Placenta, or AFTER-BIRTH, the structure which unites the unborn mammal to the womb of its mother and establishes a nutritive connection between them. The placenta is peculiarly a mammalian structure, but it is not developed in Ornithorhynchus and Echidna, which lay eggs, nor is it more than incipient in the Marsupials, which bring forth their young after a short gestation. In all other mammals it occurs in various forms, partly embryonic in its origin, partly maternal, always acting as a double vascular sponge, by means of which the blood of the mother nourishes and purifies that of her unborn young. Vague prophecies of it occur in two cartilaginous fishes and in two lizards, in which there is a connection between the yolk-sac of the embryo and the wall of the oviduct.
In the hedgehog, which is a conveniently central type of mammal, the connection between embryo and mother has the following history. (a) The outer wall of the embryonic sac is moored to the wall of the uterus by small cellular outgrowths known as the preliminary 'villi,' and minute cavities between these are bathed by the blood of the mother. (b) The growing embryo becomes ensheathed by the double folds of the Amnion (q.v.), the inner parts of which form the 'amnion proper,' while the outer form the 'sub-zonal membrane.' Part of the yolk-sac wall fuses with this sub-zonal membrane; from the united area vascular villi grow out into the wall of the uterus, which is now much modified. Thus is formed a 'yolk-sac placenta,' as exhibited for a time by Insectivores and Rodents. (c) But the most important union between mother and offspring is that due to the union of Allantois (q.v.) and sub-zonal membrane. If there has been a yolk-sac placenta it dwindles before this new and more efficient union. From the united area vascular villi grow out into depressions or crypts in the uterine wall, part of which is modified into a spongy vascular tissue. In Insectivores, Bats, and Rodents the original outer wall of the embryonic sac persists between the placental villi and the maternal blood, and mediates between them.
The final placenta thus consists (1) of a maternal part—viz. a modified region on the wall of the uterus—and (2) of an embryonic part—viz. part of the allantois, fused to the sub-zonal membrane, and giving off vascular villi, between which and the maternal blood the persistent outer wall of the embryonic sac sometimes persists and mediates.
The term 'chorion' has been used in so many senses, that it seems advisable to abandon it. It is best applied to the union of sub-zonal membrane and allantois ('true chorion'), or to the union of sub-zonal membrane and yolk-sac ('false chorion').
The embryonic part of the placenta necessarily comes away at birth, and sometimes the vascular part of the maternal placenta is also discharged when the young is born. When this is the case, the placenta is called 'deciduate,' or better 'caducous.' When the maternal part of the placenta does not come away at birth the placenta is called 'indeciduate,' or better 'non-caducous.' Of non-caducous placentation two kinds are distinguished: Diffuse, when the villi are scattered over the surface of the embryonic sac (in Manis among Edentates, in the dugong, in Cetacea, in most Ungulates except Ruminants, in Lemurs); Cotyledonary, when the villi occur in patches (in Ruminants). Of caducous placentation three kinds are distinguished: Zonary, when the villi form a partial or complete girdle round the embryo (in Orycteropus and Dasypus among Edentates, in Elephants and Hyrax, in Carnivora); Discoidal, when the villi occur on a circular cake-like disc (in most Edentates, in Insectivores and Bats, in Rodents); Meta-discoidal, when the villi are at first scattered, but are afterwards restricted to a disc (in Monkeys and in Man). Sir William Turner, the 'grand-master of placental research,' allots the lowest place to such diffuse forms of placenta as that of the pig, but others maintain that the discoidal type as illustrated in the Insectivora is the most primitive. In Botany 'placenta' usually means the portion of the Ovary (q.v.) which bears the ovules. See also AMNION, ALLANTOIS, FÆTUS, MAMMAL.