Fertilisation

Question 1 What is fertilisation?

Question 2 What is external fertilisation? Explain with example?

Question 3 What is internal fertilisation? Explain with example?

Question 4 Define the term zygote?

Question 5 How is zygote formed?

Question 6 Why does a female frog lay hundreds of eggs?

Question 7 How does external fertilisation take place in frog?

Fertilisation

The fusion of male gamete(sperm) with female gamete(egg)give rise to a new cell called zygote.

The new cell which is formed by the fusion of a male gamete and female gamete is called zygote (fertilised egg)

Zygote is a single cell which contain one nucleus. The nucleus of zygote is formed by the combination of nuclei of sperm and egg.

The fusion of male gamete with a female gamete to form a zygote during sexual reproduction is called fertilisation.

Many sperms try to enter the egg, only one sperm is successful in entering the egg. When the head of sperm enters the egg, then the nucleus of sperm cells fuses with the nucleus of egg cell to form a new nucleus. This results in the formation of fertilised egg or zygote.

All the multicellular animals start their life from a single cell called zygote.

The fertilisation of an egg by sperm can take place either inside the body of female or outside the body of female animal.

 

Two modes of fertilisation are:

(1) Internal Fertilisation

(2) External Fertilisation

Internal Fertilisation

The fertilisation which take place inside the female body is called internal fertilisation.

The female animal’s eggs are fertilised by sperm inside her body. The male animal puts his sperm into the female animal’s body. And these sperms then fertilise the eggs inside body.

Type of fertilisation in which the fusion of a male gamete and a female gamete occurs inside the body of the female animal is called internal fertilisation.

For Example: Humans, cow, dogs, cats, tigers, lions, rabbit, deer, horse, birds, reptiles etc.

External Fertilisation

The fertilisation which take place outside the female body is called external fertilisation.

The female animal’s eggs are fertilised by sperms outside her body.

In frogs and fishes, the fertilisation of eggs occurs inside the female animals’s body. Male and female animal release their sperms and eggs in water where fertilisation take place by collision between sperm and eggs.

(1) During spring season, frog move to the water in ponds and slow moving streams.

(2) When the male frog and female frog come together, the female frog lays hundreds of eggs in water.The frog’s eggs are very delicate.

(3) A layer of jelly holds the frog’s egg together and provides protection to the eggs.When the female frog lays hundred of eggs , at the same time the male frog releases millions of sperm from its body.

(4) The sperm swim randomly in water with the help of their long tails.When the sperms of frog come in contact with the eggs floating, then fertilisation takes place.

Type of fertilisation in which the fusion of male gamete and a female gamete takes place outside its body is called external fertilisation.

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. This is because:

(1) Many of these eggs and sperm are carried away by the movement of water, wind or rainfall and hence all the eggs do not get fertilised.

(2) Many of the eggs of frogs are eaten up of other animals which live in water.

(3) Many of the larvae 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 frog or fish.

Out of hundreds of eggs laid by the female frog or fish, only a few will survive to become adult frogs or fish. 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 fish only a few get fertilised and larvae survive to become adult frogs or fish.

The new cell which is formed by fertilisation is called zygote. And this zygote then grows and develops into a full organism. The method in which zygote grows and develops into a full organism also varies in different animals.