Viviparity is a form of reproduction found in most mammals and in several other species. Viviparous animals give birth to living young that have been nourished in close contact with their mothers' bodies. Humans, dogs, and cats are viviparous animals. Viviparous animals differ from egg-laying animals, such as birds and most reptiles. Egg-laying, or oviparous, animals obtain all nourishment as they develop from the yolk and the protein-rich albumen, or "white," in the egg itself, not from direct contact with the mother, as is the case with viviparous young.
The offspring of both viviparous and oviparous animals develop from fertilized eggs, but the eggs of viviparous animals lack a hard outer covering or shell like the chicken egg. Viviparous young grow in the adult female until they are able to survive on their own outside her body. In many cases, the developing fetuses of viviparous animals are connected to a placenta in the mother's body. The placenta is a special membranous organ with a rich blood supply that lines the uterus in pregnant mammals. It provides nourishment to the fetus through a supply line called an umbilical cord. The time between fertilization and birth of viviparous animals is called the gestation period.
All mammals except the platypus and the echidnas are viviparous. Only these two unusual mammals, called montremes, lay eggs. Some snakes, such as the Garter snake, are viviparous. So are some lizards and even a few insects. Ocean perch, some sharks, and a few popular aquarium fish, guppies, and mollies are also viviparous.
Although certain snakes give birth to live young, they are not viviparous. These snakes hatch from eggs which never leave the body of the parent snake. Because these young snakes hatch from eggs, and do not receive nourishment directly from the mother's body, this type of reproduction is called ovoviviparity. It is considered a more primitive form of reproduction than viviparity.
See also Embryo and embryonic development.