The chorion is the outermost 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 vitteline envelope and is developed by the follicle cells while the egg is in the ovary.he chorion is one of the four extraembryonic membranes that make up the amniotic egg that provide for the nutrients and protection needed for the embryo's survival. It is located between the albumin, which is the white of the egg, and the eggshell. It encloses the embryo and the rest of the embryonic system.