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==Chapter IX The Placenta==
The placenta is that organ in which the blood-vessels of the
embryo are brought into intimate anatomical and physiological
relation with the spaces-  which may be blood-vessels or lacunae
of quite a difierent character-  in which maternal blood is circulating. Though the apposition of foetal to maternal bloodchannels is very close, there is yet never any communication
between the two ; an injection passed into the maternal mil
not make its way into the foetal vessels, and conversely. At
the same time the tissues that separate the two sets of channels
are so thin that substances can readily travel by diffusion from
the one to the other. In this way the embryo obtains its oxygen
and probably food-stuffs, while by the same means it gets rid
of its carbon dioxide and possibly of other waste products of
its metaboHsm. The foetal blood is brought to the capillaries
of the placenta by the allantois, which carries umbiUcal arteries
and veins, while the maternal blood-supply is from the uterine
vessels. The embryonic tissue which comes immediately in contact with the uterine wall is the trophoblast-  the outer or
ectodermal layer of the false amnion or chorion, -  and it is the
trophoblast which ensures the adherence of the embryo to the
uterine waU and plays a part of conspicuous importance in
the edification of the placenta, particularly in placentas of the
so-called ' deciduate ' type.
In addition to the placenta -  this organ formed by the trophoblast and vascularized by the capillaries of the allantois -  the
embryo has frequently other means of obtaining nutrition. Thus
the trophoblast is often phagocytic-  in early stages, before the
allantois is developed, and in later stages in regions where it is not
adherent to the uterine wall, the debris of dead maternal tissues
and extravasated maternal corpuscles are devoured by it and
passed on to the embryo inside. Again, in several forms, the yolk-sac with its absorptive epithelium and area vasculosa is instrumental in securing additional nutriment for the foetus. These
processes we shall consider individually in the several groups.
Although it has been usual to separate the Eutheria as Placental
Mammals from the Marsupials or Metatheria, it must yet be
remembered that in the latter group there are arrangements by
which the trophoblast is able to secure nourishment for the
embryo from the walls of the uterus, which is handed on by
means of the area vasculosa of the yolk-sac, and that in one
case there is a true allantoic placenta, though it is of a peculiar
type, not met with anywhere else.
The Marsupials thus stand apart in this as well as in
other reproductive characters (the birth of the young in a very
undeveloped condition, the large size of the egg, the presence
of an egg-shell, the mode of segmentation, and the structure
of the blastocyst), and we shall accordingly consider them
separately.
The Marsupials
The yolk-sac, as we have seen, is large and its upper wall
invaginated by the embryo. On this upper wall is an area
vasculosa, which extends only a short way over the outer or
lower wall, the greater part of the latter being directly in contact
with the trophoblast.
In Didelphys the trophoblast opposite the area vasculosa of
the yolk-sac is a columnar epithelium, thrown into folds. These
folds fit into corresponding depressions m the uterine wall from
which they appear to absorb nutrient material, which is then
handed on to the vessels of the yolk-sac.
In Dasyurus the same region of the trophoblast is apphed
closely to the uterine wall, and there is also beyond the limits
of the area vasculosa a conspicuous annular zone of thickened
trophoblast (Fig. 122, c). Cell-boundaries disappear and the
syncytium so formed sends out pseudopodial processes which
attack the uterine epithelium, grow in and enclose portions of
it and the subjacent capillaries. The enclosed capillaries enlarge
and maternal blood passes in between the trophoblast and the
yolk-sac ; presumably it serves as food, for, as we shall see when we come to the Placentalia, maternal corpuscles are the source
from which the embryo obtains its necessary iron.
In Perameles there is an allantoic placenta (Fig. 140). Where
the trophoblast over the allantois touches the uterine wall the
epithelium of the latter thickens to form a syncytium, from
which processes grow down into the connective tissue ; the
syncytium is soon invaded by maternal capillaries. Meanwhile
the thin trophoblast has disappeared and the foetal capillaries
of the aUantois, passing into the irregular depressions on the outside of the syncytium, are brought into fairly intimate relation
with the maternal vessels.
Fig. 140.-  Section through the placenta of PeromeZes. (After Hill.) all.,
allantoic epitheUum ; m., mesoderm of allantois together with somatopleure
of false amnion ; f.h.v., foetal blood-vessel ; ep.s., syncytium of uterine
epithelium; m.6.u, maternal blood-vessels; c.«., subepithelial connective
tissue.
At birth the allantois and its blood-vessels are left behind
and absorbed by maternal leucocytes. This condition has
been termed ' contra-deciduate '. The same fate befalls the
syncytium.
The foetal tissues are similarly absorbed in Dasyurus.
==The Placentalia==
It has long been the custom to sharply distinguish two
principal types of placenta from one another as the Indeciduate
and Deciduate. In the former the connexion between foetal
and maternal tissues is so slight that at parturition the first easily separate from the second, no maternal tissue is lost or
'deciduous', and the placenta is ' indeciduate '. In the other
type, however, the union of foetal to maternal tissues was held
to be so fast that at birth a considerable quantity of the latter
was carried away by the former, and there was, in the language
of a terminology which was invented when the histology of
the placenta was not understood, a 'decidua'. This entirely
erroneous conception of the structure of certain types of placenta
(found, for example, in Rodents, Insectivora, and the human
being), was based on the structure of the placenta in Ungulates.
In the Ungulata, as was then well known, the * indeciduate '
placenta arises by the penetration of foetal (chorionic or trophoblastic) processes into crypts or depressions in the uterine wall,
from v/hich crypts the processes or villi are readily puUed out
at birth. Not unnaturally, in ignorance of the facts, it was
surmised that the ' deciduate ' placenta originated in similar
fashion, with the difference that the chorionic villi adhered so
closely to the crypt walls that at birth they dragged away not
only crypts but connective tissue and blood-vessels as well.
Thus the term ' decidua ' came to be applied to the tissue of the
uterine wall, whether an embryo and placenta were present or not.
Now while it is true that the placenta of the Carnivora is
developed in this kind of way, modern research has conclusively
shown that in the majority of the so-called ' Deciduates ' the
genesis of the placenta proceeds on an entirely different plan.
If, therefore, we retain the name ' deciduate ' for the placenta
of the Rodents, Insectivora, Cheiroptera, and some Primates
{Tarsim, Monkeys, and Man), it must be on the distinct understanding that the word bears its original meaning no longer.
The term ' indeciduate ' is not inapplicable to the Ungulate
placenta, and there is no objection to its use.
We shall begin with the Ungulate as exhibiting structurally
the simplest type.
==Ungulata==
In Ungulata the placenta is of the indeciduate form. At the
surface of the chorionic sac there are produced finger-shaped
processes or villi, formed of a single layer of trophoblast, and provided with a core of mesodermal tissue in whieh are the
foetal eapillaries. The endodermal epithelium of the allantois
is not continued into the villi. These villi fit into depressions
in the wall of the uterus known as crypts. The crypts are hned
by an epithelium which is perfectly continuous with the ordinary
epitheUum of the uterus and persists throughout gestation. The
persistence of the uterine epithelium is the real mark by which
the indeciduate is distinguished from other placentas. Below
the epithelium of the crypts are the maternal capillaries and
connective tissue. The villi do not adhere closely to the crypt
all.
Fig. 141. -  Diagram of a foetal and maternal cotyledon of the cow. all.,
allantoic epithelium ; ir., trophoblast ; v., villus ; e'p., uterine epithelium
continued into crypt ; c.w., wall of crypt. The maternal connective tissue
is shaded.
Avails, and at birth are easily removed without damage to the
maternal tissues.
The Ungulate Placenta may be Diffuse, or Cotyledonary, or
of an intermediate type. In the first the whole surface of the
chorionic sac is covered uniformly with villi which may be simple
(as in the pig) or branched (as in the horse) (Fig. 131). In the
cotyledonary placenta the villi are gathered together into bunches
or cotyledons, the intervening regions of the chorion being smooth
(Fig. 141). The villi -  which are much branched -  fit into crypts
of a corresponding shape, the whole aggregation of crypts for
the reception of the villi of a single cotyledon being termed
a maternal cotyledon. The points in the wall of the uterus where these maternal cotyledons will be formed are predetermined and recognizable as raised areas -  the cotyledonary
caruncles -  ^before gestation, can be seen indeed in the uterus
of the unborn calf. The foetal cotyledons are scattered all over
the surface of the chorion, except at its extreme ends, the
' diverticula allantoidis ' so called.
