Illustration: Figure 149Fig. 149. Section through the placenta and adjacent parts of a Rat one inch and a quarter long.(From Huxley.)a.uterine vein;b. uterine wall;c.cavernous portion of uterine wall;d.deciduous portion of uterus with cavernous structure;i.large vein passing to the fœtal portion of the placenta;f.false chorion supplied by vitelline vessels;k.vitelline vessel;l.allantoic vessel;g.boundary of true placenta;e,m,m,e. line of junction of the deciduate and non-deciduate parts of the uterine wall.
Fig. 149. Section through the placenta and adjacent parts of a Rat one inch and a quarter long.(From Huxley.)a.uterine vein;b. uterine wall;c.cavernous portion of uterine wall;d.deciduous portion of uterus with cavernous structure;i.large vein passing to the fœtal portion of the placenta;f.false chorion supplied by vitelline vessels;k.vitelline vessel;l.allantoic vessel;g.boundary of true placenta;e,m,m,e. line of junction of the deciduate and non-deciduate parts of the uterine wall.
In the Rat(Mus decumanus) (fig. 149) the sack of the allantois completely atrophies before the close of fœtal life[89], and there is developed, at the junction of the maternal part of the placenta and the unaltered mucous membrane of the uterus, a fold of the mucous membrane which completely encapsules the whole chorion, and forms a separate chamber for it, distinct from the general lumen of the uterus. Folds of this nature, which are specially developed in Man and Apes, are known as a decidua reflexa. The decidua reflexa of the Rat is reduced to extreme tenuity, or even vanishes before the close of gestation.
Guinea-pig. The development of the Guinea-pig is dealt with elsewhere, but, so far as its peculiarities permit a comparison with the Rabbit, the agreement between the two types appears to be fairly close.
The blastodermic vesicle of the Guinea-pig becomes completely enveloped in a capsule of the uterine wall (decidua reflexa) (fig. 150). The epithelium of the blastodermic vesicle in contact with the uterine wall is not epiblastic, but corresponds with the hypoblast of the yolk-sack of other forms, and the mesoblast of the greater part of the inner side of this becomes richly vascular (yk); the vascular area being bounded by a sinus terminalis.
Illustration: Figure 150Fig. 150. Diagrammatic longitudinal section of an ovum of a Guinea-pig and the adjacent uterine walls at an advanced stage of pregnancy.(After Bischoff.)yk.yolk-sack (umbilical vesicle) formed of an external hypoblastic layer (shaded) and an internal mesoblastic vascular layer (black). At the end of this layer is placed the sinus terminalis;all.allantois;pl.placenta.The external shaded parts are the uterine walls.
Fig. 150. Diagrammatic longitudinal section of an ovum of a Guinea-pig and the adjacent uterine walls at an advanced stage of pregnancy.(After Bischoff.)yk.yolk-sack (umbilical vesicle) formed of an external hypoblastic layer (shaded) and an internal mesoblastic vascular layer (black). At the end of this layer is placed the sinus terminalis;all.allantois;pl.placenta.The external shaded parts are the uterine walls.
The blastodermic vesicle is so situated within its uterine capsule that the embryo is attached to the part of it adjoining the free side of the uterus. From the opposite side of the uterus,viz.that to which the mesometrium is attached, there grow into the wall of the blastodermic vesicle numerous vascular processes of the uterine wall, which establish at this point an organic connection between the two (pl). The blood-vessels of the blastodermic vesicle (yolk-sack) stop short immediately around the area of attachment to the uterus; but at a late period the allantois grows towards, and fuses with this area. The blood-vessels of the allantois and of the uterus become intertwined, and a disc-like placenta more or less similar to that in the Rabbit becomes formed (pl). The cavity of the allantois, if developed, vanishes completely.
In all the Rodentia the placenta appears to be situated on the mesometric side of the uterus.
Insectivora. In the Mole (Talpa) and the Shrew (Sorex), the fœtal membranes are in the main similar to those in the rabbit, and a deciduate discoidal placenta is always present. It may be situated anywhere in the circumference of the uterine tube. The allantoic cavity persists (Owen), but the allantois only covers the placental area of the chorion. The yolk-sack is persistent, and fuses with the non-allantoic part of the subzonal membrane; which is rendered vascular by its blood-vessels. There would seem to be (Owen) a small decidua reflexa. A similar arrangement is found in the Hedgehog (Erinaceus Europæus) (Rolleston), in which the placenta occupies the typical dorsal position. It is not clear from Rolleston’s description whether the yolk-sack persists till the close of fœtal life, but it seems probable that it does so. There is a considerable reflexa which does not,however, cover the whole chorion. In the Tenrec (Centetes) the yolk-sack and non-placental part of the chorion are described by Rolleston as being absent, but it seems not impossible that this may have been owing to the bad state of preservation of the specimen. The amnion is large. In the Cheiroptera (VespertilioandPteropus), the yolk-sack is large, and coalesces with part of the chorion. The large yolk-sack has been observed in Pteropus by Rolleston, and in Vespertilio by Owen. The allantoic vessels supply the placenta only. The Cheiroptera are usually uniparous.
