sex, or reproductive, cells in animals and plants, which, by their fusing, make possible the development of a new individual and the transmission of hereditary traits from the parents to the offspring. Gametes have a single (haploid) set of chromosomes which is ensured through the complex process of gametogenesis. Two gametes belonging to individuals of different sexes fuse during fertilization to form a zygote which thus receives a full (usually double, or diploid) set of chromosomes arid which develops into a new organism.

Several types of sexual processes are distinguished according to the morphology of the gametes: heterogamy (subdivided into heterogamy proper, or anisogamy, and oogamy), isogamy, and zygogamy. In the case of heterogamy (in the broad sense) the two gametes participating in fertilization differ in form and (or) in dimensions; the female gamete is called the egg cell, the male gamete is called the sper-matozoid or sperm. The most common type of heterogamy is oogamy (occurring in all multicellular animals and in all higher and many lower plants) in which the egg cell is a large, usually nonmotile cell (macrogamete) often containing a store of nutrients or accompanied by special cells for the nourishing of the future embryo, and the sperms are small (microgametes) and capable of movement. The actively moving male gametes are called spermatozoids; they have a vibrating “tail,” or flagellum (in all vertebrates and most invertebrates), or two (in many invertebrates) or more flagella (among higher nonflowering plants and many algae). Male gametes which lack flagella and are nonmotile or which move actively by so-called amoeboid movements—that is, by forming pseudopodia into which the contents of the cell flow (in roundworms, most arthropods, and certain myriapods) or move passively as a result of the growth of a pollen tube (in gymnospermous and angiospermous plants)—are called sperms. It is possible that in many organisms with nonmotile sperms the egg cell plays an active role in fusion by grasping the sperm with its pseudopodia. In gymnospermous and angiospermous plants the sperm is the generative nucleus of the pollen tube which grows out of the pollen grain; each pollen tube contains two such sperms. In heterogamy proper, or anisogamy (in a number of green and brown algae), both gametes participating in fertilization are motile, equipped with flagella, and often indistinguishable in form but differ in size (microgamete and macrogamete). In isogamy, which occurs in certain green algae, myxomycetes, and lower fungi, both gametes which form the zygote are identical morphologically but differ physiologically and are designated as (+) and (-) gametes. In zygogamy in many lower plants the notion of gamete largely loses its meaning because the sexual process in these organisms consists of the fusing of two physiologically different sexual parts of a mycelium (in zygomycetes) or of two vegetative cells of the sexual generation (in Conjugatae and diatoms) or of specialized portions of the mycelium (in higher fungi). Either the two cell nuclei which fuse during this sexual process or the portions of the protoplast containing these nuclei may be referred to as gametes. In unicellular animals the individuals themselves which enter the phase of sexual reproduction and which fuse during fertilization can be considered gametes.