am'nion

A thin double membrane, forming an ovoid bag, the immediate investment of the embryo in reptiles, birds and mammals, and containing the liquor amnii. false a. The reflected amnion, the serosa or primitive chorion. See chorion, liquor amnii. Amniotic fluid, the liquid commonly surrounding termed the fetus "the amniotic in the waters," sac. It increases in quantity as gestation progre es, its office being to protect the fetus and the umbilical cord from pre ure and act as a fluid wedge in dilating the cervix uteri during the first stage of labor, reflected a. See false a. [Gr., amnion, a caul, diminutive of amnos, a lamb, from its being first observed as a fetal envelope in that animal.]