Simiadæ and Anthropidæ. The fœtal membranes of Apes and Man, though in their origin unlike those of the Rodentia and Insectivora, are in their ultimate form similar to them, and may be conveniently dealt with here. The early stages in the development of these membranes in the human embryo have not been satisfactorily observed; but it is known that the ovum, shortly after its entrance into the uterus, becomes attached to the uterine wall, which in the meantime has undergone considerable preparatory changes. A fold of the uterine wall appears to grow round the blastodermic vesicle, and to form a complete capsule for it, but the exact mode of formation of this capsule is a matter of inference and not of observation. During the first fortnight of pregnancy villi grow out, according to Allen Thomson over its whole surface, but according to Reichert in a ring-like fashion round the edge of the somewhat flattened ovum, and attach it to the uterus. The further history of the early stages is extremely obscure, and to a large extent a matter of speculation: what is known with reference to it will be found in a special section, but I shall here take up the history at about the fourth week.
At this stage a complete chorion has become formed, and is probably derived from a growth of the mesoblast of the allantois (unaccompanied by the hypoblast) round the whole inner surface of the subzonal membrane. From the whole surface of the chorion there project branched vascular processes, covered by an epithelium. The allantois is without a cavity, but a hypoblastic epithelium is present in the allantoic stalk, through which it does not, however, form a continuous tube. The blood-vessels of the chorion are derived from the usual allantoic arteries and vein. The general condition of the embryo and of its membranes at this period is shewn diagrammatically infig. 147, 5. Around the embryo is seen the amnion, already separated by a considerable interval from the embryo. The yolk-sack is shewn atds. Relatively to the other parts it is considerably smaller than it was at an earlier stage. The allantoic stalk is shewn atal. Both it and the stalk of the yolk-sack are enveloped by the amnion (am). The chorion with its vascular processes surrounds the whole embryo.
It may be noted that the condition of the chorion at this stage is very similar to that of the normal diffused type of placenta, described in the sequel.
While the above changes are taking place in the embryonic membranes, the blastodermic vesicle greatly increases in size, and forms a considerable projection from the upper wall of the uterus. Three regions of the uterinewall, in relation to the blastodermic vesicle, are usually distinguished; and since the superficial parts of all of these are thrown off with the afterbirth, each of them is called a decidua. They are represented at a somewhat later stage infig. 151. There is (1) the part of the wall reflected over the blastodermic vesicle, called the decidua reflexa (dr); (2) the part of the wall forming the area round which the reflexa is inserted, called the decidua serotina (ds); (3) the general wall of the uterus, not related to the embryo, called the decidua vera (du).
The decidua reflexa and serotina together envelop the chorion, the processes of which fit into crypts in them. At this period both of them are highly and nearly uniformly vascular. The general cavity of the uterus is to a large extent obliterated by the ovum, but still persists as a space filled with mucus, between the decidua reflexa and the decidua vera.
Illustration: Figure 151Fig. 151. Diagrammatic section of pregnant human uterus with contained fœtus.(From Huxley after Longet.)al.allantoic stalk;nb.umbilical vesicle;am.amnion;ch.chorion;ds.decidua serotina;du.decidua vera;dr.decidua reflexa;lFallopian tube;c.cervix uteri;u.uterus; z. fœtal villi of true placenta;z´.villi of non-placental part of chorion.
Fig. 151. Diagrammatic section of pregnant human uterus with contained fœtus.(From Huxley after Longet.)al.allantoic stalk;nb.umbilical vesicle;am.amnion;ch.chorion;ds.decidua serotina;du.decidua vera;dr.decidua reflexa;lFallopian tube;c.cervix uteri;u.uterus; z. fœtal villi of true placenta;z´.villi of non-placental part of chorion.
The changes which ensue from this period onwards are fully known. The amnion continues to dilate (its cavity being intensely filled with amniotic fluid) till it comes very close to the chorion (fig. 151,am); from which, however, it remains separated by a layer of gelatinous tissue. The villi of the chorion in the region covered by the decidua reflexa, gradually cease to be vascular, and partially atrophy, but in the region in contact with the decidua serotina increase and become more vascular and more arborescent (fig. 151,z). The former region becomes known as the chorion læve, and the latter as the chorion frondosum.The chorion frondosum, together with the decidua serotina, gives rise to the placenta.
Although the vascular supply is cut off from the chorion læve, the processes on its surface do not completely abort. It becomes, as the time of birth approaches, more and more closely united with the reflexa, till the union between the two is so close that their exact boundaries cannot be made out. The umbilical vesicle (fig. 151,nb), although it becomes greatly reduced in size and flattened, persists in a recognisable form till the time of birth.
As the embryo enlarges, the space between the decidua vera and decidua reflexa becomes reduced, and finally the two parts unite together. The decidua vera is mainly characterised by the presence of peculiar roundish cells in its subepithelial tissue, and by the disappearance of a distinct lining of epithelial cells. During the whole of pregnancy it remains highly vascular. The decidua reflexa, on the disappearance of the vessels in the chorion læve, becomes non-vascular. Its tissue undergoes changes in the main similar to those of the decidua vera, and as has been already mentioned, it fuses on the one hand with the chorion, and on the other with the decidua vera. The membrane resulting from its fusion with the latter structure becomes thinner and thinner as pregnancy advances, and is reduced to a thin layer at the time of birth.
