.METAGENESIS. Mk 04111 was proposed by ()wen iu 1848 in his Parthrnogencsis, and after wards more fully in his Lecfaris on the Compara tive Anatomy end Physiology of the In r, rl, brat" .tnimals (London, 1855). llis examples are the alternation of generations of the distomes. the hydroids, medustr, and even the metamorphoses of the echinoderms, the gemmation of Nais and other annelids, as well as the metamorphoses of the fish-lice (.Etheres, etc.). which he says "is a slightly mollified parthenogenesis:" again, he regards the phases they pass through as 'much more those of a metagenesis than a metamor phosis." (See ACTERx-\TIoN o CENERATIoss.) In his paper on the agamie reproduction of Aphis, Huxley employs the term 'agamogenesis.' which he says occurs "when the produced •zordd' is capable of development into an independent or ganism without the influence of an aet of con jugation with another 70(141 The producing zordd may be devoid of sexual organs. as in the many Ilydrozoa, and many Trematoda," in fact in the great majority of cases of agamo genesis ; and to this kind of alternation of gen erations he applies ((wen's term 'Inci restricting the term 'parthenogenesis' to cases where the parent ('protozo;ibl') possesses sexual organs (ovaries). and its hinds have all the his
tological characters of ova. Metagenesis is de fined by it Ilertwig as "alternation of genera tions in the narrower It 1110 alternation of at least two generations, of whieh one repro duces only asexually, by division or budding. the other either exclusively, or at leas( to a great e x tent, sexually. The first generation is vaned the the second, the 'sexual animal.' The best example is furnished by the mode of repro duetbm of Hydromedus.c. See Ilvntoioc.
IIETEnouoxv. This differs from metagenesis by the fact that the asexual generation is re placed by parthenogenesis. For example. inn certain Crustacea (Paphnhhml only female.: oc cur in time summer time. which increase by nn fertilized eggs. Then males appear for a short time. which fertilize the winter eggs which are now produced, from which again parthenogenetic generations arise (R. II ertwig).