Definition: Innermost of the extraembryonic membranes enveloping the embryo in utero and containing the amnionic fluid; it consists of an internal embryonic layer with its ectodermal component and an external somatic mesodermal component; in the later stages of pregnancy the amnion expands to come in contact with and partially fuse to the inner wall of the chorionic sac; derived from the trophoblast cells.
Synonym(s): amnionic sac
[G. the membrane around the fetus, fr. amnios, lamb]
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