The placenta has a somewhat discoidal form, with a slightly convex uterine surface and a concave embryonic surface. At its edge it is continuous both with the decidua reflexa and decidua vera. Near the centre of the embryonic surface is implanted the umbilical cord. As has already been mentioned, the placenta is formed of the decidua serotina and the fœtal villi of the chorion frondosum. The fœtal and maternal tissues are far more closely united (fig. 152) than in the forms described above. The villi of the chorion, which were originally comparatively simple, become more and more complicated, and assume an extremely arborescent form. Each of them contains a vein and an artery, which subdivide to enter the complicated ramifications; and are connected together by a rich anastomosis. The villi are formed mainly of connective tissue, but are covered by an epithelial layer generally believed to be derived from the subzonal membrane; but, as was first stated by Goodsir, and has since been more fully shewn by Ercolani and Turner, this epithelial layer is really a part of the cellular decidua serotina of the uterine wall, which has become adherent to the villi in the development of the placenta (fig. 161,g). The placenta is divided into a number of lobes, usually called cotyledons, by septa which pass towards the chorion. These septa, which belong to the serotina, lie between the arborescent villi of the chorion. The cotyledons themselves consist of a network of tissue permeated by large vascular spaces, formed by the dilatation of the maternal blood-vessels of the serotina, into which the ramifications of the fœtal villi project. In these spaces they partly float freely, and partly are attached to delicate trabeculæ of the maternal tissue (fig. 161,G). They are, of course, separated from the maternal blood by the uterine epithelial layer before mentioned. The blood is brought to the maternal part of the placenta by spirally coiled arteries, which do not divide into capillaries, butopen into the large blood-spaces already spoken of. From these spaces there pass off oblique utero-placental veins, which pierce the serotina, and form a system of large venous sinuses in the adjoining uterine wall (fig. 152,F), and eventually fall into the general uterine venous system. At birth the whole placenta, together with the fused decidua vera, and reflexa, with which it is continuous, is shed; and the blood-vessels thus ruptured are closed by the contraction of the uterine wall.
Illustration: Figure 152Fig. 152. Section of the human uterus and placenta at the thirtieth week of pregnancy.(From Huxley after Ecker.)A.umbilical cord;B.chorion;C.fœtal villi separated by processes of the decidua serotina,D;E,F,G. walls of uterus.
Fig. 152. Section of the human uterus and placenta at the thirtieth week of pregnancy.(From Huxley after Ecker.)A.umbilical cord;B.chorion;C.fœtal villi separated by processes of the decidua serotina,D;E,F,G. walls of uterus.
The fœtal membranes and the placenta of the Simiadæ (Turner,No.225) are in most respects closely similar to those in Man; but the placenta is, in most cases, divided into two lobes, though in the Chimpanzee, Cynocephalus, and the Apes of the New World, it appears to be single.
The types of deciduate placenta so far described, are usually classified by anatomists as discoidal placentæ, although it must be borne in mind that they differ very widely. In the Rodentia, Insectivora, and Cheiroptera there is a (usually) dorsal placenta, which is co-extensive with the area of contact between the allantois and the subzonal membrane, while the yolk-sack adheres to a large part of the subzonal membrane. In Apes and Man the allantois spreads over the whole inner surface of the subzonal membrane; the placenta is on the ventral side of the embryo, and occupies only a small part of the surface of the allantois. The placenta of Apes and Man might becalled metadiscoidal, in order to distinguish it from the primitive discoidal placenta of the Rodentia and Insectivora.
In the Armadilloes (Dasypus) the placenta is truly discoidal and deciduate (Owen and Kölliker). Alf. Milne Edwards states that in Dasypus novemcinctus the placenta is zonary, and both Kölliker and he found four embryos in the uterus, each with its own amnion, but the placenta of all four united together; and all four enclosed in a common chorion. A reflexa does not appear to be present. In the Sloths the placenta approaches the discoidal type (Turner,No.218). It occupies in Cholæpus Hoffmanni about four-fifths of the surface of the chorion, and is composed of about thirty-four discoid lobes. It is truly deciduate, and the maternal capillaries are replaced by a system of sinuses (fig. 161). The amnion is close to the inner surface of the chorion. A dome-shaped placenta is also found amongst the Edentata in Myrmecophaga and Tamandua (Milne Edwards,No.208).
Zonary Placenta. Another form of deciduate placenta is known as the zonary. This form of placenta occupies a broad zone of the chorion, leaving the two poles free. It is found in the Carnivora, Hyrax, Elephas, and Orycteropus.
It is easy to understand how the zonary placenta may be derived from the primitive arrangement of the membranes (videp.240) by the extension of adiscoidalplacental area to azonaryarea, but it is possible that some of the types of zonary placenta may have been evolved from the concentration of a diffused placenta (videp.261) to a zonary area. The absence of the placenta at the extreme poles of the chorion is explained by the fact of their not being covered by a reflection of the uterine mucous membrane. In the later periods of pregnancy the placental area becomes, however, in most forms much more restricted than the area of contact between the uterus and chorion.
In the Dog[90], which may be taken as type, there is a large vascular yolk-sack formed in the usual way, which does not however fuse with the chorion. It extends at first quite to the end of the citron-shaped ovum, and persists till birth. The allantois first grows out on the dorsal side of the embryo, where it coalesces with the subzonal membrane, over a small discoidal area.
