The development of the amniote egg freed the reptiles completely from life in the water by allowing them to fully reproduce on land. Unlike the young of amphibians, who had to go through a larval stage in the water before metamorphosing into an adult, the amniote egg acted as a sort of “private pond” for the young reptiles.

The egg itself had a hard shell, which contained numerous small pores. These pores allowed air to enter, but the shell prevented the inside from drying up as long as the surroundings were humid. The eggs were fertilized inside the mother’s body before being laid. There were three very thin bags inside the shell itself, each of which had a specific function. The first bag held the developing young and a liquid (which took the place of the pond or stream); this area was called the amnion, from which the egg gets its name. The next bag contained the yolk, the source of food for the developing embryo. The third bag was in contact with the air diffusing in through the shell. Thus, the young reptiles had food, air, protection from predators, and an aquatic environment in which to grow. The young would eventually hatch into a miniature version of its parents and was able to fend for itself. Because of the egg, the reptiles no longer had to have a source of water to reproduce and could spread out, populating the land as well as hunting for prey well away from water.