Book - Text-Book of the Embryology of Man and Mammals 12
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Foetal Membranes of Mammals
IN their early stages of development the foetal membranes of Mammals present an extraordinary correspondence with those of .Reptiles and Birds (fig. 129). We find a yolk-sac (UV) with abundant capillaries, an amnion (am), a serous membrane or serosa (sz), and an allantois (ALC) ; we find that, in the same way as before, the embryo is developed out of a small region of the blastula, and is constricted off in the same way from the extra-embryonic area, with which it remains united only by means of a dermal and intestinal yolk-stalk.
The correspondence becomes a striking one and stimulates to further reflection, when we take into consideration that the developmental processes enumerated are primarily evoked by means of the accumulation of yolk-material in the eggs of Reptiles and Birds, and that the eggs of most Mammals lack almost entirely the yolk, are of very small size, undergo total segmentation, and in all these respects resemble more the eggs of Amphioxus.
Why, then, does the mammalian germ nevertheless undergo metamorphoses which in other cases are only the result of the accumulation of yolk ? Why is there developed a yolk-sac that contains no yolk, with a system of blood-vessels that is designed for the resorption of yolk ?
For the explanation of these conditions we must have recourse to an hypothesis which can be formulated about as follows :
The Mammalia must have descended from animals which possessed large eggs with abundant yolk, which were oviparous, and in which consequently the embryonic membranes were developed in the same way as in Reptiles and Birds. The loss of the yolk-contents from the eggs of these animals must have been a supplementary event, which began at the time when the eggs were no longer deposited outside, but were developed in the uterus. For by this change there was found a new and more productive, because unlimited, source of nourishment for the developing germ in substances which were secreted by the walls of the uterus from the maternal blood. There was therefore no more need of a dower of yolk. But the enveloping structures, which were originally called into existence by the presence of yolk-contents in the eggs, were retained, because they were still of use in many other relations, and because, through a change of function, they became subservient to uterine nourishment and correspondingly underwent changes.
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Fig. 129. Diagram of the foetal membranes of a Mammal, after TUKNER.
pc, Zona pellucida with villi (prochorion) ; sz, serous membrane ; E, outer germ-layer of the embryo ; am, amnion ; AC, amniotic cavity ; M, middle germ-layer of the embryo ; H, inner germ-layer of the same ; UV, yolk-sac (vesica umbilicalis) ; ALC, allantoic cavity ; al, allantois.
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Three facts can be cited in favour of this hypothesis.
In the first place, in the lowest classes of Mammals, as in the Monotremes and Marsupials, the eggs are larger than in placenta! animals. They are characterised by a large quantity of yolk, which, as in Ornithorhynchus for instance, is deposited in closely compacted spheres of varying size and fat-like lustre. In this particular they form a transition to the eggs of Reptiles and Birds.
Secondly, it has been observed that the Monotremes, the lowest division of the Mammalia, are oviparous, like Birds and Reptiles. Quite recently two investigators, HAACKE and CALDWELL, have made the interesting discovery that Echidna and Ornithorhynchus, instead of giving birth to living young, as was hitherto assumed, lay eggs which are nearly two centimetres in diameter, and enveloped in a parchment-like shell, and which they carry about with them in their brood-pouch or mammary pocket.
Thirdly, the foetal membranes of Marsupials, which next to the Monotremes are to be considered as the lowest Mammals, remain permanently in a condition which corresponds to that of Reptiles and Birds, although the development takes place in the uterus. As we know through OWEN, the embryo, which is enclosed in a capacious amnion, possesses a very large vascular yolk-sac, which extends out to the serosa, and in addition a small allantois and a serosa. The latter lies closely applied to the walls of the uterus, but without being intimately united with it. Probably, therefore, after resorption of the yolk, substances which have been secreted by the uterus are taken up by the blood-capillaries of the yolk-sac. Thus a kind of intra-uterine nutrition begins to be established in the Marsupials ; but otherwise the embryo with its envelopes lies in the cavity of the uterus, like the Avian or Reptilian embryo with its membranes in the firm egg-shell.
Having established the hypothesis, already expressed by various authors, that the eggs of Mammals must originally have contained more yolk, let us turn to a more exact description of the foetal membranes. As regards the first stages of development, let us begin with the Rabbit, because its embryology has been the most thoroughly investigated; then, in order to facilitate our understanding of the structure of the human placenta, we shall show in a brief sketch how, in the class of Mammalia, in various ways more intimate anatomical and physiological relations are developed between the mucous membrane of the uterus and the embryonic membranes. We shall treat of the foetal membranes of Man in a special chapter.
When, in the Rabbit, the ovum, which has reached the uterus, has here become metamorphosed into the blastula already described, it is still enveloped by the zona pellucida. This in the meanwhile has been distended into a thin pellicle (prochorion), which is subsequently destroyed.
