placenta

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A membranous vascular organ that develops in female mammals during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord. Following birth, the placenta is expelled.
2. n. An organ with similar functions in some nonmammalian animals, such as certain sharks and reptiles.
3. n. Botany The part within the ovary of a flowering plant to which the ovules are attached.

Century Dictionary and Cyclopedia

1. n. In zoöl., anat., and medicine: The organ of attachment of a vertebrate embryo or fetus to the wall of the uterus or womb of the female. It is a specially modified part of the surface of the chorion or outside one of the fetal envelops, of a flattened circular form, like a plate or saucer, one side of which is closely applied to the wall of the womb, and from the other side of which proceeds the umbilical cord or navel-string. It is highly vascular, and in intimate vital connection with a similarly vascular area of the uterine walls, serving for the interchange of the constituents of the blood between the female and the fetus, and thus acting during in-tra-uterine life as the organ of circulation, respiration, and nutrition of the fetus. The human placenta is about as large as a soup-plate, and in connection with the navel-string and membranes is commonly known as the uterine cake, afterbirth, or secundines. The presence of a true placenta is necessarily restricted to viviparous vertebrates, and does not occur in all of these (the two lower subclasses of mammals, the marsupials and monotremes, being implacental). Several forms of placenta have been distinguished among placental mammals, and made a basis of classification. See also cuts under embryo and uterus.
2. n. In echinoderms, a flat discoidal sea-urchin, as a sand-dollar or cake-urchin: used in a generic sense by Klein, 1734.
3. n. [capitalized] A genus of bivalve mollusks, now called Plaruna.
4. n. In botany, that part of the ovary of flowering plants which bears the ovules. It is usually the more or less enlarged or modified margins of the carpellary leaves, and is of a soft cellular texture. When the ovary is composed of a single leaf, both margins give rise to ovules, and they are consequently in two rows. In a compound ovary there are various modifications of the placenta. Thus.when the edges of the car-pellary leaes all meet in a common axis, the placentas are said to be axile. When, by obllteration of the dissepiments, such an ovary becomes one-celled, the axile placentas remain in a column as a free central placenta. Or, when the edges of the carpellary leaves barely meet and slightly incurve, the placentas become parietal, being borne on the wall. There are all degrees of incurvation, the placentas being located accordingly. In vascular cryptogams the point giving rise to the sporangia is sometimes called the placenta. The placenta is sometimes termed the trophospermium and spermophorum. See also cut under ovary.

Wiktionary

1. n. anatomy A vascular organ in mammals, except monotremes and marsupials, present only in the female during gestation. It supplies food and oxygen from the mother to the foetus, and passes back waste. It is implanted in the wall of the uterus and links to the foetus through the umbilical cord. It is expelled after birth.
2. n. botany In flowering plants, the part of the ovary where ovules develop; in non-flowering plants where the spores develop.

GNU Webster's 1913

1. n. (Anat.) The vascular appendage which connects the fetus with the parent, and is cast off in parturition with the afterbirth.
2. n. (Bot.) The part of a pistil or fruit to which the ovules or seeds are attached.

WordNet 3.0

1. n. the vascular structure in the uterus of most mammals providing oxygen and nutrients for and transferring wastes from the developing fetus
2. n. that part of the ovary of a flowering plant where the ovules form