of the
coelom. This and the completion of the pericardium comes about in a
manner which will now be described.
 
   
 
pleural cavity
> leuro-perlcardlal septum
aorta
ventral mesogastrium '_ -r
lT'|CS€|'1E¢I'y ..
Fig. 335. ——Diagrams to illustrate the separation of the pleural, pericardial and
abdominal cavities, and the formation of the diaphragm in the Pig and other Mammals. A. Transverse section of the body just behind the septum transversum. B.
Transverse section of the body through the lung region. C. Lateral view of median
region showing forming septa in relation to heart, liver, lungs and gut.
COMPLETION OF THE DIVISION OF THE BODY CAVITY
The Diaphragm. ——The development of the pericardium and diaphragm has already been described somewhat in the case of the Bird
where, however, the strictly diaphragmal parts are incompletely formed.
Also the structures involved are somewhat different in their origin. We
shall therefore start from the beginning in the Pig.
The first part of the diaphragm to appear is the septum transversum.
In this case it consists of a layer of tissue growing dorsad from the ven634.« THE LATER DEVELOPMENT OF THE PIG
tral body wall just anterior to the liver to whose face the septum is
fused. The median part of this septum also forms the posterior wall of
the pericardial cavity, i.e., the part of the parietal pericardium separating the cavity from the coelom posterior to it. The sides of the septum, however, form the ventro-lateral parts of the diaphragm separating
the ventral portions of the pleural cavities from the coelom posterior to
them. The dorso-lateral parts of the diaphragm completing this separation are formed by a pair of membranes, the pleura-peritoneal folds,
growing out from the body walls-(Fig. 335, A). In the middle they meet
the dorsal mediastinum and complete the diaphragm. These folds also
extend anteriorly in such a way as to bound the pleural cavities (canals) ventrally and the pericardial cavity dorsally. The ventral and
caudal growth of the lungs then occurs, causing these organs to lie
more on either side of the heart than above it. As this takes place the
lungs split off more and more of the pleural-peritoneal folds from the
body walls, and push these augmented folds before them._As this occurs
on the median side next to the heart, the folds come to constitute the _
lateral and ventral as well as the dorsal pericardial wall, and likewise
the medial pleural walls. Hence these parts of the pleural-peritoneal
folds (septum) are called the pleura-pericardial septum (Fig. 335, B, C ) .
The posterior pericardial wall formed by the median part of the septum transversum has already been noted. Anteriorly where the vessels
of the heart emerge, the parts of the parietal pericardium come together, and are reflected over the heart muscle as the visceral pericardium. Here also these parts fuse to form the dorsal mesocardium,
attached to what was the ventral edge of the dorsal part of the mediastinum. It is to be noted, however, that though the pleuro-pericardial
folds meet and fuse ventrally, the pleural cavities never become coextensive. Hence the ventral wall of the parietal pericardium is attached
to the ventral body wall. Thus the pericardium and heart now form a
central mass connecting the former ventral edge of the dorsal part of
the mediastinum with the body wall. This mass might then be referred
to as the ventral part of the mediastinum. Actually because of shifts dur
ing development the various parts of the mediastinum are difierently
named, but the details of this need not be gone into here.
THE CIRCULATORY SYSTEM
When this system was previously discussed we began with a description of the blood islands, and followed with the development of the
heart, leaving the intra-embryonic blood vessels until last. Nothing furTHE ARTERIES 635
th_er need be said of course about the blood islands which soon disappear, and for various reasons it is more convenient to begin with the
blood vessels rather than the heart. We shall therefore start with the
arteries.
THE ARTERIES
The Aortic Arches and Related Vessels. -—It will be recalled
that at 10 mm. the first pair of aortic arches had disappeared, while the
third, fourth and sixth remained, the fifth being vestigial. From the base
of the third pair the external carotids were just beginning to develop,
while the sixth pair had produced rudimentary pulmonary arteries. Dorsally the arches on each side were still connected by the dorsal» aortae
which continued anteriorly as the internal carotids. Posteriorly the aortae had fused as far forward as the anterior appendages, and posteriorly
to the tail.
Subsequent to 10 mm. we find that the base of each third arch between the origin of the respective external carotid and the point of origin of the fourth arch becomes lengthened somewhat. These lengthened
bases thus come to constitute the two common carotids (Fig. 317, B, C).
Conti.r..1ing posteriorly the part of each dorsal vessel between the third
and fourth arches as usual disappears, while on the left side the fourth
arch and the dorsal aorta posterior to it enlarge and persist as the main
or great aortic arch of the adult (Fig. 319, B). At this point two important differences between Bird and Mammal are to be noted. One of
course is the fact that in the former it was the right arch which so persisted. A second difference is that whereas in the Bird the fourth arch
opposite the great aorta entirely disappeared, in the Mammal it does
not. Thus in the Mammal this arch, in this case the right, remains to
form two things. Its proximal part constitutes the brachioceplzalic artery (innominate) while its more distal parts, together with a portion of
the right dorsal aorta, comprise the proximal part of the right subclcwian artery. The rest of the right dorsal aorta disappears. The left
subclavian, it may be noted, arises directly from the distal part of what
was the left dorsal aorta, but which later becomes simply a part of the
main aortic arch. The genesis of the right subclavian distal to its aortic
portion will be referred to presently. It now remains to add in connection with the carotids that in the Pig the left common carotid usually
shifts its point of attachment so that eventually it does not arise directly
from the left (main) aortic arch. Instead it emerges from the brachiocephalic close to the right common carotid (Fig. 319).
636 THE LATER DEVELOPMENT OF THE PIG
Passing now to the sixth aortic arches we are familiar with the manner in which they take part in the formation of the pulmonary arteries
in the Frog and Chick. It has been indicated also that this same situation occurs at first in the Pig (Fig. 316, E). Subsequent to 10 mm., however, certain changes occur which are a little different from events in
the Chick, or in other Mammals. Thus in the case of the Pig the two
pulmonary branches which proceed from the upper parts of the sixth
arches to the lungs, fuse with one another in their proximal regions.
This single branch then retains the connection with the left sixth arch,
but loses the connection with the right sixth which disappears completely. In this fashion it comes about in this animal that only the left
sixth arch is involved in the permanent pulmonary circulation (Figs.
317, 319). Meanwhile there develops within the truncus arteriosus a septum dividing it into two channels. One as usual leads from the left ventricle to the systemic aorta, and the other from the right ventricle to the
single pulmonary artery. In the Bird it will be recalled.that the portion
of each sixth (pulmonary) aortic arch between it and the respective main
aorta persists until hatching as a duct of Botallo or ductus arteriosus.
In the Pig and other Mammals, however, only the left so persists. Its
embryonic function and ultimate fate are similar in the Mammal to
what they were in the Chick, and will be referred to again in connection
with the development of the heart.
The Intersegmental Aortic Branches and Their Derivatives.
—- It may he recalled that the Pig like the Chick has intersegmental arteries, and that anterior to the seventh cervical they have fused to form
the vertebral and basilar arteries. It remains to note their further development as follows:
Posterior to the seventh cervical, the intersegmentals in the anterior
part of the thorax also become fused antero-posteriorly, and disconnected from the aorta. Thus independent longitudinal vessels are produced in this region also (Fig. 317). Here, however, they come to supply the breasts, and are known as the mammary arteries. Returning now
to the seventh cervical intersegmentals, it will be recalled that at 10
mm. these vessels have started to enlarge slightly in connection with the
development of the subclavians. In fact the left one, continuing to enlarge, comes to constitute the entire left subclavian, which as noted,
thus takes its permanent origin from the dorsal aorta. The right seventh
cervical also enlarges, but only forms the distal part of the right sub
clavian. This is l)ecause’the proximal part on this side is formed from
the right fourth aortic arch, and a short portion of the right dorsal aorta
THE VEINS 637
between the arch and the origin of the right seventh cervical. The part of
the right dorsal aorta posterior to its junction with the seventh cervical
of course disappears. Reference to figure 319 will make it clear how
these developments result in the origin of both the vertebral and the
mammary arteries on either side from the subclavians.
It is of some interest in connection with this origin of the subclavians
to recall that in the Chick the so-called primary subclavians arise as
branches of the eighteenth segmental arteries. Then a shift later occurs
so that the permanent subclavians arise from the common carotids. In
the Pig, as we have seen, it is the seventh cervical intersegmentals that
are involved in the development of the subclavians, both originally and
finally.
The Aorta and Its Branches Posterior to the I-Ieart.—The
origins of the coeliac and anterior mesenteric arteries have already been
noted as occurring at 10 mm. The more anterior of these, the coeliac,
eventually comes to supply the stomach, liver, pancreas and spleen,
