Reproduction, as previously intimated, is mainly of two kinds, namely: somatogenic reproduction, which is less general and confined to the metists, and cytogenic reproduction, which is common to metists and protists, and which is the ordinary method by which new organisms originate. Reproduction is termed somatogenic, when the germ separated from the body of the parent consists of a whole mass of somatic or tissue cells not expressly set aside and specialized for reproductive purposes. Reproduction is termed cytogenic, when the germ separated from the parent or parents consists of a single cell (_e.g._ a spore, gamete, or zygote) dedicated especially to reproductive purposes. Cytogenic reproduction may be either nonsexual (agamic) or sexual, according as the cell which constitutes the germ is an agamete or a gamete. An agamete is a germ cell not specialized for union with another complementary cell, or, in other words, it is a reproductive cell incapable of syngamy, _e.g._ a spore. A gamete, on the other hand, is a reproductive cell (germ cell) specialized for the production of a zygote (a synthetic or diploid germ cell) by union with a complementary cell, _e.g._ an egg, or a sperm. Nonsexual cytogenic reproduction is of three kinds, according to the nature of the agamete. When a unicellular organism gives rise to two new individuals by simple cell-division, we have fissiparation or binary fission. When a small cell or bud is formed and separated by division from a larger parent cell, we have budding (gemmation) or unequal fission. When the nucleus of the parent cell divides many times to form a number of daughter-nuclei, which then partition the cytoplasm of the parent cell among themselves so as to form a large number of reproductive cells called spores, we have what is known as sporulation or multiple fission. The first and second kind of nonsexual reproduction are confined to the protists, but the third kind (sporulation) also occurs among the metists.