Placenta

PLACENTA, the organ by which the embryo is nourished within the womb of its mother. When the young one is born the placenta and membranes come away as the "afterbirth." The human placenta is a circular disk about 7 or 8 in. in diameter and 14 in. in thickness at its centre, while at its margin it is very thin and is continuous with the foetal membranes. It weighs about a pound.

Mode of Formation.

Before each menstrual period, during the child-bearing age of a woman, the mucous membrane of the uterus hypertrophies, and, at the period, is cast off, but if a fertilized ovum reaches the uterus the casting off is postponed until the birth of the child. From the fact that the thickened mucous membrane lining the interior of the uterus is cast off sooner or later, it is spoken of as the "decidua." The fertilized ovum, on reaching the uterus, embeds itself in the already pre pared decidua, and, as it enlarges, there is one part of the decidua lying between it and the uterine wall ("decidua serotina" or "basalis"), one part stretched over the surface of the enlarging ovum ("decidua reflexa" or "capsularis") and one part lining the rest of the uterus ("decidua vera") (see fig. ). It is the decidua basalis which is specially concerned in formation of the placenta. That part which is nearest the ovum is called the "stratum compactum," but farther away the uterine glands dilate and give a spongy appearance to the mucous membrane (stratum spongiosum). Processes from the surface of the ovum penetrate the stratum compactum and push their way into the enlarged maternal blood sinuses; these are the "chorionic villi." Later, a very immature condition, finish their development in their mother's pouch ; but although these mammals have no allantoic placenta there is an intimate connection between the walls of the yolk-sac and the uterine mucous membrane, and so an umbilical placenta exists. The name Aplacentalia therefore only means that they have no allantoic placenta. Among the Placentalia the umbilical and allantoic placentae sometimes coexist for some time, as in the hedgehog, bandicoot and mouse.

In the Carnivora, elephant, procavia (Hyrax) and aard vark (Orycteropus), there is a "zonary-placenta" which forms a girdle round the embryo. In sloths and lemurs the placenta is dome

shaped, while in rodents, insectivores and bats, it is a ventral disk or closely applied pair of disks, thus differing from the dorsal disk of the ant-eater, armadillo and higher Primates, which is known as a "metadiscoidal placenta." Thus the form of the pla centa is not an altogether trustworthy indication of the systemic position of its owner. In the diffuse and cotyledonous placentae the villi do not penetrate very deeply into the decidua, and at the "allantoic" or "abdominal stalk" grows from the mesoderm of the hind end of the embryo into the chorionic villi which enter the decidua basalis, and in this blood-vessels push their way into the maternal blood sinuses. Eventually the original walls of these sinuses and the false amnion, disappear, and nothing sep arates the maternal from the foetal blood except the delicate walls of the foetal vessels covered by undifferentiated, nucleated tissue (syncytium), derived from the chorionic epithelium, so that the embryo is able to take its supply of oxygen and materials for growth from the blood of its mother and to give up carbonic acid and excretory matters. It is the gradual enlargement of the chorionic villi in the decidua basalis together with the intervillous maternal blood sinuses that forms the placenta ; the decidua cap sularis and vera become pressed together as the embryo enlarges, and atrophy. The allantoic stalk elongates enormously, and in its later stages contains two arteries (umbilical) and one vein em bedded in loose connective tissue known as "Wharton's jelly." At first the stalk of the yolk-sac is quite distinct from this, but later the two structures become bound together (see fig. 2), as the "umbilical cord." A distinction must be made between the allantoic stalk and the allantois; the latter is an entodermal out growth from the hind end of the mesodaeum or primitive alimentary canal, which in the human subject only reaches a little way toward the placenta. The allantoic stalk is the mass of mesoderm containing blood-vessels which is pushed in front of the allantois and, as has been shown, reaches and blends with the decidua basalis to form the placenta.