while the anterior mesenteric passes mainly to the anterior and middle
intestine. Posterior to the anterior mesenteric the renal arteries grew
from the aorta at 10 mm. in connection with the mesonephros. Eventu
ally when the metanephros develops, other arteries in close association
with the original mesonephric vessels supply the new organs. The posterior or inferior m.e.senteric artery had not arisen at the 10 mm. stage,
but develops at about 12 mm., and sends branches to the posterior part
of the intestine at approximately the point where the latter emerges
from the body-stalk. It continues to supply this part of the alimentary
tract.
The largest branches of the aorta during fetal life in the Mammal
are the large umbilicals whose origin has already been mentioned. It
was also noted that even at 10 mm. each of them had given rise to a
small branch, the external iliacs. These increase in size as the hind limbs
develop, and finally at birth they become the main arteries supplying
the hind legs. At the same time parts of the former umbilicals within
the body, but distal to the point of origin of the external iliacs, persist
as small branches, the internal iliacs. The parts of the umbilicals proximal to the external and internal iliacs remain as the common iliacs.
THE VEINS
Derivatives of the Omphalomesenterics. -—-"By 10 mm. the yolksac had virtually disappeared, and with it the omphalomesenteric veins
leading to it. However, as was noted, the parts of these vessels within
638 THE LATER DEVELOPMENT OF THE PIG
the body proper altered to produce the hepatic portal system. This consisted of the two hepatic veins, the liver capillaries, and a single hepatic
portal vein, with branches draining blood from the intestine. This is essentially the adult situation.
The Umbilical Veins. ———When last noted there were two of these
within the body, though the right one was becoming smaller (Fig. 321).
Presently this latter vessel disappears anteriorly, while its caudal part
persists for a time as a small vein draining the body wall posteriorly
into the left umbilical. The latter vein increases its size within the liver
where, as noted, it forms the posterior major portion of the ductus
venosus. Also, as this occurs, it comes to lie nearer the mid-line, and
thus to pass between the two hepatic veins, which enter it at about the
same point as the hepatic section of the developing posterior vena cava.
As previously noted, the short anterior section of the ductus which empties into the sinus venosus, and was formed from the fused vitelline
veins, now receives the hepatic-s, the major part of the ductus, and the
hepatic portion of the posterior vena cava. Thus this short section becomes the 3'-'lt6I‘.l0I‘ extremity of that vessel. Therefore since the anterior
remains of the posterior cardinals empty into the ducts of Cuvier, it
comes about that the posterior vena cava is the sole vein entering the
sinus from the back part of the body. The further development of the
posterior parts of this important vessel will be considered presently.
As to the fate of the left umbilical, its function of course ceases entirely
at birth, the anterior portion of its path (the duptus venosus) being
marked by a fibrous strand, the round ligament of the liver.
The Anterior Cardinal System and Anterior Vena Cava. ———- As
described at 10 mm. the anterior cardinal system consisted of the anterior cardinal veins and their capillaries, and the external jugulars
which joined the cardinals just anterior to the ducts of Cuvier. It was
also noted that each subclavian, consisting of an enlarged intersegrnental vein, entered the posterior cardinal virtually at the point where anterior and posterior cardinals passed into the respective Cuvierian ducts
(Fig. 322, E). Continuing with the subsequent story it may now be
stated that with the caudal shift of the heart and ducts of Cuvier, these
‘parts soon come to lie posterior to the limb buds. As a result of this the
entrance of the subclavians shifts forward so that presently they definitely empty into the anterior cardinals (Fig. 322, F).
The next steps consist in the shifting of the previously symmetrically
arranged veins so that they enter the right side of the heart. This is
brought about mainly by the development of a diagonally transverse
THE POSTERIOR CARDINAL SYSTEM 639
vessel. This vessel runs from the junction of the left subclavian with the
left anterior cardinal, across to the right anterior cardinal, slightly pos-'
terior to the point where that vessel receives the right subclavian. In the
meantime the left anterior cardinal posterior to the origin of the new
vessel disappears (Fig. 322, H, I). Hence all the blood from the left anterior region, along with that from the right, now has to enter the sinus
venosus through the right anterior cardinal and duct of Cuvier. With
these changes the vessels concerned have their adult arrangement, and
may be given their adult names. The new transverse vessel is the left
innominate vein. The section of the former anterior cardinal between the
junction of the left innominate with this cardinal and the entrance of the
right subclavian, is now the right innominate vein (Fig. 322, I). The
posterior or proximal portion of the right anterior cardinal between the
entrance of the left innominate and the right duct of Cuvier, plus that
duct, is now the anterior vena cava. As will presently appear both posterior cardinals have by this time disappeared as such, though certain
remnants persist which will be described below. Finally the distal parts
of both anterior cardinals cephalad to the points of entrance of the respective subclavians and external jugulars are now termed the int-:-rnal
jugulars.
The Posterior Cardinal System, Posterior Vena Cava and Related Vessels. — It will be recalled that at about 10 mm. the posterior
cardinals had practically disappeared at the mesonephric level. Their
posterior remains, however, drained into the newly formed median anastomosis of the subcardinal sinuses through numerous capillaries. Anteriorly the left subcardinal had almost lost its connection with the anterior
part of the left posterior cardinal‘. At the same time the right subcardinal had established a connection with the newly formed median vessel
passing through the liver to the sinus venosus. This vessel, together with
the subcardinal sinus and remains of the right subcardinal then constituted the anterior part of the posterior vena cava. Its establishment, as
noted, has thus produced the essentials ‘of a renal portal system. The
final step in this process is the complete severance of the connection of
the left subcardinal vein with the posterior cardinal which occurs very
shortly after the 10 mm. stage (Figs. 320, 322, C, D, E). The further development of the posterior venous system then proceeds as follows:
The posterior parts of the posterior cardinals have from an early period received the external and internal iliac veins which form in con
nection with the posterior limb buds. These cardinals, however, are
gradually replaced by a new pair of cardinals close to the dorsal body
640 THE LATER DEVELOPMENT OF THE PIG
wall, and hence called the supracardinals (Fig. 322, F). The external
and internal iliacs then become attached to these new supracardinals
(Fig. 322, F, H) through the stumps of the old posterior cardinals, now
termed the common iliacs. In the region of the subcardinal sinus_ (the
present end of the posterior vena cava) the supracardinals become connected, at first through capillaries, and then by larger channels, with
this sinus. Just anterior to this region the supracardinals are ‘slightly
developed and presently disappear, though still further forward they
continue to exist and to connect with the anterior remains of the old posterior cardinals (Fig. 322, I ). We shall return to this situation presently. Continuing with the account of the more caudal region, however,
we find that the final steps here are: (1) the degeneration of the left
supracardinal, (2) the connection of the left common iliac with the end
of the right supracardinal, and (3) the shift of the latter to the median
line. The result of this is to make the surviving supracardinal the posterior extension of the posterior vena cava, thus completing that vessel in
its caudal extent (Fig. 322, H, I, I) . Anteriorly the portion of it within
the liver finally works its way to the dorsal surface where it becomes
quite conspicuous before opening into the right atrium of the heart in a
manner to be indicated presently.
Returning now to the more anterior parts of the supracardinals, and
the remnants of the posterior cardinals into which they drain, we find
that these vessels persist somewhat irregularly as the azygos veins. Generally the latter are united transversely, one or the other loses its anterior connection, and both drain into the anterior vena cava through
the remains of a posterior cardinal, now termed the cervico thoracic,
though in the Pig this may not occur (Fig. 322, J) . Hence it may happen
that the left duct of Cuvier is left with no (or in the Pig, few) tributaries. In any event it does not disappear, but instead becomes imbedded
in the heart muscle as the coronary sinus.
In conclusion of this discussion it remains to state that while these
changes have been going on both anteriorly and posteriorly the sinus
Venosus has been absorbed into the right atrium of the heart. Hence,
since the sinus previously received the anterior and posterior vena
cavae and the coronary sinus, this final change means that these three
vessels ultimately open separately into the right atrium.
The Pulmonary Veins.——It will be recalled that at 10 mm. the
‘pulmonary veins ehtered the left atrium of the heart by a common
trunk. It now remains to state that eventually this trunk is incorporated
into the atrium, and its two or more branches achieve separate openings.
THE ‘HEART 641
The Heart. —--When last described at 10 mm. this organ consisted
of a ventro-posteriorly directed ventricle and antero-dorsally directed
atrium. The walls of the former were lined by spongy tissue, the trabeculae carneae, and the chamber was partly divided by a septum growing
toward the atrio-ventricular canal. In the latter the fusion of the
 
