1. Fertilisation that takes place outside the body of mother is called external fertilisation. This is most common in animals like fishes and amphibians. As the chance of fertilisation is controlled by nature it becomes necessary to give rise to vast number of eggs and sperms by these animals.
2. Fertilisation that takes place inside the body of mother is called internal fertilisation. This is common in most of the land animals. As the chance of fertilisation is not controlled by the nature, these animals generally produce less number of eggs.
