amniotic

views updated May 08 2018

amniotic 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.