   
   
   
anterior vena cava
septum secundum(Il)
' ena cava ‘
P°“"'°r V pulmonary vein
right atrium _ I (I)
septum pr mum
septum secundum (ll)
. left atrium
//2%
f 5; mitralvalves
_ i left ventricle
tricuspid valves
chordae tendineae
right ventricle
foramen ovale
septum II V
Fig. 336. ——-Drawing of fetal Pig heart at nearly full term, opened from the ventral side. B. Semidiagrammatic view of the foramen ovale and septa I and II from
the right side. C. Same from the left side. Arrows in all cases represent directions
of blood flow according to the most recent conclusions. In B and C the dashed
parts of the arrows indicate that a membrane lies between the arrow and _the ob»
server. For a complete discussion of the flow of blood in the embryo of the Chick
and the Mammal see the text on this topic in the account of the Chick, and Fig.
235X.
cushion septa had almost, or quite, completed the division of this orifice
into right and left channels. At the same time the atrium had been
nearly divided by the septum primum growing from -its antero-dorsal
wall. As was indicated, however, this septum had already developed an
opening in its antero-dorsal region called the interatrial foramen secundum. The right atrium received the sinus venosus, and the left the
single pulmonary vein. Further development may now be described as
follows: _
The completion of the cushion septum if not accoinplished at 10mm.
642 . THE LATER DEVELOPMENT OF THE PIG
soon takes place, This is then quickly followed by the completion of the
interventricular septum, and also that of the interatrial septum primum.
This latter event closes the interatrial foramen primum, but leaves wide
open the recently developed interatrial foramen secundum. The heart
therefore is now completely divided into right and left parts except for
this latter opening. Meanwhile there has developed another atrial septum just to the right of the first, called the septum secundum, the beginning of which was shown at 10 mm. (Fig. 313). It too is a crescentshapecl sheet extending from the antero-dorsal wall of the atrium along
its dorsal and ventral walls. Presently it extends all around these walls
and fuses with the septum primum near the atrio-ventricular cushion septum. The new septum secundum, however, fails to become complete in its central region just ventral to the interatrial foramen secundum of the septum primum. This opening in the new septum is called
simply the foramen ovale. As reference to Figure 336 will show its position is such that the middle part of the septum primum acts as a valve
which can functionally close the foramen ovale. Such closure would obviously occur if pressure were applied to the valve from the left side.
We shall return to this matter presently.
Meanwhile as the septa have been thus completed certain further
events have taken place. On the sides of the atrio-ventricular canals flaps
of tissue have developed, two on the left side and three on the right.
These form the atria-ventricular valves (tricuspid right, and mitral left)
which hang downward into the respective ventricles. Here their edges
have remained attached to some of the traheculae carneae, which in these
particular instances become drawn out into strands, the chordae. tendineae, continuous ventrally with the papillary muscles. These, however,
are not all the valves of the heart. As previously noted, the truncus arteriosus also becomes divided by a septum into two channels, the systemic and pulmonary, whifli lead respectively from the left and right
ventricles. It now remains to state that at its union with the heart the
truncus, previous to its division, develops upon its walls two thickenings. Then with the growth of the dividing septum these thickenings are
transformed into six semilunar valves, three in each channel.
Finally in the atrial region it has already been remarked that the sinus venosus has been incorporated into the heart on- the right side, and
the single pulmonary trunk on the left. This of course causes the separate veins previously opening respectively into the sinus and pulmonary
trunk to open directly into the right and left atria. In connection with
this it remains to state that as this occurs portions of the right valvula
THE EXCRETORY SYSTEM 643
venosa of the sinus are retained as valves of the caval and coronary’
openings. Also in the later stages of development the atria of the Pig
and other Mammals acquire the more or less earlike appendages which
have given rise to the name auricle. These it may be recalled occur in
the Bird, but only to a slight extent, and not at all in the Frog.
The Fetal and Adult Circulation.—This topic was discussed
at considerable length in the case of the Chick, and since essentially the
same situation is involved in the Mammal we shall not repeat it here.
The student is urged to reread that section at this point. If this advice is
followed it will be noted that the chief item of difference cited between
the Bird and the Mammal concerned the character of the interatrial
opening and its method of closure. There was only one septum in the
Bird, corresponding to the mammalian septum primum, and instead of
a single opening it contained several. These were closed at hatching by
the equalization of pressure on the two sides of the septum which took
the stretch out of it, and allowed the perforations to close by contraction. In the Mammal there is the same equalization of pressure at birth.
In this case, however, the result is to press the valvelike part of the
septum primum against the foramen ovale in the septum secundum, and
thus functionally to close that opening. The actual fusion of the parts
of the two septa does not occur for several weeks and sometimes several
months post partum. Indeed a probe patency may exist permanently,
but so long as equal pressure in the atria is maintained, this is of no
consequence. The closure of the duct of Botallo was also noted in the
discussion of this topic in the section on the Chick, and it was indicated
that its permanent closure in the Mammal might occur in about a
month. As a matter of fact the time varies in different animals, being
3-4 weeks in the Pig and 6-7 weeks in Man. The relation of the failure
of the closure of the septum or of the duct to infantile cyanosis in Man
was indicated in the discussion of this topic in the Chick (Figs. 236X,
- 336).
THE URINOGENITAL SYSTEM
THE EXCRETORY SYSTEM
The Mesonephros. —— When the excretory system was last discussed
the pronephros had entirely disappeared, and the .mesonephros was
well developed and functional. Indeed it is relatively larger at this and
immediately subsequent stages than when it reaches its peak in absolute
size and activity. Thus it continues to grow and funhtion for some time
644 THE LATER DEVELOPMENT OF THE PIG
beyond the 60 mm. stage, when it is replaced by the metanephros. In the
male of course certain parts of the mesonephros persist permanently in
connection with the reproductive system as will be indicated presently.
The Metanephros. —The origin of the permanent kidney or metanephros has already been indicated. Thus at 10 mm. each of these organs consists of a short tubular outgrowth from the postero-dorsal side
of the respective mesonephric duct just short of the point where the latter enters the cloaca. At its anterior end this outgrowth, the future ureter, has an enlargement, the anlage of the future pelvis of the kidney.
Surrounding this is a concentration of nephrogenic mesoderm (Figs.
296, 323). ‘ '
Further development consists in the forward growth of the ureter and
its pelvic enlargement, which carries with it the nephrogenic mesoderm
to a position dorso-lateral to the middle of the mesonephros. Meanwhile
from the pelvic enlargement there have grown out into the surrounding
nephrogenic substance numerous outgrowths which soon become hollow, and which represent the collecting ducts. At the same time concentrations within the nephrogenic mesoderm have become vesicular, and
the vesicles send forth outgrowths which become tubular and connect
with the collecting tubules. Later these outgrowing secreting tubules become even more convoluted than in the case of those of the mesonephros. Finally, each vesicle becomes invaginated by a glomerulus,
and thus is transformed into a Bowman’s capsule. The blood supply to
both glomeruli and tubules is entirely arterial in the metanephros. This
supply also differs from that to the mesonephros in that it is furnished
to each permanent kidney by one main renal artery instead of by several
smaller branches.‘
The details of development of the caudal outlets of the ureters and
mesonephric ducts can best be described in connection with related parts
of the reproductive systems, and will be taken up presently. Before proceeding to that topic, however, there remains a word to say about certain other organs closely connected with the kidneys, though not excretory.
* The Adrenals.—As we have seen in the case of the Frog and
Chick-, these structures vary considerably in form, but always consist of
two parts having specific origins. The medullary substance develops
from cells which: have their origin in the neural crests. These cells migrate from the crests along with some of the cells which are to form the
sympathetic ganglia, and many of them, after acquiring a special staining capacity, become associated with these ganglia. Others, now called
I
I
i
THE REPRODUCTIVE SYSTEM‘ 645
8'“ 10°?
mesoneph ros
 
 
 
   
yolk stalk .- V
. Hantolc suIkl""b"'°' "I"
mesonephros
2 urinary bladder
 
urinary bladder
Fig. 337. —Semi-diagrammatic illustrations of the development of the 1netanephros, the adult ureters and gonoducts, and the separation of the cloaca into anal and
urino-genital regions in the Pig. A. Unseparated cloaca with no indication of sex
differentiation (about a 10 mm. embryo). B and D. Progressive separations of the
cloaca. and development of the urino-genital ducts of the male. C and E. The same
process in the female.
chromafiin. cells, come to lie beneath the mesoderm of the coelom. The
larger number of these chromafiin cells, however, form a mass adjacent
to the cephalic end of the kidneys, where they form the adrenal medulla.
Around this medullary substance which becomes arranged in cords,
there then accumulate mesodermal cells which constitute the adrenal
cortex.
THE REPRODUCTIVE SYSTEM
The Gonads.—The later development of both, testes and ovaries
has been previously described at some length in general and in connec646 THE LATER DEVELOPMENT OF THE PIG
tion with specific forms. It is essentially similar in all these cases, except in regard to certain aspects of the mammalian ovary, which were
also considered previously when mammalian oiigenesis was discussed.
diaphragmatic ligament
metaneph ric kidney
 
       
 
 
 
inguinal ligament
portion of gubernaculum
..‘.'»».' ’"“" — —.‘\._L__ _ inguinal ligament
scrotal ligament
scrotal sac
tunita vaginalis
vas deleren: uh‘
   
 
 
scrotal ligament
processus vaginalls
inguinal canal
rectum
K “ ejaculatory duct
" , -1
seminal vesicle _.l,'§4~ g prostate
V35 d°f°l‘¢“‘,r/ a\  bulbo-urethral
‘ pubis
         
   
tunica vaginalis
, inguinal canal
   
Fig. 338.-— Diagrams representing the descent of a Pig testis. A. Before the testis
has started to move. B. The testis about to enter the scrotum. C. The testis in the
scrotum.
We shall not therefore go into this subject again in connection with the
Pig.
The Male Urinogenital Ducts. — As we have seen in the case of
the Bird, so in the Mammal, the mesonephric duct when no longer
needed as a ureter is pressed into service as a sperm duct, 'or 12:15 deferens. Anteriorly the connection between this duct and the respective testis
is made through certain mesonephric tubules which are retained for this
purpose. They, together with the immediately. adjacent portion of the
THE REPRODUCTIVE SYSTEM 647
 
   
   
 
 
I‘
. . _  '\y‘\‘\>\\‘“ \\
kidney metanephros mm, id
ov uct
urinary bladder n  "nth...
 
   
broad ligament ‘ix ‘
- .1. ureter r
UIETUS
§\\\
;- , ll
‘ - "‘.“'.‘—“.”W.*‘!'i\. ' "‘  ectum
kidney oviduct  §;..i~\ *~..a‘ vagina
x...-. . i  '*-"Pr-.'~'-' ~95
,u:erus —- - th y-,’/4r,‘r:...,~:~§-g vestibule
'°”"‘.i “83m¢"‘ °f\ ovary U e I.’.( T ‘fig: labium minus
ovary us .
 
