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Amnion Summary


For the alien race in Stephen R. Donaldson's The Gap Cycle, see Amnion (Gap Cycle).The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus. The amnion, along with the chorion, the yolk sac and the allantois form a protective sac around the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus. The term amnion is from Ancient Greek ἀμνίον 'little lamb', diminutive of ἀμνός 'lamb'. it is cognate with the English verb 'yean', bring forth young (usually lambs). The amnion is a feature of the vertebrate clade Amniota, which includes reptiles, birds, and mammals. Amphibians and fish are not amniotes and thus lack the amnion. The amnion stems from the extra-embryonic somatic mesoderm on the outer side and the extra-embryonic ectoderm or trophoblast on the inner side.

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