The process of fusion of male and female gametes to form a zygote is called fertilization. The zygote develops into an embryo.

 

So we can classify these fertilizations into:

 

1. Internal fertilization
2. External fertilization
3. In-Vitro Fertilization

 

Internal Fertilization

 

Millions of sperm released by the male, swim in the oviduct with the help of tails to reach the egg. Only one fuse with the egg. When sperms come in contact with the egg, the nuclei of one sperm fuses with the nuclei of the egg and form a single nucleus. This results in the formation of a fertilized egg called a zygote. A zygote is the beginning of a new individual. So the fertilized egg is formed as a result of the fusion of, egg from female and sperm from the male, and therefore the new individual inherits some characteristics from the mother & some from the father.

 

Internal fertilization: Fertilization which takes place inside the body of the female is called internal fertilization.

 

Internal fertilization occurs in -
� Humans
� Cows
� Dogs
� Hens

 

External Fertilization: The fertilization in which the fusion of a male and a female gamete takes place outside the body of the female is called External Fertilization. As in the case of frogs. When a male and a female frog come together in water the female frog lays hundreds of eggs.

 

A layer of jelly holds the eggs together and provides protection to the eggs. As the eggs are laid, the male frog deposits sperms over the laid eggs. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs, which results in fertilization. This type of fertilization also occurs in aquatic animals such as fish, starfish, etc.

 

�In-Vitro Fertilization�

 

In some cases, women are unable to conceive a child because of some blockage in their oviducts and hence sperm cannot reach the egg for fertilization. In such cases, doctors collect freshly released eggs and sperms and keep them together for few hours.

 

This way fertilization occurs and a zygote is allowed to develop for about a week and then it is placed in the mother�s uterus. Complete development takes place in the uterus and the baby is born like any other baby. Babies born through this technique are called �Test-Tube Babies�. This type of fertilization is also known as �In-Vitro Fertilization.

 

Have you heard of test tube babies?

 

In real life, there are cases in some women, where their oviducts are blocked. These women are unable to bear babies because sperm cannot reach the egg for fertilization. In such cases, doctors collect freshly released eggs and sperms and keep them together for a few hours for IVF or in Vitro fertilization (fertilization outside the body).

 

In case fertilization occurs, the zygote is allowed to develop for about a week, and then it is placed in the mother�s uterus. Complete development takes place in the uterus and the baby is born like any other baby. Babies born through this technique are called test-tube babies. This term is actually misleading because babies cannot grow in test tubes.

 

Development of Embryo in the hen

 

In hen, soon after fertilization, the zygote divides repeatedly and travels down the oviduct. As it travels down many protective layers are formed around it The hard shell that you see in a hen�s egg is one of such protective layer. After a hard shell is formed around the developing embryo, the hen finally lays an egg.

 

Hen sits on an egg to provide sufficient warmth required for the development of the chick inside the egg. The embryo takes about 3 weeks to develop into a chick; after the complete development, it comes out from the shell.

 

For the animals which undergo external fertilization, the development of the embryo takes place outside the body of the female and after complete development, young ones come out from the egg.

 

Read More: Sexual Reproduction In Plants: Process, Pollination, and Dispersion