The amniotic egg was an evolutionary invention that allowed the first reptiles to colonize dry land more than 300 million years ago. Fishes and amphibians must lay their eggs in water and therefore cannot live far from water. But thanks to the amniotic egg, reptiles can lay their eggs nearly anywhere on dry land.
The amniotic egg of reptiles and birds is surrounded by a tough outer shell that protects the egg from predators, pathogens , damage, and drying. Oxygen passes through tiny pores in the shell, so the embryo doesn't suffocate. Inside the shell are four sacs. The first sac inside the shell is the chorion, which carries oxygen from the shell to the embryo and waste carbon dioxide from the embryo to the shell. Within the chorion is the amnion, the membrane for which the amniotic egg is named. The amnion keeps the embryo from drying out, so it's critical to living on land. A third sac, the allantois, stores wastes from the embryo and also fuses with the chorion to form the chorioallantoic membrane, which carries oxygen and carbon dioxide to and from the embryo, just like a lung. A fourth membrane, the yolk sac, holds and digests nutritious yolk for the developing embryo.
Together, the shell and membranes create a safe watery environment in which an embryo can develop from a few cells to an animal with eyes and ears, brain, and heart. Because reptiles, birds, and mammals all have amniotic eggs, they are called amniotes.
The duck-billed platypus and some other mammals also lay eggs. But most mammals have evolved amniotic eggs that develop inside the mother's womb, or uterus, and so lack a shell. In humans and other mammals, the chorion fuses with the lining of the mother's uterus to form an organ called the placenta. The placenta transports oxygen and carbon dioxide to and from the embryo and delivers nutrients from the mother's blood.
Browder, Leon W., Carol A. Erickson, and William R. Jeffery. Developmental Biology, 3rd ed. Philadelphia, PA: Harcourt College Publishing, 1991.
Dorit, Robert L., Warren F. Walker, and Robert D. Barnes. Zoology. Philadelphia, PA: Harcourt College Publishing, 1991.