a vascular organ, developed within the uterus of most mammals during gestation from the chorion of the embryo and a part of the maternal uterine wall, that is connected to the embryo by the umbilical cord and that is discharged shortly after birth, which serves as the structure through which nourishment for the foetus is received from, and wastes of the foetus are eliminated into, the circulatory system of the mother; the placenta.