The blastula, or blastodermic vesicle, expands rapidly, and from the fifth to the seventh day grows from 1-5 mm. to 5 mm. in diameter.
In consequence of this increase in size the prochorion on the seventh and eighth days is so closely applied to the inner surface of the uterus that it becomes more and more difficult, and finally impossible, to detach the eggs without injury. For by the rupturing of the prochorion, which adheres to the walls of the uterus, the delicate blastula, which is in close contact with it, generally becomes injured and torn open, and thereupon collapses, owing to the escape of its contents. The latter have also suffered changes which make the investigation more difficult, having increased in consistency until they equal in density the albumen of the Hen's egg.
During the process of attaching itself, the embryonic fundament, which at first is round, increases in size and takes on a more elongated form. On the seventh day it becomes oval (fig. 130 ag), then pear-shaped, and on the eighth day acquires a more and more marked sole-like form; meanwhile it grows to a length of about 3 - 5 mm. (fig. 131).
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Fig. 130. Embryonic fundament of the ovum of a Rabbit of seven days, from KOLLIKER. o, Vascular area (area opaca) ; ay, embryonic fundament ; pr, primitive streak ; rf, dorsal furrow.
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As has been already described in the previous chapter, at this time the middle germ-layer spreads out in the embryonic fundament, the medullary groove (figs. 130 and 131 rf), the chorda, and a number of primitive segments are formed, and, on the eighth day, the first trace of the vessels and blood appears in the vascular area (o). On the ninth and tenth days the embryonic fundament is by a process of folding converted into the body of the embryo, and is constricted off from the remaining part of the blastodermic vesicle, out of which at the same time various foetal membranes begin to be developed. The initial stages of all these processes are the same in Mammals as in Birds and Reptiles, so that we can express ourselves very briefly in describing them. We shall connect the description with the diagrammatic drawings which KOLLIKER has made, and which have found a place in many text-books (fig. 132, 1-5).
Diagram 1 shows a blastodermic vesicle which in the Rabbit would correspond to about the seventh or eighth clay. It is still enclosed from without by the very much attenuated vitelline membrane (cZ), which is now also called prochorion, since in many Mammals flakes and shreds of albumen have been precipitated on its outer surface out of the fluid secreted by the mucous membrane of the uterus. The inner germ-layer (i) which in a slightly
younger blastula, such as is represented in figure 62 B, reaches only to the line ye, and still leaves uncovered a third of the inner surface of the sphere has now entirely grown around to the vegetative pole. The middle germ-layer (m) is in full process of development, and embraces about a fourth part of the surface of the sphere. A small portion of this three-layered region contains the embryonic fundament, which would be in about that stage of development which we have
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Fig. 131. Embryonic fundament of a Rabbit of nine days with a portion of the area pellucida, from KOLLIKER.
Ap, Area pellucida ; ao, area opaca ; h', h", h'", medullary plate in the region of the first, second, and third cerebral vesicles ; stz, stem-zone (Stammzone) ; pz, parietal zone ; rf, dorsal furrow ; pr, primitive streak.
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Fig 132. Five diagrammatic figures illustrating the development of the foetal egg-membranes of a Mammal, after KOLLIKER.
In figures 1 to 4 the embryo is represented in longitudinal section.
(1) Ovum with zona pellucida, blastula, embryonic area, and embryonic fundament.
(2) Ovum in which the yolk-sac and the arnnion are beginning to develop.
(3) Ovum in which, by the fusion of the amniotic folds, the amuiotic sac and the serous membrane are formed, and the allantois makes its appearance.
(4) Ovum with serous membrane, which has developed villi, with a large allantois and an embryo, in which the oral and anal openings have arisen.
(5) Diagrammatic representation of a young human ovum, in which the vascular layer of the allantois has become applied to the serous membrane on all sides, and has grown into its villi. The serous membrane from this time forward takes the name of ohoriou. The cavity before us in the surface- view in figure 130. It is ovate, and shows the primitive streak (JJT) in the posterior half, and in front of it a deep dorsal furrow (rf) ; the extra-embryonic part of the middle germ-layer can be designated as the vascular area (o), since the first traces of the formation of the vessels and the blood are noticeable in it.
In the much further developed embryo figured in diagram 2 (at about the ninth day in the Rabbit) the middle germ-layer has spread out over about the third part of the blastula, and now encloses an easily distinguishable body-cavity, since the parietal and visceral middle layers have separated from each other in the embryonic as well as extra-embryonic regions. It extends as far as the place marked st, where the sinus fcerminalis is found as the outer limit of the now clearly denned vascular area.
