The amniotic egg contains several membranes that allowed animals to live a fully terrestrial life style.

• The allantois holds metabolic wastes produced by the embryo.
• The chorion and the allantois exchange gases between the embryo and the air.
• The amnion protects the embryo in a fluid-filled cavity that serves as a cushion.
• The yolk sac provides nutrients via blood vessels; other nutrients are stored in the albumen (egg white).