A cotyledonary placenta is characteristic of the Ruminants.
In some cases (Cervus, Giraffa, Oreas, Tetraceros) tho placenta
is of an intermediate type, simple villi being found between the
cotyledons.
As examples of indeciduate placentation we may take the
cow and sheep.
Before describing the anatomy and physiology of the actual
placenta it will be convenient first to consider the changes that
take place in the wall of the uterus preparatory to the reception
of the embryo, as well as the nutrition of the embryo while it
is still free in the uterine cavity.
In the period known as the ' pro-oestrus ', which precedes •
heat or 'oestrus', the subepithehal connective tissue of the uterus
becomes h5rpertrophied, while the capillaries increase in number
and become enlarged. Numbers of corpuscles -  ^both haematids
and leucocytes -  are now extra vasated from these swollen blood vessels into the surrounding stroma of connective tissue, where
many of the haematids are devoured and digested by leucocytes
with the resultant deposition of pigment in the cytoplasm of
the latter. This brown jjigment, derived from the haemoglobin
of the extravasated corpuscles, may remain in the wall of the
uterus for a considerable time. Meanwhile, as a result possibly
of the pressure exerted by the congested capillaries, the uterine
epithelium has given way ; patches of it degenerate and are
cast into the uterine lumen along with some debris of subepithelial cells, haematids, and leucocytes. The fluid in the uterus
already contains proteid, glycogen, and fat secreted by the
uterine epithelium and glands. In this fat-secretion the outer
ends of the cells, containing fat-globules, are nipped off and
ejected. There are also present (in the sheep) rod-like or needleshaped bodies, composed of an albuminous substance and secreted
by the epitheUum. Iron, too, is found, derived from the digested
haemoglobin of the extravasated haematids. To all this must
be added the products of the cellular secretion of the glands
(Fig 142) Small tracts of the epithelial wall become invagmated
into the gland-lumen, are cut off, degenerate, and are thrown
out by the mouths. The secretion of fat and proteid, of the
albuminous rod-shaped bodies and of cell-masses by the glands,
is not confined to the period of ' pro-oestrus ' but occurs
throughout gestation.
Fig. 142.-  Cellular secretion in the glands of the viterus. a, horse (after
Kolster) ; B, clog (after Bonnet). In A a piece of the epithelium is being
invaginated into the lumen of the gland. In B this has been nipped off. In
A the secretion of fat (black globules) and pieces of cells is also shown.
The material thus provided is of a thick, viscid consistency and
of a yellow colour, like pus, and is known as ' uterine milk ' . It is
of the greatest importance for the nutrition of the embryo.
Ovulation or the escape of the ovum from the Graafian foUicle
occurs in ' oestrus ' : the ovum passes into the Fallopian tube,
where fertihzation takes place. Development begins and the
blastocyst enters the uterus. Here the trophoblast at once
begins to absorb the nutriment prepared for it. The cells are
phagocytic and ingest solid particles of the uterine milk : they
also absorb fat and possibly iron.
The blastocyst next becomes attached by its trophoblast to
the uterine wall, and the placenta is formed.
The uterine epithelium, where destroyed, has now been
restored. In the cotyledonary caruncles it is continued into the
crypts, which are now developed. If we may judge of what
happens now by what is known of the manner in which accessory
maternal cotyledons are formed in the later stages (in the cow), the
crypts 'arise (Fig. 143) by a pitting of the columnar cihated
epitheUum, the cells which are at the bottom of the pits becoming
shorter than the ordinary columnar cells around {a), followed
by the outfolding of the epithelium between the pits (6) ; into
these folds connective tissue soon penetrates (c), and later blood vessels.
Villi or finger-shaped processes of the trophoblast are now
formed and enter the crypts. The epithelium lining the latter
soon becomes modified, the cilia are lost, and the cells become
cubical (d, e) (in the cow) or very flat (in the sheep). Even in
the latter case, however, small patches of cubical cells remain,
from which fresh crypts are formed by downgrowth into the
subepithelial tissue (Fig. 144).
Fig 143. -  a-e. Five stages in the formation of a crypt in tlie cow.
a-c, pitting and folding of the epithelium; cl, the epithelium becomes
cubical ; e, the cilia are lost.
Fig. 144. -  Formation of accospory crypts in tlto sheep. In a and & the
(lowngrowth of epithchuni i.s still solid ; in c it i« becoming hollow ; in e it
is open to the old crypt.
With continued development the villi and crypts elongate and
branch repeatedly, and the maternal cotyledon is raised above
the level of the uterine wall (Fig. 141). The free surface is
convex in the cow, but deeply concave in the sheep, where also
the base of attachment is constricted to a narrow stalk.
In the cow the trophoblast covering the villi is composed of
rounded or cubical elements, amongst which are gland-cells and
curious oval binucleate cells (found also in the sheep). The core
of each villus is occupied by connective tissue (somatopleure of
the false amnion plus splanchnopleure of the allantois) and foetal
capillaries, the latter very close to the epithehal cells (Fig. 145, 1).
The crypt is lined by cubical cells which secrete fat and proteid,
the ends of the cells with the contained fat-globules being protruded, pinched off, and thrown into the space between crypt
and villus. Fat can be demonstrated in the trophoblast, which
doubtless absorbs the proteid also. The gland-cells may be of
importance in this respect.
Below the epithelium in the crypt-walls are maternal connective tissue and maternal capillaries. The foetal and parental
blood-streams are thus separated by the endothelium of the
foetal capillaries, some connective tissue (not always), the trophoblast, the epithelium of the crypts, the cormective tissue, and
the endothelium of the maternal vessels. Through these layers
oxygen diffuses from maternal to foetal blood, and carbon
dioxide in the reverse direction ; other substances may also
pass. In the cotyledons, therefore, the respiratory exchange
takes place and the absorption of fat and proteid.
It is, however, not merely by means of its cotyledonary villi
that the embryo obtains nutrition. At the bases of the villi
and therefore opposite the summits of the walls between the
crypts the trophoblast is very tall and columnar (Fig. 145, 2).
The outer ends of the cells are pseudopodial and ingest quantities
of cell-debris and maternal red blood-corpuscles. The capillaries
at the summits of the crypt-walls are gorged, blood is extravasated, and together with the remains of epithehal and subepitheUal cells eagerly devoured by the trophoblast, and digested.
The ingested haematids get clumped together in the cells, and
often surrounded by a food-vacuole (Fig. 145, 3). As intra
Fig. 145. -  Histology of the placenta in the cow and sheep.
1, Foetal and maternal tissues in a cotyledon, tr., trophoblast of a
villus ; the cells are absorbing fat (black). In the trophoblast are two
binucleate cells. Behind the trophoblast are the connective tissue and
Tilood-vessels of the allantois. ep., uterine epithelium lining a crypt.
Fat secretion is going on, the ends of the cells with fat-globules being
pinched off and thrown into the lumen of the crypt. Below the epithelium
are the maternal capillaries and connective tissue.
2, Columnar trophoblast cells from between the bases of the cotyledonary
villi. The cells are full of ingested matter (corpuscles, nuclear, and cell
debris).
3, Ingestion of extravasated maternal corpuscles by the trophoblast in
the sheep. The corpuscles are seen inside the cells. The cells also contain
pigment.
4, Deposition of pigment after digestion of the haemoglobin of ingested
corpuscles in trophoblast cells of the cow. The pigment-granules (black)
are seen to be deposited on irregular masses in the cells. cellular digestion proceeds little granules of yellow-brown pigment appear on the surface of the mass, and gradually the whole
assumes the same coloiar (Fig. 145, 4). The pigment probably
contains no iron, at least when digestion is completed, the iron
of the haemoglobin having been separated and carried o£E to
the embryo by the blood of the allantois. Thus the foetus
obtains its necessary iron in this as in earlier stages from the
haemoglobin of extravasated maternal corpuscles, devoured by
the trophoblast. The pigment remains in the trophoblast, where
large quantities of it are accumulated by the end of gestation.
