Internal Fertilization

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Fertilization is the process of fusion of male and female gametes. The nuclei of the sperm fuse with the nuclei of the egg and form a single nucleus which results in the formation of a zygote. Fertilization can be external or internal.

What is Internal Fertilization?

Internal fertilization is the process of fertilization that occurs inside the body of an individual. It occurs mostly in land animals. For internal fertilization to occur there needs to be a method for the introduction of male sperms into the female reproductive tract. Different animals have different methods of transferring the sperms. For eg., in mammals, reptiles, and some fish, this is done by copulation. In birds, this is done by the cloacal kiss.

There are three ways in which internal fertilization produces offsprings. They are:

  • Oviparity

  • Ovoviviparity

  • Viviparity


The fertilized eggs are laid outside the body of the mother. The egg receives nourishment from the yolk. For eg., fish, amphibians, reptiles and all birds are all oviparous.


The fertilized eggs are retained in the female, and the embryo receives nourishment from the yolk. When they hatch, the young ones are fully developed. This is seen in some bony fish, sharks, lizards, snakes, etc.


In this, the young one develops inside the mother and receives nourishment through the placenta. Most mammals, a few reptiles, and cartilaginous fish are viviparous.

Advantages of Internal Fertilization

The advantages of internal fertilization are:

  • Chances of fusion of gametes are more.

  • The probability of successful fertilization is increased.

  • The mates are selective.

  • The chances of dehydration of gametes decreases.

  • The young one is protected against predators and the outside environment and hence chances of survival are increased.

Internal Fertilization in Humans

Fertilization in humans is internal. The male introduces the sperms into the female reproductive tract. The sperm fuses with the egg in the ampulla of the fallopian tube. This results in the formation of a zygote which undergoes mitosis to form an embryo. The embryo is then implanted into the uterus. The entire process of development of the child takes place inside the uterus. Once the child is completely developed, it is delivered.

Internal fertilization, however, increases the risk of sexually transmitted diseases in individuals. But, it is considered to be one of the most favourable methods of fertilization.

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Frequently Asked Questions on Internal fertilization

What animals use internal fertilization?

Internal fertilization is observed in reptiles, birds and mammals.

How does internal fertilization occur?

Internal fertilization occurs by the transfer of male gametes into the female body. The fusion of male and female gamete takes place inside the female body.

What is external fertilization?

The fertilization of male and female gamete outside the female body is termed external fertilization. E.g. Amphibians

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