(a) What may be called path ological parthenogenesis is illustrated when the ovum, of a bird for instance, exhibits without fertilization a number of cleavages, but with no further development. (b) The term casual partheno genesis may be used for exceptional aspermic development, as in silk-moths. (c) The workers of ants, bees and wasps, not normally reproductive, may exhibit occasional parthenogenesis, the eggs apparently always developing into males. (d) The queen bee exhibits partial parthenogenesis, for it rests with her whether the deposited egg is fertilized from her store of spermatozoa, received during the nuptial flight from the inseminating drone. (e) The term seasonal parthenogenesis may be applied to cases like green-flies and some Entomostraca. (f) In a few strange cases, as in some species of the gall-midge (Miastor) the larval form becomes precociously reproductive, and illustrates juvenile or paedogenic parthenogenesis. The female lays a few, very large eggs, which develop into larvae. But inside these there arise other larvae which eat their way out. The same thing happens through several generations which succeed one another through autumn, winter and spring. In the following summer, however, the last set of larvae become pupae, which metamorphose into sexually perfect midges. The parthenogenetically-reproducing lar vae have no ovaries, and their offspring arise from peculiar cells which occur in association with the fatty body. (g) This leads on to what is familiar in the life-history of the liver-fluke and re lated forms, where rediae arise inside the sporocyst, and cercariae within the rediae. The precociously reproductive sporocyst is a transformation of the sexually-produced free-swimming larva or miracidium, while the rediae and cercariae arise from "spore cells," which are undoubtedly primitive germ-cells homologous with ova, but requiring no fertilization. (h) Such cases lead on to the sporogony common in plants, and familiar in ferns. But spore f ormation is probably a persistent primitive mode of uni-parental reproduction, whereas the parthenogenetic development of ova seems in all cases to be secondary and derivative. (i) The series
ends in total parthenogenesis—for one generation, as in many Cynipidae ; for several successive generations, as in many Entomostraca ; or perpetually, as in many rotifers.
The animal ovum typically undergoes a process of maturation, in which, by meiotic or reducing division, the normal number of ordinary chromosomes (q.v.) is halved (see CYTOLOGY). A similar reduction occurs in the maturation of the sperm-cells, and thus in fertilization the normal number will be restored (see SEx). Now in regard to parthenogenetic development, the broad fact is that the reduction process occurs in some types and not in others. It may be noted further that parthenogenesis without reduction may be due (a) to the fact that only one polar-body is formed and no reducing di vision occurs (Cypris reptans) ; or (b) to the return of the polar body nucleus to fuse with the ovum-nucleus (some individuals of Artemia) ; or (c) to the fact that both the maturation divisions are equational, neither meiotic (saw-fly Nematus ; gall-wasp Rhodites). In a case like the dandelion, the egg-cell undergoes no reduction of chromosomes, while in Chara crinita the number is in the parthenogenetic strain permanently half the normal.
There is no known general peculiarity character izing those ova able to develop without fertilization. There is no necessary degeneration associated with long-continued or perpetual parthenogenesis. Besides much non-inherited varia bility, mutations (i.e., inherited variations) are known to occur in parthogenetically reproduced strains. Although parthenogene sis may favour rapid multiplication, so that forms exhibiting it would tend to survive, and although it may be of use in species where males are few or where fertilization is difficult, it is not at present possible to find a utilitarian justification of every occurrence of this departure.