In neutral solution it shows two bands very, nearly in the
position of those of oxyhaemoglobin, in acid solution two bands
almost exactly in the position of those of acid haematoporphyrin,
but in alkaline solution not the four bands of alkaline haematoporphyrin, but only the two seen in the neutral solution. It is
probably related to haematoporphyrin : it is certainly a haemoglobin derivative, and from it bile-pigments may be formed.
We have already had occasion to notice the curious roimded
or elongated, often flattened bodies, sometimes soft, sometimes
hard and brittle, found floating in the allantoic fluid and familiar
for many centuries under the title of ' hippomanes '.
In the cow they are white or pale yellow, in the sheep a dirty
brown. In the sheep they are formed by local accumulations
of the viscid uterine milk, which get into pockets of the trophoblast, between the cotyledons. Gradually, pushing the trophoblast and allantois in front of them, they make their way into
the cavity of the latter, in which they lie attached by a stalk
to the wall ; the stalk narrows and breaks, and they are free
in the cavity. At first they are surrounded by a membrane - 
the remains of their covering of allantois and trophoblast -  and
are soft : they are composed of a granular coagulable material,
full of cell-detritus, degenerating nuclei, globules of fat and
glycogen, and leucocytes. Later the membrane disappears, and
the bodies become hard by being saturated with calcium oxalate
in the form of ' envelope ' crystals. In the cow, when outside
the chorion and still soft, they are a bright orange colour, due
to the presence of bilirubin, doubtless derived from the extravasated corpuscles eaten by the trophoblast ; they are, indeed, found at the bases of the villi, just where these extravasations
occur.
Large allantoic bodies impregnated with calcium oxalate are
found in the horse.
Small quantities of glycogen are found in the uterine epithehura and subepithelial tissues, and in the uterine milk. Much
larger quantities are found stored up in the amniotic thickenings
-  ^masses of stratified epithelium on the inner surface of the
amnion. Towards the end of gestation the glycogen diminishes,
and the cells undergo fatty degeneration and are impregnated
with calcium oxalate. As the glycogen diminishes the dextrose
in the amniotic fluid increases (from 1 % to 0-37 %).
Glycogen also occurs in the trophoblast, in the connective
tissue of the chorion and in the umbilical cord round the
blood-vessels and allantoic epithelium. In the body of the
embryo it is abundant in all tissues, except in the liver, where
it only appears late, when it is disappearing from the others.
The glycogen in the amniotic bodies appears to be a reserve
store. We shall find a similar storage of glycogen in other cases.
Besides dextrose the amniotic fluid contains albumin, mucus,
and chlorides of sodium and potassium.
In the allantoic fluid are dextrose (0-3 %), albumin, mucin,
magnesium, sodium, and calcium phosphates, sodium chloride
and sulphate, and ' envelope ' crystals of calcium oxalate ;
further, a yellow pigment, and allantoin, the embryonic representation of urea.
It appears, therefore, that the allantois is a receptacle for the
waste products of foetal metabohsm.
Cetacea. In Orca the placenta appears to be indeciduate and
diffuse, uniformly studded with villi. The chorionic sac extends
into both cornua of the uterus. The villi, which are branched,
are only absent at the ends, opposite the Fallopian tubes, and
again opposite the os uteri,
Sirenia. Halicore possesses an indeciduate, diffuse placenta.
It is known that the uterine epithelium persists in the crypts.
The villi, which are slightly branched, are Umited to an annular
area surrounding the chorionic sac, not qmte in the middle of
the latter. When the region of the trophoblast, which enters into such intimate relations with the uterus as to form a placenta,
is of this annular shape, the placenta is spoken of as zonary.
(A zonary placenta may be of the deciduate type.)
Hippomanes are found in Hcdicore, but there are no amniotic
bodies.
Proboscidea. In the elephant the shape of the placenta is
zonary, though diffuse villi occur at the ends of the chorionic sac.
These villi appear to be of the nature of those found in Ungulates.
In the zonary region the villi appear to have become embedded
in the wall of the uterus by their ends, while maternal blood is
extravasated between their bases. In the absence of more exact
information this placenta cannot be properly classified. Brown
pigment abounds in the trophoblast of the villi, presumably
a haemoglobin derivative.
Hyracoidea. In Hyrax the placenta is zonary in shape, with
villous patches at the poles.
Edentata
The placenta is stated to be bell-shaped in Myrmecophaga and
Tatnandua, zonary in Orycteropus, oval in Dasypus, diffuse in
Manis and Choloepus, but we have no knowledge of its minute
structure.
Lemuroidea
In this, the lower division of the Primates, the placenta is of
the diffuse indeciduate tjrpe (except in Tarsius, which must
certainly be placed with Monkeys and Man).
In Galago (Fig. 146) the chorionic sac is large and occupies
both horns of the uterus. It is covered with short simple villi
at the extremity of each of which is a sUght pit, the cells of which
contain granular greenish masses (? haemoglobin derivatives).
The villi lie in grooves lined by a persistent epithelium, from
which they are easily pulled out. The chorionic vesicles are
invaginations of the trophoblast opposite depressions in the
uterine wall at the bottom of which glands open. Both chorionic
vesicles and depressions are filled with a granular material - 
uterine milk -  which appears to be absorbed by the villi which
spring from the floor of the vesicle.
Fig. 146. -  Placenta of the lemur Galago. (After Strahl.)
A, Section of a villus with the crypt in which it is lodged. The uteruie
epithelium (ep.) lining the crypt persists ; m.h.v., maternal blood-vessel ;
all, allantois epithelium ; f.b.v., foetal blood-vessel ; tr., trophoblast.
B, Section through a chorionic vesicle and the opposed depression of the
uterine wall ; tr., trophoblast ; v., small branching vilh protruding into the
chorionic vesicle ; gl., glands opening into the uterine depression ; m.h.v.,
maternal blood-vessels.
In the Carnivora we meet with a group which is from the
comparative anatomical point of view of the greatest importance,
since the placenta here holds an intermediate position between
the Indeciduate and Deciduate (so-called) types. While in the
disappearance of the uterine epithelium it must be ranked with
the latter, it differs widely from them in the fact that the
channels in which the blood of the mother circulates are the
capillaries of the uterine wall, between which the trophoblast
has penetrated after the destruction of the superficial epithelium.
In this mutual apposition of foetal vessels and maternal vessels
the Carnivorous does indeed resemble the Ungulate type, from
which it may conceivably have been derived, and comes very
near to fulfilling the original definition of a ' deciduate ' placenta,
since at birth the maternal vessels and connective tissue are
removed along with the foetal constituents. The shape of the
placenta is always zonary. The genera most carefully investigated
are the dog and cat.
We begin with a description of the processes preparatory to
the reception of the embryo.
In the period of pro-oestrus prior to ' heat ' the uterus becomes
swollen and hjrperaemic owing to the multiplication and enlargement of the blood-vessels and capillaries. Blood is extravasated
first into the subepithelial tissue, and masses of brown pigment
appear, as the result, presumably, of the digestion of the haemoglobin by the abundant leucocytes. Soon the superficial columnar
epithelium gives way, and quantities of haematids with a certain
amount of destroyed epithelial and connective tissue are
discharged into the lumen of the uterus.
The uterine glands are long and twisted, and branch ; they
apparently secrete some proteid material and masses of cells
in the way already described in the Ungulata (Fig. 142). In
addition there are the crypts, short tubular downgrowths of the
epitheUum.
During the following period of oestrus a regeneration of the
destroyed epithelium takes place. Should fertilization have
occurred the blastocyst is developed and makes its way into the uterus, in the placental regions of which the following changes
now occur.
While the blastocyst is still unattached fat appears in the superficial epithelium of the uterus, and in that of the necks of the glands
and crypts. The necks of the glands widen, so giving rise to the
' spongy ' layer. A thick layer of dense subepithelial tissue is
formed, in which run the capillaries. The surface epithelium next
becomes lower, while its nuclei begin to degenerate, and eventually
the whole epithelium disappears. The openings of the crypts and
glands are closed by masses of enlarged epithelial cells which, uniting to form syncytia, soon show signs of degeneration (Fig. 147, a).
Attachment now occurs. In the zonary placental region the
trophoblast is produced into finger-shaped villi -  which may be
solid or provided with a core of mesoderm -  and these villi make
their way into the connective tissue from which the epithelium
has now been removed, as well as into the plugs of degenerating
syncytia closing the mouths of the crypts and glands. To
these syncytia are added the cellular secretions of the glands.
