The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish (Ichthyopsida), which are consequently termed “Anamniota”. The primary role of this is the protection of the embryo for its development. It stems from the extraembryonic somatic mesoderm on the outer side and the extraembryonic ectoderm on the inner side.
Read more about Amnion: In Humans, In Reptiles, Birds, and Many Mammals, Extra-amniotic Pregnancy, Additional Images