 
lobium moius
clitoris
   
Fig. 339. — Diagrams representing the partial descent of a Pig ovary.
A. Before the ovary has started to move. B. After it has reached its
definitive position.
mesonephric duct become the e piclidymis. The extreme anterior remnant of the mesonephros may persist as the appendix to the epididymis,
while the vestigial caudal remainder occurs as the paradidymis. '
At its caudal end the mesonephric duct when last noted was emptying into the antero-ventral part of the cloaca, which was being separated ofl' as the urinogenital sinus. This division of the cloaca into urinogenital and rectal portions by the urorectal fold is presently completed,
and shortly thereafter the cloacal membrane is ruptured. This of course
648 THE LATER DEVELOPMENT OF THE PIG
puts both cloacal parts in communication with the proctodaeum, the
opening of the urinogenital sinus being termed the ostium urogenitale,
and that of the rectum, the anus (Figs. 337, 340) At the same time that
this has been going on the part of the allantois inside the body has been
dilating to form the urinary bladder. Presently when the urinogenital sinus, into which the allantois opens, becomes completely separated from
the rectum, the cephalic part of the sinus also expands somewhat. Thus
this part is in efiect simply added to the posterior end of the bladder,
forming its proximal portion. The more caudal portion of the sinus,
however, is narrowed instead of dilated, and becomes the urethra.
While this has been taking place the end of the mesonephric duct into
which the metanephric duct opened has been drawn into the urinogenital
sinus, so that these ducts now open separately. Furthermore, the cephalic
growth of the metanephros seems to have pulled its duct forward somewhat. The result is that when the separate openings are achieved, that of
the metanepliric duct is into the antero-lateral part of the old urinogenital sinus, now forming the base of the bladder. The opening of the old
mesonephric duct, however, now the vas deferens, is further posterior
into the part of the sinus whichgnow forms the urethra (Figs. 337, 338).
' It remains to add that slightly anterior to the point where the vas
deferentia enter the urethra each becomes dilated, and the dilation
drawn out slightly to form a small sac, the seminal vesicle. The short
remaining part of the vas deferens between the vesicle and its entrance
into the urethra is termed the ejaculatory duct. Finally the urethral
epithelium gives rise to two glands on the outside of the urethral lumen.
but with openings into it, the prostate and the bulbo-urethral or Cowper’.s gland (Figs. 337, 338). This concludes the part of the male urinogenital duct system which is, so to speak, within the body. The remain‘ing portion, together with a description of the ultimate disposition of
the testes, will be taken up presently. Before doing that, however, we
must return for a moment to the development of the ducts of the female,
and certain other considerations.
The Female Urinogenital Ducts. -—- The oviduct originates in_the
Mammal, .as it has been seen to in the Frog and Chick, from a thickened
ridge of mesoderm lying along each side of the mesonephric duct. This
_ ridge becomes tubular and pulls away from the body wall, to which it
‘ remains attached by’ a fold of peritoneum supporting both ovary and
duct. This fold or double sheet of tissue, homologue of the Chick mesovarium, is called‘ the broad ligament, of which more will be said later
(Fig. 339). There are of course two oviducts, one on either side, and they
THE REPRODUCTIVE SYSTEM 649
at first open separately into the urinogenital sinus. Very shortly, however.
their caudal ends fuse to form the vagina. Anterior to this each duct becomes differentiated histologically into a part called the uterus, and still
further forward into the definitive oviduct or Fallopian tube. As has already‘ been indicated in our introductory discussion of the Mammal, the
degree to which the uterine portions of each duct later fuse to form a single uterus varies in different kinds of animals. In all but the most primitive, however, a slight fusion always occurs to form a region known as the
cervix opening into the vagina by a single orifice. In the Sow and other
Ungulates this fusion continues a short distance anterior to the cervix to
produce a typical uterus bicornis; in Man, of course, the fusion of the uterine parts is complete, giving a uterus simplex. At their anterior ends each
oviduct, as has been seen, develops a funnel or infundibulum which
may or may not embrace the ovary. In the Sow it does, but in Man it
does not. In any event it is of interest to find that this anterior opening
develops. not quite at the anterior tip of the original tube, but slightly
caudal to it. I
So far as the excretory ducts of the female are concerned the ureter
comes to open into the base of the bladder following the division of the
cloaca, just as it does in the male. The mesonephric duct naturally has
no function in the female, but does persist, along with parts of the mesonephros as a vestige. There are asa matter of fact several of these vestiges in both sexes in addition to those already indicated. Some of these
are outside the body, and will be referred to later. Confining ourselves
for the moment, however, to those within, it will be well at this point to
make some further reference to these remnants.
Internal Vestiges of the Reproductive Systems.——The vestigial appendix of the epididymis and the paradidymis respectively have
already been noted. In addition to these in the male, a vestige of the
oviduct may be found in the tissue investing the testis, where it is called
the appendix of the testis. Posteriorly also a further vestige of the fused
parts of -the oviducts may occur as the uterus masculinus. In the female
the.undi~fferentiated anterior tip of the oviduct often remains as a small
vesicle attached to the duct. Also a vestige of the mesonephros is usually
embedded in the broad ligament (mesovariurn) as the epoiiphoron, a
structure previously mentioned as occurring in the Chick. Finally ves
"tiges of the mesonephric duct. or parts of it, may renriain near the uterus
and vagina as the» canals of Gdrtner.
We are now prepared to return to a consideration of the migration of
the gonads, and to the development of external features connected with
650 THE LATER DEVELOPMENT OF THE PIG
both male and female systems. We shall consider the movement of the
gonads first, and we shall begin with the testes.
The Descent of the Testes. -— The student is well aware of course
that in the lower animals, such as the Fishes, Amphibia and Reptiles
the testes remain within the body at their places of origin. Indeed this
is even true in the Birds, which in their way are quite as “ high ” or
specialized as the Mammals. It is only within the latter group, however, that the testes radically alter their position so that in most cases
they are actually outside the original body cavity all or part of the
time. How this comes about is now to be considered.
Both the mesonephros and adjacent testes’ are held against the body
wall by a covering of peritoneum. As they grow they push this covering out into the coelom, but the covering does not cut in above them to
form a mesentery-like sheet. Instead they simply remain beneath it, such
a position being described as retroperitoneal. As development goes on
the testis becomes relatively larger and the mesonephros relatively, and
finally absolutely smaller, so that the former occupies more and more
of the retroperitoneal space. Meanwhile, though the peritoneum (mesodermal epithelium plus connective tissue) does not cut in above the
testis and mesonephros, anterior and posterior to them it is drawn out
into a longitudinal fold within whose layers runs a bundle of connective tissue fibers. Anteriorly the fold and its bundle of fibers extends
from the mesonephros to the diaphragm, and is known as the diaphragmatic ligament (Fig. 338, A). The posterior section of the fold and
fibers reaches to the extreme caudal end of the coelom, this section being
termed the inguinal ligament of the mesonephros. Here a pair of coelomic
evaginations occur, the scrotal sacs or pouches, the cavity in each being termed the processus vaginalis. From the distal wall of each pouch
a fibrous strand, the scrotal ligament, proceeds beneath the epithelium
to the coelom prop,er. There each scrotal ligament becomes united to the
caudal end of the respective inguinal ligament of the mesonephros (Fig.
338). Here it should be incidentally noted that this inguinal ligament
has nothing at all to do in origin or function with the inguinal ligament
of the adult, known in Man as Poupart’s ligament.
While this is occurring posteriorly the testis is outstripping the mesonephros in growth, and as it does so the attachments of the diaphragmatic and inguinal ligaments of the latter organ become transferred to
the former. When this has taken place the united inguinal and scrotal
ligaments are given a single name, the gubernaculum. Thus it comes
about that a gubernaculum extends from the caudal end of each testis
THE REPRODUCTIVE SYSTEM 651
and adjacent epididymis to the bottom of each scrotal sac. We might
now briefly complete the story by simply saying that while the diaphragmatic ligament stretches the gubernaculum contracts, thus pulling
the testis and epididymis back and down into the scrotal sac. Essentially
this is what happens, but as a matter of fact the gubernaculum does not
contract. It merely fails to grow, while the other parts do, so that the
effect is the same as if it did contract. (It is like the case of the boy holding the cat’s tail. He does not pull it. The cat does that.) In the course
of this movement the vas deferens is bent into a loop which passes
across the permanent ureter.
It must now be pointed out that since the testis is retroperitoneal it
does not actually lie in the coelomic space of the scrotal pouch (processus vaginalis) any more than it lay in the general body coelom. Instead it is pulled down all the way beneath the peritoneal covering
which within the pouch is, reflected over it as the tunica vaginalis. Of
course in this process the coelomic space within the scrotal sac is elimil
nated. While this space existed, however, it was connected with the general coelom by the inguinal canal. From what has just been said it must
also be clear that the testes do not really pass into the pouches through
the canals, though the existence of the canals permits the movement.
They pass back of the canals underneath the peritoneum. After the testes
have thus gone into the scrotal sacs the inguinal canals fuse completely
shut, except in a few animals to be indicated presently. Nevertheless, it
is of interest that this spot evidently comprises a point of weakness
which accounts for the occurrence of inguinal hernia in Man. The fact
that it occurs in this case, but seldom if at all in the lower animals is
probably the result of Man’s erect position. There seem still to be certain advantages in walking on all fours.
It remains to state that the movement of the testes just described does
not occur in all Mammals. Thus in the Elephant the testes remain permanently within the body, while in the Rat they pass back and forth,
descending during sexual activity. In this connection it is significant
that the temperature of the scrotum has been shown to be lower than
that of the body cavity. Furthermore, experiment has proven that in
animals in which the testes normally remain permanently in the scrotum
the retention of the testes within the body results in sterility. Lastly, if
in such animals the temperature of the scrotum is‘ artificially raised to
that of the body, sterility also results. Thus it appears that in these
cases the temperature conducive to spermatogenesis and (or) sperm
survival is lower than the normal body temperature.
umbilical stalk
 
