From the fifth week, the ovum is composed of three distinct layers. An internal, the amnion, which, growing away from the embryo, forms a cavity which fills up, more and more, with the amniotic fluid. An external, the chorion, furnished with ramifying villi, which cover the whole of the ovum, and penetrate into the mucous membrane of the uterus. A middle, the allantois, which, thinning out more and more as it tends t,o disappear, becomes a vesicle, a cellular layer, which brings the umbilical vessels to the finest radicles of the chorionic The whole is surrounded by the uterine decidua. From this time forth, the vital conditions of the ovum change. While up to now, it is the umbilie,a1 vesicle which has furnished nourishment t,o the ovum and the embryo, from the moment when, through the allantois, the umbilical vessels reach the mucous membrane of the uterus, it is these which supply nourishment to the fcetus, and we are going to witness the formation of a new organ, the placenta, which becomes the centre of fcetal nutrition. The umbilical vesicle atrophies and collapses, and it is to be found, be tween the amnion and the chorion, as a little vesicle adhering to the embryo by a long obliterated pedicle, the omphalo-mesenteric cord, in which are seen traces of the primitive vessels of the embryo. Before, however, the placenta is formed, the ovum undergoes many other changes, 4which concern in particular the amnion and the chorion.
At first adhering to the embryo, from which it is an offshoot, except at the level of the ventral opening, the amnion recedes more and more towards the dorsal and ventral portion, forming thus a complete sac in which the embryo is plunged, a sac, which, at its ventral surface, is going to form an addition t,o the organs which issue from the abdominal cavity of the fcetus to form the umbilical cord. It thus gradually meets the chorion, and is separated from it only by the cellular layer, which is the remnant of the allantois. The amnion and chorion then grow simulta neously, and, the amniotic fluid increasing, the decidua reflexa is, in turn, pushed towards the parietal decidua, and, towards the fourth month, this decidua joins the parietal and merges into it.
During this period, what becomes of the villosities which we have seen covered the entire ovum ? While those which cover the side of the ovum, towards the parietal decidua develop further to form the placenta, those which correspond to the decidua, reilexa, pushed aside by the growth of the ovum, are flattened out, and their vessels obliterated. These villi atrophy, and, when the decidua reflexa and parietal meet, these villi exist no longer, so to speak, and the ovum remains in communication with the mother only by the placental villi, which, bathing in the uterine sinuses, become the medium of exchange in nutritive substances which go towards the development of the fcetus. As the ovum developes, these
utero-placental adhesions become the stronger, so strong in certain in stances that we are obliged to detach the placenta even at term. The nearer we approach term the feebler, normally, become these adhesions, and consequently, the greater the ease with which the ovum detaches itself from the uterus.
.After three months and a half to four months, miscarriage becomes a labor in miniature. The placenta is fully formed, the ovum has definite structure. The uterine mucous membrane has sent solid epithelial pro longations between the villi, so as to intimately attach the ovum to the uterus, and protect it against destruction. While prior to this, the ovum lived, so to speak, by its entire periphery, under the influence of the de velopment of the amnion, and of its greater distension by the liquor amnii, the distended chorion loses its villi, and thins out, and, at the same time, displaces before it the decidua reflexa, which becomes similarly thinned out. At the end of the fourth month, the parietal decidua and the re flexa unite, and the ovum exists as it remains till the end of pregnancy.
Attached to the uterus by the placenta, the ovum is compmed of the amnion and the chorion,. thin membranes, of great tenuity, and it is covered by the decidua. In the liquor ammi swims the fcetus, attached to the cord, which gains in size and in length. It is especially important to remember that the placenta is relatively larger than the fcetus, and, above all, more solid and resisting.
The second factor consists in the modifications of the uterus. These, we have seen, concern both the cervix and the body, and are characterized by increase in size, in capacity, in weight, by change in form, in situation, in consistency, in thickness, and above all by modifications in the serous, muscular, and mucous layers. These changes, and modifications, we have already sufficiently noted. Let us recall only the transformation of the inter-utero placental mucous membrane into a species of spongy tissue, the whole of the in terstices of which are filled with blood in which the villi of the placenta are plunged, and are united to this tissue by epithelial bands from the mucous membrane; and further, let us recall the progressive develop ment of the muscular tissue, which, through the exercise of its funda mental property, contractility, becomes the active, essential agent, which expels the ketus. Let us add, finally, the shedding of the uterine mucous membrane, a process which to-day, it is admitted, concerns not alone the parietal decidua, but also the superficial part of the inter-utero-placental mucous membrane.