The reptiles were the first vertebrates to become fully terrestrial in so far as they had no need to return to water to breed. This was achieved through the evolution of the cleidoic (closed box) egg. Such an egg is large and has a tough leathery or calcareous shell. The shell protects the developing embryo from mechanical damage and, being porous, permits the entry of air for respiration and, at the same time, impedes loss of water by evaporation. The embryo is provided with a large amount of yolk and albumen giving food and water for growth. As it grows the embryo becomes covered with a fluid-filled bladder, the amnion, so that it develops, in a sense, within its own private pool and is thus further protected against mechanical shock and temperature change. A second bladder, the allantois, grows from the embryo’s hind gut and forms a highly vascular surface closely applied to the shell for gaseous exchange. The allantois also serves as a container for the uric acid nitrogenous waste of the embryo and is left behind when the young animal hatches. It is clear that covering the egg cell proper with a shell and albumen would prevent external fertilisation.
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