   
   
pl'£'pfLlC¢
Iabium mains
 
_ _ ; .,, “ lcbium moius
lobuum mmus *’ cum.-5,
Stfclllm _ “"3
   
90 mm. 0
Fig. 340.——Drawings of ‘stages in the development of the Pig external genitalia.
A and B. The same indifferent stage preceding sexual differentiation. In A the
genital tubercle and related parts are turned posteriorly. In B these parts are reflected anteriorly to show their ventral aspects. C’, E and G represent the progressive development of the genitalia of the male at the stages indicated, while D. F
and H represent corresponding development in the female.
652
THE REPRODUCTIVE SYSTEM 653
The DESCEINC Of the Ovary. — In the case of the ovary and oviduct
we have noted that these organs are attached to the coelomic wall by a
fold of mesothelium and connective tissue called in the Mammal the
broad ligament. Within this fold is enclosed the fibrous inguinal liga~
merit of the mesonephros along with the vestiges of the epididymis
(epoiiphoron) and vas deferens (canals of Giirtner) . In this instance as
development proceeds the inguinal ligament (anterior part of the guber-naculum of the male) apparently exerts no traction. Rather the ovary
and oviduct, pulled downward by their weight, stretch both the broad
ligament and inguinal ligament within it. Shortly the ovary has moved
so far posteriorly that both the oviduct and the ligament are bent
around at a considerable angle. When this has occurred the part of the
inguinal ligament between the ovary and the bend is called the round
ligament of the ovary, and that part between. the bend and the uterus the
round ligament of the uterus. In this manner the ovaries come to lie
much further back in the body than their point of origin, but unlike the
testes they never pass outside (Fig. 339). 3.
The External Genitalia, Indifferent Stage. ——As in the case of
the very early stages of the gonads themselves so also in this case an in»
dilferent stage exists during which sex is indistinguishable. Also, as will
presently appear, we find that the same fundamental structures occur in
both sexes. It is only with later development beyond the 25 mm. stage
that they begin to become differentiated to form the external urinogenital parts of the adult male and female. The parts concerned and
their locations are as follows:
As the urorectal fold is dividing the cloaca into the urinogenital sinus
and the rectum, the proctodaeum surrounding the original common orifice essentially disappears as such (Fig. 337). Thus the orifice of the
urinogenital sinus (the ostium. urogenitale), the edge of the urorectal
fold (the rudiment of the perineum) and the anus are brought virtually
to the surface in this region. Just anterior to the ostium urogenitale
there meanwhile appears a slight elevation known as the genital eminence, which shortly becomes more prominent, and is then called the
genital tubercle. Immediately on either side of this tubercle lie a pair
of folds called the genital folds. These folds lie not only at the sides of
the tubercle, but also extend caudad enough to flank the ostium urogenitale causing the latter to become slit-like. Somewhat further to ei~
ther side of the genital folds are another pair of elevations, the genital
swellings (Fig. 340, A, B).
The External Genitalia, Male.—-—The genital tubercle becomes
elongated, and grows forward toform the penis. The genital folds from
654 THE LATER DEVELOPMENT OF THE PIG
either side then grow around the penis to form the prepuce, while more
posteriorly and to the sides the genital swellings are pushed out by the
coelomic evaginations to form the coverings of the scrotal sacs. These
presently fuse in the mid-line to produce the single scrotum, the line of
fusion constituting a ridge called the scrotal raphe. Up to this point it
will be noted that the penis lacks a canal. This is formed by a groove
developing along its ventral side, the edges of which soon fuse, and thus
is formed the penile urethra, extending from the tip of the penis to the
urinogenital sinus. The part of this sinus between this point and the bladder then comprises the prostatic urethra. The line of fusion of the edges
of the ostium urogenitale and those of the groove along the ventral or
caudal side of the penis forms an extension of the scrotal raphe called
the penile raphe (Fig. 340, C, E, G).
The External Genitalia, Female. — The situation in the female is
considerably simpler. Starting from the same structures in the indifferent stage we find the tubercle forming a vestigial part; at the anterior
border of the ostium urogenitale. It is called the clitoris, and is obviously the homologue of the male penis. The urinogenital sinus itself becomes the vestibule which leads into the vagina formed from the fused
ends of the uteri. Upon either side the ostium urogenitale of the vestibule is flanked by the genital folds which have become the labia minora,
and slightly more laterally by the genital swellings which have become
the labia majora. The former are of course the homologues of the male
prepuce and the latter of the scrotal sac coverings. The term vulva includes all the parts just mentioned (Fig. 340, D, F, [17 ).
’l7
HE SKELETON, TEETH, HAIR, HOOFS AND HORNS
THE SKELETON
I T is not the intention to undertake for the Pig, anymore than we
have done for previous forms, a detailed description of skeletal development. It does seem worthwhile, however, to point out a few of the outstanding similarities and differences in this development as it occurs in
this animal and in the Frog and Chick.
The Skull. —As in the case of the Frog and Chick the bones of the
Pig skeleton may be divided into membrane or dermal bones and cartilaginous bones. On this basis we find in the cranial part of the skull
of this animal the same embryonic cartilaginous foundation which we
have previously noted, i.e., the basilar plate (fused parachordals and
notochord) and the trabeculae. Later of course these develop ossification centers giving rise to the ethmoid and certain of the sphenoid bones.
Also added to the cranium from cartilage are the occipitals and the
various bones forming the otic and nasal capsules such respectively as
the periotics and the naso-turbinals. It will be recalled, however, that
the primitive cartilaginous element of the upper jaw, the palato quadrate, still represented in the Bird by the quadrate, has in the Mammal
apparently moved into the middle car as the incus. Likewise in the
lower jaw a portion of Meckel’s cartilage, in the Mammal is thought to
constitute the malleus. All the dermal bones, i.e., those ossifying directly from membrane which occurred in the Bird, exist also in the Pig,
with the exception of the quadrato-jugals and parasphenoids. In the
lower jaw dermal elements replacing the main remnants of Meckel’s
cartilage become ossified and fused together to form the single mandible.
The Vertebrae, Ribs and Sternum.——The concentrations of
mesenchyme which are to form the vertebrae alternate with the original
_ somites just as they did in the Frog and Chick, and surround the noto
chord. Cartilage forming centers then develop, one about the remains
of the notochord, i.e., the future centrum, one in each neural arch and
one in each costal process. The cartilage soon spreads from these centers to form a continuous cartilaginous structure for each future vertebra. Then ossification begins in the same centers ivhere cartilage forma656 OTHER MAMMALIAN STRUCTURES
tion began, and spreads until each vertebra consists entirely of bone.
The rib cartilage is at first continuous with that of the costal processes,
but when ossification begins, the cartilage of the ribs becomes separated from that of the vertebrae, and each rib has its own ossification
center. It is of interest that in correlation with the adult condition the
cartilage in each rib of the Pig consists of a single piece, instead of two
as in some. of the ribs of the Bird. Although the cartilage of each rib is
in this case in a single piece, this cartilage ultimately contains more
than one ossification center. Thus the ribs in the Pig and other Mammals are like the long bones of the appendages in this class, in that the
ends ossify separately from the shafts, forming the so-called epiphyses.
As in the Bird the sternum has two cartilage centers attached to the rib
cartilage on either side. Later these fuse in the median line.
The Appendicular Skeleton.———Considering the fore limbs first,
we find the Pig shoulder girdle differing from that of the Bird in lacking both clavicle and coracoid. The only member of the girdle bones it
does possess is the scapula, and this of course is a bone ossified from
cartilage.
As regards the long bones of the fore limb (humerus, radius and
ulna) we find that in the Mammal the method of ossification in all such
bones differs somewhat from that in either the Frog or the Chick. Development begins as usual by the differentiation of cartilage from membrane. Around the middle (diaphyseal region) of this cartilaginous core
the former perichondrium, now periosteum, starts to erode the cartilage
and to deposit a band of bone. Sincethis band is soon thicker at its middie than at its ends, the remaining central cartilage presently becomes
hour-glass shaped. Almost simultaneous with this outer deposit by the
periosteum, the cartilage in the middle of the diaphyseal core also begins to be eroded by invading chondrioblasts, and its place is taken by
bone deposited by osteoblasts. Soon this endochondral bone and that
produced peripherally by the periosteum meet, and the diaphysis is entirely ossified. This bone, however, is all cancellous, and within it three
changes occur. First, in the central axis of the diaphysis or shaft the
bone is shortly removed and replaced by marrow. Second, about the
periphery the original cancellous bone of both central and periosteal
origin is also constantly removed and replaced as the diaphysis grows in
diameter. Finally, as growth is completed the inner cancellous bone remaining at that time is remade by processes previously described, into
compact Haversian systems. Likewise the outer cancellous periosteal
bone is replaced by layers of compact periosteal bone. On the basis of
THE SKELETON V 657
thisdescription it might be questioned whether any of the ultimate diaphyseal bone is really endochondral, and it would appear probable
that at least what occurs near the mid-region of the diaphysis is not.
Nearer the ends, however, the case is different, and for the same reason
that this was true in the Chick, i.e., because of the method of longitudinal growth. This method, though fundamentally similar to that in
the Bird, differs in certain significant details, and is as follows:
While the processes described above are occurring toward the midregion of the diaphysis each cartilaginous epiphysis is also undergoing
ossification in one and sometimes two centers. In this manner there is
presently produced in it a single disc of cancellous endochondral bone.
At either end of the diaphysis, however, between the bone earlier formed
in that location and the respective epiphyseal bony disc, there persists
during growth a plate of cartilage known as the epiphyseal plate. These
plates correspond in function to the cartilaginous ends of the growing
bones of the Chick, i.e., they continue to produce cartilage distally and
endochondral bone proximally on the side of each adjacent to the marrow cavity of the diaphysis. Finally, when growth ceases, the epiphyseal
plate becomes entirely ossified, and thus joins the already formed bony
epiphyses to the ends of the diaphysis. Hence it comes about that, as in
the Bird, all of every epiphysis is endochondral. Also somewhat more
of the mammalian diaphysis is endochondral because not so much of its
interior is ultimately removed as is true in the Bird. For further de
tails of bone histogenesis the reader is referred to the account of this
process under the Frog, and to the accompanying figures. .
The behavior of the digits has already been referred to in the Pig
and we have noted that, as in the Bird, five digits are present in mem
brane. In the Pig, of course, the third and fourth are well developed
while the first disappears and the second and fifth remain vestigial. The
ossification of the znetacarpals and phalanges occurs in these cases from
cartilage in the same manner as in other mammalian long bones.
Posteriorly the pelvic girdle is ossified from three cartilages representing the ilium, ischium and pubis. As in the Bird they extend respectively
anteriorly, posteriorly'and antero-ventrally. In the Pig, however, the
antero-ventrally extending pubic cartilages remain in this position, instead of rotating caudad to lie parallel with the ischia, as in the Chick.
Thus when ossification occurs the pubic bones meet one another in the
median ventral line, and are held firmly together by ligaments in the
manner characteristic of Mammals. The long bones of the Pig hind
limb are ossified in the same way as the long bones of the fore limb,
658 OTHER MAMMALIAN STRUCTURES
and consist of course of the femur, tibia and fibula. The four digits, two
vestigial, are also formed as in the anterior appendages.
THE TEETH
As previously noted, although the Frog does develop teeth, they are
small and late in forming so that nothing was said about them, while
modern Birds have no teeth at all. It therefore seemed best to postpone
stellate cells of dental papilla ameloblascs
4 enamel epithelium ofenamel organ
A.-. ,5. ‘ pp . _.‘
   
