Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
Those vertebrate animals, sometimes called Anamniota, which lack an amnion in development. The amnion is a protective embryonic envelope that encloses the embryo and its surrounding liquid, the amniotic fluid, during fetal life. An amnion is present in mammals, birds, and reptiles (collectively called the Amniota), but is absent in fishes and amphibians. See Amnion, Amniota, Amphibia, Pisces (zoology)