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14. Reproduction eLearn.Punjab

14.4.2 Fertilization

After the formation of gametes, fertilization occurs. There are two mechanisms by which fertilization
can take place i.e. external fertilization and internal fertilization.

In external fertilization, egg cells are fertilized outside of body. External fertilization occurs mostly
in aquatic environment. It requires both the male and the female animals to release their gametes
into their surroundings at almost the same time. For external fertilization, the animals have to
release great number of gametes. In external fertilization, there is risk of loss of gametes due to
environmental hazards such as predators. External fertilization occurs in many invertebrates and
the first two groups of vertebrates i.e. fishes and amphibians.

In internal fertilization, egg cells are fertilized
within the reproductive tract of female. It
occurs in reptiles, birds and mammals. Such
animals provide protection to the developing
embryo. After fertilization, reptiles and birds
make protective shells around their egg cells
and then lay them.

Figure 14.20: External fertilization in fish

The shell is resistant to water loss and damage. In
mammals (with the exception of egg-laying mammals) the
development of fertilized egg into new baby takes place
within mother body. In this case, there is extra protection
to the embryo and mother also supplies everything that
embryo needs.

Figure 14.21: Reptiles and Bird’s egg provides
protection and food to embryo

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