A gamete is a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm. Ova and sperm are haploid cells, meaning that each cell carries only one copy of each chromosome. During fertilization, a sperm and ovum unite to form a new diploid organism. In animals, ova mature in the ovaries of females, and sperm develop in the testes of males. Each sperm cell, or spermatozoon, is small and motile due to the flagellum, a tail-shaped structure that allows the cell to propel and move. In contrast, each egg cell, or ovum, is relatively large and non-motile.