The cells of the trophoblast are phagocytic and devour all this
detritus. Where the trophoblast invades the connective tissue
between the crypts and gland it comes into intimate relation
with the capillaries there, and as soon as the villi have been
penetrated by the foetal blood-vessels the placenta may be
said to have been established (Fig. 147, b).
•Below the placenta are the wide parts of the glands, separated
only by thin lamellae of connective tissue in which run the larger
blood-vessels. By these lamellae the placenta is attached to the
muscularis.
The placenta so formed is at first thin, but soon grows in
thickness by the simultaneous elongation of the trophoblastic
villi and of the connective tissue which covers them and includes
the maternal capillaries (Fig. 147, c). The villi meanwhile branch,
the branches being thin sheets (perpendicular to the surface)
and radiating out from the original villi : the foetal blood vessels of course branch correspondingly, as do, on the other
side, the maternal connective tissue and capillaries. In the cat
certain large cells are present in between the maternal capillaries
which are possibly hypertrophied connective tissue-cells, but they may be trophoblastic (Fig. 148). The trophoblast at the
base of the placenta continues to ingest and absorb the celldebris and fat supplied by the glands up to the end of gestation.
A feature of great physiological interest is the ' green border
a system of pockets in the trophoblast along both edges of the
placenta filled with masses of extravasated maternal blood, to
which extravasation indeed the formation of the pockets is due
(Pig. 147, d). Leucocytes are present, fibrin, and a green pigment (haematochlorine), a derivative of haemoglobin ; what its
relation is to biliverdin is not known. There is also a yellow brown pigment, and, at the end of gestation, a black one. All
this material is ingested by the trophoblast. The green border
is poorly developed in the cat.
It will be evident that the placenta we have just considered
is made of a compound tissue, foetal in the trophoblast and
connective tissue and capillaries of the allantois, maternal in the
connective tissue and capillaries surrounded and engulfed by the
invading vilH.
The placentas we have still to study are not constructed on
this plan, for though they have this much in common with that
of the Carnivora, that the uterine epithelium disappears, yet they
differ wholly from it in that the maternal blood circidates not
in blood-vessels enclosed by the trophoblast but in lacunae, excavated in that tissue, into which extravasated maternal blood
is poured. No maternal tissue, therefore, is lost at birth except
the blood, apart from fragments of connective tissue adherent
to the maternal side of the placenta.
A placenta of this kind is found in the Rodents, Insectivora,
Cheiroptera, and, amongst the Primates, in Tarsius, Monkeys,
and Man. We shall begin with the Rodents.
==RODENTIA==
The placenta is always discoidal in shape, and attached to
the mesometric side of the uterus.
As an example we may take the mouse. The uterine cavity
is bounded by a columnar eiVitheUum in which fat is secreted.
Into it open glands with long necks. These secrete a coagulable,
presumably proteid, material. These secretions are absorbed by the free blastocyst (Figs. 149, A ; 150, a). There is prepared
for the reception of the blastocyst a pit on the antiinesonietric
side of the uterus. This pit lies in the middle of a pronounced
swelling, due to the hyi)ertroj)hy of the subepithelial connective
tissue and enlargement and multiplication of the blood-vessels.
To this tissue may be applied Hubrecht's term ' trophospongia '.
By it the glands are driven away towards the muscularis, their
necks stretched and eventually broken. The mouse produces
a large litter of young, and there is a correspondingly large series
of these swellings along the uterus. The pit in the middle of
each swelling is open freely to the main cavity of the uterus
(towards the mesometrium), and in each pit a blastocyst is lodged
with its embryonic pole towards the opening of the pit. It is at
this pole that the trophoblast will thicken to form the placenta,
which is therefore on the mesometric side (Figs. 149, b ; 150, b).
Fig. 148. -  Histology of the placenta of the cat. a, Earlier ; b, later
stage (full time), f.c.t., foetal connection tissue ; tr., trophoblast (pale) ;
m.c.t., maternal connective tissue (dark) ; f.h.v., foetal capillary ; m.b.v.,
maternal capillary.
Where the trophoblast touches the sides of the pit the epithelium, clothing the latter, now disappears, the cells becoming
cubical, then flat, and finally vanishing. The nuclei are resolved
into spherules of chromatin, the cytoplasm undergoes fatty
degeneration. The fat is absorbed by the trophoblast. The
trophoblast is thus brought into immediate contact with the
subepithelial tissues.
The same degradation later attacks the epithelium at the
bottom of the pit, and later still extends to that hning the
main uterine lumen above it. This lumen then disappears and
each embryonic pit is isolated, as also are the inter-embryonic
regions of the viterus. At a subsequent stage a fresh lumen will
be formed on the anti-mesometric side of the embryo, and this
re-unites the inter-embryonic regions with one another and once
more there is a continuous uterine lumen.
At the embryonic pole the trophoblast now thickens and
drives the embryonic knob towards the opposite end, so mvagmating the upper wall of the yolk-sac ; the amnion is then developed
and separated from the temporary cavity in the trophoblast as
the extra-embryonic coelom extends between the two in the
fashion already described.
This thickening is the precursor of the placenta. It extends
towards the mesometrium and is at first conical (Figs. 149, c ; 150, c), but soon becomes discoidal as the embryo in its amnion
and extra-embryonic coelom enlarge. It is in contact with the
distended uterine capillaries, and very quickly these burst and
the extra vasated maternal blood is poured into irregular spaces
or lacunae excavated in the trophoblast. Many of the haematids
are phagocytically devoured by the trophoblast (Fig. 151, 8).
The blood enters these spaces in the centre, leaves them by
a number of wide vessels at the periphery. At its base this trophoblast remains cellular, but elsewhere it becomes syncytial by the
disappearance of cell-boundaries ; the two regions have been
termed respectively cyto- and plasmodi-trophoblast. Between
the blood-vessels that supply these trophoblastic lacunae is
the subepithehal connective tissue (Fig. 151, 7), and this soon
undergoes an important modification. While some of the cells
remain unaltered -  ^fusiform or stellate in shape -  as a supporting
tissue, others become rounded and filled with globules of
glycogen. The cells, though fairly closely packed, are distinct
from one another. The nucleus is spherical, not very chromatic,
and has one nucleolus. We shall speak of this tissue as the
maternal glycogenic tissue (Fig. 151, 4). It is at about the zenith
of its development by the time the foetal blood-vessels reach
the placenta.
The future placental region of the trophoblast may be distinguished as ' allantoidean ' from the ' omphaloidean ' region,
which lies immediately against the distal wall of the yolk-sac
and therefore on the anti-mesometric side. The cells here become
enormously hypertrophied and theix nuclei correspondingly
enlarged (hence the term ' megalokaryocytes ') : in the nuclei
there are large nucleoli, and the chromatin is in irregular strings.
They are incapable of mitosis. In contact with the subepithehal
tissues they eagerly devour debris of degenerate cells, leucocytes
and the haematids, which are abundantly extravasated in this
region also (Fig. 151, 6, 9). They apparently play an important
role in the nutrition of the embryo during this stage, prior to the
development of the allantois, but later they are less important
and disappear long before the end of gestation. Presumably
the stuffs they have digested are passed on by means of the area
vasculosa of the yolk-sac to the embryo.
V)
Fig. 151. -  Histology of the placenta of the mouse.
1, Foetal capillaries (with nucleated corpuscles) lying alongside the
lacunae of the trophoblast (stage e).
2, 3, Early and late stages of glycogenesis of the trophoblast (stages E and
later).
4, Maternal glycogenic cells with intervening connective tissue-cells
(stage d).
5, Fold of epithelium on the proximal wall of the yolk-sac with bloodvessel (stage E).
6, Megalokaryocyte from the omphaloidean trophoblast. On the right
extravasated maternal corpuscles, on the left the flat epithelium of the
distal wall of the yolk-sac (stage d).
7, Closely packed maternal sub-epithelial tissue with blood-vessels
(stage c).
8, Allantoidean trophoblast, the cells ingesting maternal corpuscles
(stage c).
9, Melagokaryocyte cf the omphaloidean trophoblast ingesting corpuscles
and detritus of maternal cells (stage c).
