Amniotic eggs are laid by reptiles, mammals, and birds. Within the egg are four extra-embryonic membranes: amnion, allantois, chorion, and yolk sac. The amnion serves to protect the embryo. The allantois manages wastes and gas exchange. The chorion also deals with gas exchange. The yolk sac contains yolk, which provides the embryo with nutrients. The amniotic egg as a whole provides an environment suitable for the development of the embryo. Eggs are used as opposed to laying the egg in water (water is needed for oxygen and diffusion of wastes). With amniotic eggs, these functions can be performed on land. The shell is very important since it prevents the egg from drying out.