» For the alien race in Stephen R. Donaldson's The Gap Cycle, see Amnion (Gap Cycle). The amnion's a membranous sac that surrounds and protects an embryo. It's 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's the protection of the embryo for its development. In humans In the human embryo the earliest stages of the formation… (More on Amnion)