P. 230
1
jX THE PLACENTA 237
In the next stage (Figs. 149, d ; 150, d) the allantois is developed, grows with its blood-vessels across the coelom, reaches
the so'matopleure at the base of the allantoidean trophoblast, and
sends its capillaries into the latter in between the lacunae. The
necessary relation between the foetal and maternal circulations
which constitutes a placenta is now established. Further change
is mainly one of growth.
Firmly fixed in the trophoblast the capillaries soon elongate
and branch, mostly parallel to one another and perpendicular
to the surface of the placenta. The trophoblast with its lacunae
keeps pace, and so the whole organ, attaining a thickness many
times greater than that which it originally possessed, comes
ultimately to project button-like towards the centre of the
uterus (Figs. 149, e ; 150, b). The trophoblast lining the lacunae
becomes finally much attenuated except for the protrusions due
to the rather large nuclei (Fig. 151, 1).
On the foetal side of the placenta are somewhat large lacunae
to which blood is brought by chaimels passing directly through
the centre of the placenta ; hence it passes into the smaller
lacunae round the foetal capillaries and so into the efferent
maternal vessels which leate the organ peripherally. The
capillaries of the allantois, however, never penetrate the whole
thickness of the trophoblast. On the maternal side there is
a layer, increasingly broad, between the ends of the foetal vessels
and the maternal tissues, a layer only traversed by the large
channels which lead to and from the smaller lacunae (Fig. 150, e).
In this layer the secretion of glycogen begins at the stage when
the allantois has just reached the trophoblast, and soon attains
enormous dimensions (Fig. 151, 2, 3). The whole tissue consequently appears highly vacuolated. The cells -  ^if we may
indeed speak of cell-boundaries -  are oblong, the nuclei oval,
rich in chromatin and provided with several nucleoli, thus
differing from the maternal glycogen cells. We shall speak of
this as the trophoblastic glycogenic tissue.
The previously differentiated maternal glycogenic tissue ceases
to grow further, mth the enlargement of the whole uterus the
constituent cells get separated, the glycogen cells having given
up their glycogen collapse, disintegrate, and disappear, and only
238
THE PLACENTA
IX
the supporting cells are left between the maternal blood-vessels.
Upon the space so left vacant the trophoblastic glycogen tissue
encroaches, engulfing the blood-vessels as it does so, and finally
extends as far as the muscularis.
There can be no doubt that this tissue holds in reserve a store
of food material for the use of the embryo . As sugar the glycogen
passes into the maternal vessels and into the lacunae, and so is
absorbed by the foetal capillaries. When the glycogen is used
up the cells collapse, and their collapse may be a factor in
determining the moment of parturition, since it is across this
layer that the placenta breaks away. The trophoblastic is much
more voluminous than the maternal glycogenic tissue ever was.
In the omphaloidean regions important changes have meanwhile occurred. A new lumen has been formed on the antimesometric side, placing the inter -placental portions of the
uterus once more in communication with one another. This
new lumen (Fig. 149, d, e, I'.u') is separated from the cavity of
the yolk-sac by (1) the distal wall of the yolk-sac, (2) the omphaloidean trophoblast, (3) the subepithelial tissues, and (4) the
epithelium. All these layers cease to grow, become passively
stretched, and finally rupture, disintegrate, and disappear.
The 5^olk-sac now opens freely into the uterine lumen, and the
richly folded columnar epithelium (Fig. 151, 5) of the upper wall
is able to absorb the fat and proteid material secreted by the
uterine epithelium and glands. Thus the yolk-sac acts and
continues to act till the end of gestation as an accessory organ of
nutrition. It also forms a protective envelope, since its edge
is inserted into the margin of the placenta. This edge is
l^ter carried up some little way on the outer surface of the
placenta, the base of attachment of the latter to the uterine
wall being narrowed, while at the same time the yolk-sac
is inflected on the foetal side towards the insertion of the
umbilical cord.
In a placenta of this type the foetal is only separated from
the maternal blood by the endothelium of the capillaries and the
trophoblastic lining of the lacunae, the foetal connective tissue
being in the last stages negligible. There is no penetration of
foetal tissues into maternal (except for the encroachment of
jX THE PLACENTA 239
the glycogenic tissue of the trophoblast on the space between the
maternal blood-vessels), and there is no maternal tissue in the
organ but the blood in the lacunae (except again the blood vessels in the glycogenic region). The relation between maternal
and foetal blood-streams is brought about by the fastening of
the trophoblast upon the subepithelial tissues after the destruction of the uterine epithelium ; once fixed there lacunae are
excavated in it in which extravasated maternal blood circulates,
and it is finally vascularized from the foetal side by the capillaries
of the allantois.
In the guinea-pig {Cavia) the blastocyst is placed in a pit on
the anti-mesometric side ; it comes into contact with the subepitheUal tissues by burrowing beneath the epithelium, which
is then destroyed. The original lumen of the uterus is obliterated
in the embryonic swellings ; a new lumen is formed anti-mesometrically, and the tissues between it and the upper wall of
the yolk-sac are distended and disintegrate, thereby placing the
yolk-sac in contmuity with the uterine cavity, precisely in the
way akeady described for the mouse, except that the lower wall
of the yolk-sac has never been present. The placenta is discoidal
and mesometricaUy placed ; it is developed from a thickening
of trophoblast at the embryonic pole of the blastocyst. On its
maternal side is an abundant glycogenic tissue, but whether
this is of maternal or foetal origin, or both, has not been
determined.
In the rabbit and squirrel no anti-mesometric pit is formed
for the reception of the blastocyst. In the rabbit there are
on the mesometric side two prominent folds, the placental folds,
and in the future embryonic regions these become greatly
thickened by the proliferation of the subepitheUal tissue and
blood-vessels (trophospongia). They have been termed 'cotyledons ', but the expression is here inapplicable. To these two
swellings the blastocyst attaches itself by the trophoblast behind
and at the sides of the embryonic plate ; the latter is at the
surface when Rauber's cells have disappeared, but sinks inside
when the amnion closes (Fig. 152).
The uterine epithelium, where touched by the trophoblast
now disappears, and the latter is brought into immediate contact
240
THE PLACENTA
IX
with the subepithelial tissue and blood-vessels. The blood vessels are to a very slight extent engulfed by the growing
trophoblast, but their endothelial walls soon break down and
their extravasated blood is discharged into lacunae excavated
in the trophoblast, now much thickened and syncytial (plasmodi
FiG. 162. -  Foetal membranes and placenta of the rabbit, pr.am., proamnion. Other letters as before. (Aiter Duval and Van Beneden.)
trophoblast), except at its base, where cell-bomidaries remain
(cyto-trophoblast). The allantoic capillaries then make their
way into the trophoblast and the placenta is established.
The trophoblast with its lacunae and the foetal tissues grow
pari passu ; the placenta thus increases in thickness and projects
IX
THE PLACENTA
241
into the uterine cavity. In shape it is discoidal, but made up
of two distinct halves or lobes, due to the attachment of the
trophoblast to the two enlarged placental folds.
There is a voluminous glycogenic tissue on the maternal side,
stated to be entirely of maternal origin. A good deal of it is,
however, probably trophoblastic. It has been shown that the
glycogen of the placenta increases up to the twenty-first day of
gestation, but then diminishes till the end (twenty-ninth day).
The glycogen in the foetal liver, which is at first almost negligible,
increases rapidly during the last week of pregnancy. A glycogen
splitting ferment has also been isolated from the placenta ; it
is found, too, though less active, in the overlying maternal
tissues. In the placenta, therefore, the embryo has a means of
controlUng the glycogen metabolism ; but this function is taken
on by the foetal liver towards the close of gestation. The yolksac in these forms also is an accessory organ of nutrition. The
lower wall disappears, the cells of the upper wall then absorb
material from the uterine cavity, and pass it on to the embryo
by means of the area vasculosa.
Cheiroptera
In Vespertilio there is a discoidal placenta, or rather, since it
is concave, saucer-shaped or bell-shaped (Fig. 153).
The blastocyst attaches itself by its embryonic pole to the
anti-mesometric side of the right cornu of the uterus : only one
is present at a time.
Below the epithelium the connective tissue has thickened, and
the blood-vessels have increased in number and size. The uterine
epithehum disappears, and the trophoblast then fixes itself by
invading the subepithelial tissue and engulfing some of the
superficial capUlaries. The endothelium of these capillaries then
degenerates, and they are indistinguishable from the lacunae
formed in the way with which we have become familiar in the
Rodent placenta.
