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==Introduction==
 
==Introduction==
  
IN taking up the development of the Mammal in a book of this
+
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.
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
+
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.
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
+
We shall then proceed with the comparisons of the early embryos of selected orders of Mammals with special emphasis upon the development 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.
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
+
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.
  
, THE MALE
 
  
The Testes and Their Ducts. —— In the adult male Mammal there
+
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.  
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
+
==The Reproductive Organs of the Adult==
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 Male===
  
The Ovary.——In the female Mammal there is a single pair of
+
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.
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.
+
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 0viducts.—As in the Bird, the ovaries are not directly connected with the Miillerian ducts or oviducts. The latter, sometimes
+
===The Female===
OOGENESIS" 439
 
  
known as the Fallopian tubes, are, however, provided as usual with a
+
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.
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
+
The Genital Tract.
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—
+
The 0viducts.—As in the Bird, the ovaries are not directly connected with the Miillerian ducts or oviducts. The latter, sometimes OOGENESIS" 439
TION, AND THE SEXUAL CYCLE
 
  
OOGENESIS ,
+
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.
  
The O6gonia.——-The embryonic ovary of the Mammal contains the
+
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.
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
+
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.
Development). After Waldeyer.
 
  
a. “ Germinal epithelium.” b. Ovigerous cords. c. Small ovarian follicles. :1. Older
+
==The Development of the Ovum up to Segmentation, and the Sexual Cycle==
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
+
===Oogenesis===
  
growths which push into the deeper mesenchyme. These outgrowths are
+
The Oogonia.—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 most accounts this cortical epithelium thickens and then produces out-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 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.
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
+
Fig. 247. — Section through part of the ovary of a Dog. From Kellicott (Chordate Development). After Waldeyer.
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
+
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.
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~
+
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.  
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
+
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 follicle.'The young ovum now enters upon the growth period as an oiicyte.
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.
+
===The Oocyte and the Graafian Follicle===
It is surrounded, however, by a thick transparent substance which is
+
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).
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
+
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 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.
  
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
+
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.).
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
+
===The Mature Ovum And Ovulation===
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 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, 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 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)
+
* 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 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
 
  
 
  
 
+
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) .
 
  
pmoeurn-I dloumm pmoenruml an-«run wuourua
+
===Ovulation===
 +
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.
  
1 Mulldnzil 1 °"" '"° 1 °"“'l“°"
+
===The Sexual Cycle in the Female===
  
our
+
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, moreover, may be divided into two main groups with respect to these uterine changes, i.e., the Primates and the non-Primates.
uwpusluuun
 
  
1 __ /, _________ -  
+
The Non—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:
  
9.. \
+
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.
  
nnlaua
+
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)
  
+
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 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) .
  
cm
 
xl“ mu
 
  
 
  
- eyu
 
  
Fig. 249.—Diagrams comparing the events of the oestrus cycles of the Pig and
+
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.
  
.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
+
Variations in the Non-Primate Cycle.—-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).
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
+
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.
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
+
The Primate Cycle.—In discussing this group it should at once he pointed out that the peculiarities about to be described do not actually 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.
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
+
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:
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
+
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,
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
+
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
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
 
presumably homologous with the dioestrum in the lower animals, we
  
find conditions at that stage in the Human subject about the same as in
+
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 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 ”
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
+
* 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.  
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
+
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.
  
3 Just what parts of the follicle are responsible for this hormone is not alto»
+
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 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.
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
+
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.  
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
+
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.
  
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,
+
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.,
+
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?
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
+
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
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
 
  
+
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.
 
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
+
acciassory in aiding the oestrogens to prepare the breasts for final stimulation by pro actin. -n.7<ma.t:..a
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,
+
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: .
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
+
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.
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
+
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 hormone 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.
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.
+
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) .
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
+
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.
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
+
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
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
+
- reduction of progesterone, which quiets uterine contraction, and an in crease in oestrogens, which are known to stimulate it: play a part. This,
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
+
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
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
+
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) .
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
+
==The Sexual Cycle in the Male==
  
As regards the male among Mammals, it is found that here also there
+
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.
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
+
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.’  
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
+
==Maturation And Fertilization==
  
Although in Mammals the first maturation division often occurs before ovulation and fertilization, the second, with apparently only a few
+
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.
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.
 
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
+
ward. Hence it has seemed best to mention both divisions in connection with the latter phenomenon.
with the latter phenomenon.
 
  
The First Maturation Division.—At some time during the
+
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
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,
+
large, i_.e., often as much as one fourth the diameter of the ovum itself, 506 EARLY MAMMALIAN DEVELOPMENT
506 EARLY MAMMALIAN DEVELOPMENT
 
  
and in abnormal cases sometimes equal to .the latter. The fate of these
+
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.
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
+
Fertilization. —— Sperm introduced into the vagina of the Mammal rapidly make their way into the uterus and up the oviducts. A few hours
rapidly make their way into the uterus and up the oviducts. A few hours
 
  
 
0
 
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
+
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
polar bodies separated. B. Four-cell stage, 25% hours after coitus. C. Eight-cell
 
  
stage.
+
stage. a. Albumenous layer derived from the wall of the oviduct. z. Zona radiata.
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
+
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 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.
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,
+
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.
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
+
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 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 Rat (Blandau and Jordan, ’41), and for twenty hours in the Guinea Pig (Blandau and Young, ’39) .
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
 
  
   
 
 
  
 
+
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.
  
«. _‘ ' A
 
future trophoblast
 
  
Inner cell mass
 
  
Fig. 252.—Semi-diagrammatic sections through stages of
+
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.  
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
+
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).
(Blandau and Young, ’39) .
 
  
From these data it will be evident that even though ovulation may
+
==Segmentation, Gastrulation, Amnion Formation, And The Primitive Streak==
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
+
===Segmentation===
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
+
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. _
  
SEGMENTATI ON
+
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 Type of Cleavage. — Segmentation in the placental Mammals
+
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,
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
+
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 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).
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
+
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.
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
+
Within the uterus the cleaving egg, or morula, soon becomes a blastocyst, if it is not already one, and this begins to enlarge through
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
+
Fig. 253.—-Section through the fully
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
+
formed blastodermic vesicle of the Rabbit,’ From Quain’s Anatomy, after Van Beneden.
egg, or morula, soon becomes a
 
blastocyst, if it is not already one,
 
and this begins to enlarge through
 
  
+
f.c.m. Granular cells of the inner cell mass. troph. Trophoblast. zp. Zona pellucida.
  
