Biology » The Reproductive System » Fertilization

Summarizing Fertilization


Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can occur either outside the bodies or inside the female. Both methods have advantages and disadvantages. Once fertilized, the eggs can develop inside the female or outside. If the egg develops outside the body, it usually has a protective covering over it. Animal anatomy evolved various ways to fertilize, hold, or expel the egg. The method of fertilization varies among animals. Some species release the egg and sperm into the environment, some species retain the egg and receive the sperm into the female body and then expel the developing embryo covered with shell, while still other species retain the developing offspring through the gestation period.



common body opening for the digestive, excretory, and reproductive systems found in non-mammals, such as birds

external fertilization

fertilization of egg by sperm outside animal body, often during spawning

internal fertilization

fertilization of egg by sperm inside the body of the female


process by which fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg


process by which fertilized eggs are retained within the female; the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched


specialized sac that stores sperm for later use


process in which the young develop within the female, receiving nourishment from the mother’s blood through a placenta

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