The blood-vessels of the yolk-sac are at first applied to this
mass of trophoblast, but as soon as the allantois is developed
it pushes the yolk-sac away and sends its capillaries into the
trophoblast. The placenta increases in thickness by the simul
Q
242 THE PLACENTA IX
taneous growth of capillaries and lacunar troplioblast, and in
area by an extension at the edges of the same process by which
it was formed. After the first fixation there is no further penetration of the maternal by the foetal tissues.
Fig. 153.-  Foetal membranes and placenta of the bat (V espertilio).
(After Nolf.) Letters as before.
On the anti-embryonic side (mesometric) the uterme epithehum
also disappears, the fatty debris, together with that of the
underlying connective tissue, being eaten up by the trophoblast.
In Pteropus the placenta is discoidal but mesometric : the
uterine epithehum seems to disappear.
IX
THE PLACENTA
243
Insectivora
In this order the placenta is again discoidal, and usually
concave ; but in Tupaia there are two placentas, one right,
the other left, at the sides of the uterus, and in Cenietes a large
niimber. Where there is only one {Erinacells, Sorex, Taljia) it
is anti-mesometric in position.
In all cases the uterine epithelium disappears in that region
where the placenta is formed : the thickened trophoblast fastens
on the subepithelial tissues, and lacunae are formed in it ; in
Fig. 154. -  Two stages in the formation of the decidua reflexa of the
hedgehog. (After Hubrecht.) d.r., decidua reflexa. Letters as before.
these the maternal blood circulates. The whole is then vascularized from the foetal side by the allantoic capillaries.
In Erinacells, the hedgehog, the most interesting feature is
the formation of a ' decidua reflexa ' or ' capsularis ' resembling
the structure known by that name in human embryology. ^
On the anti-mesometric side of the uterus there are formed two
thick folds by the proliferation of the subepithelial vascular
tissue (trophospongia). Between these two folds the blastocyst
is lodged with its embryonic pole turned away from the mesometrium (Eig. 154, a). By the closure of the lips of the folds
1 It is highly probable, however, that the human ' reflexa ' is formed
in a different manner. (See below.)
Q 2
244
THE PLACENTA
IX
and obliteration of the cavity in front and behind this point
the blastocyst is securely shut up in a coat of maternal tissue,
the ' decidua reflexa ' (Fig. 154, b). The whole of the trophoblast
now thickens enormously, becomes syncytial, destroys and
devours the epithelium lining the cavity which lodges it, while
into the lacunae hollowed out in it quantities of maternal blood
are soon discharged from the adjacent swollen capillaries. The
-am.c.
Fig. 155. -  Foetal membranes and placenta of the hedgehog. (After
Hubrecht.) l.u., lumen uteri ; d.r., decidua reflexa. Other letters as before.
yolk-sac and omphaloidean trophoblast, against which its lower
wall lies, are at the anti-embryonic end, that is, towards the
covering ' decidua reflexa ', while towards the opposite end the
allantois grows out and reaches the ' allantoidean ' trophoblast.
It is from this part that the placenta is formed (Fig. 155), the
foetal capiUaries being driven into the trophoblast between the
lacunae. The whole grows in thickness.
The ' deciduofracts ' are phagocytic trophoblastic cells which
eat up the maternal tissues adjoining the placenta.
In the omphaloidean region relations are at first established
IX
THE PLACENTA
245
between the yolk-sac and the trophoblast with its lacunae. But
as the allantoic placenta becomes increasingly functional the
yolk-sac dwindles in importance and is folded up under the
' decidua reflexa '. By the extension of the uterine cavity round
Fig. 156. -  Foetal membranes and placenta of the shrew {Sorex). (After
Hubrecht.) x, point where the omphaloidean trophoblast is in contact
with the maternal tissues ; tr.an., trophoblastic annvdus, or thickening of
trophoblast below x.
the base of the placenta the ' reflexa ' is enlarged, and surrounds
the embryo on all sides except at the placenta. It becomes
stretched, and the trophoblast beneath it much attenuated.
In the toole {Talpa) the uterine epitheUum is also destroyed
on the mesometric (non-placental) side ; the trophoblast comes
into immediate contact with the subepithelial tissues. At birth
246
THE PLACENTA
IX
the allantoic capillaries are pulled out of the placental trophoblast, which remains behind to be absorbed by the leucocytes of
the mother. This arrangement is known as ' contra-deciduate '.
In Sorex (Fig. 156) there is, prior to the attachment of the
trophoblast in the placental region, a conspicuous proliferation
of the uterine epithelium with concomitant development of crypts
between the glands on the anti-mesometric side. Into these the
syncytial trophoblast makes its way, and then the epithelium is
destroyed. The further stages in the development of the placenta
are similar to those occurring in other forms.
Laterally there are also independent proliferations of the
uterine epithelium to which the trophoblast becomes attached.
The fused maternal and foetal tissues afterwards degenerate
together and are dehisced from the wall ; the continuity of the
uterine lumen is then restored. The area vasculosa of the yolksac which had been applied to this region is at the same time
detached. Further towards the anti-embryonic polp there is an
annular thickening of the trophoblast. The cells are here phagocytic and ingest quantities of extra vasated maternal haematids.
Digestion of these presumably takes place in the trophoblast,
since a bright-green pigment (? haemoglobin derivative) fills the
yolk-sac. The iron would then be carried o£E by the blood vessels of the area vasculosa. At the anti-embryonic pole the
trophoblast is thin and not attached to the uterus ; here the
epithelium persists.
In Tupaia the yolk-sac, which has at first relations with the
placental regions of the trophoblast, is later displaced by the
allantois.
Tarsiits, Monkeys, and Man
As we have already had occasion to see, the aberrant Lemur
Tarsius agrees with Monkeys and the human being in the possession of a diminutive yolk-sac (provided, nevertheless, with an area
vasculosa), a large and precociously developed extra-embryonic
coelom, and a rudimentary allantois which only extends far
enough outside the body of the embryo to penetrate the base
of the ventral or body-stalk, which connects the embryo in its
amnion and with its yolk-sac to the wall of the blastocyst and
IX
THE PLACENTA
247
is to be developed into the umbilical cord. Such an arrangement
of the foetal membranes is found nowhere else amongst the
Mammalia. We have now to inquire whether in the origin and
minute structure of the placenta there is an equally complete
agreement.
Fig. 157. -  Foetal membranes and placenta of Tarsius. (After Hubrecht.)
Letters as above.
In Tarsius alone is the complete history of the placenta kno^vn,
and there is no doubt whatever here at any rate that the placenta
is of that type which prevails in Rodents, Insectivores, and
Cheiroptera. In form it is discoidal, or rather button-shaped,
protruding into the uterine cavity ; it is developed at the antiembryonic pole of the blastocyst, and is placed on the mesometric side of the uterus (Fig. 157). Here there is, prior to
fixation, a ' trophospongia ' or area of proliferating connective
248
THE PLACENTA
IX
tissue and enlarged blood-vessels, and with this the placental
trophoblast comes in contact as soon as, under its influence, the
epithelium has been destroyed. Firmly fixed here, the tropho
blast becomes hollowed out by lacunae, in which maternal blood
circulates and is invaded from the other side by the foetal
capillaries. The whole then grows into the lumen of the uterus
until the complete thickness of the placenta is attained. An
interesting feature is the conversion of much of the trophoblast
into * megalokaryocytes ', large cells with enormous nuclei containing big nucleoli, similar to those seen in the omphaloidean
trophoblast of the mouse.
Unfortunately, we have no such thorough knowledge of the
genesis of the placenta of Man and Apes, but the structure of
the fully formed organ is known, and such early stages as have
been described are comparable, without difficulty, with stages
in the development of such placentas as those of Tarsius,
Insectivores, and so on.
When completed, the placenta is discoidal in shape. Amongst
the Platyrhine (New World) Monkeys it is double in Cebus,
single (occasionally double) in Mycetes. The two placentas are
placed respectively on the dorsal and ventral walls of the utenis,
and are connected, of course, by blood-vessels. Where only one
is present it is ventral, but there is on the dorsal wall a placentoid -  a thickened region of widened blood-vessels -  as though
for the reception of a second placenta.