The embryonic fundament is in the act of bring constricted off from the blastodermic vesicle. The head- and tail-ends of the embryo, by foldings of the separate layers, have been elevated from the area pellucida in the same way as in the Chick. As there, a cephalic and pelvic part of the intestinal tract (fore and hind gut) have arisen, with an anterior and posterior intestinal portal, which open toward the cavity of the blastodermic vesicle.
At the same time occms the development of the amnion, which was first recognised in the Mammalia by BAER and BiSCHOFF. On the diagrammatic section one sees that the extra-embryonic body-cavity has become very capacious, in that the outer germ-layer with the closely applied parietal middle layer has risen up in the vicinity of the embryo and formed itself into the folds ks and ss. The anterior fold of the amnion (ks) has bent over the head, and the posterior fold (ss) over the tail. The two sheaths lie so close to the embryo in the Mammalia, that in looking from the surface they are not easily recognised, especially as they are extraordinarily. transparent.
On the third diagram the anmiotic folds have greatly enlarged, and have grown toward each other over the back of the embryo till their of the ailantois has diminished aud the yolk-sac has become very small, but the amniotic cavity is in the act of increasing.
</, Vitelliue membrane (zoiia pellucida) ; d', villi of the same; sh, serous membrane [serosaj , ch, chorion ; ch.z, villi of the chorion ; am, aninion ; L >-, ss, cephalic and caudal folds of the amnion ; a, outer germ-layer ; a', the same in the extra-embryonic region of the blastula ; ,,,, middle germ-layer ; ,,J , the same in the extra-embryonic region ; dd, inner germ-layci ; /, the same in the extra-embryonic region ; df, vascular area ; at, sinus terrninalis ; kh, cavity of the blastula, which later on becomes the cavity of the yolk-sac (ds) ; dy, stalk of the yolkaac (vitelline duct) ; ai, allantois ; <, embryo ; .-, space between chorion ;ind amnion. extraembryonic part of the body-cavity, rilled with albuminous fluid ; i-l, ventral body- wall ; Jih, pericardia! cavity.
edges are in mutual contact. The closure of the sac takes place in a somewhat different manner from that of the Chick. Instead of meeting in a longitudinal suture, the edges of the amniotic folds meet, in the Rabbit at least, approximately in the middle of the back in a small spot, where for a considerable time a circular opening in the sac is retained. The outer layer of the amniotic fold, which in diagram 3 is still in connection with the amniotic sac at the point of fusion, but which later entirely separates from it, represents, as in the Chick, the serosa. It first appears as an independent structure in the vicinity of the embryo, whereas farther downwards it is still firmly united with the entoblast, and together with it constitutes the wall of the original blastula, which is here only two-layered.
In the third diagram, furthermore, we can recognise the first trace of the allantois (), which grows out from the anterior w r all of the hind gut in the manner already described (p. 217), and which in the Rabbit is seen as early as the ninth day in the form of a small, pedunculated, exceedingly vascular sac.
The fourth diagram shows the development of the foetal membranes much further advanced. The prochorion has become ruptured by the distension of the entire blastoderrnic vesicle, and is no longer recognisable as a separate membrane. What we see on the outside is the serosa, which has been changed in a striking manner. In the first place, it has become completely detached from the amnion ; however, it should be remarked in this connection that in certain Mammals, and especially in Man, a stalk uniting the two membranes is retained for a considerable time at the amniotic suture. Secondly, the serosa is everywhere separated from the yolk-sac, and loosely surrounds the embryo and its remaining membranes as a thin sac. This condition has been brought about in the following manner : the middle germ -layer, which in diagram 3 had grown over only one half of the original blastula, has now spread over the other half also, and has become divided into its t\vo layers. By this means the extraembryonic part of the blastula is now completely split, as in the Chick, into an outer sac, the serosa, and the yolk-sac, separated from it only by the body-cavity.
Moreover, there exist in this respect differences among the Mammalia, since in some the serosa remains to a greater or less extent permanently united with the yolk-sac. This is the case, for example, in the Rabbit.
In the Rabbit, in which the yolk-sac at first fills the greater part of the blastoclerrnic vesicle, the middle germ-layer spreads out over that half of the yolk-sac only which is turned toward the embryo. There is developed in it a system of capillaries, which ends abruptly in a marginal vein. The other half of the yolk-sac is without vessels, and is everywhere firmly united with the serosa. When, after the resorption of its contents, the yolk-sac commences to shrivel, it begins to take on a mushroom-like form (fig. 133 ds), owing to the folding i n o f the vascular half (fd) against the non-vascular part (ed"), which is fused with the serosa (*//). It remains united with the umbilicus of the embryo by means of an elongated intestinal stalk (or f / . vitelline duct), which is comparable to the stalk of the mushroom.
