Fertilization - Internal And External Fertilization

Living organisms ensure their continuance on the earth by reproduction. Reproduction may occur either asexually or sexually. There is an evident difference between sexual reproduction and asexual reproduction. The sexual mode is more complex process than the asexual mode. One crucial difference is the fertilization. It is the main stage of sexual reproduction which is absent in asexual mode. Let’s learn more about types of fertilization.

Fertilization in Animals

The process of fusion of sperm with egg (ovum) to produce zygote is called fertilization. It is the crucial and primary stage of sexual reproduction.During sexual intercourse, penis ejaculates millions of sperms into the vagina of the woman. Sperms will travel through the uterus to oviducts. At oviduct, one out of million sperms fertilizes the released ovum. Fertilized egg develops into a zygote. Without fusion of gametes, sexual reproduction is futile. It doesn’t occur in asexual reproduction.


Fertilization in most of the animals is similar to that in humans. Animals also produce gametes for fusion. But the fusion of gametes may take place inside or outside the body. Based on this, fertilization is of two types – internal and external fertilizations.

Internal fertilization

In sexual reproduction, the male inserts the sperms into the female reproductive tract to fuse with the egg. If the fusion takes place within the female parent, it is called internal fertilization. In humans, most of the animals like cats, lions, pigs, dogs, hens, etc., the fusion of gametes takes place internally. In this type, a zygote is formed within the mother and gets its nourishment from her.

External fertilization

When the fusion of sperm and egg takes place outside the female parent, it is called external fertilization. Only a minority of organisms exhibit this type of gamete fusion. For example, fish, frogs, etc. Here the female parent deposits her eggs in a place and later, the male parent ejects his sperms over them, then fusion takes place.

Gametes that fuse externally has to face many challenges. Since eggs and sperms are deposited in the external environment, the chances of fusion are very less. Predators may eat eggs and the zygote that is formed. To compensate this loss, organisms like fish and frogs lay hundreds of eggs at a time.

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Practise This Question

In human beings, males attain puberty a little later than females.