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amniote A vertebrate whose embryos are totally enclosed in a fluid-filled sac – the amnion. The evolution of the amnion provided the necessary fluid environment for the developing embryo and therefore allowed animals to breed away from water. Amniotes comprise the reptiles, birds, and mammals. Compare anamniote.

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amniote Applied to a type of development, typical of higher vertebrates (reptiles, birds, and mammals), in which the amnion surrounds the embryo in a bag of (amniotic) fluid. The amnion is primitively associated with a shell, and is capable of gaseous exchange; its development thus enabled eggs to be laid on dry land for the first time in vertebrate evolution.