From Big Medical Encyclopedia

METAGENESIS (grech, meta between, later, through + genesis an origin, generation) — one of forms of digenesis, at a cut sexual and vegetative ways of reproduction alternate.

M — one of forms of evolution of ways of reproduction which provided as adaptation to the habitat (e.g., at hydroids — a colonial way of life) and a possibility of enrichment by genetic information at a gametic syngamy from different individuals (see. Reproduction ). Distinguish vegetative reproduction and a tsitogoniya. At vegetative reproduction (see) rise the new organism is given by not a separate cell, but multicellular complexes, sometimes difficult differentiated. The syngenesis is carried to a tsitogoniya, i.e. to a way of reproduction, at Krom the new organism develops as a result of merge of two separate cells — gametes, various on structure and function (a spermatozoon and an ovum'). Sexual process as one of forms of a tsitogoniya is resisted by other its form — asexual reproduction (see), i.e. reproduction due to cell fission and formation of specialized cells — a dispute.

In an animal and flora at organisms of the same look various ways of reproduction meet and alternate. This alternation is connected, as a rule, with digenesis and received the name of alternation of generations. Refer change of sexual and asexual reproduction to primary alternation of generations, eurysynusic at plants. The main forms of secondary alternation of generations are M. and a heterogony. At a heterogony the typical syngenesis alternates not with vegetative, as at M., and with parthenogenesis (see), i.e. development of an organism from one gamete (ovum) without its merge to a spermatozoon.

The m is characteristic of coelenterates, nek-ry flat and annlides and tunicates (salp). So, e.g., in a development cycle of an echinococcus (Echinococcus granulosus) at a syngenesis of the tape form living in intestines of the final owner eggs are formed. From them in a body of the intermediate owner there is a bubbly form capable in the sexless way to form many secondary bubbles with heads. Each bubble in a body of the new owner gives rise to a new tape form.

See also Life cycle .

V. N. Pavlova.