The development of eggs with protective shells was the single thing that freed reptiles from the watery environment which amphibians needed to reproduce. The egg of a reptile contains membranes, which surround the developing embryo. They provide a protective pool of liquid and food stores for use while the embryo grows. The young reptile completes its development inside the egg, and emerges perfectly formed as a miniature version of the adult.
Reptiles also differ in having a scaly skin that helps protect them from predators, in contrast to the thin naked skin of amphibians. Their heart and circulation is also improved to allow them to have a more active lifestyle.
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