FISSION

in biology, the general process of cell division. If two daughter cells are formed, the process is called binary fission. The production of more than two cells is multiple fission. Binary fission represents the major reproductive procedure of unicellular organisms, but it also occurs in the process of embryonic development and tissue growth and repair of higher plants and animals. It generally follows nuclear doubling, whether in mitosis or in meiosis (qq.v.). Bacteria reproduce principally by transverse binary fission. The mother cell enlarges until it divides into two identical daughter cells. Certain metazoan animals, e.g., some planarian species, regularly reproduce asexually by a separation of the body into two portions, a method also referred to as fission. Regular transverse fission at the neck region of the tapeworm and at the oral end of the scyphostome polyp is distinguished as strobilation. Commonly, this results in a chain, called a strobilus, of the fission productsthe proglottids of tapeworms and the ephyrae of scyphozoan jellyfish; each proglottid or ephyra matures in turn and separates from the end of the strobilus. A few metazoan species regularly undergo a body division into several units simultaneously, a process called fragmentation. Planarian fission and fragmentation generally represent direct reproduction in which each portion regenerates missing parts to become a complete, new animal. Strobilation products, however, are only indirectly reproductive; proglottids are not regenerative and die but carry and release great numbers of eggs; ephyra do not produce new polyps but mature into sexually reproducing medusae, the larvae of which become polyps.