Book - Embryology of the Pig 6: Difference between revisions

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==I. The Formation of the Extra-embryonic Membranes==
==I. The Formation of the Extra-embryonic Membranes==


In dealing with the establishment of the embryonic body we have  
In dealing with the establishment of the embryonic body we have already seen that the germ layers extend far beyond the region in which the embryo itself develops. These peripheral portions of the germ layers give rise to membranes which are of service as a means of protection and as a means of securing food from the blood of the mother. Because they are not incorporated in thd body of the embryo but discarded at the time of birth, they are called extra-embryonic membranes. These membranes are the yolk-sac, the amnion, the serosa, and the allantois. Their designation as extra-embryonic should not lead us to lose sight of their vital importance to the embryo during its intra-uterine existence.
already seen that the germ layers extend far beyond the region in which  
the embryo itself develops. These peripheral portions of the germ  
layers give rise to membranes which are of service as a means of  
protection and as a means of securing food from the blood of the  
mother. Because they are not incorporated in thd body of the embryo  
but discarded at the time of birth, they are called extra-embryonic  
membranes. These membranes are the yolk-sac, the amnion, the  
serosa, and the allantois. Their designation as extra-embryonic should  
not lead us to lose sight of their vital importance to the embryo during  
its intra-uterine existence.  


The Yolk-sac. In mammals, although there is virtually no yolk  
===The Yolk-sac===
accumulated in the ovum, a large yolk-sac is formed just as if yolk  
In mammals, although there is virtually no yolk accumulated in the ovum, a large yolk-sac is formed just as if yolk were present. Such persistence of a structure, in spite of the loss of its original function, is not an uncommon phenomenon in evolution and has given rise to the biological aphorism that ‘‘morphology is more conservative than physiology.” Not only, does the yolk-sac itself persist, even the blood vessels characteristically associated with it in its functional condition appear in the empty yolk-sac of mammalian embryos (Fig. 45). When the mammalian yolk-sac is spoken of as a vestigial structure, therefore, we must bear in mind that such a statement has reference primarily to its function. Morphologically it is of considerable size in young embryos.
were present. Such persistence of a structure, in spite of the loss of its  
original function, is not an uncommon phenomenon in evolution and  
has given rise to the biological aphorism that ‘‘morphology is more  
conservative than physiology.” Not only, does the yolk-sac itself  
persist, even the blood vessels characteristically associated with it in  
its functional condition appear in the empty yolk-sac of mammalian  
embryos (Fig. 45). When the mammalian yolk-sac is spoken of as a  
vestigial structure, therefore, we must bear in mind that such a statement has reference primarily to its function. Morphologically it is of  
considerable size in young embryos.  


The yolk-sac may be defined as that part of the primitive gut
which is not included within the body when the embryo is “folded
off.” In dealing with the formation of the digestive tube we have
already sufficiently considered the processes which thus establish the
boundaries between the yolk-sac and the intra-cmbryonic portion
of the original gut cavity (Fig. 37). The peripheral extent of the yolk sac is unusually great in young pig embryos correlated with enormous
elongation of the blastocyst (Fig. 18). From the yolk-st^lk, the yolk-sac
extends beneath the embryo, beyond its head in one direction and
beyond its tail in the other, nearly to the ends of the blastocyst
(Fig. 49, A).


The yolk-sac may be defined as that part of the primitive gut which is not included within the body when the embryo is “folded off.” In dealing with the formation of the digestive tube we have already sufficiently considered the processes which thus establish the boundaries between the yolk-sac and the intra-cmbryonic portion of the original gut cavity (Fig. 37). The peripheral extent of the yolk sac is unusually great in young pig embryos correlated with enormous elongation of the blastocyst (Fig. 18). From the yolk-st^lk, the yolk-sac extends beneath the embryo, beyond its head in one direction and beyond its tail in the other, nearly to the ends of the blastocyst (Fig. 49, A).


Fig. 49. Schematic diagrams showing the relations of the extra-embryonic
membranes. The great length of the chorionic vesicle of the pig precludes
showing the entire vesicle in correct proportions. In A, sections of the vesicle
have been omitted. In B and C, the vesicle has been shortened from its true
proportions.


A represents conditions in embryos of 15-20 somites; B, conditions in
Fig. 49. Schematic diagrams showing the relations of the extra-embryonic membranes. The great length of the chorionic vesicle of the pig precludes showing the entire vesicle in correct proportions. In A, sections of the vesicle have been omitted. In B and C, the vesicle has been shortened from its true proportions.
embryos of 4-6 mm. (after flexion); and C, conditions in embryos of 30 mm.
or more.  


A represents conditions in embryos of 15-20 somites; B, conditions in embryos of 4-6 mm. (after flexion); and C, conditions in embryos of 30 mm. or more.




During this, the period of its greatest relative development, the  
During this, the period of its greatest relative development, the yolk-sac serves as an organ purveying nutritive material to the embryo. Separated from the uterine wall only by the thin outer layer of the blastocyst, its abundant vessels are in a position which maker absorption from the uterus readily possible. The food material and oxygen thus absorbed are transported by way of the vitelline circulation (Fig. 45) to the growing embryo. When, somewhat later, the allantois becomes highly developed it takes over this function and the yolk-sac decreases rapidly in size (Fig. 49, B), finally becoming a shriveled sac buried in the belly-stalk (Figs. 49, C, and 65). The yolk-sac, therefore, exhibits another point of special interest in addition to being an organ which persists after the cessation of its original function. The persistent yolk-sac and its associated vessels are utilized by the embryo in a new manner. Containing no stored food materials, the mammalian yolk-sac turns about, so to speak, and absorbs from the uterus through its external surface. The fact that the function is temporary, later being taken over by the allantois, makes this physiological opportunism in seizing on a new source of supplies none the less striking.
yolk-sac serves as an organ purveying nutritive material to the  
embryo. Separated from the uterine wall only by the thin outer layer  
of the blastocyst, its abundant vessels are in a position which maker  
absorption from the uterus readily possible. The food material and  
oxygen thus absorbed are transported by way of the vitelline circulation (Fig. 45) to the growing embryo. When, somewhat later, the  
allantois becomes highly developed it takes over this function and the  
yolk-sac decreases rapidly in size (Fig. 49, B), finally becoming a  
shriveled sac buried in the belly-stalk (Figs. 49, C, and 65). The  
yolk-sac, therefore, exhibits another point of special interest in addition to being an organ which persists after the cessation of its original  
function. The persistent yolk-sac and its associated vessels are utilized  
by the embryo in a new manner. Containing no stored food materials,  
the mammalian yolk-sac turns about, so to speak, and absorbs from  
the uterus through its external surface. The fact that the function is  
temporary, later being taken over by the allantois, makes this physiological opportunism in seizing on a new source of supplies none the  
less striking.  


