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a means of reproducing offspring in which the embryo develops in the mother’s body, is nourished directly by the mother through a placenta, and is born more or less developed and free of any egg covering.
Viviparity is counterposed to oviparity, in which the development of the embryo and its release from the egg covering occur outside the mother’s body (after the eggs are laid). The historical connection between viviparity and oviparity is indicated by rare cases of ovoviviparity, in which the embryo develops from an egg in the mother’s body and is nourished chiefly on substances stored in the egg itself, but also is secreted by the mother’s body and is freed of the egg covering, usually after the egg is laid.
Among invertebrates, viviparity is characteristic of some coelenterates, arthropods, mollusks, worms, and echinoderms. Among chordates, it is characteristic of salpas, many sharks and rays, some cyprinodonts, frogs, Caeciliidae, salamanders, tortoises, lizards, and snakes, and of the majority of mammals (except Echidna, Proechidna, and the platypus), including man. Development of the embryo in viviparity may occur in the ovary, the oviducts or dilations of them converted into a uterus, or in the vagina. In many viviparous animals embryonic membranes are formed around the embryo.