The space (;) which is produced in the blastodermic vesicle by the shrinking of the yolk-sac does not become filled out by compensating growths of the amnion (a) and allantois (al), both <>f which remain small. Therefore a large amount of fluid collects between the separate foetal membranes. The space filled with fluid is none other than the extra-embryonic part of the body-cavity, winch in the Rabbit, as in no other Mammal, is highly developed. The allantois (al) hangs freely in this space as a stalked vesicle, a part of its surface having applied itself to that portion of the serosa (.?//) which is not united with the yolk-sac, and which is circumscribed by the sinus terminalis (st). It is gradually
metamorphosed into an organ of nutrition for the embryo, the placenta inasmuch as it receives a rich supply of blood through the vessels of the allantois, the umbilical vessels.
Subsequently the remaining surface of the blastodermic vesicle, over which the umbilical vessels do not extend, also becomes' vascular. This is due to the fact that the albuminous fluid still contained in the mushroom-like yolk-sac becomes entirely absorbed, and that consequently its outer non-vascular and inner, invaginated vascular walls come to lie on each other and to fuse into a single membrane. In this manner the blastodermic vesicle in the Rabbit becomes provided with blood on its entire surface, but from two different sides the placental portion from the vessels of the allantois, and the larger part of the surface from the degenerating vitelline vessels.
In regard to the formation of the amnion in the Rabbit, upon which VAN BENEDEN ET JULIN have made very thorough investigations, it is to be added that the middle germ-layer is wanting in the region of the anterior amniotic fold to a greater degree in this case than in the Chick. The anterior amniotic fold therefore consists during a considerable period of only the two primitive germlayers, closely joined together. VAN BENEDEN has therefore given to the cephalic sheath, as long as the inner germ-layer takes part in its formation, the name of proamnion. Later on, however, a separation of the amnion i'mm the entoblast takes place also in the head-region in the Rabbit.
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Fig. 133. Diagrammatic longitudinal section through the ovum of a Rabbit at an advanced stage of pregnancy, after BISCHOFF.
f t Embryo ; a, amnion ; u, urachus ; al, allantois with blood-vessels ; sit, subzonal membrane ; pi, villi of the placenta ; fJ, vascular layer of the yolk-sac ; ed, entoblast of the yolk-sac ; ed', ed", inner and outer lamella of the entoblast which lines the flattened cavity of the yolk-sac ; ds, cavity of the yolk-sac ; st, sinus terminalis ; r, the space between amnion, allantois, and yolk-sac that is filled with fluid.
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Finally, in our fourth diagram, still a third change has appeared in the serosa. By rapid growth of the epithelium large numbers of small evaginations or villi have arisen on its outer surface. On this account the name of chorion or mllous layer has been applied to it when these changes have been completed. It should also be added here that in the development of the villi uniformity among all Mammals by no means prevails. In the lowest orders (Monotremes, Marsupials) the surface of the blastodermic vesicle remains almost smooth, as in Reptiles and Birds. Ill them, therefore, the serosa is permanently retained during embr} T onic life, whereas in other Mammalia it is transformed into a villous membrane. By reason of these differences KOLLIKER has divided Mammals into Mammalia achoria and Mammalia choriata.
On the other embryonic membranes of fig. 132, 4, it is principally changes in size only that have been effected. The yolk-sac (ds), over the entire surface of which the vitelline vessels now spread, has become considerably smaller, and is continuous with the embryonic intestine by means of a long slender stalk, the vitelline duct (da). The amniotic sac (am) has already enlarged and is filled with fluid, the liquor amnii. Its walls are continuous at the umbilicus with the ventral wall of the embryo. The allantois (at) has become a vascular pear-shaped sac, which has grown out between the dermal stalk and umbilicus into the extra-embryonic part of the body-cavity, and soon after reaches the serosa.
The accurate representation of an embryo Dog of twenty-five days (fig. 134) affords us, better than the diagram (fig. 132, 4), a view of the connection of the two vascular sacs, the allantois and yolk-sac, with the intestinal canal.
The embryo is removed from the chorion and amnion. The ventral belly- wall is partly removed, and thereby the dermal umbilicus, which about this time has become rather narrow, has been destroyed. The intestinal canal, now to be seen in its entire length. is already converted throughout into a tube (d) ; near its middle it is continuous, by means of a short vitelline duct, with the yolk-sac which was cut open in the process of preparation. The allantois (al) is attached to the very end of the intestinal canal by means of the attenuated stalk-like urachus.
Up to this stage the correspondence in the development of the embryonic membranes in Mammals, Birds, and Reptiles is clear. But from now on the course of development in the Mammalia becomes more and more divergent, since one portion of the embryonic membranes enters into closer relations with tie mucous membrane of the uterus, < i nd is thus converted into an organ of nutrition for the embryo. In this manner a compensation is provided for the loss of the yolk.
