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The intimate association or fusion of a tissue or organ of the embryonic stage of an animal to its parent for physiological exchange to promote the growth and development of the young. It enables the young, retained within the body or tissues of the mother, to respire, acquire nourishment, and eliminate wastes by bringing the bloodstreams of mother and young into close association but never into direct connection. Placentation characterizes the early development of all mammals except the egg-laying duckbill platypus and spiny anteater. It occurs in some species of all other orders of vertebrates except the birds. In fact, in certain sharks and reptiles it is almost as well developed as in mammals. A few examples are also known among invertebrates (Peripatus, certain tunicates, and insects). See Fetal membrane
Efficient interchange depends on close proximity of large areas of fetal tissues to maternal blood and glandular areas. This is provided in mammals by a remarkable regulatory cooperation between the developing outer layer (trophoblast) of the chorion, together with the vascular yolk sac or allantois or both, and the mother's uterine lining (endometrium). In the typical mammalian placenta, which is always formed by the chorion and the allantoic vessels, the fetal and maternal bloodstreams are as close as a few thousandths of a millimeter from each other (see illustration). The surface area of the fetal villi which contain the functional fetal capillaries is probably several times larger than the body surface of the female. In humans this ratio is known to be about 8:1.
the location of the placenta in the ovary of an angiospermous plant. A distinction is made between laminar placentation, in which the placentas are located in the inner surface of the carpel (on the sides or along the midrib), and marginal placentation, in which the placentas are located near the margins of the carpel.
Syncarpous gynoecia are marked by central marginal placentation: the placentas are located near the margins of the carpel, in the corners of the locules of the ovary, and along the ovary’s longitudinal axis. Parietal marginal placentation—the disposition of the placentas on the ovary wall near the margins of the carpel—is characteristic of paracarpous gynoecia. With free central placentation the ovules are borne by a central column; this arrangement characterizes lysicarpous gynoecia. In apical and basal placentation, the placentas are located at the apex or base of the ovary, respectively. The most primitive type of placentation is laminar, with placentas scattered over the inner surfaces of the carpel.