 
tcmmnn
odontoblast Tomes’ processes
dentine enamel pulp of enamel organ
Fig. 341.——A sagittal section through a developing tooth, showing
the cells responsible for the secretion of enamel and dentine, and
the relations of these cells to those products.
an account of the origin of these structures until we came to the Mammal in which class they attain their fullest development. We shall not
attempt to describe the development of any particular tooth since what
is true for one is true for all in forms like the Pig or Man, save for variations in shape. '
The Enamel Organs.———As has been previously indicated, at 30
mm. or shortly thereafter the originally single epithelial thickening
termed the labio-dental ledge, has divided into two parts. The outer
part presently forms the labio-gingival lamina or groove, and the inner
one the dental ledge (Fig. 329). This ledge runs along the surface of an
elevation which represents the gum, and at intervals along it the formation of the teeth occurs as follows: ‘
_ At each point in the gum region where a tooth is to develop, there occurs a special ingrowth from the dental ledge which penetrates further
into the mesenchymé than the non-tooth-forming part of the ledge. The
THE TEETH 659
lower part of this ingrowth is expanded into a double-walled inverted
cup, known as the enamel organ, which remains connected with the
dental ledge for a time by a fairly stout neck (_ Fig. 329). The ledge in
turn is also temporarily connected with the oral epithelium by a considerably narrower neck. The cells on the inner wall of the cup are co
llumnar in shape, and are destined to secrete the enamel of the tooth.
Hence they are called ameloblasts. Those in the outer wall are at first
polyhedral, but soon become flattened, and are known as the epithelium.
of the enamel organ. The rather extensive space between the inner and
outer walls of the cup is filled with a loose reticulate tissue termed the
enamel pulp. Though all enamel organs start out with the relatively
simple cap shape that has been indicated, each later assumes the contours characteristic of the crown of the tooth whose enamel it is to form
(Five. 329, 341) . .
The Dental Papiila. — As the enamel organ pushes into the mesenchyme the latter necessarily comes to occupy the cup which the organ
forms, by which process this mesenchyrne comes to constitute the dental
papilla. Of course where the tooth is to have several cusps and roots the
enamel organ develops more than one cup, and therefore gives rise to
more than one dental papilla and parts subsequently related to it. Presently through multiplication the cells constituting the bulk of a papilla
form a rather dense aggregation. At the same time those at its surface
adjacent to the ameloblasts of the enamel organ. become columnar like
the ameloblasts. These columnar cells of the papilla are then ready for
the secretion of their special product, the dentine, and are termed odontoblasts. It thus presently comes about that while the ameloblasts of the
enamel organ secrete enamel to form the surface of the tooth, the odontoblasts secrete dentine beneath and adjacent to the enamel. As this activity begins to get under way the enamel pulp lying between the outer
epithelium of the enamel organ and its ameloblasts, largely disappears,
thus placing these two layers almost in contact. Probably this is significant in bringing the now active ameloblasts that much closer to their
external blood supply. At the same time nerves and blood vessels penetrate the central tissue of the dental papilla, which gradually becomes
transformed into the pulp cavity of the completed tooth. By the time
these processes are under way, the enamel organ has lost all connection
with the dental ledge. V A 1
Formation of Dentine.~———The formation of the dentine by the
odontoblasts is in some respects similar to the formation of circumferential bone by periosteum. In both cases it involves the deposition of
660 OTHER MAMMALIAN STRUCTURES
calcium salts about organic fibers (ossein fibers). In the case of-the
dentine, however, the product is not laminated, i.e., in layers, but is
continuous. Also no cells are left entrapped within the calcareous substance, and the organic material is less abundant, about 28 percent in
dentine as compared with 45 percent in bone. Hence the dentine is
harder even than compact bone. Otherwise the materials are similar in
that the calcium salts are permeated with ossein fibers, both fibers and
salts being produced by the odontoblasts. Likewise there are processes
of the odontoblasts which extend into the hard matrix just as the living
processes of osteoblasts extend into bone. In this instance, however, the
processes all come from the layer of odontoblasts at the inner surface
of the dentine, since none are embedded within it, and they are known
‘as the fibers of Tomes (shown but not labeled in Fig. 341,) . They are in
general at right angles to the secreted ossein fibers. Obviously the continued production of dentine forces the odontoblasts away from’ the
enamel, and also reduces the size of the original pulp cavity, until it becomes not much more than a canal. This canal continues to contain
blood vessels and nerve fibers in intimate contact with the odontoblast
layer which ultimately becomes inactive and simply lines the pulp
canal. Since these inactive odontoblasts send the living fibers of Tomes~
clear through the dentine, it is easy to understand why this substance is
sensitive when injured by decay or bored into by a‘dental drill.
The Formation of Enamel.-The enamel, as already indicated.
is produced by the ameloblasts of the enamel organ. Because of the relation of these cells to the odontoblasts, moreover, the layer of enamel
will necessarily lie adjacent to, and on the outside of, the dentine, or
rather a part of it. As will shortly appear, and as reference to Figure 329
will show, the enamel organ, and hence the enamel, only covers the future crown of the tooth, not its roots These are covered by other material whose origin will be described presently. In the region of the crown
where the ameloblasts are at work we find that the layer they produce
consists of microscopic prisms of very hard calcium salt crystals called
dahlite. These are held together by small amounts of a different substance called cement. It seems to be clear that each prism of the enamel
is produced by a single ameloblast, and therefore extends all the way
from one side of the layer to the other. Since the prisms are not straight,
or precisely parallel to one another, however, this is difficult to demonstrate in section. Organic matter is present, but in even smaller amounts
than in the dentine, about 5 percent of the total substance being so constituted. It apparenlly consists mainly of fine protoplasmic processes
THE TEETH 661
from the ameloblasts which are often called the processes of Tomes
(Fig. 341). They evidently correspond to the similarly named processes or fibers put out into the dentine by the odontoblasts. Finally it is
obvious that as the tooth grows outward due to the formation of more
dentine underneath, the crown will presently be forced through the surface of the gum with the concomitant destruction of the enamel organ.
When this has occurred it is evident that no more enamel can ever be
formed, and that what has formed will extend only to the gum line.
Hence if this hard covering of the exposed surface is later destroyed in
any way it is gone forever. Dentine, on the other hand can be, and
often is, added to from within, if in later life some of it is removed, as
is the case when a tooth is filled. From what has just been said it also
follows that unlike the processes of Tomes in the dentine, those of the
enamel must disappear when the ameloblasts cease to exist.
The Formation of Cementum.— It has already been noted that
only the crown of the tooth is covered by enamel, and that a different
material covers the dentine of the root. This material is called cementum, and is produced by the mesenchyme which surrounds the entire
tooth and enamel organ previous to eruption. This rnesenchyme is said
to constitute the dental sac (Fig. 329). It is only in the neighborhood
of the root, however, that the tissue of the sac produces cementum. Here
its cells behave almost exactly like the osteoblasts of any periosteum,
and the cementum with which they cover the root is essentially the same
as periosteal bone. Indeed on its outer side where the cells of the sac
are in contact with the jaw bone instead of the teeth, they do in fact add
to that bone in the manner of any periosteum. As will be recalled ossein
fibers are produced by the cells of such periosteum, and such is the case
here, both on the side of the jaw bone, and on that of the cementum. It
thus comes about that these fibers actually extend out of the cementum
right into the bone of the jaw. In this manner therefore the tooth is very
firmly anchored in its socket.
The Permanent Teeth. —- Thus far no mention has been made of
more than one type of dentition. As everyone is aware, however, the
first set of so-called milk teeth is later replaced by the permanent teeth.
This process, however, need not detain us long. The enamel organ for
each second or permanent looth arises from the dental ledge near that
of the milk tooth. When the ledge disappears, the organ in question lies
in a depression of the alveolar socket on the lingual side of the growing
milk tooth, but develops no further at this time. Later this “tooth
germ ” goes through the same processes as occurred in the case of the
662 OTHER MAMMALIAN STRUCTURES
milk tooth. Meantime the root of the latter is absorbed, and the crown
is pushed off by the growing permanent tooth beneath it.
Teeth with Open Roots.— It is of some interest to note that in
some animals, notably the Rodents, the incisor teeth continue to grow
throughout life. This is made possible by the persistence of a wide root
canal and the constant addition of more dentine. To compensate for
this the outer end of these teeth is continually worn down by the gnaw
   
