Fertilization is the process by which the male and female gametes fuse to form the zygote. There are two types of fertilization- External fertilization- It is the fertilization where the eggs are fertilized outside the body of the organisms. They occur mostly in wet environments. It requires both the male and the female to release their gametes into their surroundings (usually water) and this process is called spawning. Eg- amphibians and fishes. Internal fertilization- In this type of fertilization the eggs are fertilized within the female reproductive tract. It helps in the protection of the developing egg.  Eg- mammals and birds.