In the Catarrhines (Old World tailed Monkeys) there are
usually two placentas, dorsal and ventral, as in Semnopithecus
and Gercocebus (Macacus) (Fig. 136), but one (the ventral) may
be absent. Either of the two may be the primary one. The
umbilical cord in Gercocebus passes to the ventral placenta,
whence blood-vessels travel to the other.
In the Simiidae {Hylobates, the gibbon) and Simia (the orang)
and in Man there is but a single discoidal placenta, placed in
the two Apes on the anterior (ventral) waU of the uterus, in Man
usually on the posterior wall, though the position is variable.
Further, in these forms the blastocyst or chorionic sac is always
embedded in maternal tissue which forms, between it and the
lumen uteri, a layer known as the ' decidua reflexa ', or, in more
IX THE PLACENTA 249
modern parlance, the ' capsularis ' (Fig. 158). What has been
regarded as a precursor of this structure-  a ridge running round
the placenta -  has been observed in Mycetes and Cercocebus.
Human anatomists distinguish from the ' decidua reflexa ' or
' capsularis ' that maternal tissue to which the placenta is
attached as ' decidua serotina ' or ' basalis while the opposite
Fig. 158. -  ^Human foetal membranes and placenta. (After Balfour, after
Longet.) The amniotic cavity (am.c.) has enlarged and occupies nearly
the whole of the extra-embryonic coelom (c), the amnion being reflected over
the umbilical cord (u.c.) and yolk-sac (y.s.). d.b., decidua basalis (serotina) ;
d.r., decidua capsvdaris (reflexa) ; d.v., decidua vera ; l.u., lumen uteri ;
am., amnion ; pi., placenta ; o.d., oviduct.
wall of the uterus is known as the ' decidua vera '. The application of the term ' decidua ' to maternal tissues has already been
alluded to ; it dates from the time when the type of placenta
we are considering was supposed to include, and carry away at
parturition, a considerable portion of the uterine wall.
Structurally all these placentas resemble one another very
closely. The maternal blood circulates in large spaces known
as sinuses, which are supplied by the blood-vessels of the uterine
250
THE PLACENTA
IX
wall (the decidua serotina or basalis in the Simiidae and
in Man) (Fig. 158*). These sinuses are lined everywhere-  not
only over the foetal blood-vessels, but also on the maternal and
on the foetal sides -  by a syncytial layer, usually referred to as
the syncytium, below which is a layer of cells-  the cell-layer of
Langhans of human embryology. These two layers separate the
maternal blood in the sinuses from the foetal connective tissue
and blood-capillaries (Figs. 159, 160). The more usual way,
Fig. 158*. -  Diagram of the structure of the human placenta, m.b.v.,
maternal blood-vessels in the decidua basalis {d.b.) opening into the sinuses
of the placenta (s) in which the villi branch. The villi are covered and
the sinuses lined on all sides by trophoblast (ir.) (syncytial layer and cell
layer of Langhans). am., epithelium (ectoderm) of the amnion.
perhaps, of describing this arrangement is to say that the foetal
villi -  meaning by that the. capillaries, and coimective tissue and
the cell-layer and syncytium covering them -  branch about in
sinuses filled with maternal blood. The expression ' villi ' dates,
however, from the older conception of the origin of these structures from villi similar to those seen in an Ungulate, a conception
which is almost certainly erroneous. The foetal capillaries do
branch very considerably it is true, but the sjmcjTtium and celllayer are continued over the outer walls of the sinuses, next the
tissues of the serotina. The sinuses, in fact, are lined everywhere
by these two layers.
5.
<^'L -  /
4»
â– (v'
sy
A
r. 1^,
â– 7>
Fig. 160. -  Structure of the insertion of
a ' villus ' into the ' decidua basalis ' of
the human placenta. d.b., large cells
(' decidual cells ') of the basalis ; m.b.v.,
maternal blood-vessels ; s., sinus of the
placenta ; si/., syncytial layer, and c, celllayer covering villus ; f.h.v., foetal bloodvessel in villus ; c'., mass of vacuolated
cells continuous with the cell-layer and
covering the extremity of the villus. The
fat globules in the syncytium are rendered
in black.
A, Large glycogen cells from tlia
maternal side of the human placenta
(5 months).
Fig. 159.-  Middle strip of a section
through the middle of the human placenta
at 5 months, d.b., decidua basalis ; v'.,
-  ua. "^i'li inserted into basalis; s., sinus; v.,
villi in sinus ; f.b.v., foetal vessel in villus ;
- urn umbilical vein ; w.a., umbilical artery ;
am., epithelium of amnion.
The syncjiiium and cell-layer covering the villi and lining the
inuses are stippled. Notice that this cell-layer is found between
he end of the villus and the maternal tissue of the basalis.
200
jX THE PLACENTA 251
Further, the cell-layer at the outer extremities of the villi is
continued into a mass of cells which separate the villus from
the tissue of the decidua basalis. These cells are vacuolated,
containing glj^cogen.
In Mycetes there is a syncytial network between the ' villi
cutting up the sinuses into smaller cavities (? lacunae) : there
is no cell-layer.
In the human placenta the sjmcytium contains fat ; in late
stages the cell-boundaries vanish in the layer of Langhans also.
On the maternal side of the placenta in the ' basalis ' there
are in man, Simia, Hylobates, and the Catarrhines, enlarged
coimective tissue-cells, known as ' decidual ' cells (Fig. 160).
These decidual cells get intermingled with the masses of cells
which, continuous with the layer of Langhans, cover the outer
extremities of the villi and contain glycogen, the two together
being disposed in a sheet known as the chorio-basahs. In
Simia and in man there are also septa, that is, peninsulae of
basalis tissue projecting into the placenta proper.
In man the layer of the basalis next the placenta is known
as the compacta. In this are the necks of glands. As gestation proceeds the epithelial lining of these glands degenerates,
the inter -glandular tissue undergoes a fibrinous degeneration,
and there are extravasations of blood in between these cells and
into the glands. Similar extravasations occur in Hylobates and
Simia. The whole layer becomes stretched and thinned. Beyond
the compacta is the spongiosa, a layer of maternal tissue
in which the gland -necks are much enlarged. There is slight
degeneration here also. A spongiosa is found in Simia, but
not in Hylobates.
In the lower Monkeys which possess no decidua capsularis
the chorion is smooth except in the placental region or regions,
but in Hylobates, Simia, and Man the chorion which is covered
by the capsularis is in an early stage produced into ' villi '
(which become poorly vascularized), as well as that opposite
the basalis. Later the ' villi ' disappear, and this part of the
chorion is then, to use an old term, the ' chorion laeve ', as
distinct from the ' chorion frondosum ' of the placenta.
The capsularis is covered by a cubical epithelium (Fig. 158).
262
THE PLACENTA
IX
In it, at the sides only, are a few glands with openings
into the lumen uteri. There are blood-vessels and extravasations. The whole layer gets distended by the growth
of the embryo and eventually its tissues wholly degenerate.;
the chorion is then immediately apposed to and united with
the vera on the opposite side, and the uterine cavity Is
m.h.v. dJ>. tr.
Fig. 161. -  Early human embryo with its membranes. (After Pet«rs.)
d.h., decidua basalis (serotina); d.r.ep., uterine epithelium covering the
decidua reflexa or capsularis ; I., lacuna in trophoblast (tr.) ; gl., uterine
gland ; m.b.v., maternal blood-vessels opening here and there into lacunae ;
cl., clot marking (probably) the point of entrance of the blastocyst ; here
the epithelium is interrupted. Other letters as before.
obliterated. Only in the condition known as placenta reflexalis
does the maternal circulation continue on this side.
In the decidua vera the epithelium ultimately disappears,
the compacta is stretched and attenuated, and there are
slight degenerative changes.
Such is the structure of the placenta in Man and Apes. We
have still to discuss the mode of formation of the capsularis
and the nature of the ' villi ' and ' sinuses '.
<3&
<1SS
®
5y.
- c
Fig. 162.-  a small portion of the wall of the embryonic sac-  on the side
of the decidua basalis-  of the human embryo shown in the last figure. (After
Peters.) d.b., maternal connective tissue of decidua basalis ; end., endothelium of maternal capillary (m.c), opening into lacuna (/.) ; sy-^ sjaicytium (plasmodi-trophoblast) ; tr., cellular trophoblast (cyto-trophoblast) :
the syncytium is pale, the cyto-trophoblast more deeply staimng ; m.,
somatopleure lining the extra-embryonic coelom (c).