Fig. 253.—-Section through the fully
+
the multiplication and flattening of the cells of the subzonal layer
  
formed blastodermic vesicle of the Rabbit,’ From Quain’s Anatomy, after Van
+
(Fig. 253). There is considerable variation in the size and shape
Beneden.
 
  
f.c.m. Granular cells of the inner cell
+
_ 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) .
mass. troph. Trophoblast. zp. Zona pellucida.
 
  
the multiplication and flattening
+
===Gastrulation===
of the cells of the subzonal layer
 
  
(Fig. 253). There is considerable
+
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  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 man
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
+
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.
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
+
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. Embryo oniclepiblast forming a Hat disc on the surface of the blastodermic ‘. vesic e.
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.
+
E. Inner cells mass, now embryonic knob. ec. Embryonic epiblast. en. Hypohlast. tr. Trophoblast.
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
+
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 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.
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
+
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.
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
+
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
+
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 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 activity of the trophoblast, aided by certain changes in the uterine
 
wall itself.
 
  
 
THE AMNION
 
THE AMNION
  
There are two chief methods by which the amnion is formed in the
+
There are two chief methods by which the amnion is formed in the Mammal: ‘
Mammal: ‘
+
 
 +
I. The First Method of Amnion Formation.——-This method may be defined briefly as the method of amnion formation by folds. The
  
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.
  
Fig. 256.-—Formation of the amnion in the Rabbit (Lepus). From Jenkinson
+
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.
(Vertebrate Embryology). After Assheton.
 
  
i.m. Inner cell. mass. Ll. Lower layer (i.e., hypoblast) . e.p. Embryonic plate (i.e..
+
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.  
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.
+
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. '
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
+
‘ 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 manner 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).
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
+
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.
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).
+
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.  
  
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
+
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.
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
+
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).
(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
+
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). '
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
+
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 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 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.
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.
 
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.
+
am. Amnion. c. Extra-embryonic coelom. ec. Ectoderm. e.k. Embryonic knob. l. Lacuna. m. Mesoderrn. n. Notochord. tr. Trophoblast. y.s. Yolk-sac.
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
+
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: _
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
+
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 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
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
 
  
+
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.
  
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.
+
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.
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
+
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.
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
+
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
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
+
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.  
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
+
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.
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
+
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.
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.
 
  
+
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 endoderm
  
 
 
  
beginning
+
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.
  
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.
+
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.
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
+
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 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
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.
  
Fig. 263.--Later primitive streak and mesoblast formation in the Pig. After
+
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
Streeter.
 
  
folding, and further that most of the (chorionic) portion of the folds is
+
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.
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
+
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
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,
+
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
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
 
those cases where it does
  
not (Method II) are mere
+
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.
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
+
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
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.
 
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
+
more cogent and reasonable than those opposed to it and it is the one which is more widely held.
which is more widely held.
 
  
 
THE PRIMITIVE STREAK AND RELATED STRUCTURES
 
THE PRIMITIVE STREAK AND RELATED STRUCTURES
  
It will have been noted that during the process of amnion formation
+
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.
(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
+
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
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,
+
' Henserrs node 526 EARLY MAMMALIAN DEVELOPMENT
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.
+
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.
t. Trophoderm.
 
  
the following conclusions seem justified. There first.appears a thickened crescent of epiblast about what proves to be the posterior margin
+
ant. Amnion. b. or bl. Blastopore. ce. Chorda endoderm. ec. Ectoderm. en. Endoderm. nc. Neurenteric canal. prm. Peristomial mesoderm. ps. Primitive streak. t. Trophoderm.
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
+
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
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.
 
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 archenteron (Fig. 266). In some others the pit merely pushes into THE PRIMITIVE STREAK 527
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).
  
the notochord where it is known as the notochordal canal. In either case
+
Origin of Mesoderm and Notochord. -As in the Chick, so in the Pig, and presumably in other Mam primitive streak
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
 
ectoderm
Line 1,378: Line 404:
 
mals, the streak is again the Fig. 267.—-‘Transverse section of one side of v
 
mals, the streak is again the Fig. 267.—-‘Transverse section of one side of v
  
d a Pig blastoderm similar to one from which
+
d a Pig blastoderm similar to one from which 5°urce Of the meso erm’ surface reconstruction in Fig. 262, C, was
5°urce Of the meso erm’ surface reconstruction in Fig. 262, C, was
 
  
which is proliferated from made. After Streeter. Long axis measurement
+
which is proliferated from made. After Streeter. Long axis measurement . . of the blastoderm from which this section was its sides, and spreads out on
. . of the blastoderm from which this section was
 
its sides, and spreads out on
 
  
 
taken was .5 mm.
 
taken was .5 mm.
Line 1,389: Line 412:
 
either hand and posteriorly
 
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
+
(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
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
+
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
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
+
“-‘9..3'.~‘4‘:-"W ,
,
 
  
 
  
Fig. 268.—A. Transverse section through the primitive streak of
+
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
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
+
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.
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
+
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
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
+
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).
(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
+
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
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-.
 
difliculties experimental observations on the behavior of materials dur-.
  
ing and immediately after the formation of the primitive streak are not
+
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. '
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
+
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. '
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:
+
THE YOLK-SAC, THE ALLANTOIS, AND THE PLACENTA: THEIR STRUCTURE AND FUNCTIONS IN THE MAMMAL
THEIR STRUCTURE AND FUNCTIONS IN THE MAMMAL
 
  
Among the Amniotes of which the Chick is a type, i.e., the Birds, the.
+
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
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
+
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.)
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
+
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.
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.
 
