Ans. 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.
External 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 environment hazards such as predators. External fertilization occurs in many invertebrates and the first two groups of vertebrates i.e. fishes and amphibians.
Internal Fertilization
In Internal fertilization egg cells are fertilized within the reproductive tract female. It occurs in reptiles, birds and mammals. Such animals provide protection to the developing embryo. After fertilization, reptiles and birds make proti shells around their egg cells and then lay them. The shell is resistant to loss and damage. In mammals (with the exception of egg-laying mammals development of fertilized egg into new baby takes place within mother's body. I this case, there is extra protection to the embryo and mother also supplk everything that embryo needs. Figure -14.18