The Amnion. The amnion arises as a layer of somatopleure which
enfolds the developing embryo. When the amniotic sac is completed
it becomes filled with a watery fluid in which the embryo is suspended.
This suspension of the embryo in a fluid-filled sac, by equalizing the
pressure about it, serves as a protection against mechanical injury.
At the same time the soft tissues of the growing embryo, being bathed
in fluid, do not tend to form adhesions with consequent malformations. The functional significance of the amnion is emphasized by the
fact than an amnion appears only in the embryos of non-water-living
forms. This has led to designating the embryos of fishes and Amphibia,
which form no amnion, as ‘‘anamniotes,” and those of birds, reptiles,
and mammals, which do form an amniotic sac during development, as
‘"amniotes.”


In all mammalian embryos the amnion is formed at an exceedingly early stage of development. In some forms (e.g., man) the  
===The Amnion===
amniotic cavity appears even before the body of the embryo has taken definite shape. When the amnion is thus precociously established the  
The amnion arises as a layer of somatopleure which enfolds the developing embryo. When the amniotic sac is completed it becomes filled with a watery fluid in which the embryo is suspended. This suspension of the embryo in a fluid-filled sac, by equalizing the pressure about it, serves as a protection against mechanical injury. At the same time the soft tissues of the growing embryo, being bathed in fluid, do not tend to form adhesions with consequent malformations. The functional significance of the amnion is emphasized by the fact than an amnion appears only in the embryos of non-water-living forms. This has led to designating the embryos of fishes and Amphibia, which form no amnion, as ‘‘anamniotes,” and those of birds, reptiles, and mammals, which do form an amniotic sac during development, as ‘"amniotes.”
processes in its formation are, as it were, hurried through and consequently they are difficult to analyze. In the pig the amnion is formed
in more leisurely fashion, the process being quite similar to that in
reptiles and birds.  


The first indication of amnion formation in pig embryos becomes
evident shortly after the primitive streak stage. The embryonic disk
appears to settle into the blastocyst so that it is overhung on all sides
by the extra-embryonic somatopleure (Figs. 25 and 26). As the
embryo grows in length its head and tail push deeper and deeper
into these folds of somatopleure (cf. Fig. 37, B, C). At the same time
the folds themselves become more voluminous, growing centripetally
over the embryo from the region of its head, its sides, and its tail.
These circumferential folds of somatopleure are known as the amniotic
folds. For convenience in description we recognize cephalic, caudal,
and lateral regions of the amniotic folds, although in reality they are
all directly continuous with each other.


Progress of the amniotic folds soon brings them together above the
In all mammalian embryos the amnion is formed at an exceedingly early stage of development. In some forms (e.g., man) the amniotic cavity appears even before the body of the embryo has taken definite shape. When the amnion is thus precociously established the processes in its formation are, as it were, hurried through and consequently they are difficult to analyze. In the pig the amnion is formed in more leisurely fashion, the process being quite similar to that in reptiles and birds.
mid-dorsal region of the embryo to complete the amniotic sac (Figs.  
37, C, and 49). Where the amniotic folds close, there persists for a
time a cord-like mass of tissue between the amnion and the outer layer
of the blastocyst (Fig. 49, B). In time even this trace of the fusion is
obliterated and the embryo in its amnion lies free in the blastocyst
(Fig. 49, C).  


The amnion is attached to the body of the embryo where the bodywall opens ventrally in the region of the yolk-stalk (Fig. 50). As
development advances this ventral opening becomes progressively
smaller, its margins being known finally as the umbilical ring. Meanwhile the yolk-stalk and the allantoic stalk, in the same process of
ventral closure, are brought close to each other, to constitute, with
their associated vessels, the belly-stalk (Fig. 49, C). The amnion is now
reflected from its point of continuity with the skin of the body at the
umbilical ring, to constitute the covering of the belly-stalk, before it is
recurved as a free membrane enclosing a fluid-filled space about the
embryo.


The Serosa. The outer layer of the mammalian blastocyst has
The first indication of amnion formation in pig embryos becomes evident shortly after the primitive streak stage. The embryonic disk appears to settle into the blastocyst so that it is overhung on all sides by the extra-embryonic somatopleure (Figs. 25 and 26). As the embryo grows in length its head and tail push deeper and deeper into these folds of somatopleure (cf. Fig. 37, B, C). At the same time the folds themselves become more voluminous, growing centripetally over the embryo from the region of its head, its sides, and its tail. These circumferential folds of somatopleure are known as the amniotic folds. For convenience in description we recognize cephalic, caudal, and lateral regions of the amniotic folds, although in reality they are all directly continuous with each other.
various names. In the inner-celFmass stage, the outer layer of the
blastocyst is most commonly called the trophoblast (Fig. 15, C). Since
the primary germ layers are not at this time differentiated, this term
is far more appropriate than ectoderm, which implies a layer having




Progress of the amniotic folds soon brings them together above the mid-dorsal region of the embryo to complete the amniotic sac (Figs. 37, C, and 49). Where the amniotic folds close, there persists for a time a cord-like mass of tissue between the amnion and the outer layer of the blastocyst (Fig. 49, B). In time even this trace of the fusion is obliterated and the embryo in its amnion lies free in the blastocyst (Fig. 49, C).


Fig. 50. Drawing (X 12) of pig embryo having about 28 pairs of somites
(crown-rump length 4 mm.; age approximately 17 days). The embryo has
been removed from the chorionic vesicle with the amnion and allantois intact
but the distal portion of the yolk-sac has been cut off. (Cf. Fig. 49, .)


The amnion is attached to the body of the embryo where the bodywall opens ventrally in the region of the yolk-stalk (Fig. 50). As development advances this ventral opening becomes progressively smaller, its margins being known finally as the umbilical ring. Meanwhile the yolk-stalk and the allantoic stalk, in the same process of ventral closure, are brought close to each other, to constitute, with their associated vessels, the belly-stalk (Fig. 49, C). The amnion is now reflected from its point of continuity with the skin of the body at the umbilical ring, to constitute the covering of the belly-stalk, before it is recurved as a free membrane enclosing a fluid-filled space about the embryo.


relations which make it stand in contrast with entoderm, or with
entoderm and mesoderm. The premature use of the term ectoderm
would seem to have nothing to recommend it, and has been responsible for much confusion in thinking and much pointless discussion.


When the germ layers have been differentiated, the outer layer  
The Serosa. The outer layer of the mammalian blastocyst has various names. In the inner-celFmass stage, the outer layer of the blastocyst is most commonly called the trophoblast (Fig. 15, C). Since the primary germ layers are not at this time differentiated, this term is far more appropriate than ectoderm, which implies a layer having relations which make it stand in contrast with entoderm, or with entoderm and mesoderm. The premature use of the term ectoderm would seem to have nothing to recommend it, and has been responsible for much confusion in thinking and much pointless discussion.
of the blastocyst is directly continuous with the ectoderm of the
embryo (Fig. 19). There is then justification for calling it more  
specifically trophectoderm. The prefix troph- (Greek root, to nourish)
suggests the part this layer plays in the acquisition of food materials
from the uterus and at the same time sets it off from the ectoderm of
the embryonic disk.  