dermal (connective
outer mo, Sheath tissue) root ‘sheath
glassy membrane of -_
outer root sheath
' inner root sheath
V ——cortex
hairpapma  ~’ V V  N p ‘_ at 'V;~—-hair matrix
Fig. 342.—Photomicrograph of a mid-sagittal section through a hair
root and papilla under high magnification.
ing activities of these animals. This furthermore is made possible by the
fact that only the front side of the tooth is covered with enamel. The
back side is dentine. Hence since enamel is much harder than dentine
the wear is uneven, which gives the end of the tooth a constantly renewed chisel edge. Of course this process makes a continuance of enamel
formation also necessary on the front surface of the teeth by the perina
nent existence of ameloblasts within the gum in this region, not ‘found
in other cases.
HAIR
Since hair idevelflops long before the Mammal is born, and is one of
the most characteristic features of the class, occurring nowhere else, it
seems appropriate tci refer at least briefly to its development.
THE HAIR 663
As previously noted, hair like feathers is an epidermal structure, and
again it actually consists of cells, not of a secretion by them like teeth.
In this case the cellular character of hair is evident if it is examined
under the microscope. Under these conditions its surface lcuticle) reveals transverse rows of wavy lines, which represent the edges of flat
cells which overlap one another like the shingles of a roof. Beneath this
cuticle are cornified layers of spindle shaped cells and their products,
including pigment, which are termed the cortex (Fig. 3452). In many
types of hair, including that on the human head, the cuticle and the cortex constitute the entire substance of the shaft. In others, e.g., those of
the heard, there is a restricted central region, the medulla, occupied by
a few shrunken cells and numerous air spaces. The latter rfive such hairs
a more silvery appearance when the pigment disappears with age. The
base of each completed hair is contained in a tubular invagination of
the epidermis. This invagination is called the hair follicle, and all of
the parts which lie beneath the surface of the skin together comprise the
root. The walls of this follicle consist oi modified cells of the Malpighian layer of the epidermis, those next to the dermis constituting the
ouzer root sheath, and those next to the hair the inner rooi. sheath. The
latter is itself usually divided into three separate cell layers, but these
need not concern us here. At the base of the root these sheaths merge
into dividing cells which are producing the substance of the hair, and
pushing it upward through the lumen of the follicle. This mass of dividing cells is itself invaginated by an up-pushing bulblike portion of
the dermis containing a blood vessel and known as the hrzir papilla. It
is quite similar to the dermal invagination at the base of a leather called
the feather pulp, and the function in both cases is to nourish the growing structure (Fig. 342).
Again, as in the case of the feather, the hair originates as a downgrowth of the Malpighian layer termed the hair gernz. A small upgrowth
of the dermis invaginates the base or proximal part of this hair germ
and constitutes the beginning of 'the hair papilla. Presently the central
cells of the germ distal to the base become cornifiecl and thus form the
hair. The more peripheral cells of the distal part of the germ soon differentiate into the inner and outer root sheaths of the follicle indicated
above. As growth continues the hair presently comes to extend beyond
the surface of the skin, until much more of it is outside the follicle than
in it. At a point on the follicle near the surface certiain cells of the Malpighian layer constituting the sheaths bu‘d off groups of cells in which
fat droplets accumulate, and which constitute the sebaceous glands (Fig.
, ._.o_...-.......¢.....
l
4
l
l
l
664 OTHER MAMMALIAN STRUCTURES
343) . Just proximal to these there also develop, within the dermis, muscle cells which are attached at one end to the outer root sheath and at the
other to the under surface of the adjacent epidermis. They are called
the erectile muscles of the hair, and serve to ruflle it. This helps to keep
the animal warm, or probably in other cases to frighten its enemies by
making it appear larger, as in the Cat (Fig. 343).
Although not essentially
is of interest to note that all
types of hairs have relatively
fixed periods of life. At the
end of this period the hair is
shed, and its place taken by
a new one. As the time for
shedding approaches the epidermal cells at the base of
the hair shaft and inner root
sheath cease dividing. At the
same time those constituting
the base of the hair become
cornified like those in the
main part of the shaft. The
hair is then detached from the
' papilla, and easily comes out
F18; 343-—‘1,’h°‘°‘_‘1i°1'€’g"‘Pl‘ °f the ?‘‘me of the follicle. Later the new
section of hair as in Fig. 342, taken with a . . . .
lower magnification to show relations to ha” 15 f91'med In the Same fol‘
neighboring hairs and also to a sebaceous 1ic]e_ The papilla which has
gland and erectile muscle. .
shrunken 1S restored, and the
remaining live epidermal cells which cover it start to multiply. The
latter presently give rise to both a new inner sheath and hair shaft in
a manner similar to the original process.
 
:1
NAILS, HOOFS, AND HORNS
It is not feasible to give a discussion of the development of these
structures in a volume of this size and character. However it may be
noted that once more both nails (claws) and hoofs arise as modifications of epidermal cells, involving mainly their cornification. Horns of
one type such as those of the Cow are cornified epidermal sheaths supported by bony cores. The antlers of deer on the other hand are mostly
bone covered by a layer of skin (dermis and epidermis) which soon dies
REFERENCES T0 LITERATURE 665
and is rubbed off. The bony horn itself is shed annually, and renewed
by a remarkably rapid growth of nomcartilaginous bone. The two lastnoted structures are not strictly speaking embryological since they never
appear until after birth. Because of their developmental similarity in
some respects to the other dermal and epidermal appendages, however,
it was thought worth while to mention their origins.
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668 THE MAMMAL
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Webster, C., Human. P/acentalian, Clticago, 1901.
Weller, G. L., .Ir., “ Development of the Thyroid, Parathyroid and Thymus Glands
in Man," Carnegie Inst. Cont. to Emb., XXIV, 1933.
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Reproductive Tract of the Bat,” Anat. Rec., XXCVIII, I944.
Winiwarter, H. YOU, “ Recherches sur lbvogenese et Yorganogenese de l’ovaire des
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Wislocki, G. B., “ Ht‘-mopoiesis in the Chorionic Villi of the Placenta of Platyrrhine
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185-194, 1924-.


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McEwen RS. Vertebrate Embryology. (1949) IBH Publishing Co., New Delhi.

   Vertebrate Embryology 1949: 1 Germ Cells and Amphioxus | 2 Frog | 3 Teleosts and Gymnophiona | 4 Chick | 5 Mammal | 1949 Vertebrate Embryology
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This 1949 third edition textbook by McEwen describes embryonic development.



1923 Edition

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Vertebrate Embryology

Robert S. McEwen

Professor Emeritus of Zoology, Oberlin College

Contents

Part I The Germ Cells and Early Development of Amphioxus

  • 1. Introduction
  • 2. Fertilization and Early Stages in Development
  • The Early Development of Amphioxus

Part II The Development of the Frog

  • The Frog: from the Production of the Germ Cells through Gastrulation
  • The Frog: Early or Embryonic Development Subsequent to Gastrulation
  • The Frog: Later or Larval Development

Part III The Teleosts and Gymnophiona

  • The Teleosts and Gymnophiona: their Segmentation and Gastrulation

Part IV The Development of the Chick

  • The Chick: the Adult Reproductive Organs, and the Development of the Egg Previous to Gastrulation
  • Gastrulation and Development through the First Day of Incubation
  • The Chick: Development during the Second Day of Incubation
  • The Chick: Development during the Third Day of Incubation
  • The Chick: Development during the Fourth Day of Incubation
  • The Chick: Development during the Fifth and Subsequent Days

Part V The Development of the Mammal

  • The Early Development of the Mammal and its Embryonic Appenclages
  • Development of the Pig to the Ten Millimeter Stage
  • The Later Development of the Pig
  • The Skeleton, Teeth, Hair, Hoofs and Horns

Index

Preface to the Fourth Edition

As in previous revisions, the fourth edition of this text does not purport to be a new book. It again frankly retains the fundamental plan and character of the older editions, in that it is primarily descriptive. but with enough experimental results interwoven with the descriptive material to stimulate interest, and to elucidate such principles of development as have been firmly established.

Though not radically altered, the older book has nevertheless been carefully gone over page by page, and, as before, changes have been made. wherever it was thought desirable in order to bring the subject matter up to date, to clarify statements, or to correct errors. In some cases. whole pages have been entirely rewritten, and in certain instances, as in the section on maturation of the germ cells, this has involved several successive pages. Mistakes in figures have also been corrected, and in a few cases. as in the diagram of frog gastrulation, the figure has been completely modified and, the writer believes, greatly improved.

Thanks are due to various colleagues who have made suggestions and pointed out errors. Especial gratitude is felt by the author to Dr. Roland Walker for his meticulous notations of errors both large and small, and for his constructive eiiorts to aid in their correction.

