External fertilization is limited essentially to animals living in aquatic environments. The flagellated sperm must have fluid in which to swim, and the eggs lack a protective coat or shell and would dry out in the air. Almost all aquatic invertebrates, most fish, and many amphibians use external fertilization.
Most land animals, both invertebrate and vertebrate, use internal fertilization. In effect, the sperm cells are provided with the sort of fluid environment that is no longer available to them outside the animals’ bodies. The sperm can remain aquatic, swimming through the film of fluid present on the walls of the female reproductive tract. Once fertilized, the egg is either enclosed in a protective shell and released by the female, or held within the females’ body until the embryonic stages of development have been completed. Internal fertilization requires close physiological and behavioral synchronization of the sexes, which involves extensive hormonal control.