» For the alien race in Stephen R. Donaldson's The Gap Cycle, see Amnion (Gap Cycle). 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 function of this is the protection of the embryo for its development. It stems from parts of the mesoderm on the o… (More on Amnion)