the extraembryonic membrane that helps in the formation of the placenta is the
Chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles. It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline membrane in other animals. In insects it is developed by the follicle cells while the egg is in the ovary.[1] The main function of the chorion is to protect and nurture the embryo.[2]