Still later in development when the mesoderm splits and its
somatic layer becomes associated with the ectoderm, the name
trophectoderm is no longer appropriate (Fig. 22). Three different
names are in common use to cover this double layer. Extra-embryonic
somatopleure is a term following the general embryological usage in
designating by the suffix -pleure the double layers formed by the
secondaiy association of somatic and splanchnic mesoderm with
ectoderm and entoderm respectively. Trophoderm is widely used by those working especially in mammalian embryology. Those especially
interested in comparative embryology are quite likely to use the term
serosa for this layer, thus emphasizing its very evident homology with
the part of the extra-embryonic somatopleure so designated in birds
and reptiles. This multiplicity of names is not difficult to master if we
understand they are all appropriate terms selected to emphasize
somewhat different aspects of the same structure. Trophoblast, then,
means the outer layer of the blastocyst before ectoderm, entoderm, and
mesoderm are differentiated. Trophectoderm is the extra-embryonic
part of the ectoderm when the germ layers have been differentiated,
and trophoderm (serosa) (extra-embryonic somatopleure) is trophectoderm reinforced by a layer of somatic mesoderm.


In the formation of the amnion, the somatopleure is thrown into
folds surrounding the embryo. The inner limb of the fold, as we have
seen, becomes the amnion (Fig, 37, C). The outer limb of the fold is
commonly called the serosa. It is, perhaps, in just this designation
of the outer limb of the amniotic fold that the term serosa is most often
encountered. After the amnion has been completed, interest in this
outer layer centers about its role in food absorption and the term
trophoderm is most commonly used to designate it.


The Allantois. Almost as soon as the hind-gut of the embryo is
Fig. 50. Drawing (X 12) of pig embryo having about 28 pairs of somites (crown-rump length 4 mm.; age approximately 17 days). The embryo has been removed from the chorionic vesicle with the amnion and allantois intact but the distal portion of the yolk-sac has been cut off. (Cf. Fig. 49, .)
established, there arises from it a diverticulum known as the allantois
(Fig. 37). The allantoic wall, because of its manner of origin, is necessarily composed of splanchnopleure. As the allantois increases in
Gxtent its original communication with the hind-gut is narrowed to a
cylindrical stalk, while its distal portion becomes enormously dilated
(Figs. 37 and 49).


Externally the developing allantois appears at first as a crescentic
enlargement directly continuous with the caudal part of the embryo
(Figs. 29 and 30). Its dilated distal portion continues to grow with
great rapidity until it is a large semilunar sac free of the caudal end
of the embryo except for the stalk by which it maintains its original
connection with the hind-gut (Fig. 50).


The allantois early acquires an abundant blood supply by way of
large branches from the caudal end of the aorta (Figs. 45 and 51). In
the allantoic wall these arteries break up into a maze of thin-walled
vessels. As the yolk-sac circulation undergoes retrogressive changes,
this plexus of allantoic vessels becomes progressively more highly
developed and soon takes over entirely the function of metabolic interchange between fetus and mother.


When the germ layers have been differentiated, the outer layer of the blastocyst is directly continuous with the ectoderm of the embryo (Fig. 19). There is then justification for calling it more specifically trophectoderm. The prefix troph- (Greek root, to nourish) suggests the part this layer plays in the acquisition of food materials from the uterus and at the same time sets it off from the ectoderm of the embryonic disk.


Still later in development when the mesoderm splits and its somatic layer becomes associated with the ectoderm, the name trophectoderm is no longer appropriate (Fig. 22). Three different names are in common use to cover this double layer. Extra-embryonic somatopleure is a term following the general embryological usage in designating by the suffix -pleure the double layers formed by the secondaiy association of somatic and splanchnic mesoderm with ectoderm and entoderm respectively. Trophoderm is widely used by those working especially in mammalian embryology. Those especially interested in comparative embryology are quite likely to use the term serosa for this layer, thus emphasizing its very evident homology with the part of the extra-embryonic somatopleure so designated in birds and reptiles. This multiplicity of names is not difficult to master if we understand they are all appropriate terms selected to emphasize somewhat different aspects of the same structure. Trophoblast, then, means the outer layer of the blastocyst before ectoderm, entoderm, and mesoderm are differentiated. Trophectoderm is the extra-embryonic part of the ectoderm when the germ layers have been differentiated, and trophoderm (serosa) (extra-embryonic somatopleure) is trophectoderm reinforced by a layer of somatic mesoderm.


Fig. 51. Drawing showing the main vascular channels in a pig embryo of
about 17 days (27 pairs of somites). Based on cleared preparations of injected
embryos loaned by Dr. C. H. Heuscr, and on Sabin’s figures of similar
material.


In the formation of the amnion, the somatopleure is thrown into folds surrounding the embryo. The inner limb of the fold, as we have seen, becomes the amnion (Fig, 37, C). The outer limb of the fold is commonly called the serosa. It is, perhaps, in just this designation of the outer limb of the amniotic fold that the term serosa is most often encountered. After the amnion has been completed, interest in this outer layer centers about its role in food absorption and the term trophoderm is most commonly used to designate it.


n. The Relation of the Embryo and Its Membranes to the Uterus


Having seen the manner of origin and the relations of the various
===The Allantois===
extra-embryonic membranes to the embryo, we must now turn our
Almost as soon as the hind-gut of the embryo is established, there arises from it a diverticulum known as the allantois (Fig. 37). The allantoic wall, because of its manner of origin, is necessarily composed of splanchnopleure. As the allantois increases in Gxtent its original communication with the hind-gut is narrowed to a cylindrical stalk, while its distal portion becomes enormously dilated (Figs. 37 and 49).
attention to their relations to the uterus. It will be recalled that the ova


Externally the developing allantois appears at first as a crescentic enlargement directly continuous with the caudal part of the embryo (Figs. 29 and 30). Its dilated distal portion continues to grow with great rapidity until it is a large semilunar sac free of the caudal end of the embryo except for the stalk by which it maintains its original connection with the hind-gut (Fig. 50).


The allantois early acquires an abundant blood supply by way of large branches from the caudal end of the aorta (Figs. 45 and 51). In the allantoic wall these arteries break up into a maze of thin-walled vessels. As the yolk-sac circulation undergoes retrogressive changes, this plexus of allantoic vessels becomes progressively more highly developed and soon takes over entirely the function of metabolic interchange between fetus and mother.


