Types of Fertilisation

Internal Fertilisation and External Fertilisation

We have just studied that the fusion of a sperm with an egg is called fentilisation Now, the egg (or ovum) is made in the body of a female animal. So, the fertilisation of an egg by a sperm can take place either inside the body of the female animal or outside the body of female animal. This leads to two modes of fertilisation in animals : internal fertilisation, and extenal fertilisation.

The fertilisation which takes place inside the female body is called internal fertilisation. In internal fertilisation, the female animal's eggs are fertilised by sperms inside her body. In internal fertilisation, the male animal puts his sperms into the female animal's body And these sperms then fertilise the eggs inside her body. For example, a  man puts his sperms inside a woman's body. These sperms then fertilise the egg inside the  woman's body. This type of fertilisation in which the fusion of a male gamete (sperm and a female gamete (egg) occurs inside the body of the female animal, is called internal fertilisation. Internal fertilisation takes place in a very large number of animals such humans, cows, dogs, cats, tigers, lions, rabbits, deer, horse, birds (such as hen, sparrow crow, pigeon, etc.), reptiles (such as lizard, snake and crocodile, etc.), and insects (such as silk moth, housefly and butterfly, etc.). In all these animals, fertilisation occurs inside the body of the female animal.

The fertilisation which takes place outside the female body is called external fertilisation. In external fertilisation, the female animal's eggs are fertilised by sperm outside her body. In frogs and most fishes, the fertilisation of eggs occurs outside the female animal's hody. In extemal 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. External fertilisation is very common in aquatic animals such as frog, fish and star fish, etc. In these animals, fertilisation takes place in water. We will now describe the external fertilisation in frogs in somewhat detail.

During spring season (or rainy season). frogs move to the water in ponds and slow- moving streams. When the male frog and female frog come together, the female frog lays hundreds of eggs in water (see Figure 4). Unlike the hen's eggs, frog's eggs are not covered by hard shells. The frog's eggs are very delicate. A layer of jelly holds the frog's eggs together and provides protection to the eggs. When the female frog lays hundreds of eggs, at the same time the male frog releases millions of sperms from its body (see  Figure 4). The sperms swim randomly in water with the help of their long tails. When the sperms of frog come in contact with the eggs floating in water, then fertilisation takes place. This type of fertilisation in which the fusion of a male gamete (sperm) and a female gamete (egg) takes place ouside the body of the female animal, is called external fertilisation. Just like frogs, external fertilisation also takes place in fish. When the male Gish and female fish come together in water, the female fish lays hundreds of eggs in water and at the same time, the male fish releases millions of sperms in water. When the Sperms come in contact with eggs, the eggs get fertilised. In this way, a female fish's eggs are fertilised by sperms outside its body. This is also an example of external fertilisation.

We will now explain why the animals like frog and fish lay hundreds of eggs at a time (whereas a hen lays only one egg at a time). Though the animals like frog (and fish) which undergo external fertilisation, lay hundreds of eggs, all the eggs do not get fertilised and hence do not develop into new frogs (or fishes). This is because of the following reasons

(i) Frog (and fish) lay eggs and release sperms in water. Many of these eggs and sperms are carried away by the movement of water, wind or rainfall, and hence all the eggs do not get fertilised.

(ii) Many of the eggs of frogs (and fish) are eaten up by other animals which live in water.

(iii) Many of the larvae (young ones) of frog and fish which are hatched from the fertilised eggs are eaten up by other animals in water and fail to develop into adult frogs (or fish).

So, out of hundreds of eggs laid by the female frog (or female fish), only a few will survive to become adult frogs (or adult fish). Since many of the eggs of frogs and fish are carried away by moving water, wind or rain, and many are eaten up by other animals, therefore, the production of a large number of eggs is necessary to ensure the fertilisation of at least a few of them. Out of hundreds of eggs laid by female frog (or female fish) only a few get fertilised and their larvae survive to become adult frogs (or adult fish).