V. 253
IX
THE PLACENTA
253
In the hedgehog and mouse Ave have seen the blastocyst
embedded in a pit in the uterine wall, in which it becomes
securely enclosed. The pit is lined by a continuation of the
uterine epithelium, which, however, soon disappears. The
relation of the blastocyst to the enveloping maternal tissues is
then very similar to the relation between the human chorionic
sac and its capsularis, and it has not unnaturally been suggested that the latter is really developed in the same way.
There is, however, knoisTi to us another way by which the
mammalian blastocyst may come into contact with subepithelial
tissue, for, as we have seen, the blastocyst of the guinea-pig
bores its way through the epithelium. In the earliest human
embryos knoAvn to us -  those described by Peters and Bryce ^ - 
there are very strong indications that the human capsularis
is. formed in this way, for in both cases there is in the centre of
this layer an interruption in the continuity of the epithelium,
marked, in Peters's case, by a blood-clot (Fig. 161). This
would then be the spot where the ovum effected its entrance.
If so then there can never have been any uterine epithelium on
the other side of the blastocyst, the side of the basalis where
the placenta is developed. This should be borne in mind in
considering the next question, the origin of the ' villi ' and
' sinuses '. In the embryos described by Peters and Bryce the
somatopleuric wall of the extra-embryonic coelom is covered
on the outside by a layer of cubical cells. Next to and perfectly
continuous with this layer is a thick mass, composed of similar
but polyhedral cells or in some places of a sjnacytium, with nuclei
similar to those of the cellular tissue. In this mass are lacunae,
and in these are maternal blood-corpuscles (Fig. 162). Outside
all this is the subepithelial connective tissue of the uterus, with
glands and blood-vessels : the latter open into the lacunae.
There can be no reasonable doubt that the whole of this
lacunated mass, with a basal cellular layer next the somatopleure, is the trophoblast, which has become thickened and
^ The embryo described by Bryce is perhaps shghtly the younger of the
two, as the extra-embryonic coelom appears to be not yet ])roperly formed.
That described by Peters was, however, obtained mi silii in the uterus, and
so gives us more information as to the relation between the foetal and
maternal tissues.
254 THE PLACENTA ix
hollowed out for the reception of extravasated maternal blood.
We have seen this occurring in a part only of the trophoblasl^
as in the mouse-  or in the whole of it-  as in the hedgehog,
and there is no reason why any other interpretation should be
put upon this stage in human ontogeny. The steps in its formation have, however, not yet been observed. The comparative
anatomy of the placenta has taught us that in cases of this kind
the necessary relation between foetal and maternal blood-streams
is brought about by the penetration of the allantoic capillaries
into the trophoblast. Exactly the same process occurs presumably
in the human being : the embryonic blood-vessels, with their
surrounding connective tissue, make their way into the trophoblast between the lacunae. There they branch repeatedly and
become the ' villi ' of the completed placenta, while the sinuses
are the transformed lacunae. The syncytial and cellular layers
lining the sinuses and covering the villi are then both trophoblastic in origin, and similar to the plasmodi-trophoblast and
cy to -trophoblast seen in other placentas of this type, in the
mouse for example ; they may be derived respectively from
the outer and inner layers of trophoblast in the early stage. It is
now possible to understand why the sinuses are Uned throughout,
on the maternal side against the basahs, as well as over the
' villi ' and on the foetal side, by the syncytium and cell-layer,
and why the outer extremities of the villi are separated from the
decidual cells of the basahs by the cell-masses continuous
with the layer of Langhans.
If this interpretation is correct then such hypotheses as that
the sinuses are enlarged veins and the syncytium the endothehum of those veins, or that the sync3rtium is derived from
uterine epitheHum, must evidently be discarded ; the second of
these views is indeed already ruled out of cotu-t if the implantation of the blastocyst is really effected in the way we have
suggested, for there could be in that case no uterine epithehum
on the side of the decidua basahs.
Such views as these date from the period when it was beheved
that the human, like other ' deciduate ' placentas, was derived
from the condition found in Ungulates by a simple adherence
of the villi to the crypt-walls ; and this beUef was supported by the existence of ' chorionic villi that is, branching processes
of the trophoblast, all over the outer surface of the early blastocyst, before the foetal vessels had appeared. But these ' chorionic vilh ' Avere observed only in blastocysts removed violently,
perhaps after post-mortem changes, from the sac of the capsularis,
and a proper histological examination of foetal and maternal
tissues together has revealed their true nature ; they are not
' vilh' or processes plungmg into maternal tissues, but the irregular
walls between the lacunae excavated in a thickened trophoblast.
The placenta of Man and Monkeys is, then, of the same kind as
that seen in Tarsius, and in Rodents, Insectivora, and Cheiroptera,
It contains no maternal tissue except the blood circulating
in the sinuses or enlarged trophoblastic lacunae, and, in addition
to the blood, no maternal tissue is lost at birth beyond the thin
layer of the degenerate compacta -  in both the deciduae basalis
and vera -  across which the break occurs, and such septa as
may have forced their way into the placenta.
We may now briefly review the genesis of the Mammahan
placenta in its varied types.
In Marsupials the placenta is wholly dissimilar from anything
met with elsewhere, since the trophoblast degenerates while the
syncytium is of uterine epithelial origin.
The Ungulates possess a typical Indeciduate placenta, with
villi dipping loosely into crjrpts hned by a persistent epitheUum,
from which they may be readUy withdrawn without injury to
the maternal tissues. Haemorrhage from the uterine blood vessels does, however, occur during gestation, and is of physiological importance in foetal nutrition.
The placenta is similar in Cetacea, Sirenia, and in the Lemuroidea (except Tarsius).
In the Proboscidea these haemorrhages are perhaps more
extensive.
In the Camivora the conditions are different, for here the
trophoblast does not send vilh into specially prepared crypts,
but, after the destruction of the uterine epitheUum, eats its way
into the tissues, engulfing the maternal capillaries. These and
the surrounding connective tissue grow pari passu with the trophoblast to produce the full thickness of the placenta. The
foetal capillaries grow into the trophoblast. The placenta is
therefore compounded of foetal and maternal tissues.
In the remaining orders this is no longer the case, for, after
the destruction of the epithelium, the trophoblast merely fastens
on to the underlying tissues ; only occasionally are the immediately adjacent capillaries engulfed (in the rabbit and in the
bat), and even here their endothelium soon vanishes. Once fixed
to the uterine wall the trophoblast grows not into the wall but
from it towards the centre of the uterus, receiving into its
lacunae the stream of maternal blood ; from the other side it
is vascularized by the allantois.
But distinct though these three types of placentation are, it
is yet possible that the third might have been derived from the
second -  if we inxagine the centripetal growth of the trophoblast
to occur before the ingrowth into the maternal tissues has taken
place -  and the shght-' enclosure of maternal capillaries in the bat
and rabbit almost demonstrates the change, while the insertion
of the trophoblast into the newly formed cr3rpts in Sorex recalls
another Carnivorous character. The second, in turn, may have
sprung from the first by the suppression of the uterine epitheUum.
These, however, are mere speculations, for which alternative
hypotheses may without difficulty be substituted.
One other point requires brief consideration. It has been held
that the characters of the placenta are a valuable criterion of
genetic relationship, and may accordingly be used for classificatory purposes. Now while it must be pointed out that single
characters in regard either to the gross or the minute anatomy
cannot be employed legitimately in this way -  there is no justification, for example, in grouping together the elephant, Hyrax,
the Sirenia, Orycteropus, and the Carnivora, because they all
possess a zonary placenta, nor on the other hand do we beUeve
it is yet proposed to separate the Lemuroid Primates, with their
typically Indeciduate, from the Anthropoids, with their Deciduate
placenta -  yet a combination of characters is often found to be
a constant mark of a natural order -  for instance, the large
yolk-sac with its lower Avail lost and the mesometric discoidal
placenta of Rodents, the zonary shape of the (histologically) peculiar placenta of Carnivora-  £vnd it is for this reason that
we hold that the remarkable structure of its foetal membranes
and its placenta entitle Tarsius to be separated from the Lemurs
and ranked with Monkeys and Man.
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