Fertilization << `fur` tuh luh ZAY shuhn >> is the process by which male and female sex cells unite to form a new individual. It is the first step in sexual reproduction. The male reproductive system produces sex cells called spermatozoa, or sperm. The female reproductive system produces sex cells called eggs, or ova. A fertilized egg, which results from the union of a sperm and an egg, is called a zygote. As the zygote develops further, it becomes an embryo. This article discusses fertilization among animals. For information about fertilization among plants, see Plant (The reproduction of plants).
Among animals, fertilization may be external or internal. During external fertilization, male and female sex cells unite outside the female’s body. This process occurs in water. The male releases sperm into the water at about the same time that the female releases eggs. Some sperm come into contact with eggs and fertilize them. Almost all aquatic invertebrates and some vertebrates, including the majority of fish and amphibians, reproduce in this way.
Internal fertilization occurs when the male deposits sperm directly into the female’s body. Most land animals, including insects, reptiles, birds, and mammals, reproduce in this way. In almost all mammals and some reptiles, the embryo develops inside the female’s body after fertilization. However, in birds and most reptiles, the embryo develops outside her body. The female lays an egg in which the embryo develops. The egg is protected by a shell and contains material that nourishes the growing embryo.
Scientists have developed methods of promoting fertilization in mammals, including human beings. For example, fertility drugs increase a female’s chances of becoming pregnant. In addition, eggs may be fertilized by a process called artificial insemination. In this process, sperm are collected from the male and later injected into the female by artificial methods.
Scientists have also united mammalian sperm and eggs in vitro—that is, in an artificial environment outside the body. The zygote must then be transferred into the female reproductive system to develop. The human infants that have resulted from in vitro fertilization are sometimes called “test-tube babies.”