Fig. 52. Uterus of pregnant sow opened to show distribution of embryos.
(After Corner.) Note the approximately uniform spacing of the embryos in
the two horns of the uterus in spite of the fact that the corpora lutea indicate
the origin of two ova from one ovary and seven from the other.


Fig. 51. Drawing showing the main vascular channels in a pig embryo of about 17 days (27 pairs of somites). Based on cleared preparations of injected embryos loaned by Dr. C. H. Heuscr, and on Sabin’s figures of similar material.




==II. The Relation of the Embryo and Its Membranes to the Uterus==


Fig. 53. Schematic diagrams indicating the intra-uterine relations of the  
Having seen the manner of origin and the relations of the various extra-embryonic membranes to the embryo, we must now turn our attention to their relations to the uterus. It will be recalled that the ova are fertilized in the uterine tube shortly after their discharge from the ovary. It takes from three to four days for the ova to pass through the uterine tube so that by the time the embryos reach the uterine cavity the cleavage divisions are well under way (Figs. 14, C, D, and 15, A).
embryos and their membranes: A, in the elongated blastocyst stage; B, when
the blastocysts have been dilated to form the chorionic vesicles characteristic
of the later stages of pregnancy.  




Fig. 52. Uterus of pregnant sow opened to show distribution of embryos. (After Corner.) Note the approximately uniform spacing of the embryos in the two horns of the uterus in spite of the fact that the corpora lutea indicate the origin of two ova from one ovary and seven from the other.


Fig. 54. Drawing of pig embryo in unruptured chorionic vesicle. (After
Grosser.) Compare with figures 49, C., and 53, B.




are fertilized in the uterine tube shortly after their discharge from the  
Fig. 53. Schematic diagrams indicating the intra-uterine relations of the embryos and their membranes: A, in the elongated blastocyst stage; B, when the blastocysts have been dilated to form the chorionic vesicles characteristic of the later stages of pregnancy.
ovary. It takes from three to four days for the ova to pass through
the uterine tube so that by the time the embryos reach the uterine
cavity the cleavage divisions are well under way (Figs. 14, C, D, and
15, A).  


The Spacing of Embryos in the Uterus. Normally, of course,
some of the ova fertilized were formed in one ovary and some in
the other. The corpora lutea which develop from the ruptured
ovarian follicles leave unmistakable evidence of the number of ova
contributed by each ovary. Usually the numbers are approximately
equal, but it not infrequently happens that, at a given estrus, one
ovary is far more prolific than the other. If all the ova originating on
one side remained in the corresponding horn of the uterus there
would be crowding of the embryos on one side and unutilized space
on the other. Such a condition does not ordinarily occur. The
mechanism by which the embryos are arranged within the uterus is
not known, but the careful observations of Comer show that they do
tend to become uniformly distributed in the two horns of the uterus
regardless of the side on which the ova were liberated (Fig. 52).


Fig. 54. Drawing of pig embryo in unruptured chorionic vesicle. (After Grosser.) Compare with figures 49, C., and 53, B.


This spacing of the embryos apparently takes place before the
elongation of the blastocysts occurs. Even when elongated, the blastocysts do not spread over as much space in the uterus as one might
suppose. The lining of the uterus is extensively folded and the threadlike blastocysts follow along these folds, so that a blastocyst a meter
long will occupy perhaps no more than 10-15 cm. of a uterine horn
(Fig. 53, A).


The attenuated condition of the blastocyst does not persist long.
With the increase in the extent of the allantois and growth of the
embryo, the blastocyst becomes greatly dilated and somewhat
shortened. Thus altered in shape and appearance it is commonly
called a chorionic vesicle. The allantois never grows to quite the entire
length of the chorionic vesicle and there remains an abrupt narrowing
at either extremity of the vesicle where the allantois ends (Figs. 49,
C, and 54).


Where these undeveloped terminal portions of neighboring
chorionic vesicles lie close to each other the uterus remains undilated,
sharply marking off the region where each embryo is located (Fig.
53, B). One of the local enlargements of the uterus containing an
embryo and its membranes is known as a loculus.


The Chorion. While these changes have been occurring in the  
===The Spacing of Embryos in the Uterus===
general relations of the embryo and its membranes to the uterus there
Normally, of course, some of the ova fertilized were formed in one ovary and some in the other. The corpora lutea which develop from the ruptured ovarian follicles leave unmistakable evidence of the number of ova contributed by each ovary. Usually the numbers are approximately equal, but it not infrequently happens that, at a given estrus, one ovary is far more prolific than the other. If all the ova originating on one side remained in the corresponding horn of the uterus there would be crowding of the embryos on one side and unutilized space on the other. Such a condition does not ordinarily occur. The mechanism by which the embryos are arranged within the uterus is not known, but the careful observations of Comer show that they do tend to become uniformly distributed in the two horns of the uterus regardless of the side on which the ova were liberated (Fig. 52).
have been further specializations of the fetal membranes themselves.  
In its peripheral growth the allantois comes into contact with the  
trophoderm (extra-embryonic somatopleure) (serosa) and becomes
fused with it. The new layer thus formed by the fusion of allantoic
splanchnopleure with extra-embryonic somatopleure is the chorion
(Fig. 49, C). Lying in intimate contact with the uterine mucosa
externally, and having in its double mesodermal layer the rich plexus
of allantoic vessels communicating with the fetal circulation, it is well
situated to carry on the functions of metabolic interchange between
the fetus and the uterine circulation of the mother.  


The Placenta. In many mammals the chorion becomes fused
with the uterine mucosa. In the pig, although the chorion and the
uterine mucosa lie in close contact with each other, they can always
be peeled apart (Fig. 57, A). When the uterine mucosa and the
chorion do not actually grow together and can readily be separated
from one another without the tearing of either, they are said to constitute a contact placenta (semi-placenta). Most mammals show a
more highly specialized condition in which the fetal (chorionic) and


This spacing of the embryos apparently takes place before the elongation of the blastocysts occurs. Even when elongated, the blastocysts do not spread over as much space in the uterus as one might suppose. The lining of the uterus is extensively folded and the threadlike blastocysts follow along these folds, so that a blastocyst a meter long will occupy perhaps no more than 10-15 cm. of a uterine horn (Fig. 53, A).


The attenuated condition of the blastocyst does not persist long. With the increase in the extent of the allantois and growth of the embryo, the blastocyst becomes greatly dilated and somewhat shortened. Thus altered in shape and appearance it is commonly called a chorionic vesicle. The allantois never grows to quite the entire length of the chorionic vesicle and there remains an abrupt narrowing at either extremity of the vesicle where the allantois ends (Figs. 49, C, and 54).


Fig. 55. Semischernatic diagram showing the structure of the chorion and  
Where these undeveloped terminal portions of neighboring chorionic vesicles lie close to each other the uterus remains undilated, sharply marking off the region where each embryo is located (Fig. 53, B). One of the local enlargements of the uterus containing an embryo and its membranes is known as a loculus.
its relation to the uterine wall. Reference to figure 49, C, will show the general relations of the region here depicted. The area indicated in this figure
by the rectangle is shown in detail in figure 56.  


