develops from chorion The chorion is one of the membranes that exists during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood. chorion (vertebrates) chorion frondosum chorionic sac embryonic chorion fetal chorion uterine chorion mesoderm-derived structure extraembryonic membrane amniotic fold