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
+
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
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:
 
noes-2 ax:
  
‘ma.
+
‘ma. THE MARSUPIALS 531
THE MARSUPIALS 531
 
  
of allantoic placenta, and perhaps suggest its method of evolution. The
+
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.
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
 
THE MONOTREMES
  
These curious mammalian forms comprise the Spiny Ant Eater
+
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)-.
(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
 
THE MARSUPIALS
  
This group comprises the Kangaroos (Macropodidae), the Opossums
+
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.
(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 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 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
+
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
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 '
 
exact means by which the embryonic blood is aerated, therefore, is a '
  
little uncertain. Very possibly, however, it also is accomplished through
+
little uncertain. Very possibly, however, it also is accomplished through the contact of yolk-sac and maternal tissues.
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
+
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.
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
+
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
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
 
THE PLACENT-ALIA 533
  
wall, whose epithelium in this region is transformed into a vascular
+
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.
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/
 
4/
  
 
 
  
 
f. b. v.
 
f. b. v.
  
:73. mt.
+
73. mt.
 
 
 
Fig. 270.——Section through the placenta of Perameles. From Jenlcinson (Vertebrate Embryology). After Hill.
 
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
+
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.
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
+
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.
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
+
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. '
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
 
THE PLACENTALIA OR TRUE PLACENTAL MAMMALS
  
Within this large group, the embryonic appendages whose condition
+
Within this large group, the embryonic appendages whose condition is being considered are probably in their-most primitive form among 534 EARLY MAMMALIAN DEVELOPMENT
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 Ungulates, and this ‘order, therefore, will be treated first with special reference to the Mammal; we have selected for later detailed study,
Line 1,630: Line 503:
 
the Pig.
 
the Pig.
  
The Ungulates (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
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.
+
Fig. 271.—Diagram of a fetal and maternal cotyledon of the Cow. From Jenkinson (Vertebrate Embryology).
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
+
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.
live tissue is shaded.
 
  
dioestrum periods as explained in cohnection with the oestrus cycle. As
+
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. '
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
+
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
will be remembered, soon becomes greatly elongated, reaching a length
 
1
 
1
 
i
 
I
 
I
 
I
 
  
 
THE PLACENTALIA 535
 
THE PLACENTALIA 535
  
of as much as a meter. It is not, however, to be understood from this
+
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
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
+
blastodermic vesicle amnion ¢mb")'°
amnion ¢mb")'°
 
  
   
 
  
l‘ I horlonlc crophoblast
+
l‘ I horlonlc crophoblast
  
 
  
 
  
diagrammatic x section r
+
diagrammatic x section r of blastodermlc vesicle _.- '
of blastodermlc vesicle _.- '
 
  
 
 
  
Fig. 272.—-Drawing of a Pig blastodermic vesicle measuring about 350 mm. in
+
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.
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
+
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
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
+
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
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
+
Tailed Weasel and the Martin the blastocysts are said to lie dormant in the uterus for many weeks (Wright, '42). - 536 - EARLY MAMMALIAN DEVELOPMENT
for many weeks (Wright, '42).
 
- 536 - EARLY MAMMALIAN DEVELOPMENT
 
  
blastocyst, though not to the extent that it was at its greatest length.
+
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
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
 
atrial part posterior ardinti vein
  
 
 
 
 
  
 
 
  
 
 
  
ventricular area .
 
temporary viteiime and intcstlnai arteries
 
  
 
+
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.
 
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
+
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.
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
+
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
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
+
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.
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
+
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.
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
+
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.
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 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
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
+
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
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. '
 
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
+
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.
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
+
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) .
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,
+
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
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___,.
+
‘7‘;;-~;-‘'7‘/=‘7‘-—--— 8"’:/’ %g___,.
  
 
tr.
 
tr.
Line 1,808: Line 578:
 
rn.b.c.
 
rn.b.c.
  
f. c. t.
+
f. c. t. f. b. c.
f. b. c.
 
  
 
In .b.v.
 
In .b.v.
  
Fig. 275.—Section through the placenta and uterine
+
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).
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
+
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
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
 
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
+
is called zonary. The villi of the chorion, which may contain a core of 540 EARLY MAMMALIAN DEVELOPMENT
540 EARLY MAMMALIAN DEVELOPMENT
 
  
mesoderm, now push their way directly iillio the mucous tissue of the
+
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).
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
+
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
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-.
 
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
+
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.
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
+
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. _
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
+
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 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 PLACENTALIA 54.1
  
the coelomic wall, the latter region being termed the mesometric side.
+
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
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»
+
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.
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
+
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. ’ ..
obtains attachment at every point, and completely obliterates the cavity
+
 
of the‘ ‘uterus where it is situated. At every place where contact is thus
+
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.
 
97- am.
  
Fig. 277.--Fetal membranes and placenta of the Rabbit. From Jenlrinson (Vertebrate Embryology). After Duval and Van Beneden.
+
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.
pr.am. Proamnion. Other letters as in Fig. 276.
 
  
established, i.e., on the bottom and sides of the original pit, and also
+
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_
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
 
THE PLACENTALIA 543
  
, 276, D). Inside the latter, the yolk-sac has meanwhile formed, and on its
+
, 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
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
+
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).
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
+
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.
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
+
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
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
+
” 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
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.
 
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
+
seem to indicate that such a function is negligible, at least in this animal (Noer’47). 544_._ EARLY MAMMALIAN DEVELOPMENT
(Noer’47).
 
544_._ EARLY MAMMALIAN DEVELOPMENT
 
  
 
  
 
a. m. f. b. v.
 
a. m. f. b. v.
  
 
 
  
 
\\\\
 
\\\\
  
vs‘
+
vs‘ \§.\ .\
\§.\
 
.\
 
  
 
it
 
it
  
\\\\ ( ‘
+
\\\\ ( ‘ .t\\\ r ‘~" ‘L
.t\\\ r ‘~" ‘L
 
  
 
  
Fig. 278.—Placentation of the Mouse. Details of the five stages of
+
Fig. 278.—Placentation of the Mouse. Details of the five stages of Fig. 276. From Jenkinson (Vertebrate Embryology».
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. 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
+
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.
lumen of the uterus. m.g.c. Maternal glycogen tissue.
 
  
 
B. Similar strip of the same parts in stage D, Fig. 276.
 
B. Similar strip of the same parts in stage D, Fig. 276.
  
_ fjmv. Fetal blood-vessel. a.m. Allantoic mesoderm. Other letters as
+
_ fjmv. Fetal blood-vessel. a.m. Allantoic mesoderm. Other letters as in .
in .
 
