The amniotic egg allows reptiles, birds and mammals to lay eggs on land without drying out. The fluid-filled cavity containing the embryo is separated from the external environment by an amniotic sac that is impermeable to water. It is protected by an external hard shell.
Amniotic eggs are the evolutionary separation between amphibians and reptiles. Amphibians lay eggs that must remain moist. Lacking a protected fluid-filled cavity requires the non-amniotic eggs of amphibians to remain in water. With the development of the amniotic egg, reptiles were able to lay their eggs on land. This birthing shift from land to water cultivated other differences that made reptiles suitable for terrestrial instead of aquatic life.
The amniotic egg contains a series of fluid-filled membranes including the amnion, allantois, yolk sac and chorion. The amnion is the layer closes to the embryo. It is filled with amniotic fluid that cushions the embryo with moisture. The allantois layer allows gases and waste to escape from the inner part of the egg. Nutrition is supplied to the developing embryo through the yolk sac. The chorion provides the final enclosure.
As reptiles laying amniotic eggs transitioned from water to land they gave rise to other species. Besides the amniotic egg, differences in skull size and shape evolved. Skull shape is one factor used to differentiate reptiles, mammals and birds.