We have just studied that the fusion of a sperm with an ovum (or egg) is called fertilisation. Now, the ovum (or egg cell) is made in the body of the female animal. So, the fertilisation of an egg by a sperm can takes place either inside the body of the female animal or outside its body. This leads to two modes of (fertilisation in animals: internal fertilisation and external fertilisation.
The fertilisation which occurs inside the female body is called internal fertilisation. In internal fertilisation, the female animal’s eggs are fertilised by sperms inside her body. In mammals (including human beings), birds and reptiles, the fertilisation occurs inside the female body. In other words, internal fertilisation takes place in mammals (including human beings), birds and reptiles.
In internal fertilisation, the male animal puts his sperms into the female animal’s body. This transfer of sperms from the testes oil the male animal into female animal’s body occurs at the time of copulation (or mating). Copulation is act by which the male animal transfers his sperms into the female animal’s body.
During copulation, very large numbers of sperms are discharged into the female body. These sperms fertilise the eggs inside body. For example, during copulation a man puts his sperms inside a woman’s body through and or called penis. These sperms then fertilise the egg inside the woman’s body. So, this is a case of internal fertilisation.
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The fertilisation which occurs outside the female body is called external fertilisation. In external fertilisation, the female animal’s eggs are fertilised by sperms outside its body. In amphibians (like frogs and toads) and fishes, the fertilisation of eggs occurs outside the female animal’s body.
In other words, in amphibians (like frogs and toads) and fishes, external fertilisation takes place. In external fertilisation, the male and female animals release their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs.
For example, the males and females of frogs and fishes release their sperms and eggs in water in which they live. The sperms then collide with the eggs and fertilise them outside the body of female frog or fish.
From the above discussion we conclude that there are two different modes of fertilisation in nature: internal fertilisation and external fertilisation. The fertilisation in humans, cats, dogs and birds are examples of internal fertilisation. The fertilisation in frogs and fishes are the examples of external fertilisation
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We know that the new cell which is formed by fertilisation is called ‘zygote’. And this zygote then grows and develops into a full organism (or baby animal). The method in which a zygote grows and develops into a full organism also varies in different animals. For example, in human beings the zygote grows and develops into a baby inside the female body (mother’s body).
And then the mother gives birth to the baby. Just like humans, the animals like cats and dogs also give birth to their young ones. But the process is entirely different in the animals (like birds) which lay eggs.
For example, a hen sits on its fertilised eggs for a considerable time to give them warmth. During this period, the zygote grows and develops form a complete chick. This chick then comes out of the egg by breaking its shell. It is clear from discussion that all the organisms do not give birth to individuals like humans do.