  
 
C. Similar strip of the last stage, Fig. 276.
 
C. Similar strip of the last stage, Fig. 276.
Line 2,010: Line 667:
 
tr.g.c. Trophodermal glycogen tissue. Other letters as in 3.
 
tr.g.c. Trophodermal glycogen tissue. Other letters as in 3.
  
Note that ,ultimately this placenta is very largely composed of
+
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
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
 
THE PLACENTALIA 545
  
penetrate the trophoderm so as to come near to the cavities containing
+
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.
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,
+
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.
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,
+
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,
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- ‘
 
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
+
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
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
+
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.
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
+
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.
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.
+
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.
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
+
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
in its activity, differs markedly from the types previously studied within
 
the strictly placental group.
 
  
The Primates.
+
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
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
+
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
(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  
+
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
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
+
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.
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
+
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
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‘
 
Trophcblosl‘
Line 2,122: Line 707:
 
.. Ex traembryonic
 
.. Ex traembryonic
  
mesoblusi
+
mesoblusi Pfimmve Exccoelo "c cndodmn ,,,,,,,,,,;“,;
Pfimmve Exccoelo "c
 
cndodmn ,,,,,,,,,,;“,;
 
  
Extrcemrycmc mdoderm Uterus
+
Extrcemrycmc mdoderm Uterus ring,“ 5"," db“ Trophoblufl mesoblast Amnion Gerrfldigk
ring,“ 5"," db“ Trophoblufl mesoblast Amnion Gerrfldigk
 
  
 
  
xh-cembryonic
+
xh-cembryonic ‘"‘l°d"’"‘ mesobtast
‘"‘l°d"’"‘ mesobtast
 
  
   
 
  
E 1 A Primitive
+
E 1 A Primitive ",:.::ni,:::. endodtrm
",:.::ni,:::. endodtrm
 
  
Fig. 281.—Mid-sagittal sections through four Human blastocysts
+
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)
(“ 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
+
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:
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
+
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
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
+
,Ԥ THE PLACENTALIA 54,9
THE PLACENTALIA 54,9
 
  
occupied by a reticulate material, the magma reticulare, which probably
+
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
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
+
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
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
+
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.
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
 
brane (Fig. 281). Dorsally this space is lined by the endoderm of the
Line 2,179: Line 736:
 
blastoderm, and it has therefore been interpreted by some as the yolk— '
 
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
+
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
later, about the 13th day. It is difiicult, however, to distinguish the
 
  
 
_ “endoderm” of this later yolk-sac from the exocoelomic membrane
 
_ “endoderm” of this later yolk-sac from the exocoelomic membrane
  
bounding the central “ exocoelomic space ” of the earlier embryos. At
+
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
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
+
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.”
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
+
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.“
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
+
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.
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
+
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.
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
+
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
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
+
" 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
the stalk shifts its point of attachment to the trophoblast accordingly. V
 
  
‘3 In a more recent human specimen. the Martin-Falkiner blastocyst C38),
+
‘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.
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
+
“ 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
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
 
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
Line 2,242: Line 762:
 
Fig. 283. —— Development of the fetal membranes in Tarsius. From Jenkinson (Vertebrate Embryology). After I-lubrecht.
 
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
+
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
(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-'
 
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
+
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.).
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
+
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). ‘
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
+
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
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
+
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
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.
 
m. b.v. _ d. b. tr.
  
 
  
 
d. r. ep.
 
d. r. ep.
Line 2,275: Line 781:
 
Fig. 284.——Early Human embryo with its membranes. From Jenkinson (Vertebrate Embryology). After Peters. "
 
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
+
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.
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
+
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
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
+
perhaps open to question: but they are certainly not “ ova ” as they have sometimes been designated. ' ' v »
been designated. ' '
 
v
 
»
 
  
 
  
 
 
__ .... _,.,, A,.,._
 
  
 +
__ .... _,.,, A,.,._
 
THE PLACENTALIA 553
 
THE PLACENTALIA 553
  
covered (Hertig and Rock, ’4l; Figs. 281, 282). The additional data
+
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:
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)
+
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.
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
+
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
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
+
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.
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
+
_ 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.
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.
+
Syncytiotrophoderm. t. Trophoderm. 1:. Villi. vf. Fixation villi, i.e., those which extend clear across a sinus. THE PLACENTALIA 555
THE PLACENTALIA 555
 
  
Fig. 286. —A. A diagram of an idealized section through the inner portion of the
+
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.
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
+
.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.
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
+
1 556 EARLY MAMMALIAN DEVELOPMENT
556 EARLY MAMMALIAN DEVELOPMENT
 
  
and are hence covered by, the trophodermal material in the Mouse or
+
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
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’
 
Fig. 287.——-Human embryo of the fourth uterine wall‘ A5 growth con’
  
month in ulero, showing the arrangement of tinugs, this epithelium is even.
+
month in ulero, showing the arrangement of tinugs, this epithelium is even. the membranes and placenta. From Kellicott
the membranes and placenta. From Kellicott
 
  
(Chonlate Development). After Strahl. many bound to come in con‘
+
(Chonlate Development). After Strahl. many bound to come in con‘ c. Chorion and amnion. p. Placenta. LL. tact with that which lines the
c. Chorion and amnion. p. Placenta. LL. tact with that which lines the
 
  
Umbilical Cord‘ walls of the uterus at other
+
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
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 .
 
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 sul)-epithelial mucosa of the uterine wall. It is only in the region THE PLACENTALIA 557
THE PLACENTALIA 557
 
  
 
  
 
I.-u.
 
I.-u.
Line 2,418: Line 829:
 
d. v.
 
d. v.
  
Fig. 288.—Diagrammatic section through the pregnant human uterus and embryo at the seventh or eighth week. From Jenltinson (Vertebrate Embryology).
+
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.
After Balfour, after Longet.
 
  
am. Amnion. a.m.c. Amniotic cavity. The latter has enlarged until it occupies
+
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.
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
+
of the placenta, however, that the chorion normally continues to be vascularized and to thicken by the growth of villi.
vascularized and to thicken by the growth of villi.
 
