During sexual reproduction, males and females produce specialized cells which fuse with one another and lead to the origin of a new organism. These cells are known as reproductive cells or sex cells or gametes. Sperm and ovum are examples of gametes.

Fertilisation is the process by which male and female gametes are fused together, initiating the development of a new organism. A zygote is formed when an egg cell is fertilised due to the union of a female gamete called the ovum with a male gamete called sperm. In the embryonic development of humans and other animals, the zygote stage is brief and is followed by cleavage, when the single cell becomes subdivided into smaller cells.