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Amnion

(Science: anatomy, obstetrics) 1. a thin but tough extraembryonic membrane of reptiles, birds and mammals that lines the chorion and contains the foetus and the amniotic fluid around it. In mammals, it is derived from trophoblast by folding or splitting. 2. Thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals). The amnion is a fluid filled sac that forms around a foetus; its main function is to provide protection to the foetus.