  
The placenta, as so far described, consists then essentially of a greatly
+
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
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
 
EARLY MAMMALIAN ‘DEVELOPMENT
  
the uterine mucosa, i.e., that part of the mucosa which the trophoderm
+
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
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
+
Fig. 289. —— Reconstruction of a human embryo of 2.6 mm. From Minot (Laboratory Text-Book of Embryology). After His.
(Laboratory Text-Book of Embryology). After His.
 
  
 
/1. Aortic limb of heart. All. Bodystalk. A0. Dorsal aorta. Au. Umbilical arteries. Car. Posterior cardinal
 
/1. Aortic limb of heart. All. Bodystalk. A0. Dorsal aorta. Au. Umbilical arteries. Car. Posterior cardinal
  
vein. Jg. Anterior cardinal vein
+
vein. Jg. Anterior cardinal vein (internal jugular). Om. 0mphalomesenteric vein. op. Optic vesicle. or. Otocyst. V It. Right umbilical vein.
(internal jugular). Om. 0mphalomesenteric vein. op. Optic vesicle.
 
or. Otocyst. V It. Right umbilical vein.
 
  
 
This completes the description
 
This completes the description
  
into two main layers. The outermost
+
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
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
+
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.
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
+
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
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
+
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:
(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
+
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
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
+
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
(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.
 
arches.
  
the thin chorion, now lack
+
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.
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
+
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
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
+
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
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
 
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
+
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.
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
+
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
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
+
and we shall take those up at this point, together with a few comments on other external features.
blastocyst is becoming implanted. The embryo itself is represented by a
+
 
blastoderm in which a primitive groove and notochord are evident, and
+
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
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
 
T‘ eural fold
  
Cgtedgfi “ ous curvatures begin to
+
Cgtedgfi “ ous curvatures begin to o ammon .
o ammon .
 
 
 
:- r , , develop, and certain flex
 
; smus rhomboldahs . _ d
 
  
 +
- r , , develop, and certain flex
 +
smus rhomboldahs . _ d
 
pmnmve streak: ures are again recognize .
 
pmnmve streak: ures are again recognize .
  
The cranial and cervical
+
The cranial and cervical flexures are the same as
flexures are the same as
 
  
in the Chick, and in addi
+
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'
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
 
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
+
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.,
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
+
3 EXTERNAL FEATURES 563
EXTERNAL FEATURES 563
 
  
somewhat posterior to a point dorsal to the origin of the hind-limbs.
+
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
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
+
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.
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
+
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
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
 
burying of the anterior end Fig. 292. -— Surface view of a Pig embryo with
Line 2,662: Line 911:
 
. about 16 somites (4.5 mm.), showing outpush“1 the yolk‘ In the Mam‘ ing of allantois beneath chorion. After Keibel.
 
. about 16 somites (4.5 mm.), showing outpush“1 the yolk‘ In the Mam‘ ing of allantois beneath chorion. After Keibel.
  
cut edge
+
cut edge
of yolk sac
+
of yolk sac
  
 
 
 
  
 
mal of course there is no
 
mal of course there is no
  
yolk, but it is an interesting fact that the lateral torsion still takes place
+
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.
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-.
 
Other External Features.—Finally before proceeding to a dis-.
  
cussion of the specific systems a few further remarks are pertinent with
+
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
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"
+
refer to them as such. Also from the figure itmight at first be supposed" 564 THE PIC TO TEN MILLIMETERS
564 THE PIC TO TEN MILLIMETERS
 
  
that there are five-clefts and five arches rather than four. The apparent
+
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
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
 
Chick and that of a 10
  
 
 
  
hyomndibuhr def‘. ':- mm. Pig is the much
+
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;
auditory Pit Jolt ,3; greater size of the eye in
+
 
2nd optic vgfldg the Bird.
+
ther be seen (Fig. 295i _‘ that antero-ventral_t.o,the
"l“°"l‘l°f“ 3rd 0" i Viewing the embryo
 
somites' llnmd‘ from the front it will fur.
 
amnion;  
 
  
ther be seen (Fig. 295i
+
eyes, between them and
_‘ that antero-ventral_t.o,the
+
‘ 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
; 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
+
somites. Both of course are highly reminiscent of the appearance of these structures in the Chick in a corresponding stage.
these structures in the Chick in a corresponding stage.
 
  
 
 
  
 
Fig. 293.-—Surface view of a 3.5 mm. Fig embryo
 
Fig. 293.-—Surface view of a 3.5 mm. Fig embryo
  
with chorion removed to show allantois. After
+
with chorion removed to show allantois. After Keibel. NERVOUS SYSTEM: EARLY DIFFERENTIATION 565
Keibel.
 
NERVOUS SYSTEM: EARLY DIFFERENTIATION 565
 
  
 
THE NERVOUS SYSTEM
 
THE NERVOUS SYSTEM
  
As in the case of the Chick, much of the general form of the early
+
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.
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
 
Illrd viscera! arch
  
h 'd
+
h 'd Nth visceral arch yo. arch
Nth visceral arch yo. arch
+
 
  
 
 
 
 
 
 
 
 
 
  
'mandibu|ar arch
+
 
cervical sinus
+
'mandibu|ar arch cervical sinus
  
 
forblimb bud - maxillary process‘
 
forblimb bud - maxillary process‘
  
 
  
 
33% 5. :2 6””
 
33% 5. :2 6””
Line 2,764: Line 964:
 
7 mm. erribryo
 
7 mm. erribryo
  
Fig. 294.—Lateml View ‘of a 7 mm. Pig embryo with amnion and
+
Fig. 294.—Lateml View ‘of a 7 mm. Pig embryo with amnion and chorion removed.
chorion removed.
 