Before the fusion of the allantois with the subzonal membrane, there grow out from the whole surface of the external covering of the ovum, except the poles, numerous non-vascular villi, which fit into uterine crypts. When the allantois adheres to the subzonal membrane vascular processes grow out from it into these villi. The vascular villi so formed are of course at first confined to the disc-shaped area of adhesion between the allantois and the subzonal membrane;and there is thus formed a rudimentary discoidal placenta, closely resembling that of the Rodentia. The view previously stated, that the zonary placenta is derived from the discoidal one, receives from this fact a strong support.
The cavity of the allantois is large, and its inner part is in contact withthe amnion. The area of adhesion between the outer part of the allantois and subzonal membrane gradually spreads over the whole interior of the subzonal membrane, and vascular villi are formed over the whole area of adhesion except at the two extreme poles of the egg. The last part to be covered is the ventral side where the yolk-sack adjoins the subzonal membrane.
During the extension of the allantois its cavity persists, and its inner part covers not only the amnion, but also the yolk-sack. It adheres to the amnion and supplies it with blood-vessels (Bischoff).
With the full growth of the allantois there is formed a broad placental zone, with numerous branched villi, fitting into corresponding pits which become developed in the uterine walls. The maternal and fœtal structures become closely interlocked and highly vascular; and at birth a large part of the maternal part is carried away with the placenta; some of it however still remains attached to the muscular wall of the uterus. The villi of the chorion do not fit into uterine glands. The zone of the placenta diminishes greatly in proportion to the chorion as the latter elongates, and at the full time the breadth of the zone is not more than about one-fifth of the whole length of the chorion.
At the edge of the placental zone there is a very small portion of the uterine mucous membrane reflected over the non-placental part of the chorion, which forms a small reflexa analogous with the reflexa in Man.
The Carnivora generally closely resemble the Dog, but in the Cat the whole of the maternal part of the placenta is carried away with the fœtal parts, so that the placenta is more completely deciduate than in the Dog. In the Grey Seal (Halichœrus gryphus, Turner,No.219) the general arrangement of the fœtal membranes is the same as in the other groups of the Carnivora, but there is a considerable reflexa developed at the edge of the placenta. The fœtal part of the placenta is divided by a series of primary fissures which give off secondary and tertiary fissures. Into the fissures there pass vascular laminæ of the uterine wall. The general surface of the fœtal part of the placenta between the fissures is covered by a greyish membrane formed of the coalesced terminations of the fœtal villi.
The structure of the placenta in Hyrax is stated by Turner (No.221) to be very similar to that in the Felidæ. The allantoic sack is large, and covers the whole surface of the subzonal membrane. The amnion is also large, but the yolk-sack would seem to disappear at an early stage, instead of persisting, as in the Carnivora, till the close of fœtal life.
The Elephant (Owen, Turner, Chapman) is provided with a zonary deciduate placenta, though a villous patch is present near each pole of the chorion.
Turner (No.220) has shewn that in Orycteropus there is present a zonary placenta, which differs however in several particulars from the normal zonary placenta of the Carnivora; and it is even doubtful whether it is truly deciduate. There is a single embryo, which fills up the body of the uterus and also projects into only one of the horns. The placenta forms abroad median zone, leaving the two poles free. The breadth of the zone is considerably greater than is usual in Carnivora, one-half or more of the whole longitudinal diameter of the chorion being occupied by the placenta. The chorionic villi are arborescent, and diffusely scattered, and though the maternal and fœtal parts are closely interwoven, it has not been ascertained whether the adhesion between them is sufficient to cause the maternal subepithelial tissue to be carried away with the fœtal part of the placenta at birth. The allantois is adherent to the whole chorion, the non-placental parts of which are vascular. In the umbilical cord a remnant of the allantoic vesicle was present in the embryos observed by Turner, but in the absence of a large allantoic cavity the Cape Ant-eater differs greatly from the Carnivora. The amnion and allantois were in contact, but no yolk sack was observed.
Non-deciduate placenta. The remaining Mammalia are characterized by a non-deciduate placenta; or at least by a placenta in which only parts of the maternal epithelium and no vascular maternal structures are carried away at parturition. The non-deciduate placentæ are divided into two groups: (1) The polycotyledonary placenta, characteristic of the true Ruminantia (Cervidæ, Antilopidæ, Bovidæ, Camelopardalidæ); (2) the diffused placenta found in the other non-deciduate Mammalia,viz.the Perissodactyla, the Suidæ, the Hippopotamidæ, the Tylopoda, the Tragulidæ, the Sirenia, the Cetacea, Manis amongst the Edentata, and the Lemuridæ. The polycotyledonary form is the most differentiated; and is probably a modification of the diffused form. The diffused non-deciduate placenta is very easily derived from the primitive type (p.240) by an extension of the allantoic portion of the chorion; and the exclusion of the yolk-sack from any participation in forming the chorion.
The possession in common of a diffused type of placenta is by no means to be regarded as a necessary proof of affinity between two groups, and there are often, even amongst animals possessing a diffused form of placenta, considerable differences in the general arrangement of the embryonic membranes.
Ungulata. Although the Ungulata include forms with both cotyledonary and diffused placentæ, the general arrangement of the embryonic membranes is so similar throughout the group, that it will be convenient to commence with a description of them, which will fairly apply both to the Ruminantia and to the other forms.
