In case of external fertilisation, the number of eggs produced is more than the number of eggs produced in internal fertilisation. External fertilisation usually occurs in aquatic animals. These animals lay hundreds of eggs and release millions of sperms. However, many of the eggs do not get fertilised (fuse with sperm) to develop into new individuals. This is because the eggs and sperm get exposed to water currents and wind. Also, there are other animals (predators) which may feed on these eggs. Thus, production of a large number of eggs and sperm is necessary to ensure fertilisation of at least a few of them.