===The Chorion===
While these changes have been occurring in the general relations of the embryo and its membranes to the uterus there have been further specializations of the fetal membranes themselves. In its peripheral growth the allantois comes into contact with the trophoderm (extra-embryonic somatopleure) (serosa) and becomes fused with it. The new layer thus formed by the fusion of allantoic splanchnopleure with extra-embryonic somatopleure is the chorion (Fig. 49, C). Lying in intimate contact with the uterine mucosa externally, and having in its double mesodermal layer the rich plexus of allantoic vessels communicating with the fetal circulation, it is well situated to carry on the functions of metabolic interchange between the fetus and the uterine circulation of the mother.


===The Placenta===
In many mammals the chorion becomes fused with the uterine mucosa. In the pig, although the chorion and the uterine mucosa lie in close contact with each other, they can always be peeled apart (Fig. 57, A). When the uterine mucosa and the chorion do not actually grow together and can readily be separated from one another without the tearing of either, they are said to constitute a contact placenta (semi-placenta). Most mammals show a more highly specialized condition in which the fetal (chorionic) and maternal (mucosal) portions of the placenta actually grow together so they cannot be pulled apart without hemorrhage. So intimately do they become fused in fact, that when, shortly following the birth of the fetus itself, the extra-embryonic membranes are delivered as the so-called ^^after-birth,” a large part of the uterine mucous membrane is pulled away with the chorion. In contrast with the primitive contact type such a placenta is called a burrowing or “true” placenta. An understanding of placental relationships, in such a form as the pig where all the component parts are clearly identifiable furnishes the best possible background for interpreting the more complex types of placentae where there are extensive fusions between fetal and maternal portions.


Fig. 56. Detailed drawing of small area of placenta of 27 mm. pig embryo
showing the structure of its chorionic and uterine portions where they lie in
contact. (After Grosser.) For relations of area drawn see figure 55.


Fig. 55. Semischernatic diagram showing the structure of the chorion and its relation to the uterine wall. Reference to figure 49, C, will show the general relations of the region here depicted. The area indicated in this figure by the rectangle is shown in detail in figure 56.




Fig. 57. A, Small portion of pig placenta drawn about natural size, to
Fig. 56. Detailed drawing of small area of placenta of 27 mm. pig embryo showing the structure of its chorionic and uterine portions where they lie in contact. (After Grosser.) For relations of area drawn see figure 55.
show the manner in which the chorion may readily be separated from the
uterine mucosa without injury to either layer. (After Grosser.)  


B, Small area of chorion of 19 cm. pig embryo showing its uterine surface
magnified. (After Grosser.) For location of area depicted see rectangle in A.
Note the minute irregular local elevations (chorionic villi of primitive type)
which are superimposed on the major folds shown in A, further increasing
the surface of contact between chorion and uterine mucosa.


C, Semischematic diagram of section in region of areola. (After Zeitzschmann.) The areolae fit into depressions in the uterine mucosa near the orifice
Fig. 57. A, Small portion of pig placenta drawn about natural size, to show the manner in which the chorion may readily be separated from the uterine mucosa without injury to either layer. (After Grosser.)
of uterine glands. The villi of the areolae are somewhat more highly developed than those of the general chorionic surface and are believed to be  
especially active in absorbing nutritive materials from the uterine secretions.  


B, Small area of chorion of 19 cm. pig embryo showing its uterine surface magnified. (After Grosser.) For location of area depicted see rectangle in A. Note the minute irregular local elevations (chorionic villi of primitive type) which are superimposed on the major folds shown in A, further increasing the surface of contact between chorion and uterine mucosa.


maternal (mucosal) portions of the placenta actually grow together
C, Semischematic diagram of section in region of areola. (After Zeitzschmann.) The areolae fit into depressions in the uterine mucosa near the orifice of uterine glands. The villi of the areolae are somewhat more highly developed than those of the general chorionic surface and are believed to be especially active in absorbing nutritive materials from the uterine secretions.
so they cannot be pulled apart without hemorrhage. So intimately do
they become fused in fact, that when, shortly following the birth of
the fetus itself, the extra-embryonic membranes are delivered as the
so-called ^^after-birth,” a large part of the uterine mucous membrane
is pulled away with the chorion. In contrast with the primitive contact type such a placenta is called a burrowing or “true” placenta. An
understanding of placental relationships, in such a form as the pig
where all the component parts are clearly identifiable furnishes the
best possible background for interpreting the more complex types of  
placentae where there are extensive fusions between fetal and maternal
portions.  




Even in its fully developed condition the chorion of the pig is a
relatively simple structure. The entodermal lining of the allantoic
cavity becomes much reduced in thickness, forming a delicate singlelayered epithelial covering on the internal face of the chorion. The
mesoderm of the allantoic wall fuses With that of the overlying
trophoderm and this combined layer becomes differentiated into a
primitive type of gelatinous connective tissue (mucoid connective
tissue) (Fig. 55). In this connective-tissue layer the blood vessels of the
allantoic plexus are located. With the fusion of the allantoic mesoderm
to the trophoderm these vessels invade the trophoderm and their
terminal arborizations come to lie close to the trophectoderm (Fig.
55). The trophectoderm becomes a highly developed simple columnar
epithelium with its surface exposure enormously increased by abundant foldings.


This plicated epithelial surface of the chorion follows the folds in  
Even in its fully developed condition the chorion of the pig is a relatively simple structure. The entodermal lining of the allantoic cavity becomes much reduced in thickness, forming a delicate singlelayered epithelial covering on the internal face of the chorion. The mesoderm of the allantoic wall fuses With that of the overlying trophoderm and this combined layer becomes differentiated into a primitive type of gelatinous connective tissue (mucoid connective tissue) (Fig. 55). In this connective-tissue layer the blood vessels of the allantoic plexus are located. With the fusion of the allantoic mesoderm to the trophoderm these vessels invade the trophoderm and their terminal arborizations come to lie close to the trophectoderm (Fig. 55). The trophectoderm becomes a highly developed simple columnar epithelium with its surface exposure enormously increased by abundant foldings.
the surface of the uterine mucosa so that the chorionic and the uterine
epithelium lie in contact with each other except for a thin film of  
uterine secretion (Fig. 56). Under the uterine epithelium is a loosely
woven layer of connective tissue richly supplied with maternal vessels.  
Thus there intervenes between fetal and maternal vessels only a scanty
amount of connective tissue and the epithelial layers of the uterine
mucosa and of the chorion. ^ Through these layers food materials and  
oxygen are passed on from maternal to fetal vessels and the waste
products of metabolism are absorbed from the fetal blood stream by
the maternal blood stream. What part the epithelial layers may play
in this transfer is uncertain. It seems not unlikely that the chorionic
epithelium is active in absorption as is the case with the yolk-sac
epithelium in Sauropsida. Whether or not this is the case it certainly
constitutes no serious impediment to the reciprocal transfer of materials between fetus and mother which is the function of the placenta.  




