The placenta is an ephemeral organ present in placental vertebrates, such as some mammals and sharks during gestation (pregnancy).The placenta develops from the same sperm and egg cells that form the fetus, and functions as a fetomaternal organ with two components, the fetal part (Chorion Frondosum), and the maternal part (Decidua Basailis).
By the beginning of fourth month, the placenta has two components:
Maternal Portion: Decidua Basalis
Fetal Portion: Chorion Frondosum.
Placenta is bordered on the fetal side by Chorionic plate, and on the maternal side by Decidual plate (decidua basailis). Between the chorionic and decidual plates are intervillous spaces, which are filled with maternal blood.During the fourth and fifth months, the decidua forms a number of decidual septa, which project into intervillous spaces, but do not reach the chorionic plate. As a result of this septum formation, the placenta is divided into a number of compartments or cotyledons.