The blastodermic vesicle during the early stages of development lies freely in the uterus; and no non-vascular villi, similar to those of the Dog or the Rabbit, are formed before the appearance of the allantois. The blastodermic vesicle has at first the usual spherical form, but it grows out at an early period, and with prodigious rapidity, into two immensely long horns; which in cases where there is only one embryo are eventually prolonged for the whole length of the two horns of the uterus. The embryonic area is formed in the usual way, and its long axis is placed at right angles to that of the vesicle. On the formation of an amnion thereis formed the usual subzonal membrane, which soon becomes separated by a considerable space from the yolk-sack (fig. 153). The yolk-sack is, however, continued into two elongated processes (yk), which pass to the two extremities of the subzonal membrane. It is supplied with the normal blood-vessels. As soon as the allantois appears (fig. 153all), it grows out into a right and a left process, which rapidly fill the whole free space within the subzonal membrane and in many cases,e.g.the Pig (Von Baer), break through the ends of the membrane, from which they project as the diverticula allantoidis. The cavity of the allantois remains large, but the lining of hypoblast becomes separated from the mesoblast, owing to the more rapid growth of the latter. The mesoblast of the allantois applies itself externally to the subzonal membrane to form the chorion[91], and internally to the amnion, the cavity of which remains very small. The chorionic portion of the allantoic mesoblast is very vascular, and that applied to the amnion also becomes vascular in the later developmental periods.
Illustration: Figure 153Fig. 153. Embryo and fœtal membranes of a young embryo Roe-deer.(After Bischoff.)yk.yolk-sack;all.allantois just sprouting as a bilobed sack.
Fig. 153. Embryo and fœtal membranes of a young embryo Roe-deer.(After Bischoff.)yk.yolk-sack;all.allantois just sprouting as a bilobed sack.
The horns of the yolk-sack gradually atrophy, and the whole yolk-sack disappears some time before birth.
Where two or more embryos are present in the uterus, the chorions of the several embryos may unite where they are in contact.
From the chorion there grow out numerous vascular villi, which fit into corresponding pits in the uterine walls. According to the distribution of these villi, the allantois is either diffused or polycotyledonary.
Illustration: Figure 154Fig. 154. Portion of the injected chorion of a Pig, slightly magnified.(From Turner.)The figure shews a minute circular spot (b) (enclosed by a vascular ring) from which villous ridges (r) radiate.
Fig. 154. Portion of the injected chorion of a Pig, slightly magnified.(From Turner.)The figure shews a minute circular spot (b) (enclosed by a vascular ring) from which villous ridges (r) radiate.
Illustration: Figure 155Fig. 155. Surface-view of the injected uterine mucosa of a gravid Pig.(From Turner.)The fig. shews a circular non-vascular spot where a gland opens (g) surrounded by numerous vascular crypts (cr).
Fig. 155. Surface-view of the injected uterine mucosa of a gravid Pig.(From Turner.)The fig. shews a circular non-vascular spot where a gland opens (g) surrounded by numerous vascular crypts (cr).
The pig presents the simplest type of diffused placenta. The villi ofthe surface of the chorion cover a broad zone, leaving only the two poles free; their arrangement differs therefore from that in a zonary placenta in the greater breadth of the zone covered by them. The villi have the form of simple papillæ, arranged on a series of ridges, which are highly vascular as compared with the intervening valleys. If an injected chorion is examined (fig. 154), certain clear non-vascular spots are to be seen (b), from which the ridges of villi radiate. The surface of the uterus adapts itself exactly to the elevations of the chorion; and the furrows which receive thechorionic ridges are highly vascular (fig. 155). On the other hand, there are non-vascular circular depressions corresponding to the non-vascular areas on the chorion; and in these areas, and in these alone, the glands of the uterus open (fig. 155g) (Turner). The maternal and fœtal parts of the placenta in the pig separate with very great ease.
Illustration: Figure 156Fig. 156. Vertical section through the injected placenta of a Mare.(From Turner.)ch.chorion with its villi partlyin situand partly drawn out of the crypts (cr);E.loose epithelial cells which formed the lining of the crypt;g.uterine glands;v.blood-vessels.
Fig. 156. Vertical section through the injected placenta of a Mare.(From Turner.)ch.chorion with its villi partlyin situand partly drawn out of the crypts (cr);E.loose epithelial cells which formed the lining of the crypt;g.uterine glands;v.blood-vessels.
In the mare (Turner), the fœtal villi are arranged in a less definite zonary band than in the pig, though still absent for a very small area at both poles of the chorion, and also opposite the os uteri. The filiform villi, though to the naked eye uniformly scattered, are, when magnified, found to be clustered together in minute cotyledons, which fit into corresponding uterine crypts (fig. 156). Surrounding the uterine crypts are reticulate ridges on which are placed the openings of the uterine glands. The remaining Ungulata with diffused placentæ do not differ in any important particulars from those already described.
The polycotyledonary form of placenta is found in the Ruminantia alone. Its essential character consists in the fœtal villi not being uniformly distributed, but collected into patches or cotyledons which form as it were so many small placentæ (fig. 157). The fœtal villi of these patches fit into corresponding pits in thickened patches of the wall of the uterus (figs.158and159). In many cases (Turner), the interlocking of the maternal and fœtal structures is so close that large parts of the maternalepithelium are carried away when the fœtal villi are separated from the uterus. The glands of the uterus open in the intervals between the cotyledons. The character of the cotyledons differs greatly in different types. The maternal parts are cup-shaped in the sheep, and mushroom-shaped in the cow. There are from 60-100 in the cow and sheep, butonly about five or six in the Roe-deer. In the Giraffe there are, in addition to larger and smaller cotyledons, rows and clusters of short villi, so that the placenta is more or less intermediate between the polycotyledonary and diffused types (Turner). A similarly intermediate type of placenta is found in Cervus mexicanus (Turner).
