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amnion
A tough membrane or sac that surrounds the embryo and fetus in the uterus. The outside of the amnion is covered by another membrane called the chorion.
Amnions occur in mammals, birds, and reptiles, which are for this reason called amniotes. All land-laid eggs contain amnions; those of fish and amphibians do not, and thus must be laid in moist surroundings or water.
See also amniotic sac, amniotic fluid, and placenta.
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