External fertilization

Background
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the body of the female. Thus, the fertilization is said to occur "externally". This is distinct from internal fertilization where the union of the egg and sperm occur inside the female after insemination through copulation.

Many aquatic animals reproduce by external fertilization, including fish, whose females spawn ova which are fertilized by the sperm that male fish secrete into the water.

In sexual reproduction, there must be some way of getting the sperm to the egg. Since sperm are designed to be motile in a watery environment, aquatic animals can make use of the water in which they live. In many aquatic animals such as coral or Hydra, eggs and sperm are simultaneously shed into the water, and the sperm swim through the water to fertilize the egg in a process known as broadcast fertilization. In many fish species, including salmon, the female will deposit unfertilized eggs in the substrate and the male will swim by and fertilize them.

Many land plants make use of external fertilization as well. For example, bees and butterflies brush against pollen when gathering nectar from flowers and spread them to another flower of the same species, pollinating that plant.

体外受精 (生物)