Illustration: Figure 157Fig. 157.Uterus of a Cow in the middle of pregnancy laid open.(From Huxley after Colin.)V.vagina;U.uterus;Ch.chorion;C1. uterine cotyledons;C2. fœtal cotyledons.
Fig. 157.Uterus of a Cow in the middle of pregnancy laid open.(From Huxley after Colin.)V.vagina;U.uterus;Ch.chorion;C1. uterine cotyledons;C2. fœtal cotyledons.
Illustration: Figure 158Fig. 158. Cotyledon of a Cow, the fœtal and maternal parts half separated.(From Huxley after Colin.)u.uterus;Ch.chorion;C1. maternal part of cotyledon;C2. fœtal part.
Fig. 158. Cotyledon of a Cow, the fœtal and maternal parts half separated.(From Huxley after Colin.)u.uterus;Ch.chorion;C1. maternal part of cotyledon;C2. fœtal part.
Illustration: Figure 159Fig. 159. Semi-diagrammatic vertical section through a portion of a maternal cotyledon of a Sheep.(From Turner.)cr.crypts;e.epithelial lining of crypts;v.veins andc.curling arteries of subepithelial connective tissue.
Fig. 159. Semi-diagrammatic vertical section through a portion of a maternal cotyledon of a Sheep.(From Turner.)cr.crypts;e.epithelial lining of crypts;v.veins andc.curling arteries of subepithelial connective tissue.
The groups not belonging to the Ungulata which are characterized by the possession of a diffused placenta are the Sirenia, the Cetacea, Manis, and the Lemuridæ.
Sirenia. Of the Sirenia, the placentation of the Dugong is known from some observations of Harting (No.201).
It is provided with a diffuse and non-deciduate placenta; with the villi generally scattered except at the poles. The umbilical vesicle vanishes early.
Cetacea. In the Cetacea, if we may generalize from Turner’s observations on Orca Gladiator and the Narwhal, and those of Anderson (No.191) on Platanista and Orcella, the blastodermic vesicle is very much elongated, and prolonged unsymmetrically into two horns. The mesoblast (fig. 160) of the allantois would appear to grow round the whole inner surface of the subzonal membrane, but the cavity of the allantois only persists as a widish sack on the ventral aspect of the embryo (al). The amnion (am) is enormous, and is dorsally in apposition with, and apparently coalesces with the chorion, and ventrally covers the inner wall of the persistent allantoic sack. The chorion, except for a small area at the two poles and opposite the os uteri, is nearly uniformly covered with villi, which are more numerousthan infig. 160. In the large size of the amnion, and small dimensions of the persistent allantoic sack, the Cetacea differ considerably from the Ungulata.
Illustration: Figure 160Fig. 160. Diagram of the fœtal membranes in Orca gladiator.(From Turner.)ch.chorion;am.amnion;al.allantois;E.embryo.
Fig. 160. Diagram of the fœtal membranes in Orca gladiator.(From Turner.)ch.chorion;am.amnion;al.allantois;E.embryo.
Manis. Manis amongst the Edentata presents a type of diffused placenta[92]. The villi are arranged in ridges which radiate from a non-villous longitudinal strip on the concave surface of the chorion.
Manis presents us with the third type of placenta found amongst the Edentata. On this subject, I may quote the following sentence from Turner (Journal of Anat. and Phys.,vol.X., p.706).
“The Armadilloes (Dasypus), according to Professor Owen, possess a single, thin, oblong, disc-shaped placenta; a specimen, probably Dasypus gymnurus, recently described by Kölliker[93], had a transversely oval placenta, which occupied the upper2⁄3rds of the uterus. In Manis, as Dr Sharpey has shewn, the placenta is diffused over the surfaces of the chorion and uterine mucosa. In Myrmecophaga and Tamandua, asMM.Milne Edwards have pointed out, the placenta is set on the chorion in a dome-like manner. In the Sloths, as I have elsewhere described, the placenta is dome-like in its general form, and consists of a number of aggregated, discoid lobes. In Orycteropus, as I have now shewn, the placenta is broadly zonular.”
Lemuridæ. The Lemurs in spite of their affinities with the Primates and Insectivora have, as has been shewn by Milne Edwards and Turner, an apparently very different form of placenta. There is only one embryo, which occupies the body and one of the cornua of the uterus. The yolk-sack disappears early, and the allantois (Turner) bulges out into a right and left lobe, which meet above the back of the embryo. The cavity of the allantois persists, and the mesoblast of the outer wall fuses with the subzonal membrane (the hypoblastic epithelium remaining distinct) to give rise to the chorion.
On the surface of the chorion are numerous vascular villi, which fit into uterine crypts. They are generally distributed, though absent at the twoends of the chorion and opposite the os uteri. Their distribution accords with Turner’s diffused type. Patches bare of villi correspond with smooth areas on the surface of the uterine mucosa in which numerous utricular glands open. There is no reflexa.
