A zoological term referring to animals or vertebrates that lack an amnion (a membrane surrounding the embryo).
From Greek an- 'without' + amnion 'membrane.' The amnion is a protective sac around embryos; animals without it include fish and amphibians. This is a technical classification term from comparative embryology.
The evolution of the amnion was a game-changer in vertebrate history—once animals developed this protective membrane, they could lay eggs on land instead of needing water, opening up entire continents for colonization.
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