Fertilization, the union of a male and a female sex cell to form a new individual. If the male and female sex cells are from different individuals of the same species, the union is called cross-fertilization. Fertilization is essential to reproduction in most higher plants and animals. The female sex cell, or female gamete, is called an egg, or ovum; the male gamete, a sperm, or spermatozoon. Fertilization is accomplished when a sperm enters an egg. The fertilized egg is called a zygote. The entrance of a sperm into an egg usually causes a chemical reaction that prevents other sperms from invading the egg. The zygote immediately begins to divide, and is then called an embryo.

Gametes, unlike other cells, contain only half the normal number of chromosomes (bodies made up of the genes through which hereditary traits are transmitted from parents to offspring). Hence the new individual inherits half of its traits from each parent.

Both gametes also make other contributions to the zygote. The egg contributes most of the cytoplasm, the substance that surrounds the nucleus of the cell. In addition, the egg provides yolk as food for the zygote. The sperm supplies a little of the cytoplasm and the acrosome, which initiates the cell divisions through which the zygote develops into an embryo.