  
 
EARLY DIFFERENTIATION
 
EARLY DIFFERENTIATION
  
The System as a Who1e.—The nervous system first appears in
+
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
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
+
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,
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
+
frontal PTOCCSS olfactory plt
  
 
naso-lateral process _ '
 
naso-lateral process _ '
  
- naso~medlaI process
+
- naso~medlaI process maxillary process
maxillary process
 
  
 
mandibular arch
 
mandibular arch
  
hyomandibular clef:
+
hyomandibular clef: hyoid arch
hyoid arch
 
  
 
lllrd viscera! arch 1
 
lllrd viscera! arch 1
Line 2,800: Line 988:
 
from 7 mm. embryo
 
from 7 mm. embryo
  
Fig. 295.——Antero-ventral view of the head of a 7 mm. Pig embryo
+
Fig. 295.——Antero-ventral view of the head of a 7 mm. Pig embryo showing parts constituting jaws and face.
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
+
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
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.
 
the figures, these vesicles, at their earlier stages, are also somewhat differently shaped from those of the Bird.
Line 2,811: Line 996:
 
DIFFERENTIATION TO TEN MILLIMETERS
 
DIFFERENTIATION TO TEN MILLIMETERS
  
The Brain. —— Following this early condition the cranial flexure
+
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
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
+
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).
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,
+
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
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
+
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
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
 
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
+
lX“(‘ 8‘"8l:)°" audkor), “Sid, metencephalon Xth gangllon jugula . '¢ Vth(Gasserian) ganglion
' OptIC cup
+
 
Fis_ 3°“ ‘ ' Fig. 301
+
' A lVth nerve
_mng telencephalon
+
Xlth spina'laccess_ory nerve '5ml°'::;";‘°:;:l°" Fig.1” Frorlep sgangluo . F3’. 2” xnth-he.-.,¢.,-00;; - ophthalmic nerve petrosal ganglion
F‘ 305 W c:narndIib‘ular n rveF;'_ 305
+
 
---?{7;--— ° 3 0 Y P yuzel Inelyolldstalk
+
. maxillar nerve ganglion nodosum Y ‘ 'diencephalon
533.306 Fin» 306
+
 
t ch " - _
+
 
*a:t":..a 3» 3;;
+
 
d°rsa s allantoie stalk
+
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 " - _
, Fig. 3l0
+
 
FIg.3l0
+
a:t":..a 3» 3;;
l ncreas .
+
d°rsa s allantoie stalk , Fig. 3l0 FIg.3l0 l ncreas . ventrzamabladder postncloacal gut. Fig. an “9~ 3"
ventrzamabladder postncloacal gut.
+
 
Fig. an “9~ 3"
+
a,.:m “-9- 313
:a,.:m “-9- 313
+
l spinal ganglion C mm
l
 
spinal ganglion C mm
 
  
 
mesonephros
 
mesonephros
  
nephrogenous tissue of metancphr
+
nephrogenous tissue of metancphr
 
mcsonephricvduct
 
mcsonephricvduct
  
Fig. 296.——Reconstruction of a 10 mm. Pig embryo, designed to show primarily
+
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. *
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
+
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).
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
+
Aside from the cord itself it will be found, as in the case of the Frog NERVOUS SYSTEM: TO TEN MILLIMETERS 569
NERVOUS SYSTEM: TO TEN MILLIMETERS 569
 
  
and Chick, that as the neural folds come together a hand of cells is
+
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
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
 
pharynx metencephalon
  
 
+
 
 
+
 
 
+
 
 
+
 
   
+
 
 
 
 
 
 
 
 
 
 
  
 
notochord
 
notochord
Line 2,949: Line 1,055:
 
mesencephalon
 
mesencephalon
  
Ra:h$<e's pocket
+
Ra:h$<e's pocket v Seesell’s pocket
v Seesell’s pocket
 
  
./ optic chiasma
+
./ optic chiasma , Optic recess
, Optic recess
 
  
 
lamina terminalis
 
lamina terminalis
  
um bi Iical
+
um bi Iical artery
artery
 
  
 
vltelline vein
 
vltelline vein
  
posterio
+
posterio vena cava
vena cava
 
  
 
  
 
mesoneph ros
 
mesoneph ros
Line 2,973: Line 1,074:
 
Fig. 297.-—Mid-sagittal section of a 10 mm. Fig embryo.
 
Fig. 297.-—Mid-sagittal section of a 10 mm. Fig embryo.
  
to form the groups of neuroblasts which develop into the spinal ganglia.
+
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
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
+
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
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
+
external llmltlng membrane
prlmordlum of
+
 
 +
lumen of neural tube
 +
 
 +
mantle layer prlmordlum of
  
ventral horn
+
ventral horn blood vessel
blood vessel
 
  
 
K
 
K
  
 
I .- 1.1‘
 
I .- 1.1‘
»‘3 ventralnerve root
 
  
 +
»‘3 ventralnerve root
 
~:
 
~:
  
  
internal Ilmltlng Vf
+
internal Ilmltlng Vf membrane
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
+
Fig. 298.———-Transverse section of the center and right side of the nerve cord and a spinal ganglion of a 10 mm. Pig embryo.
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
+
-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 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,
+
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
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.
+
lXth Xth
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
 
}cranlal nerve ganglion
  
hind-brain (metencephalon)
+
hind-brain (metencephalon) 1.3-‘
1.3-‘
 
  
 
branches of anterior cardinal velni
 
branches of anterior cardinal velni
Line 3,062: Line 1,123:
 
cndolymphatic duct
 
cndolymphatic duct
  
Fig. 299.—Transverse section through the brain region, including some of the
+
Fig. 299.—Transverse section through the brain region, including some of the spinal ganglia, of a 10 mm. Fig embryo. See reconstruction Fig. 296.
spinal ganglia, of a 10 mm. Fig embryo. See reconstruction Fig. 296.
 
  
nodosum. The former is so closely in contact with the superior ganglion
+
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
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‘:
+
J‘: 572 THE PIG TO TEN MILLIMETERS
572 THE PIG TO TEN MILLIMETERS
 
  
hence has no ganglion. It arises as a g oup of fibers ventral to the spinal
+
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).
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 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.
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
+
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
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
+
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.
nerve continues to follow the diaphragm as it moves posteriorly.
 
