The extraembryonic (foetal) membranes of a mammalian embryo are the amnion, chorion, allantois and yolk sac.Amnion develops from the somatopleure, which consists of ectoderm on the inner side and extraembryonic mesoderm on the outer side.Therefore, the amnion is derived from ectoderm and mesoderm.For comparison, chorion is formed from trophoblast (trophectoderm) and extraembryonic mesoderm, while the allantois and yolk sac membrane have endoderm on the inner side and extraembryonic mesoderm on the outer side.
