Vertebrate groups with amniotic eggs include reptiles, birds and mammals that lay eggs. In addition, the placenta of placental mammals is a modified version of the membranes within amniotic eggs. Animals in these groups are known as amniotes based on the amnion, a membrane that protects the embryo in eggs.
The amnion is filled with amniotic fluid, which provides the embryo with a stable fluid environment. It is the innermost of four important membranes in the egg. The other three are the allantois, yolk sac and chorion. The allantois aids with gas exchange and receives wastes from the embryo. The yolk sac contains the embryo's food source. The chorion encloses them all, and it is surrounded by the albumin and shell. The shell protects the egg and prevents water loss while allowing for the exchange of gases.
The development of the amniotic membrane and other aspects of amniote eggs was crucial for animals to fully colonize land. The eggs of the other vertebrate groups, amphibians and fish, have only a single membrane and take in oxygen and remove waste by simple diffusion. This requires the animals to lay their eggs in water. Eggs in air require special structures for gas exchange, waste management and the prevention of dessication.