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Fig. 134. Embryo Dog of 25 days, extended and seen from in front. Magnified 25 /diameters. After BISCIIOFF.
d. Intestine ; </>:, yolk-sac; al, allantois, urinary sac; im, primitive kidney ; I, the two lobes of the liver, with the lumen of the omphalomesenteric vein between them ; re, he. anterior and posterior appendages ; h, heart ; m, mouth ; ax, eye ; g, olfactory pit.
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The interesting adaptations for intra-uterine nutrition they have been studied especially by the English anatomist TURNER in a series of profound comparative-embryological works present very great differences in the separate orders of Mammalia : sometimes they are of a simple kind, ;it other times they are more complicated organs, which have been designated as the after-birth, or placenta. Since a knowledge of them will facilitate our comprehension of the human placenta, we shall consider them somewhat at length.
// is most e,i-/n'<lient to <7 /*//////>/ As-// three different modifications in the n-ruj in which the surface of the blastodermic vesicle comes into relation with the mucous membrane of the nterns, an,d accordingly to divide the Mammals into three groups.
In one the serosa is retained nearly in its simple primitive condition, In the second it is transformed into a villous layer or chorion, and In the third a placenta arises out of one or more portions of the chorion.
To the first group belong, among the Mammalia, only the Monotremes and the Marsupials, whose embryonic membranes are in the main constituted like those of Birds and Reptiles. Ordinarily in the Marsupials the serosa retains its smooth surface. Inasmuch as it lies in close contact with the vascular mucous membrane of the uterus, it can absorb nourishment from the latter and transmit it to the deeper-lying embryonic parts.
In the second group of Mammals an improvement in the intrauterine nourishment is effected by important changes in the organisation of the serosa, which is converted into a villous layer or chorion. In the first place, it is provided with blood-vessels by the allantois, which grows out into contact with it, and whose connective-tissue layer, containing the ramifications of the umbilical vessels, grows over its entire inner surface.
Secondly, the epithelial membrane begins to grow out into folds and villi, into which there soon penetrate vascular outgrowths of the connective-tissue layer. By this process a larger resorbing surface is provided.
Thirdly, the mucous membrane of the uterus and the chorion unite more intimately and firmly with each other, while the former also increases its surface and acquires pits and depressions into which the processes of the latter penetrate.
All these changes have simply the purpose of facilitating and rendering more perfect the interchange of materials between the tissues of the mother and those of the offspring.
We meet with membranes thus constituted in the Suidse, the Perissodactyla, Hippopotamidge, Tylopoda, Traguliclse, Sirenia, and Cetacea. In. the Pig, which shall serve as an example, the blastodermic vesicle, in adaptation to the form of the uterus, is transformed into a spindle-shaped sac. The inner embryonic appendages, the yolk-sac and allantois, are also drawn out in the same manner into two long tapering ends.
On the entire surface of the chorion, with the exception of the two ends of the sac, there have arisen rows of very vascular pads, which radiate from separate smooth round spots of the membrane, and are covered at their edges with small simple papillse. The mucous membrane of the uterus is exactly fitted into the elevations and depressions of the chorion. There are also found on it circular smooth places similar to those of the chorion, which are further noteworthy from the fact that it is only on them that the tubular uterine glands open out. At birth the interlocking surfaces of contact separate from each other without any loss of substance on the part of the mucous membrane of the uterus ; for the pads and small papillae are easily withdrawn from the depressions which serve for their reception.
In the third group a special organ, the placenta, or after- birth, has been developed for the purpose of intra-uterine nutrition. Its origin was brought about by separate portions of the chorion having assumed different characters, owing to the unequal size and distribution of the villi.
One part exhibits a condition in which the villi are entirely gone or much stunted, so that the surface of the membrane feels smooth ; moreover, it possesses few blood-vessels or is entirely destitute of them.
Another part of the chorion contains, closely packed together, villi which are extremely long and covered with many ramifying lateral branches ; furthermore, it receives large blood-vessels, which approach the tufts of villi and distribute their terminal capillaries to the finest lateral ramifications of the latter ; finally, it has entered into the most intimate relations with the mucous membrane of the uterus. Wherever the latter comes in contact with the tufts of villi it is much thickened, very vascular, and in a state of active growth. It encloses numerous branched cavities of varying size, into which the villi of the chorion exactly fit.
The entire structure is called a placenta, in which the part of the chorion which is covered with villi is distinguished as the placenta foetalis, and the part of the mucous membrane of the uterus which is united with and adapted to the latter as the placenta, uterina. Both parts together constitute an organ for the nutrition of the embryo.
The term placenta has often been extended to the kind of chorion which is evenly covered with small villi, such as exists in the Suidse, etc., and the designation of diffuse placenta has been created for it. But in the interest of a more precise definition it is advisable
to use the name only in the restricted sense in which it has. been employed in this chapter, and in other cases to speak of a villous membrane or chorion only. The formation of the placenta presents in its details important modifications.
