Fertilisation is the process of fusion of male and female gametes to form a zygote. The two types of fertilisation are internal and external fertilisation. Internal fertilisation occurs when the fusion of male and female gamete takes place inside the body of a female animal. For example, in humans, dogs, cows, birds, etc. External fertilisation occurs when the fusion of sperm and egg takes place outside the body of the female parent. It is very common in aquatic animals, such as fish, starfish, and amphibians. In humans, the sperm travels through the uterus to the oviducts, where one out of millions of sperm fertilizes the released ovum. The fertilized egg develops into a zygote. Without the fusion of gametes, sexual reproduction is futile.