The placenta is an ephemeral organ present in placental vertebrates, such as eutherial mammals and sharks during gestation (pregnancy). The term placenta comes from the Latin for cake, from Greek plakoenta, accusative of plakoeis - πλακοείς, "flat", referencing its appearance in humans. Protherial (egg-laying) and metatherial (marsupial) mammals do not produce a placenta. The placenta develops from the same sperm and egg cells that form the fetus, and functions as a foetomaternal organ with two components, the foetal part (Chorion frondosum), and the maternal part (Decidua basalis).
In humans, the placenta averages 22 cm (8.66 in.) in length and 2-2.5 cm (0.79-0.98 in.) in thickness. It typically weighs approximately 500 grams (17.64 oz.). It has a dark reddish/blue or maroon color. It connects to the fetus by an umbilical cord of approximately 55-60 cm (21.65-23.62 in.) in length that contains two arteries and one vein. The umbilical cord inserts into the chorionic plate. Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotelydons. In humans the placenta usually has a disc shape but different mammalian species have widely varying shapes.
