An organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble, but not particulate, blood-borne substances through an apposition of uterine and trophoblastic vascularized parts. According to species, the area of vascular apposition may be diffuse, cotyledonary, zonary or discoid; the nature of apposition may be labyrinthine or villous; the intimacy of apposition may vary according to what layers are lost of those originally interposed between maternal and foetal blood (maternal endothelium, uterine connective tissue, uterine epithelium, chorion, extraembryonic mesoderm, and endothelium of villous capillary). The chorion may be joined by and receive blood vessels from either the yolk sac or the allantois, and the uterine lining may be largely shed with the chorion at birth (deciduate) or may separate from the chorion and remain (nondeciduate). The human placenta is discoid, villous, haemochorial, chorioallantoic, and deciduate. After birth, it weighs about 600 gm. and is about 16 cm. in diameter and 2 cm. thick, discounting a principal functional part, the maternal blood in the intervillous space (which leaks out at birth) into which the chorionic villi dip. The villi are grouped into adjoining cotyledons making about 20 velvety bumps on the side of the placenta facing outward to the uterus; the inner side of the placenta facing the foetus is smooth, being covered with amnion, a thin avascular layer that continues past the edges of the placenta to line the entire hollow sphere of chorion except where it is reflected to cover the umbilical cord, which joins foetus and placenta. The cord usually joins the placenta near the centre but may insert at the edge, on the nonplacental chorion, or on an accessory placenta.