Metagenesis

METAGENESIS (from Greek meta—after and genesis—origin, birth), a complex method of reproduction consisting of the fact that the complete life cycle of an animal proceeds with an alternation of methods of reproduction, usually in two generations, which sequentially replace one another. The malaria plasmodium reproduces in human blood by asexual means (schizogony), while in the body of an Anopheles mosquito it undergoes sexual reproduction. Hydroids, by budding (asexual reproduction), form a colony in which, in addition to vegetative individuals (polypoids), medusae are formed, which detach and swim freely. In the medusae, germ cells develop, which in subsequent cleavage (sexual reproduction) produce larvae that form a colony shape again by budding, and so on. Metagenesis is widespread among many parasitic worms; for example, the sexual form of the echinococcus (Taenia echinococcus)—a tapeworm—produces a multitude of eggs, from which bladder worms are formed; these latter (asexual reproduction) produce secondary bladders and brood capsules with numerous heads. In metagenesis, the sexual generation alternates with the asexual one and vice versa; hence the term "alternation of generations." In parasites that reproduce metagenetically, the alternation of generations is associated with a change of hosts.