  
The Organs of Special Sense. — As inthe case of the parts of the
+
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
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
+
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
 
‘ EARLY STAGES
  
The Primitive Gut and Related Parts. —— We have already noted
+
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
, 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
+
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
+
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
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
+
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
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
+
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
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
 
,7
  
"method Mung PI“: amniotic tail fold
+
"method Mung PI“: amniotic tail fold ' anal plate
' anal plate
+
 
  
   
 
  
 
 
 
 
  
   
 
  
A °"3lPlfl€ periczrdtal coelorn Ik mflodflm
+
A °"3lPlfl€ periczrdtal coelorn Ik mflodflm yo 5“ endoderm chcfionk uaphabhn amniotic head fold neural tube amniotic nail fold amnion
yo 5“ endoderm
 
chcfionk uaphabhn amniotic head fold neural tube amniotic nail fold amnion
 
  
 
 
  
eczoder   chorionic trophohlasl
+
eczoder chorionic trophohlasl ,,,,m°n notochord . Fla“ cmdum
,,,,m°n notochord . Fla“ cmdum
 
  
 
\ mesoderm
 
\ mesoderm
  
mesoderm ‘_ ‘O \‘
+
mesoderm ‘_ ‘O \‘
hind-gut
+
 
 +
hind-gut
  
   
 
 
  
yolk sac mesoderm
+
yolk sac mesoderm yolk sac endod:rm/ perlardial coelom
yolk sac endod:rm/
 
perlardial coelom
 
  
 
B
 
B
  
Fig. 300. —-— Diagrammatic mid-sagittal sections through early Pig embryos to
+
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.
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
+
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
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
+
it may be recalled, almost immediately disappears. In the Mammal, on the other hand, this part persists much longer. Indeed in the latter, as
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
+
we shall see, some of it exists permanently, and we shall have occasion to return to it later on.
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
 
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
Line 3,227: Line 1,190:
 
mcdullary plate
 
mcdullary plate
  
 
 
 
 
  
splanchnlc _
+
 
mesoderm ‘
+
splanchnlc _ mesoderm ‘
  
 
somatic mesoderm
 
somatic mesoderm
Line 3,237: Line 1,198:
 
chorlonlc trophoblast
 
chorlonlc trophoblast
  
Fig. 301.——-Transverse section through a Pig blastocyst cutting the blastoderm
+
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.
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
+
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.
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
+
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
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~
+
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
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
 
end of 4th visceral pouch
  
+
 
+
 
+
 
 
+
 
 
 
 
 
 
 
 
  
 
mandibular arch
 
mandibular arch
Line 3,280: Line 1,217:
 
maxillary process
 
maxillary process
  
Pl‘3")’"* _/ i « _ -. , ' i ‘ portion of
+
Pl‘3")’"* _/ i « _ -. , ' i ‘ portion of ' ' ' ' ‘ ' « cerebral hemisphert,
' ' ' ' ‘ ' « cerebral hemisphert,
 
  
 
nerve I'O0C
 
nerve I'O0C
  
dorsal spinal ,
+
dorsal spinal , nerve root ganglia ‘
nerve root ganglia ‘
 
  
 
cervical nerve
 
cervical nerve
  
 
 
  
 
anterlor cardinal veln
 
anterlor cardinal veln
Line 3,296: Line 1,230:
 
3rd vlsceral clef: hyommdl I "I" dd‘
 
3rd vlsceral clef: hyommdl I "I" dd‘
  
Fig. 302.—Transverse section through the eye and visceral arch region of a 10
+
Fig. 302.—Transverse section through the eye and visceral arch region of a 10 mm. Pig. See reconstruction Figs. 296, 318, 320.
mm. Pig. See reconstruction Figs. 296, 318, 320.
 
  
gion gives rise to an anterior outgrowth which is entirely allantoic. In
+
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
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
+
outpushing, and grows posteriorly above the latter to form the definitive hind-gut (Fig. 300). '
hind-gut (Fig. 300). '
 
  
 
FURTHER DEVELOPMENT OF THE GUT
 
FURTHER DEVELOPMENT OF THE GUT
  
The Stomodaeum. — As in the Chick the fore-gut does not at first
+
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
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
+
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,
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
+
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.
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.
+
and is of course, the primordium of the anterior part of the pituitary. (See footnote on this topic in the section on the Frog.)
(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
+
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
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.
 
, the last (fourth) pair aresmall and sometimes entirely lacking (Fig.
  
302). In a 10 mm. specimen all the pairs destined to appear are well
+
302). In a 10 mm. specimen all the pairs destined to appear are well developed, and have come in contact with the corresponding ectodermal
developed, and have come in contact with the corresponding ectodermal
 
  
“ clefts ” (Figs. 294, 296). As already indicated, in the case of the Pig,
+
“ 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
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
 
Xth cranial nerve endocardi! cushion
  
 
  
 
ductus Cuvier valvulae venosae
 
ductus Cuvier valvulae venosae
Line 3,353: Line 1,263:
 
esophagus
 
esophagus
  
   
 
  
 
mesonephros
 
mesonephros
  
“mg posterior vena cava
+
“mg posterior vena cava subcardinal vein
subcardinal vein
 
  
Fig. 305. ——Transverse section through posterior of heart and the
+
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
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.
+
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).
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
+
The Trachea and Bronchi. — Just posterior to the visceral pouches the pharynx develops a deep ventral groove which, as in the
pouches the pharynx develops a deep ventral groove which, as in the
 
  
stomach
+
stomach fore- limb bud
fore- limb bud
 
  
     
 
  
 
left umbilical vein(ductus venosu§
 
left umbilical vein(ductus venosu§
  
coelom
+
coelom ericardial cavity
ericardial cavity
 
  
 
ventricle
 
ventricle
Line 3,387: Line 1,287:
 
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.
 
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.
+
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).
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
+
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
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
+
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
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 ,
 
shortly gives rise to several anteriorly directed buds which grow out ,
  
into numerous hepatic ducts. The posterior part of the same outgrowth
+
into numerous hepatic ducts. The posterior part of the same outgrowth becomes extended as the cystic duct while its end enlarges as the gall
becomes extended as the cystic duct while its end enlarges as the gall
 
  
 
  
 
Fig. 307.—Reconstruction of the stomach, dorsal
 
Fig. 307.—Reconstruction of the stomach, dorsal
  
and ventral pancreas and gall bladder of a 10 mm.
+
and ventral pancreas and gall bladder of a 10 mm. Pig, enlarged from Fig. 296.
Pig, enlarged from Fig. 296.
 
  
bladder. The anteriorly growing hepatic ducts and the posterior cystic
+
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
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
 
_ beenipreviously explained. Their development to the pointindicated
Line 3,427: Line 1,308:
 
occurs between the 5-10 mm. stages.
 
occurs between the 5-10 mm. stages.
  
