Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization: oviparity, ovoviparity, and viviparity.
In oviparity, fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. These animals are classified as oviparous.
In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk; the young are fully developed when they are hatched. This occurs in some bony fish (such as the guppy, Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake, Thamnophis sirtalis), some vipers, and some invertebrate animals (such as the Madagascar hissing cockroach, Gromphadorhina portentosa).
In viviparity, the young develop within the female, receiving nourishment from the mother’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles, making these animals viviparous.
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