^ Because the fetal and maternal vessels both retain the integrity of their endothelium, and because both chorionic and uterine epithelium persist, a placenta of  
This plicated epithelial surface of the chorion follows the folds in the surface of the uterine mucosa so that the chorionic and the uterine epithelium lie in contact with each other except for a thin film of uterine secretion (Fig. 56). Under the uterine epithelium is a loosely woven layer of connective tissue richly supplied with maternal vessels. Thus there intervenes between fetal and maternal vessels only a scanty amount of connective tissue and the epithelial layers of the uterine mucosa and of the chorion. ^ Through these layers food materials and oxygen are passed on from maternal to fetal vessels and the waste products of metabolism are absorbed from the fetal blood stream by the maternal blood stream. What part the epithelial layers may play in this transfer is uncertain. It seems not unlikely that the chorionic epithelium is active in absorption as is the case with the yolk-sac epithelium in Sauropsida. Whether or not this is the case it certainly constitutes no serious impediment to the reciprocal transfer of materials between fetus and mother which is the function of the placenta.
this type may be designated technically z&^epitheliochorial. The term is used by students  
 
of comparative placentation in contrast with endotheliochorial which designates a type  
 
of placenta such as that occurring in the cat or dog where the uterine epitheliuln is  
^ Because the fetal and maternal vessels both retain the integrity of their endothelium, and because both chorionic and uterine epithelium persist, a placenta of this type may be designated technically epitheliochorial. The term is used by students of comparative placentation in contrast with endotheliochorial which designates a type of placenta such as that occurring in the cat or dog where the uterine epitheliuln is lost but the endothelium of the uterine vessels is retained; and in contrast also with the term hemochorial which designates the type of placenta occurring in man and a number of other mammals in which maternal blood comes in direct contact with chorionic tissue without the intervention of either uterine epithelium or the endothelium of maternal vessels.
lost but the endothelium of the uterine vessels is retained; and in contrast also with  
 
the term hemochorial which designates the type of placenta occurring in man and a  
number of other mammals in which maternal blood comes in direct contact with  
chorionic tissue without the intervention of either uterine epithelium or the endothelium of maternal vessels.




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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.

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


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

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


Modern Notes

pig

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

Chapter 6. The Extra-Embryonic Membranes and the Relation of the Embryo to the Uterus

I. The Formation of the Extra-embryonic Membranes

In dealing with the establishment of the embryonic body we have already seen that the germ layers extend far beyond the region in which the embryo itself develops. These peripheral portions of the germ layers give rise to membranes which are of service as a means of protection and as a means of securing food from the blood of the mother. Because they are not incorporated in thd body of the embryo but discarded at the time of birth, they are called extra-embryonic membranes. These membranes are the yolk-sac, the amnion, the serosa, and the allantois. Their designation as extra-embryonic should not lead us to lose sight of their vital importance to the embryo during its intra-uterine existence.

The Yolk-sac

In mammals, although there is virtually no yolk accumulated in the ovum, a large yolk-sac is formed just as if yolk were present. Such persistence of a structure, in spite of the loss of its original function, is not an uncommon phenomenon in evolution and has given rise to the biological aphorism that ‘‘morphology is more conservative than physiology.” Not only, does the yolk-sac itself persist, even the blood vessels characteristically associated with it in its functional condition appear in the empty yolk-sac of mammalian embryos (Fig. 45). When the mammalian yolk-sac is spoken of as a vestigial structure, therefore, we must bear in mind that such a statement has reference primarily to its function. Morphologically it is of considerable size in young embryos.


The yolk-sac may be defined as that part of the primitive gut which is not included within the body when the embryo is “folded off.” In dealing with the formation of the digestive tube we have already sufficiently considered the processes which thus establish the boundaries between the yolk-sac and the intra-cmbryonic portion of the original gut cavity (Fig. 37). The peripheral extent of the yolk sac is unusually great in young pig embryos correlated with enormous elongation of the blastocyst (Fig. 18). From the yolk-st^lk, the yolk-sac extends beneath the embryo, beyond its head in one direction and beyond its tail in the other, nearly to the ends of the blastocyst (Fig. 49, A).


Fig. 49. Schematic diagrams showing the relations of the extra-embryonic membranes. The great length of the chorionic vesicle of the pig precludes showing the entire vesicle in correct proportions. In A, sections of the vesicle have been omitted. In B and C, the vesicle has been shortened from its true proportions.

A represents conditions in embryos of 15-20 somites; B, conditions in embryos of 4-6 mm. (after flexion); and C, conditions in embryos of 30 mm. or more.


During this, the period of its greatest relative development, the yolk-sac serves as an organ purveying nutritive material to the embryo. Separated from the uterine wall only by the thin outer layer of the blastocyst, its abundant vessels are in a position which maker absorption from the uterus readily possible. The food material and oxygen thus absorbed are transported by way of the vitelline circulation (Fig. 45) to the growing embryo. When, somewhat later, the allantois becomes highly developed it takes over this function and the yolk-sac decreases rapidly in size (Fig. 49, B), finally becoming a shriveled sac buried in the belly-stalk (Figs. 49, C, and 65). The yolk-sac, therefore, exhibits another point of special interest in addition to being an organ which persists after the cessation of its original function. The persistent yolk-sac and its associated vessels are utilized by the embryo in a new manner. Containing no stored food materials, the mammalian yolk-sac turns about, so to speak, and absorbs from the uterus through its external surface. The fact that the function is temporary, later being taken over by the allantois, makes this physiological opportunism in seizing on a new source of supplies none the less striking.


The Amnion

The amnion arises as a layer of somatopleure which enfolds the developing embryo. When the amniotic sac is completed it becomes filled with a watery fluid in which the embryo is suspended. This suspension of the embryo in a fluid-filled sac, by equalizing the pressure about it, serves as a protection against mechanical injury. At the same time the soft tissues of the growing embryo, being bathed in fluid, do not tend to form adhesions with consequent malformations. The functional significance of the amnion is emphasized by the fact than an amnion appears only in the embryos of non-water-living forms. This has led to designating the embryos of fishes and Amphibia, which form no amnion, as ‘‘anamniotes,” and those of birds, reptiles, and mammals, which do form an amniotic sac during development, as ‘"amniotes.”


In all mammalian embryos the amnion is formed at an exceedingly early stage of development. In some forms (e.g., man) the amniotic cavity appears even before the body of the embryo has taken definite shape. When the amnion is thus precociously established the processes in its formation are, as it were, hurried through and consequently they are difficult to analyze. In the pig the amnion is formed in more leisurely fashion, the process being quite similar to that in reptiles and birds.


