<Tr> <Th> TE </Th> <Td> E5. 11.3. 1.1. 0.3 </Td> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> 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 . The main function of the chorion is to protect and nurture the embryo . </P> <P> In humans and other mammals (excluding monotremes), the chorion is one of the fetal membranes that exist during pregnancy between the developing fetus and mother . The chorion and the amnion together form the amniotic sac . It is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes . The chorionic villi emerge from the chorion, invade the endometrium, and allow the transfer of nutrients from maternal blood to fetal blood . </P>

The extraembryonic membrane that helps in the formation of the placenta is the