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Fig. 135a. Uterus of a Cow laid open, in the middle of the period of The Tfo^tt'fc gestation. From BALFOUR, after COLIN.
T', Vagina ; 7, uterus ; C'k, chorion ; C\ cotyledons of the uterus ; C", festal nants, in which cotyledons. , -, i i the blastoder mic vesicle is drawn out into two tips, as in the Pig, present a special type (fig. 135a). On their chorion (CK) have been developed very many small placentse 2 }, which here are also called cotyledons. The number of the latter is exceedingly variable in the in the Sheep and Cow, and only from five to six in the Doe. They are united with corresponding thickenings of the uterine mucous membrane, the placentae uterinse (C* 1 ), though only in a loose manner, so that a little pulling is sufficient to produce a separation, and to draw the chorionic villi out of the depressions which serve for their reception, as one draws the hand out of a glove. In fact, in the preparation which serves as the basis of our figure 135a the cotyledons of offspring and mother (C- and C n ) are separated from each other, since the uterus ( U) has been opened by means of an incision and drawn back from the chorion (Oh) for a little distance.
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Fig. 135b. Cotyledon of a Cow, the fostal and maternal parts half detached from each other. After COLIN, from BALFOUR.
C'ies, from sixty u, Uterus; C\ maternal part of the cotyledon (placenta uterina) ; f^ . 1-v, ^v-L/1 c ^. chorion of the embryo; C", fcetal part of the cotyledon
to one iiuiKiiecL ,, , ,. x (chorion frondosnm or placenta roetalis).
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Figure 135b shows a single cotyledon of figure 135a somewhat larger than the natural size. The wall of the uterus (?f) is drawn back a little from the chorion (Ch). As a result of this, the maternal (f l ) and foetal parts (f 2 ) of the cotyledon are partially separated from each other. On the placenta uterina (6 rl ) one perceives many small pits, on the placenta f<i-t:ili> (f/ 2 ) the closely packed dendritically branching chorionic villi, which have been withdrawn from the pits.
As the diagrammatic section iigure l^O teaches, the foetal and maternal tissues abut immediately on each other. The villi are covered with flattened cells, and the depressions of the mucous membrane are lined with cylindrical cells ; the latter develop within them granules of fat and albumen ; they disintegrate in part, and thereby contribute to the formation of a milky fluid, the so-called uterine milk, which can be pressed out of the placenta uterina and serves for the nutrition of the foetus. It is to be noticed also that in the Ruminants the uterine glands have openings on the mucous membrane only between the cotyledons.
In all other Mammals that are provided with a placenta the intergrowth of the foetal and maternal tissue is still more intimate. At the same time there is formed in this way such a close union, that a separation of the chorion without '< njnnj to the mucous membrane of the uterus is now no loiif/er possible. At birth therefore a more or less considerable superficial layer of tie mucous membrane of the uterus is cast off with the fu-tal plac< uta. The part that is cast off is called tlte caducous membrane, or the decidna.
In accordance with HUXLEY'S proposal, all Mammals in which, in consequence of the special growth of the placenta, such a membrane is formed are now grouped together as Mammalia deciduata, or briefly Deciduata, in contradistinction to the remaining Mammals the Indeciduata, the formation of whose placenta? has just been discussed.
In the Mammalia with a decidua we must distinguish two subtypes of placenta, a ring~like and a disc-like, a placenta zonaria and a placenta discoidea.
The placenta zonaria is characteristic of I lie < 'arnivora. The hlastodermic vesicle in this ca.se generally has the shape of a, cask. With the exception of both poles, which retain a smooth surface, the chorion is covered with numerous villi arranged in a girdle-shaped zone ; the villi are furnished with lateral branches, like a tre >.
The branched villi of the chorion sink into the thickened mucous membrane of the uterus in various directions, so that in sections there arises the appearance of an irregular interlacing (fig. 137). However, according to the concurrent accounts of TURNER and ERCOLANI, there is no penetration into the uterine glands in this case, any more than in the case of the Indeciduata.
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Fig. 136. Diagrammatic representation of the finer structure of the placenta of a Cow, after TURNER. F, Foetal, M, maternal placenta ; V, villus'; c, epithelium of the chorionic villus ; e', epithelium of the maternal placenta ; d, fcetal, il', maternal blood-vessels.
Fig. 137. Diagrammatic representation of the finer structure of the placenta of a Cat, after TURNER. Explanation of letters as in fig. 13<>.
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The epithelium (e') of the maternal mucous membrane (J/) persists and forms a boundary between the villi (V) and the maternal bloodvessels (cZ'), which latter have enlarged to cavities from three to four times as wide as the f<ctal capillaries (d). This enlargement of the maternal blood-passages is full of significance for the formation of the placenta in the Decicluata as opposed to that of the Incleciduata.