The first indication of amnion formation in pig embryos becomes evident shortly after the primitive streak stage. The embryonic disk appears to settle into the blastocyst so that it is overhung on all sides by the extra-embryonic somatopleure (Figs. 25 and 26). As the embryo grows in length its head and tail push deeper and deeper into these folds of somatopleure (cf. Fig. 37, B, C). At the same time the folds themselves become more voluminous, growing centripetally over the embryo from the region of its head, its sides, and its tail. These circumferential folds of somatopleure are known as the amniotic folds. For convenience in description we recognize cephalic, caudal, and lateral regions of the amniotic folds, although in reality they are all directly continuous with each other.


Progress of the amniotic folds soon brings them together above the mid-dorsal region of the embryo to complete the amniotic sac (Figs. 37, C, and 49). Where the amniotic folds close, there persists for a time a cord-like mass of tissue between the amnion and the outer layer of the blastocyst (Fig. 49, B). In time even this trace of the fusion is obliterated and the embryo in its amnion lies free in the blastocyst (Fig. 49, C).


The amnion is attached to the body of the embryo where the bodywall opens ventrally in the region of the yolk-stalk (Fig. 50). As development advances this ventral opening becomes progressively smaller, its margins being known finally as the umbilical ring. Meanwhile the yolk-stalk and the allantoic stalk, in the same process of ventral closure, are brought close to each other, to constitute, with their associated vessels, the belly-stalk (Fig. 49, C). The amnion is now reflected from its point of continuity with the skin of the body at the umbilical ring, to constitute the covering of the belly-stalk, before it is recurved as a free membrane enclosing a fluid-filled space about the embryo.


The Serosa. The outer layer of the mammalian blastocyst has various names. In the inner-celFmass stage, the outer layer of the blastocyst is most commonly called the trophoblast (Fig. 15, C). Since the primary germ layers are not at this time differentiated, this term is far more appropriate than ectoderm, which implies a layer having relations which make it stand in contrast with entoderm, or with entoderm and mesoderm. The premature use of the term ectoderm would seem to have nothing to recommend it, and has been responsible for much confusion in thinking and much pointless discussion.


Fig. 50. Drawing (X 12) of pig embryo having about 28 pairs of somites (crown-rump length 4 mm.; age approximately 17 days). The embryo has been removed from the chorionic vesicle with the amnion and allantois intact but the distal portion of the yolk-sac has been cut off. (Cf. Fig. 49, .)


When the germ layers have been differentiated, the outer layer of the blastocyst is directly continuous with the ectoderm of the embryo (Fig. 19). There is then justification for calling it more specifically trophectoderm. The prefix troph- (Greek root, to nourish) suggests the part this layer plays in the acquisition of food materials from the uterus and at the same time sets it off from the ectoderm of the embryonic disk.

Still later in development when the mesoderm splits and its somatic layer becomes associated with the ectoderm, the name trophectoderm is no longer appropriate (Fig. 22). Three different names are in common use to cover this double layer. Extra-embryonic somatopleure is a term following the general embryological usage in designating by the suffix -pleure the double layers formed by the secondaiy association of somatic and splanchnic mesoderm with ectoderm and entoderm respectively. Trophoderm is widely used by those working especially in mammalian embryology. Those especially interested in comparative embryology are quite likely to use the term serosa for this layer, thus emphasizing its very evident homology with the part of the extra-embryonic somatopleure so designated in birds and reptiles. This multiplicity of names is not difficult to master if we understand they are all appropriate terms selected to emphasize somewhat different aspects of the same structure. Trophoblast, then, means the outer layer of the blastocyst before ectoderm, entoderm, and mesoderm are differentiated. Trophectoderm is the extra-embryonic part of the ectoderm when the germ layers have been differentiated, and trophoderm (serosa) (extra-embryonic somatopleure) is trophectoderm reinforced by a layer of somatic mesoderm.


In the formation of the amnion, the somatopleure is thrown into folds surrounding the embryo. The inner limb of the fold, as we have seen, becomes the amnion (Fig, 37, C). The outer limb of the fold is commonly called the serosa. It is, perhaps, in just this designation of the outer limb of the amniotic fold that the term serosa is most often encountered. After the amnion has been completed, interest in this outer layer centers about its role in food absorption and the term trophoderm is most commonly used to designate it.


The Allantois

Almost as soon as the hind-gut of the embryo is established, there arises from it a diverticulum known as the allantois (Fig. 37). The allantoic wall, because of its manner of origin, is necessarily composed of splanchnopleure. As the allantois increases in Gxtent its original communication with the hind-gut is narrowed to a cylindrical stalk, while its distal portion becomes enormously dilated (Figs. 37 and 49).

Externally the developing allantois appears at first as a crescentic enlargement directly continuous with the caudal part of the embryo (Figs. 29 and 30). Its dilated distal portion continues to grow with great rapidity until it is a large semilunar sac free of the caudal end of the embryo except for the stalk by which it maintains its original connection with the hind-gut (Fig. 50).

The allantois early acquires an abundant blood supply by way of large branches from the caudal end of the aorta (Figs. 45 and 51). In the allantoic wall these arteries break up into a maze of thin-walled vessels. As the yolk-sac circulation undergoes retrogressive changes, this plexus of allantoic vessels becomes progressively more highly developed and soon takes over entirely the function of metabolic interchange between fetus and mother.


Fig. 51. Drawing showing the main vascular channels in a pig embryo of about 17 days (27 pairs of somites). Based on cleared preparations of injected embryos loaned by Dr. C. H. Heuscr, and on Sabin’s figures of similar material.


II. The Relation of the Embryo and Its Membranes to the Uterus

Having seen the manner of origin and the relations of the various extra-embryonic membranes to the embryo, we must now turn our attention to their relations to the uterus. It will be recalled that the ova are fertilized in the uterine tube shortly after their discharge from the ovary. It takes from three to four days for the ova to pass through the uterine tube so that by the time the embryos reach the uterine cavity the cleavage divisions are well under way (Figs. 14, C, D, and 15, A).


Fig. 52. Uterus of pregnant sow opened to show distribution of embryos. (After Corner.) Note the approximately uniform spacing of the embryos in the two horns of the uterus in spite of the fact that the corpora lutea indicate the origin of two ova from one ovary and seven from the other.


Fig. 53. Schematic diagrams indicating the intra-uterine relations of the embryos and their membranes: A, in the elongated blastocyst stage; B, when the blastocysts have been dilated to form the chorionic vesicles characteristic of the later stages of pregnancy.


Fig. 54. Drawing of pig embryo in unruptured chorionic vesicle. (After Grosser.) Compare with figures 49, C., and 53, B.