The Pancreas. -— At about the same time that the liver diverticulum
+
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
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
+
numerous -budding cords of cells, and the two parts are almost fusing (Figs. 296, 307, 308, 309). FURTHER DEVELOPMENT OF THE GUT 581i
(Figs. 296, 307, 308, 309).
 
FURTHER DEVELOPMENT OF THE GUT 581i
 
  
The Mid-gut Region.——Immediately posterior to the liver and
+
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,
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
 
. and its ventral bending has become a very clear cut loop whose sides
Line 3,454: Line 1,322:
 
Pegterior cardinal v
 
Pegterior cardinal v
  
 
 
  
 
posterior vena ca
 
posterior vena ca
Line 3,460: Line 1,327:
 
8l°m hepatic portal vein
 
8l°m hepatic portal vein
  
Fig. 308.——Transverse section through the region of the
+
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.
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
+
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.
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
+
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
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
 
genital ridge dorm Pancreas
Line 3,485: Line 1,337:
 
posterior mrdinzl vein
 
posterior mrdinzl vein
  
 
+
 
+
 
 
+
 
+
 
 
 
 
 
 
  
 
I
 
I
Line 3,498: Line 1,346:
 
for
 
for
  
ventral pancreas
+
ventral pancreas ventral vein of mesonephros
ventral vein of mesonephros
 
  
 
Fig. 309.--Transverse section through the region of mesonephros, pancreas and
 
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
+
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.
the figure. See reconstruction Figs. 296, 318, 320.
 
  
vltelline vein
+
vltelline vein subcardinal veins left umbilical vein vitclline vein ’ ‘ K gut loop
subcardinal veins left umbilical vein
 
vitclline vein
 
’ ‘ K gut loop
 
  
 
 
  
 
umbilical arteries
 
umbilical arteries
  
 
  
 
ventral vcln ofmesonephros
 
ventral vcln ofmesonephros
  
right umbilical vein
+
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
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
+
p of embryo of a. 10 mm. Pig. See reconstruction Figs. 296, 313, 320. FURTHER DEVELOPMENT OF THE GUT 583
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
+
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
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
+
fused subcardinal veins 7 ’ ‘- \ umbilical veim
7 ’ ‘- \ umbilical veim
 
  
 
 
  
 
posterior cardinal vet
 
posterior cardinal vet
  
 
 
  
mesonephros 'w-- i’ - T " ‘ ’
+
mesonephros 'w-- i’ - T " ‘ ’ T \. — —* ut umbilical arteries mesonephric duct
T \. — —* ut umbilical arteries
 
mesonephric duct
 
  
Eig. 311.— Transverse section through the region of rnesonephros, gut, umbilical
+
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.
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
+
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
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
+
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.
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
 
MESODERMAL STRUCTURES
  
Under the headings of systems, we have thus far considered the nervous system, which of course is exclusively ectodermal, and the digestive
+
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.
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
+
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
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
+
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).
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
+
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.
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
+
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.
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 CIRCULATORY SYSTEM The Blood Islands. -——- It will be recalled that in the Bird one of the
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 _
 
first manifestations of the beginning of the circulatory system is the _
  
formation of blood islands in the area vasculosa, which is of course
+
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
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
+
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
586 THE PIG.TO TEN MILLIMETERS
 
  
in the Bird, however, the mesoderm from which they arise in this region
+
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. '
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
+
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
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
+
4 t t 6 dorsal acme truncus arterloxus somlte posterior cardinal veln
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
 
vitelline (omphalomesenteric) veins
Line 3,693: Line 1,412:
 
amum duct of Cuvier
 
amum duct of Cuvier
  
1 anterior cardinat vein
+
1 anterior cardinat vein i truncus arteriosus
i truncus arteriosus
+
 
 +
_' . - g”
  
_' . - g”
 
 
  
 
dorsal aortae
 
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
+
vitelline(omphalomesenteric)veins, arteries
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
+
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 is reached at about the 4.5-5 mm., or 13 somite stage. (See Chick, Fig. l 79.)
 
  
 
  
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.
 
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 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. Internally 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 septum 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.
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
+
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.
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
+
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 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.
sinus venosus—«\
 
  
posterior vena cava ' interatrial foramenll
 
  
valvulae venosae - septum i(primum)
 
  
hepatic vein interatrial foramenl
+
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.,
  
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.
+
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. '
  
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......,
+
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.  
590 THE PIG TO TEN MILLIMETERS '
 
  
 
  
 
+
the ventricles become definitely thickened, and muscular bands, the trabeculae carneae project into the ventricular lumen.
  
V: /e-///ne vem
+
The Truncus and Aortic Arches. ——.The truncus arteriosus has already been mentioned as it comes up underneath the pharynx. As in
  
 
   
 
 
 
 
 
 
  
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 '
 
  
+
F:g.3oz I RF?th|<e's pockeg pulmonary artery I 9"”: vertebralarter Ii: Interatrigfomcn V subciaviaingdrtery ’ fi"3°“
 
 
 
 
 
 
 
  
Le/I an):-rior cardinal Van
 
  
Luff dorsadaorla
 
  
 
+
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.
  
Left dorsal not-la.
+
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
5egmenlaIar/er/es
 
  
 
+
the remaining visceral arches (10 mm.) the first two aortic vessels have _
  
Fig. 315.—St_ages in the development of the aortic arches and other anterior ar
+
disappeared (Figs. 315, 316). Also again as in the Chick, the‘ fifth pair 594 THE PIG TO TEN MILLIMETERS
. 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
+
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
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
 
  
   
 
     
 
       
 
 
 
 
 
 
 
 
 
 
 
 
  
     
+
Fig . 319.——Semi-diagrammatic representation of the development of the aortic
  
/Ior//c /run/<
+
arc es and other anterior arteries of the Pig. A. Arteries at the 10 mm. stage. B. Arteries of a specimen near term.
.3 ‘Paar!/c arc/7
 
  
E3-—i. eff dorsal aar/a
+
left subclavlan artery
  
/f/gh/dorsa.’ aorla
+
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.
  
4_"'ao.'/Ic arch
+
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. '
_:‘L
 
  
 
+
By the 10 mm. stage this fusion has progressed to the tail, and as far forward as the anterior appendages (Figs. 316, 318).
  
E »/’u/manar} arc/7 '
+
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,‘