R. S. MCE. Oberlin College,

September, 1956.

Preface to the First Edition

This book is designed as an introductory text in Vertebrate Embryology, a work which seems to be justified on the following grounds: The older texts upon this subject, though in many cases excellent, do not cover exactly the field which is now covered in many colleges; these texts, moreover, are becoming somewhat out of date in various details. Among the newer books the best ones tend to do one of two things. Either, in the interest of thoroughness, they confine their attention entirely tn one form, e.g., the Chick, or else, for the sake of a broader viewpoint, they deal with a considerable number of animals, but in doing so touch only upon the earlier developmental stages of each. Now it is obvious that there is great value for the student, both in the accuracy gained by the careful intensive study of a single type, and also in the possession of less detailed knowledge of the history of other forms which are nearly related to it. Hence, what has seemed to be needed was a book which would, so far as is possible, make available both these advantages. To meet this need, the major part of the present text comprises a mo-,leratcl}‘ complete account of the development of two typical forms. i.e., the Frog and the Chick, each of which, in the writer’s opinion, has special features which justify such treatment. These relatively detailed discussions are then supplemented by chapters which present brief comparisons, not only with the Mammal, but also with certain other significant members of the Vertebrate group. Furthermore, the essentially embryological portion of the book is preceded by an optional introductory chapter dealing with the elements of cytology. Upon this basis the effort throughout the work has been to produce something adapted to the requirements of the general student of Zoology. us well as to the individual particularly interested in premedical preparation.


As i'crgzvx'tls certain details concerning the method of handling the topics involved, the following remains to be said. Because of the character of the book, the chapter upon cytology places special emphasis upon the structure, development, and function of the germ cells, with particular reference to nuclear phenomena and their genetic significance. The strictly cnibryological subject tn:-ttter is then introduced by a short general discussion of the more lundaixiierxtal and universal proc of Vertebrate development from the comparative standpoint. This includes a description of the various types of segmentation, gastrulation, and the formation of the rudiments of the nervous system and the main mesodermal structures. Following these introductory chapters,

‘Amphioxus is the first particular type to be considered lI(’('£lUSt‘. of the

relatively primitive character of most of its early history. The later development of this animal, i.e,, that following the fnrnizuion of the mt'.s'ndermal somites. is, however, quite highly distinguish it from the vast majority of Clionlates are without great significance for the general student. tliey are mniuml.

The Frog, as suggested above, is one of the two forms which have been treated at some length. The reasons for suvli extencled mnsirl<~ration in this instance and in that of the Chick are presunmbly olwious to every Zoiilogist. For the sake of the student. however. the uzlim uf these animals as subjects of enibryologitral study is lt\[llt‘txil,’il in tinparagraphs of the text which introduce them. ln the case ui lhv "I":-u;_». its early history has been presented under the head of c-ertuin fairly. well recognized stages which lend themselves well to corre-l;1tion with work in the laboratory. In further pursuance of this method the-. internal changes have been noted in alternation with those or-currin;__r cxtc-rnall_\ . This was done in order that the reader might obtain. so far as pm-s_<il»le. a correct idea of the really simultaneous character of tliese processes. It did not seem feasible, however, in a work of this St'(}pt.' to continue this plan throughout the entire course of development in this animal. The later external changes. therefore, are included under one lieading. while the more advanced details of organogeny are described in terms of particular systems.

Following the treatment of the Frog, there has been introduced a very brief account of segmentation and gastrulation in the Teleosts and the Gymnophiona. This has been done despite the realization that in the case of the latter group laboratory consideration will in most cziscs be impossible. The reason for this is the authors opinion that segnu-xi1;+ tion and gastrulation in these two classes of animals are extrem:-ly valuable in assisting the student to relate these processes in the Frog In those which he is about to study in the Bird. Experience, xnoremm‘, has seemed to indicate that the relation of avian and mammalian gztstrulzb tion to that in more primitive forms is always particularly clillicult for

i the beginner to grasp, and it is believed, therefore. that any legitinmte aid to this end is worth while.

In treating the early stages of the Chick a good deal of stress has been placed upon the method of segmentation and gastrulation. The latter especially has been emphasized because of its peculiar character, and the desirability of making clear its relationship to that in the forms already studied. The later history of this animal is then presented in daily periods, according to the well-known plan of Foster and Balfour. This has been done because it seems to the writer that at least in a beginning course, this method has certain marked advantages over that of stuclying the complete embryology of one system at a time. In the first place the Bird lends itself particularly well to treatment by periods, and secondly, the simultaneous development of all the systems is what is actually seen to occur in any animal. This latter fact it would seem well to impress upon the student when possible by the method of presentation. Finally it has appeared not only possible but easier to conduct the class work in correlation with the laboratory when development is studied by periods rather than by systems. It should be noted, nevertheless, that in this book the material has been so arranged that the student can readily follow through the complete growth of any one system if the instructor so desires.

As regards the Mamxnals, it is felt that the detailed differences between the organogeny of this group and that of the Birds are not, on the. whole, of great general biological significance. Of very considerable significance, however, are those unique characteristics of both mother and embryo connected with mammalian gestation. For this reason the discussion in this portion of the text is confined chiefly to the earlier developinental stages, which are treated largely from the comparative standpoint. The subject is introduced by a description of the structure and functions of the adult reproduetige organs in the,same manner as in the case of preceding forms. This involves the process of ovulation, and in that connection it has seemed worth while to describe briefly the peculiar cyclic phenomena which accompany this process in the mammalian female. Following this, the comparative idea is pursued with particular reference to the development of the extra-embryonic z1ppt’ll(l£lgC.‘.‘~. This is believed to be especially important from an evolutionary viewpoint because it shows how these appendages, already observed in the Chick. have been modified in the various Mammals. This discussion is naturally accompanied by a description of the structure and probable evolution of the placenta. For the general plan of treatmom of these latter topics the author frankly acknowledges his indebtedness to Professor Jenl<inson’s excellent book, Vertebrate Embryology.

Concerning bibliographical material, references to the more important literature of each subject are appended to the chapter which concludes consideration of the topic in question. As intimated, it will be quite obvious that these references make no pretense of being exhaustive. Their object is rather merely to point the way to further study for the reader who desires it. This is done, first, because the present volume is intended primarily as a text rather than as a book of reference, and, secondly, because it is felt that the beginner’s interest may be more effectively aroused in this manner than by presenting to him at once every reference available. The latter, if desired, can be readily obtained in the more advanced books which are cited.

It is recognized that illustrations constitute an extremely important feature in a text of this character, and the writer has spared no pains in the attempt to make the figures adequate both in number and quality. It will be evident, however, that the majority of them are not original. This is due to the fact that through the kindness of the authors and publishers indicated below, there were made available a large number of excellent illustrations, which it seemed hardly worth while to attempt to improve upon. Nevertheless, in every instance where it was felt that such improvement was possible, or where it appeared that a new figure would be profitable, original drawings have been inserted. Lastly. it remains to be. stated in this connection that in the case of all borrowed illustrations, great care has been taken to have the illustration and the terms used in its legend agree with the respective description and terminology in the text. The desirability of this, especially in an clexnemarj.' book, is obvious; yet, according to the writer’s observation, it is a feature which is too frequently overlooked.

In conclusion I desire to express my appreciation of the following favors. To Professor Frank R. Eillie and to Henry Holt and Co., I am indebted for their generous permission to use a large number of figures from Lillie’s Development of the Chick; to Professor T. H. Morgan. his co-authors, arid Henry Holt and Co., for certain illustrations from The Mechanism of Memlelian Heredity; to Henry Holt and Co., for numerous figures from Kellicott’s General Embryology and Chordate Development; and to the Delegates and Secretary of the Clarendon Press for a like favor as regards .lenkinson’s Vertebrate Embryology. It is also a pleasure to acknowledge a similar debt to Professor Morgan and The Columbia University Press fr;-2' figures from Heredity and Sex: to Professor J. Playfair McMurrich and P. Blakiston’s Son and Co. for cliches from McMurrich’s Development of the Human Body‘; to P. Blalcistozfs Son and Co. for further clichés from Minot’s Laboratory Text Book of Embryology; to Messrs. Longmans, Green and Co. for cliches from Quain’s Anatomy; to Messrs. G. P. Putnam and Co., for permission to use again certain figures from Marshall’s Vertebrate Embryology, copied and slightly modified by Kellicott; and to Professor 0. Van der Stricht and Dr. T. W. Todd for allowing the use of photomicrographs made in the Anatomical Department of Western Reserve University Medical School from preparations presented to that department by Professor Van der Stricht. In all cases the illustrations thus borrowed are acknowledged in the legends of the figures concerned.

I wish further to express particular gratitude to Professor T. H. Morgan for reading and criticizing the first half of the manuscript; to Professor J. H. McCregor for performing a similar service for the entire hook; to Professor M. M. Metcalf for suggestions regarding the earlier chapters: to my wife for assistance with the proof; and to Pro.fessor R. C. llarrison for the identification of the frog larvae used in niaking certain of my original drawings. Especial gratitude is also felt for the constant interest and helpfulness shown by my colleagues, Professors R. A. Budington and C. G. Rogers.

R. S. MCE.

Oberlin College, August 15, 1923.



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Mark Hill.jpg
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)
   Vertebrate Embryology 1949: 1 Germ Cells and Amphioxus | 2 Frog | 3 Teleosts and Gymnophiona | 4 Chick | 5 Mammal | 1949 Vertebrate Embryology

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