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the type of reproductive process in which fertilization occurs between sex cells, or gametes, that differ greatly in size, shape, and behavior. The female gamete, or ovum, is large, nonmotile, and nonflagellated. The male gamete—a spermatozoon in animals and a spermatozoid in plants—is considerably smaller and usually motile, with one or more flagella. More rarely, the male gamete is nonflagellated, examples being the spermatium in some lower plants and the sperm of many gymnosperms and all angiosperms. Oogamy is characteristic of all multicellular animals, many lower plants, and all higher plants. (SeeHETEROGAMY, ISOGAMY.)