The second form, the discoid placenta, is characteristic of the Rodentia, the Insectivora, the Chirbptera and Prosimise, the Apes and Man. Here the portion of the chorion devoted to the formation of the placenta is small ; but in compensation for this the tufts of villi (fig. 138 V) are very highly developed; the union between placenta uterina(J/) and placenta fcetalis (F) is most intimate ; the maternal blood-spaces (tZ'), in the case of the Apes and Man at least, are, as nowhere else, enormously distended, so that the villi of the chorion (T) appear to sink directly into them and to be bathed immediately by the maternal blood.
Since we shall occupy ourselves more at length in the next chapter with the human placenta, which belongs to this type, these few remarks may suffice for the time being.
I close this section with a reference to the high systematic significance of the embryonicaccessory organs of Vertebrates. They present, as we have seen, such great and striking differences in the separate classes, that the utilisation of them for systematic purposes which has been made by MILNE- EDWARDS, OWEN, and HUXLEY was natural.
All lower Vertebrates, Arnphioxus, Cyclostomes, Fishes, Dipnoi, and Amphibia, cither possess no accessory organs at all, or only an evagination of the intestinal tube, the yolk-sac. The embryos of Reptiles, Birds, and Mammals, on the contrary, are further enclosed in two fugitive membranes characteristic of embryonic life, the amnion and serosa. They have therefore been grouped together as atnuiotic animals or Aviniota, and the classes first mentioned have been contrasted with them as iion-anmiotic animals or Anamnia.
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Fig. 138. Diagrammatic representation of the finer structure of the human placenta according to the hypothesis of TURNER.
F, Foetal, M, maternal placenta ; e', epithelium of the maternal placenta ; d, foetal, d', maternal bloodvessels ; V, vilhis ; d*, decidua serotina of the human placenta ; t, t, trabeculae of the serotina running to the fcetal villi ; ca, convoluted artery which sinks into the blood-space d' ', up, one of the utero-placental veins conveying blood from the latter ; x, a continuation over the villus of maternal tissue lying o\itside the epithelial layer t' which represents either the endothelium of the maternal blood-vessels or a delicate connective tissue pertaining to the serotina, or both together. The layer e' consists, at all events, of maternal cells derived from the serotina. The fcetal epithelial layer is no longer to be seen on the villi of the completely formed human placenta.
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Among the ainniotic animals a further separation into two groups can be made : on the one side are the egg-laying Reptiles and Birds, which HUXLEY unites into the Sauropsida ; on the other side Mammals, in which (with the exception of the Monotremes) the eggs develop in the uterus, and the young are further nourished after birth by the secretions of milk-glands.
In the Mammalia the fetal membranes, inasmuch as they unite with the mucous membrane of the uterus to form an organ of nutrition, take on a still more complicated character, and present modifications which in turn can readily be utilised for systematic purposes.
In Monotremes and Marsupials the outer embryonic membrane retains an almost smooth surface, as in Reptiles and Birds ; in all other Mammals there arise on the surface of the chorioii villi, which grow into the maternal mucous membrane. OWEN has designated the one as Iinplaceiitalia, the other as Placentalia. The terms Achoria and Choriata introduced for these by KOLLIKER are better.
In the Choriata the union of the villi with the mucous membrane is either loose or firm ; corresponding to this there is either no detachable layer of the mucous membrane of the uterus formed, no decidua, or such a structure arises as the result of close intergrowth of the placenta uterina and placenta fcetalis. Thus we have the Mammalia indeciduata and the Mammalia deciduata. In each division there are again two sub -types in the formation of villi. In the Indeciduata the villi are either evenly distributed over the surface, or they are united into more or less numerous groups (placentae or cotyledons), which are separated from one another by smooth tracts of the chorion. In a part of the Deciduata the placenta is girdle-shaped, in another part disc-shaped.
Summary
- In the Mammalia there is developed, in the same way as in Reptiles and Birds, a yolk-sac, an amnion, a serosa., and an allantois.
- Excepting in the Monotremes and Marsupials, the serosa is metamorphosed into a chorion, in that it puts forth villi, and in that the connective-tissue layer of the allantois, which is provided with the umbilical blood-vessels, spreads out on its inner surface and penetrates into the villi.
- In a part of the Mammalia certain regions of the serous membrane, where the villi grow more vigorously and put forth lateral branches, and sink into corresponding depressions of the mucous membrane of the uterus, are converted into a placenta (when many of them have arisen on one chorion they are called cotyledons).
- On the placenta one distinguishes :
- A placenta foetalis, i.e., that part of the chorion which has developed the tufts of villi.
- A placenta uterina, i.e., that part of the mucous membrane of the uterus which has proliferated and is provided with depressions for the reception of the placenta fcetalis.
