PARTHENOGENESIS, the development of an egg-cell that has not been fertilized by a male element or sperm-cell. Thus, as Bonnet discovered in 1762, the summer generations of green-flies or aphids are all parthenogenetic, no males occurring for months. A drone-bee develops from an unfertilized egg, thus having a mother, the queen, but no father. Parthenogenesis is a secondary simplification of ordinary sexual reproduction, in which fertilization is an essential condition of development. This secon dary nature of parthenogenesis is indicated (I) by its sporadic occurrence in diverse classes of organisms; (2) by its occasional occurrence along with ovum-fertilization in the same animal. as in the queen-bee; (3) by its not uncommon alternation with typical sexual reproduction in the course of generations; (4) by the fact that there is often polar-body formation in parthenogenetically developing ova, though part of its significance is wrapped up with, the occurrence of fertilization; (5) by the facts of artificial parthenogenesis (see FERTILIZATION).
In Crustacea, among 3,00o specimens of the brine-shrimp Arte mia only one male was found; and von Siebold repeatedly investi gated every member of a colony of Apus (once over 5,000 strong) without finding a single male. At other times he found in other conditions many males were present. In daphnids there are three kinds of eggs—(a) large, thick-shelled, resistant eggs, which require fertilization and always develop into females, (b) small, thin-shelled eggs, which can develop without being fertilized, and then give rise to females, (c) eggs similar to the last, but produc ing males. Among the cyprids nearly related forms may show (I) a rare occurrence of males, parthenogenetic generations follow ing one another for months; (2) a frequent presence of males, but occasional occurrence of parthenogenesis; (3) an abundance of males all the year round, and parthenogenesis unknown.
Among insects parthenogenesis occurs in many gall-flies (Cyni pidae) and saw-flies (Tenthredinidae). In some of these no males have been found; and this negative evidence of parthenogenesis has been confirmed by isolating females and rearing.their eggs. The unfertilized eggs may develop into females only (thelyotoky), or into males only (arrhenotoky) for a limited period, or into both sexes. In aphids the parthenogenetic development of females may continue for at least four years, but no case is known where males do not eventually occur, also arising from parthenogenetic ova. Among scale-insects (Coccidae) parthenogenesis often oc curs, even though males are present. In most cases the male is still unknown, but this does not necessarily prove parthenogenesis, for the known males are very minute and short-lived. In Lecan ium hesperidum, the pigmy male was discovered in an ovarian cul de-sac within the female. In many gall-flies the successive genera tions show parthenogenetic and spermic development in regular alternation, but some species are perpetually parthenogenetic. In some saw-flies the parthenogenetic ova develop into females only; in others into males only (thus implying that fertilized ova devel oping into females also occur) ; in others the progeny are of both sexes. There are stray occurrences of parthenogenesis among other insects, as in species of Solenobia, a wingless relative of clothes moths. Among nematode worms there are many instances of parthenogenesis.