Although the Lemurian type of placenta undoubtedly differs from that of the Primates, it must be borne in mind that the placenta of the Primates may easily be conceived to be derived from a Lemurian form of placenta. It will be remembered that in Man, before the true placenta becomes developed, there is a condition with simple vascular villi scattered over the chorion. It seems very probable that this is a repetition of the condition of the placenta of the ancestors of the Primates which has probably been more or less retained by the Lemurs. It was mentioned above that the resemblance between the metadiscoidal placenta of Man and that of the Cheiroptera, Insectivora and Rodentia is rather physiological than morphological.
Comparative histology of the Placenta.
It does not fall within the province of this work to treat from a histological standpoint the changes which take place in the uterine walls during pregnancy. It will, however, be convenient to place before the reader a short statement of the relations between the maternal and fœtal tissues in the different varieties of placenta. This subject has been admirably dealt with by Turner (No.222), from whose paperfig. 161illustrating this subject is taken.
The simplest known condition of the placenta is that found in the pig (B). The papilla-like fœtal villi fit into the maternal crypts. The villi (v) are formed of a connective tissue cone with capillaries, and are covered by a layer of very flat epithelium (e) derived from the subzonal membrane. The maternal crypts are lined by the uterine epithelium (e´), immediately below which is a capillary flexus. The maternal and fœtal vessels are here separated by a double epithelial layer. The same general arrangement holds good in the diffused placentæ of other forms, and in the polycotyledonary placenta of the Ruminantia, but the fœtal villi (C) in the latter acquire an arborescent form. The maternal vessels retain the form of capillaries.
In the deciduate placenta a considerably more complicated arrangement is usually found. In the typical zonary placenta of the fox and cat (D and E), the maternal tissue is broken up into a complete trabecular mesh-work, and in the interior of the trabeculæ there run dilated maternal capillaries (d´). The trabeculæ are covered by a more or less columnar uterine epithelium (e´), and are in contact on every side with fœtal villi. The capillaries of the fœtal villi preserve their normal size, and the villi are covered by a flat epithelial layer (e).
In the sloth (F) the maternal capillaries become still more dilated, and the epithelium covering them is formed of very flat polygonal cells.
Illustration: Figure 161Fig. 161. Diagrammatic representations of the minute structure of the placenta.(From Turner.)F.the fœtal;M.the maternal placenta;e.epithelium of chorion;e´.epithelium of maternal placenta;d.fœtal blood-vessels;d´.maternal blood-vessels;v.villus.A. Placenta in its most generalized form.B. Structure of placenta of a Pig.C. Structure of placenta of a Cow.D. Structure of placenta of a Fox.E. Structure of placenta of a Cat.F. Structure of placenta of a Sloth. On the right side of the figure the flat maternal epithelial cells are shewnin situ. On the left side they are removed, and the dilated maternal vessel with its blood-corpuscles is exposed.G. Structure of Human placenta. In addition to the letters already referred tods,ds. represents the decidua serotina of the placenta;t,t. trabeculæ of serotina passing to the fœtal villi;ca.curling artery;up.utero-placental vein;x.a prolongation of maternal tissue on the exterior of th villus outside the cellular layere´, which may represent either the endothelium of the maternal blood-vessel or delicate connective tissue belonging to the serotina, or both. The layere´represents maternal cells derived from the serotina. The layer of fœtal epithelium cannot be seen on the villi of the fully-formed human placenta.
Fig. 161. Diagrammatic representations of the minute structure of the placenta.(From Turner.)F.the fœtal;M.the maternal placenta;e.epithelium of chorion;e´.epithelium of maternal placenta;d.fœtal blood-vessels;d´.maternal blood-vessels;v.villus.A. Placenta in its most generalized form.B. Structure of placenta of a Pig.C. Structure of placenta of a Cow.D. Structure of placenta of a Fox.E. Structure of placenta of a Cat.F. Structure of placenta of a Sloth. On the right side of the figure the flat maternal epithelial cells are shewnin situ. On the left side they are removed, and the dilated maternal vessel with its blood-corpuscles is exposed.G. Structure of Human placenta. In addition to the letters already referred tods,ds. represents the decidua serotina of the placenta;t,t. trabeculæ of serotina passing to the fœtal villi;ca.curling artery;up.utero-placental vein;x.a prolongation of maternal tissue on the exterior of th villus outside the cellular layere´, which may represent either the endothelium of the maternal blood-vessel or delicate connective tissue belonging to the serotina, or both. The layere´represents maternal cells derived from the serotina. The layer of fœtal epithelium cannot be seen on the villi of the fully-formed human placenta.
In the human placenta (G), as in that of Apes, the greatest modificationis found in that the maternal vessels have completely lost their capillary form, and have become expanded into large freely communicating sinuses (d´). In these sinuses the fœtal villi hang for the most part freely, though occasionally attached to their walls (t). In the late stages of fœtal life there is only one epithelial layer (e´) between the maternal and fœtal vessels, which closely invests the fœtal villi, but, as shewn by Turner and Ercolani, is part of the uterine tissue. In the fœtal villi the vessels retain their capillary form.
Evolution of the Placenta.
