Amnion
innermost membranous sac that surrounds and protects the developing embryo
The amnion (pl.: amnions or amnia) is a membrane that closely covers human and various other embryos when they first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect 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 amnion is a feature of the vertebrate clade Amniota, which includes reptiles, birds, and mammals. Amphibians and fish lack the amnion and thus are anamniotes (non-amniotes). The amnion stems from the extra-embryonic somatic mesoderm on the outer side and the extra-embryonic ectoderm or trophoblast on the inner side.
Etymology
Etymologists have traditionally assumed that the Greek term ἀμνίον (amnion) relates to Ancient Greek ἀμνίον : amníon, "little lamb", a diminutive of ἀμνός : amnós, "lamb", and cognate with the English verb yean, "to bring forth young (usually lambs)". However, an alternative etymology references an ancient Greek goddess of childbirth, Eileithyia, worshipped in Amnisos (on the island of Crete) and nicknamed Ἀμνιάς (Amnias).
The public source identifies “Amnion” as innermost membranous sac that surrounds and protects the developing embryo. This brief keeps that definition visible, then builds a research path around Amnion, innermost and membranous.
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This entry incorporates text from “Amnion” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.