sexual reproductive cell that fuses with another sexual cell in the process of fertilization. The cell resulting from the union of two gametes is called a zygote; the zygote usually undergoes a series of cell divisions until it develops into a complete organism.

Gametes, also called germ cells, vary widely in structure. The simplest sexual organisms are isogamous, that is, they produce a single kind of gamete. The identical gametes unite in pairs to produce zygotes. Although all isogametes are apparently alike in structure, they are thought to be different in physiological constitution, because gametes from the same individual do not successfully unite. Isogametes are found among lower fungi, such as molds, and in many algae. Within these groups, however, are found other species in which one gamete is larger than the other. This is the beginning of heterogamy, or anisogamy, which is sexual reproduction involving the fusion of two dissimilar gametes.

All higher plants are heterogamous; the gametes they produce are either female or male. The female gamete is called the egg; the male gamete is called the sperm. The organ of gamete production in plants is called a gametangium.

All animals and animallike lower organisms that reproduce sexually, except a few protozoans, are also heterogamous. The male gametes are called spermatozoa; female gametes, ova or eggs. The gamete-producing organs of animals are called gonads. The formation of gametes in the gonads of animals is called gametogenesis. By this process the number of chromosomes in the sex cells is reduced in number from diploid to haploid, which is half the number of chromosomes in the normal body cells of the species. The diploid number of human chromosomes, for example, is 46. When a human sex cell divides to form two gametes, each gamete receives only half, or 23, of the normal complement of chromosomes. This type of cell division is called meiosis. The normal total of chromosomes is restored during fertilization when two gametes fuse, with each contributing half of the chromosomes required by the zygote.