From Owen’s observations on the Marsupials it is clear that the yolk-sack in this group plays an important, if not the most important part, in absorbing the maternal nutriment destined for the fœtus. The fact that in Marsupials both the yolk-sack and the allantois are functional in rendering the chorion vascular makes ità prioriprobable that this was also the case in the primitive types of the Placentalia, and this deduction is supported by the fact that in the Rodentia, Insectivora and Cheiroptera this peculiarity of the fœtal membranes is actually found. In the primitive Placentalia there was probably present adiscoidalallantoic region of the chorion, from which simple fœtal villi, like those of the pig (fig. 161B), projected into uterine crypts; but it is not certain how far the umbilical part of the chorion, which was no doubt vascular, may also have beenvillous. From such a primitive type of fœtal membranes divergences in various directions have given rise to the types of fœtal membranes now existing.
In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done,viz.(1) by increasing the complexity of the fœtal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by placental villi. Various combinations of the two processes would also of course be advantageous.
The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the fœtus.
The arrangement of the fœtal parts in the Rodentia, Insectivora and Cheiroptera may be directly derived from the primitive form by supposing the villi of the discoidal placental area to have become more complex, so as to form adeciduatediscoidal placenta; while the yolk-sack still plays a part, though physiologically an unimportant part, in rendering the chorion vascular.
In the Carnivora again we have to start from the discoidal placenta, as shewn by the fact that the allantoic region of the placenta is at first discoidal (p.248). A zonarydeciduateplacenta indicates an increase both in area and in complexity. The relative diminution of the breadth of the placental zone in late fœtal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the fœtus by insuring a more intimate relation between the fœtal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but as mentioned below, there are other cases where it can be shewn that a diminution in the area of the placenta has taken place, accompanied by an increase in the complexity of its villi.
The second type of differentiation from the primitive form of discoidal placenta is illustrated by the Lemuridæ, the Suidæ, and Manis. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any greatextent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. Themetadiscoidalplacenta of Man and Apes, from its mode of ontogeny (p.248), is clearly derived from a diffused placenta—very probably similar to that of Lemurs—by a concentration of the fœtal villi, which are originally spread over the whole chorion, to a disc-shaped area, and by an increase in their arborescence.
The polycotyledonary forms of placenta are due to similar concentrations of the fœtal villi of an originally diffused placenta.
In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta, such as that found in Manis. The zonary placenta of Orycteropus is capable of being easily derived from that of Manis, by the disappearance of the fœtal villi at the two poles of the ovum. The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that in Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentæ of the Armadilloes, Myrmecophaga, and the Sloths may easily have been formed from a diffused placenta, just as the discoidal placenta of the Simiadæ and Anthropidæ appears to have been formed from a diffused placenta like that of the Lemuridæ.
The presence of zonary placentæ in Hyrax and Elephas does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may quite easily be derived from a diffused placenta; and the presence of two villous patches at the poles of the chorion in Elephas indicates that this was very probably the case with the placenta of this form.
Although it is clear from the above considerations that the placenta is capable of being used to some extent in classification, yet at the same time the striking resemblances which can exist between such essentially different forms of placenta, as for instance those of Man and the Rodentia, are likely to prevent it being employed, except in conjunction with other characters.
Special types of development.
The Guinea-pig, Cavia cobaya. Many years ago Bischoff (No.176) shewed that the development of the guinea-pig was strikingly different from that of other Mammalia. His statements, which were at first received with some doubt, have been in the main fully confirmed by Hensen (No.182) and Schäfer (No.190), but we are still as far as ever from explaining the mystery of the phenomenon.
The ovum, enclosed by the zona radiata, passes into the Fallopian tube and undergoes a segmentation which has not been studied with great detail. On the close of segmentation, about six days after impregnation, it assumes (Hensen) a vesicular form not unlike that of other Mammalia. To the inner side of one wall of this vesicle is attached a mass of granular cells similar to the hypoblastic mass in the blastodermic vesicle of the rabbit. The egg still lies freely in the uterus, and is invested by its zona radiata. The changes which next take place are in spite of Bischoff’s, Reichert’s (No.188) and Hensen’s observations still involved in great obscurity. It is certain, however, that during the course of the seventh day a ring-like thickening of the uterine mucous membrane, on the free side of the uterus, gives rise to a kind of diverticulum of the uterine cavity, in which the ovum becomes lodged. Opposite the diverticulum the mucous membrane of the mesometric side of the uterus also becomes thickened, and this thickening very soon (shortly after the seventh day) unites with the wall of the diverticulum, and completely shuts off the ovum in a closed capsule.
The history of the ovum during the earlier period of its inclusion in the diverticulum of the uterine wall is not satisfactorily elucidated. There appears in the diverticulum during the eighth and succeeding days a cylindrical body, one end of which is attached to the uterine walls at the mouth of the diverticulum. The opposite end of the cylinder is free, and contains a solid body.
With reference to the nature of this cylinder two views have been put forward. Reichert and Hensen regard it as an outgrowth of the uterine wall, while the body within its free apex is regarded as the ovum. Bischoff and Schäfer maintain that the cylinder itself is the ovum attached to the uterine wall. The observations of the latter authors, and especially those of Schäfer, appear to me to speak for the correctness of their view[94].
The cylinder gradually elongates up to the twelfth day. Before this period it becomes attached by its base to the mesometric thickening of the uterus, and enters into vascular connection with it. During its elongation itbecomes hollow, and is filled with a fluid not coagulable in alcohol, while the body within its apex remains unaltered till the tenth day.