The Spacing of Embryos in the Uterus

Normally, of course, some of the ova fertilized were formed in one ovary and some in the other. The corpora lutea which develop from the ruptured ovarian follicles leave unmistakable evidence of the number of ova contributed by each ovary. Usually the numbers are approximately equal, but it not infrequently happens that, at a given estrus, one ovary is far more prolific than the other. If all the ova originating on one side remained in the corresponding horn of the uterus there would be crowding of the embryos on one side and unutilized space on the other. Such a condition does not ordinarily occur. The mechanism by which the embryos are arranged within the uterus is not known, but the careful observations of Comer show that they do tend to become uniformly distributed in the two horns of the uterus regardless of the side on which the ova were liberated (Fig. 52).


This spacing of the embryos apparently takes place before the elongation of the blastocysts occurs. Even when elongated, the blastocysts do not spread over as much space in the uterus as one might suppose. The lining of the uterus is extensively folded and the threadlike blastocysts follow along these folds, so that a blastocyst a meter long will occupy perhaps no more than 10-15 cm. of a uterine horn (Fig. 53, A).

The attenuated condition of the blastocyst does not persist long. With the increase in the extent of the allantois and growth of the embryo, the blastocyst becomes greatly dilated and somewhat shortened. Thus altered in shape and appearance it is commonly called a chorionic vesicle. The allantois never grows to quite the entire length of the chorionic vesicle and there remains an abrupt narrowing at either extremity of the vesicle where the allantois ends (Figs. 49, C, and 54).

Where these undeveloped terminal portions of neighboring chorionic vesicles lie close to each other the uterus remains undilated, sharply marking off the region where each embryo is located (Fig. 53, B). One of the local enlargements of the uterus containing an embryo and its membranes is known as a loculus.

The Chorion

While these changes have been occurring in the general relations of the embryo and its membranes to the uterus there have been further specializations of the fetal membranes themselves. In its peripheral growth the allantois comes into contact with the trophoderm (extra-embryonic somatopleure) (serosa) and becomes fused with it. The new layer thus formed by the fusion of allantoic splanchnopleure with extra-embryonic somatopleure is the chorion (Fig. 49, C). Lying in intimate contact with the uterine mucosa externally, and having in its double mesodermal layer the rich plexus of allantoic vessels communicating with the fetal circulation, it is well situated to carry on the functions of metabolic interchange between the fetus and the uterine circulation of the mother.

The Placenta

In many mammals the chorion becomes fused with the uterine mucosa. In the pig, although the chorion and the uterine mucosa lie in close contact with each other, they can always be peeled apart (Fig. 57, A). When the uterine mucosa and the chorion do not actually grow together and can readily be separated from one another without the tearing of either, they are said to constitute a contact placenta (semi-placenta). Most mammals show a more highly specialized condition in which the fetal (chorionic) and maternal (mucosal) portions of the placenta actually grow together so they cannot be pulled apart without hemorrhage. So intimately do they become fused in fact, that when, shortly following the birth of the fetus itself, the extra-embryonic membranes are delivered as the so-called ^^after-birth,” a large part of the uterine mucous membrane is pulled away with the chorion. In contrast with the primitive contact type such a placenta is called a burrowing or “true” placenta. An understanding of placental relationships, in such a form as the pig where all the component parts are clearly identifiable furnishes the best possible background for interpreting the more complex types of placentae where there are extensive fusions between fetal and maternal portions.


Fig. 55. Semischernatic diagram showing the structure of the chorion and its relation to the uterine wall. Reference to figure 49, C, will show the general relations of the region here depicted. The area indicated in this figure by the rectangle is shown in detail in figure 56.


Fig. 56. Detailed drawing of small area of placenta of 27 mm. pig embryo showing the structure of its chorionic and uterine portions where they lie in contact. (After Grosser.) For relations of area drawn see figure 55.


Fig. 57. A, Small portion of pig placenta drawn about natural size, to show the manner in which the chorion may readily be separated from the uterine mucosa without injury to either layer. (After Grosser.)

B, Small area of chorion of 19 cm. pig embryo showing its uterine surface magnified. (After Grosser.) For location of area depicted see rectangle in A. Note the minute irregular local elevations (chorionic villi of primitive type) which are superimposed on the major folds shown in A, further increasing the surface of contact between chorion and uterine mucosa.

C, Semischematic diagram of section in region of areola. (After Zeitzschmann.) The areolae fit into depressions in the uterine mucosa near the orifice of uterine glands. The villi of the areolae are somewhat more highly developed than those of the general chorionic surface and are believed to be especially active in absorbing nutritive materials from the uterine secretions.


Even in its fully developed condition the chorion of the pig is a relatively simple structure. The entodermal lining of the allantoic cavity becomes much reduced in thickness, forming a delicate singlelayered epithelial covering on the internal face of the chorion. The mesoderm of the allantoic wall fuses With that of the overlying trophoderm and this combined layer becomes differentiated into a primitive type of gelatinous connective tissue (mucoid connective tissue) (Fig. 55). In this connective-tissue layer the blood vessels of the allantoic plexus are located. With the fusion of the allantoic mesoderm to the trophoderm these vessels invade the trophoderm and their terminal arborizations come to lie close to the trophectoderm (Fig. 55). The trophectoderm becomes a highly developed simple columnar epithelium with its surface exposure enormously increased by abundant foldings.


This plicated epithelial surface of the chorion follows the folds in the surface of the uterine mucosa so that the chorionic and the uterine epithelium lie in contact with each other except for a thin film of uterine secretion (Fig. 56). Under the uterine epithelium is a loosely woven layer of connective tissue richly supplied with maternal vessels. Thus there intervenes between fetal and maternal vessels only a scanty amount of connective tissue and the epithelial layers of the uterine mucosa and of the chorion. ^ Through these layers food materials and oxygen are passed on from maternal to fetal vessels and the waste products of metabolism are absorbed from the fetal blood stream by the maternal blood stream. What part the epithelial layers may play in this transfer is uncertain. It seems not unlikely that the chorionic epithelium is active in absorption as is the case with the yolk-sac epithelium in Sauropsida. Whether or not this is the case it certainly constitutes no serious impediment to the reciprocal transfer of materials between fetus and mother which is the function of the placenta.


^ Because the fetal and maternal vessels both retain the integrity of their endothelium, and because both chorionic and uterine epithelium persist, a placenta of this type may be designated technically epitheliochorial. The term is used by students of comparative placentation in contrast with endotheliochorial which designates a type of placenta such as that occurring in the cat or dog where the uterine epitheliuln is lost but the endothelium of the uterine vessels is retained; and in contrast also with the term hemochorial which designates the type of placenta occurring in man and a number of other mammals in which maternal blood comes in direct contact with chorionic tissue without the intervention of either uterine epithelium or the endothelium of maternal vessels.


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