Fertilisation, the second stage in sexual reproduction. During this stage, the male and female gametes fuse to form a diploid zygote. Fertilisation does not occur in every sexually reproducing organism. In certain organisms, such as turkey, rotifers, honeybees and even some lizards such as Cnemidophorus neomexicanus, a process called parthenogenesis takes place.
Fertilisation, on the other hand, occurs in a wide variety of aquatic as well as terrestrial organisms. However, the way fertilisation occurs in aquatic organisms is very different from the way it occurs in terrestrial organisms. In most aquatic organisms including algae, fish and amphibians such as frogs, fertilisation is external where the male and female release their gametes in water and the fusion of the gametes as well as the formation of the zygote occurs outside the body of the organism, that is, in an external medium such as water. Frogs and other organisms including fish that engage in external fertilisation undergo several bodily changes during the breeding season to attract mates. Such changes occur synchronously which ensures successful mating as well as perfect timing during the release of gametes. This enhances the chances of successful fertilisation.
Internal fertilisation is common in terrestrial organisms including fungi, reptiles, birds, mammals and in a majority of plants including bryophytes, pteridophytes, gymnosperms and angiosperms. In internal fertilisation, the fusion of gametes occurs inside the body of the female. While, the female gamete, which is formed inside the body of the female organism, is stationary in nature the male gamete is motile, which makes it possible for internal fertilisation to take place.
Male gametes are produced in large numbers, there is a remarkable reduction in the number of female gametes that are produced. However, not all terrestrial organisms possess a motile male gamete. In seed plants, the pollen grains present inside the anther bear non-motile male gametes. These pollen grains, therefore, depend on agents such as the wind, bees and insects to carry them to the stigma. When these pollen grains are deposited on the stigma, they germinate to form a pollen tube. This tube then travels downwards towards the ovule, where it discharges the male gamete. These male gametes unite with the female gamete to form a zygote, thus completing the fertilisation process. Fertilisation, thus, is an important process that ultimately results in the formation of a new life.