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Applied to a type of development typical of higher vertebrates (reptiles, birds, and mammals), in which the amnion (a protective membrane) surrounds the embryo in a bag of (‘amniotic’) fluid. Evolutionarily, 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. Compare anamniotic.

Subjects: Earth Sciences and Geography.

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