The chorion is a vital membrane that surrounds the developing embryo in mammals, including humans. It plays a crucial role in the formation of the placenta, which provides nutrients and oxygen to the growing fetus while removing waste products. The chorion is composed of trophoblast cells that help in the implantation of the embryo into the uterine wall. In addition to its role in nutrient exchange, the chorion also contributes to the development of the amniotic sac, which protects the embryo in the womb. This membrane is part of the extraembryonic membranes, which support the embryo's growth and development during pregnancy.