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 in [fig. 147], 5. Around the embryo is seen the amnion, already separated by a considerable interval from the embryo. The yolk-sack is shewn at ds. Relatively to the other parts it is considerably smaller than it was at an earlier stage. The allantoic stalk is shewn at al. 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 uterine wall, 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 in [fig. 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.
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; l Fallopian 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.