Amnion

n. membrane which surrounds a fetus before birth (Anatomy, Zoology)

For the alien race in Stephen Donaldson's The Gap Cycle, see Amnion (Gap Cycle). The amnion is a membranous sac which surrounds and protects the embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibia and fish, which are consequently termed “Anamnia”. The primary function of this is the protection of the embryo for its future development into a fetus and eventually an animal.
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Noun
1. thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)
(synonym) amniotic sac, amnios
(hypernym) sac
(part-holonym) amniote
(classification) mammal

n. amnion, membrane which surrounds a fetus before birth (Anatomy, Zoology)

(n.)
A thin membrane surrounding the embryos of mammals, birds, and reptiles.