Fertilization:

The fertilization is the complete and permanent fusion of two gametes from different parents or from the same parent to form a diploid zygote. This process is also called syngamy. Although the terms syngamy and fertilization are frequently used interchangeable. If syngamy does not occur, there would be no variations in the offspring.

Where does fertilization occur?

Fertilization occurs either in external medium (water) or inside the body of the organism. Thus there are two types of gametic fusion namely external fertilization and internal fertilization.

(i) External Fertilization:

When fertilization occurs outside the body of the organ­ism, this type of gametic fusion is called external fertilization or external syngamy. The external medium such as water is required for this type of fertilization. Thus in most aquatic organisms such as a majority of algae, fishes and amphibians, external fertilization occurs.

Organisms exhibiting external fertilization produce a large number of gametes in water to enhance the chances of fertilization. This happens in bony fishes and frogs where a large number of offspring are produced. A major disadvantage of this type of fertilization is that the offspring are not protected from the predators and their survival is threatened up to adulthood.

(ii) Internal Fertilization:

When egg is formed inside the female body where it fuses with the male gamete, the process is called internal fertilization or internal syngamy. Many terrestrial organisms belonging to fungi, higher animals such as reptiles, birds and mammals and majority of bryophytes, pteridophytes, gymnosperms and angiosperms are the examples where internal fertilization occurs.

Here male gamete is mobile and has to reach the egg in order to fuse with it. The number of sperms produced is very large but there is reduction in the number of eggs produced. However, in seed plants, the non-motile male gametes are carried to the female gamete by pollen tubes.