The membrane and cavity that encloses the developing embryo and fetus in fluid (essential for all vertebrates that reproduce out of the sea), cushioning the embryo and fetus from the effects of pressure. Unlike the chorion, contains cells that originate from the fetus itself.

Cavity enclosed by the amnion, which grows from the embryo within the gestational sac, gradually catching up with it in size by about 14 weeks, when the amnion and chorion fuse, and after which amniocentesis becomes practicable.

The outermost of the two membranes surrounding the gestational sac that encloses the developing embryo. Separate from the amnion until about 14 weeks of pregnancy, after which the two membranes stick together without intervening fluid. Like the placenta, with which at the margins it is joined seamlessly, derived from trophectoderm.