For the alien race in Stephen R. Donaldson's The Gap Cycle, see Amnion (Gap Cycle).

The amnion is a membrane that when first formed, closely covers the embryo. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo. It is a feature of the amniotes which includes reptiles, birds, and mammals. Amphibians and fish are excluded from this group. The amnion stems from the extraembryonic somatic mesoderm on the outer side and the extraembryonic ectoderm on the inner side.