PAR'THENOGEN'ESIS (Neo-Lat.. from Gk. irapOlvos, parthenos, virgin + -ye,ecris, genesis, pro duction, from 71-yvecrOaL, gignesthai, to become). The essential phenomenon of sexual generation is the union of a male with a female cell. Within the past half century, however, com paratively numerous cases have been discov ered where unfertilized eggs have developed into an adult organism. Such cases in certain ani mals are known to be in a degree regular, nor mal methods, and the phenomenon is called agamous, virgin reproduction, or 'parthenogen esis,' the name proposed by R. Owen. The most typical ease of parthenogenesis is that of the aphis or plant-louse. The species is represented in the aphides by eggs alone. (See Tim—INST.:CM.) In the spring females alone hatch, and DO males appear until at the close of summer. Bonnet dis covered that a virgin aphis may become the parent of millions of aphids, like itself, there be ing nine generations through the summer. Duval obtained eleven generations in seven months, while kyber even observed that a colony of Aphis dianthi, nyhIefc had been brought into a constant ly heated room, continued to propagate for four years in this manner, without the intervention of males. At the approach of cold weather males appear; they mate with the females, the latter laying eggs. There are thus two sets of females, the parthenogenetic and the normal oviparous forms. The queen bee also lays eggs in the drone or male cells of the honeycomb, which are un fertilized and give rise to males.
Cases of parthenogenesis are known to occur rarely in other Ilemiptera than aphids, namely in bark-lice, and in the silk-worm, and 25 or more other species of moths. Among Tlymenoptera, besides the honey bee and some wild bees (as Halictus), the currant worm ventri cosns) and twelve other species of saw-flies have been known to lay parthenogenetic eggs; also many gall-flies; in several species of ants, and in wasps (Polistes), the parthenogenetic eggs produce males. Among beetles, Gastrophyrea raphani and a species of eaddis-Hy are at times parthenogenous; also certain mites and other insects, and in the crustaceans Apus, Artemia, and Limnadia.
It was formerly supposed that these par thenogenetic eggs were different from normal eggs, and they were called •pseudova' by Huxley. but this view is untenable, since these •pseudova' arise just like ordinary eggs and develop like them, as they undergo cleavage of the yolk and form germ-layers. R. Hertwig claims that par thenogenesis is "a sexual reproduction in which a degeneration of fertilization has taken place, and the facts of parthenogenesis show that under change of conditions the normal mode of sexual reproduction (amphigony) may be modified for the benefit of the species."