Vertebrate adaptations for terrestrial Life.

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…shells, which do not have to worry about losing water. The amniotic cavity formed by the amnion is fluid-filled, protecting the embryo. The egg case often leathery in reptiles, and calcified in birds protects the contents, while permitting gas to be exchanged with the surroundings. This egg also prevents the evaporation of water from the embryo, since the egg cannot walk to the store and buy some Evian, it needs all to water it has.