- Foetal and maternal parts of the placenta can become more firmly united with each other; the iv.Milt is that at birth a larger or smaller tract of the mucous membrane of the uterus is also cast off, and is known as the caducous membrane, or the decidua.
- According to the character of the embryonic membranes, the following divisions of Vertebrates may be established :
I. Anamiiia, animals without an amnion. (Amphioxus, ( 'yclostomes, Fishes, Amphibia.)
II. Amiliota, animals with an amnion (with yolk-sac, amnion. serosa, and allantois). A. Sawropsida. Egg-laying, amniotic animals.
(Reptiles and Birds). Mammalia. In. all of them, except the Monotrenies, the eggs are developed in the uterus. () Achoria. The serosa develops no villi, or only a few.
(Monotremes, Marsupials.) (b] Choriata. The serosa becomes the villous membrane (chorion). '(1) With evenly distributed villi.
(Perissodactyla, Suidte, Hippopotarnidre, Tylopocla, mammalia m TI \ Iragulidse, Cetacea, etc.) ('2) Placentalia. The serosa is at intervals metamor deciduata.
pnosed into a placenta.
a. Numerous cotyledons. (Ruminantia.) b. Placenta zonaria. (Carnivora.) Mammalia .
T . -, y. Placenta discoiclea. (Man. Apes, Rodents, In deciduata. sectivores, Bats.)
Literature
Beneden, van, et Charles Julin. Recherches sur la formation des annexes foetales chez les Mammiferes (Lapin et Cheiropteres). Archives cle Biologie. T. V. 1884.
Caldwell, W. H. Eierlegen der Monotremen. Referat in Schwalbe's Jahresbericht, p. 507. 1886.
Caldwell, "W. H. On the Arrangement of the Embryonic Membranes in Marsupial Animals. Quart. Jour. Micr. Sci. Vol. XXIV. p. 655. 1884.
Edwards, Milne. Lecons sur la physiologic et I'anatomie comparee de rhonime et des animaux. Paris 1870.
Eschrieht. De organis quae nutrition! et respirationi foetus mammalium inserviunt. Hafniae 1837.
Godet. Recherches sur la structure intime du placenta du lapin. Inaugural Dissertation. Ncuveville 1877.
Haacke, W. Meine Entcleckung des Eierlegens cler Echidna hystrix. Zool. Anzeiger, p. 647. 1884.
Hoffmann, C. K. Ueber das Amnion des zweiblatterigen Keimes. Archiv f. mikr. Anat. Bd. XXIII. p. 530. 1884.
Kolliker. Entwicklungsge'schichte des Menschen uncl cler hb'heren Thiere, pp. 261-3 and 360, 361. 1879.
Mauthner, Julius. Ueber den miitterlichen Kreislauf in der Kanincheuplacenta mit Rticksicht auf die in der Menschenplacenta bis jetzt vorgefundenen anatomischen Verhiiltnisse. Sitzungsb. d. k. Akad. d. Wissensch. Math.-naturw. Classe. Bd. LXVII. Abth. 3. 1873.
Milne-Edwards. See Edwards, Milne. Osborn, H. F. Observations upon the Foetal Membranes of the Opossum and other Marsupials. Quart. Jour. Micr. Sci. Vol. XVIII. 1883.
Osborn, H. ~F. The Foetal Membranes of the Marsupials. Jour. Morphol. Vol. I. 1887.
Owen, R. Description of an Impregnated Uterus and of the Uterine Ova of Echidna hystrix. Ann. and Mag. Nat. Hist. Vol. XIV. p 373. 1884.
Slavjansky. Die regressiven Veranderungen der Epithelialzellen in cler serb'sen Hiille des Kanincheneies. Berichte liber die Verhandl. d. k. sachsischen Gesellsch. d. Wissensch. Leipzig. Math.-pb.ys. Classe. Bd. XXIV. pp. 247-52. 1872.
Strahl, H. Die Dottersackwand u. cler Parablast der Eidechse. Zeitschr. f. wiss. Zoologie. Bd. XLV. p. 282. 1887.
Turner. On the Placentation of the Apes with a Comparison of the Structure of their Placenta with that of the Human Female. Philos. Trans. Roy. Sci. London. Vol. CLXIX. Part I. 1878.
Turner. Some General Observations on the Placenta with especial reference to the Theory of Evolution. Jour. Anat. and Physiol. 1877.
Virchow, Hans. Ueber das Epithel des Dottersackes ina Hiihnerei. Dissertation. Berlin 1875.
Waldeyer, W. Die Placenta von Inutis nemestrinus. Sitzungsb. d. k. preuss. Acad. d. Wissensch. Berlin. 1889.
Numerous citations of the literature on the foetal membranes of Mammals are to be found in Hoffmann : Grondtrekken der vergelijkende ontwikkelingsgeschiedenis, etc. 1884.
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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