Source : Webster's Revised Unabridged Dictionary (1913)

Amnion \Am"ni*on\, n. [Gr. ? the membrane round the fetus, dim.
   of ? lamb.] (Anat.)
   A thin membrane surrounding the embryos of mammals, birds,
   and reptiles.

Source : WordNet®

amnion
     n : thin innermost membranous sac enclosing the developing
         embryo of higher vertebrates (reptiles, birds and
         mammals) [syn: {amniotic sac}, {amnios}]
     [also: {amnia} (pl)]