The watertight egg developed in the common ancestor of turtles, lizards, dinosaurs (including birds), mammals, and their relatives. These animals belong to a group called amniotes, named for the watertight membrane inside the egg -- the amnion. Filled with fluid, the amnion surrounds the developing embryo and keeps it from drying out. Even though humans do not lay eggs, there is still an amnion in the fertilized eggs inside human females. It was probably the watertight egg that allowed amniotes to colonize the land, since they no longer had to return to a wet environment to lay their eggs.

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