amnion An inner cellular layer (epithelium) surrounding the fluid-filled embryo sac of reptiles, birds, and mammals. The wall of this sac has two layers of epithelium, with mesoderm and coelomic space between. The outer layer is usually called the chorion. The amnion and chorion develop as folds in the ectoderm and mesoderm which grow up around the sides of the embryo, eventually covering it. The amniotic fluid within the sac provides a liquid environment for the embryo, necessary for animals that reproduce on land. It may also cushion the mammalian embryo against distortion by maternal organs pressing on it. Sometimes the term ‘amnion’ is applied to the entire embryo sac.

am·ni·on / ˈamnēˌän; -ən/ • n. (pl. - or -ni·a / -nēə/ ) the innermost membrane that encloses the embryo of a mammal, bird, or reptile.DERIVATIVES: am·ni·ot·ic / ˌamnēˈätik/ adj.

amnion A membrane that encloses the embryo of reptiles, birds, and mammals within the amniotic cavity. This cavity is filled with amniotic fluid, in which the embryo is protected from desiccation and from external pressure. See also extraembryonic membranes.

amnion Membrane or sac that encloses the embryo of a reptile, bird, or mammal. The embryo floats in the amniotic fluid within the sac. This sac usually ruptures shortly before birth to discharge the fluid, commonly referred to as ‘the waters breaking’. See also uterus

amnion (am-ni-ŏn) n. the membrane that forms initially over the dorsal part of the embryo but soon expands to enclose it completely within the amniotic cavity.
—amniotic (am-ni-ot-ik) adj.