Reproduction by budding or fission has probably arisen as a means of increasing the number of individuals produced, so that the coexistence of asexual with sexual reproduction is to be looked on as a kind of compromise for the loss of the power of rapid reproduction due to the absence of parthenogenesis. In the Arthropoda and Rotifera the place of budding has been taken by parthenogenesis, which may be a frequent, though not always a necessary occurrence, as in various Branchiopoda (_Apus_, _Limnadia_, etc.) and Lepidoptera (_Psyche helix_, etc.); or a regular occurrence for the production of one sex, as in Bees, Wasps, Nematus, etc.; or an occurrence confined to a certain stage in the cycle of development in which all the individuals reproduce their kind parthenogenetically, as in Aphis, Cecidomyia, Gall Insects (_Neuroterus_, etc.), Daphnia[34]. [34] Mr J. A. Osborne has recently shewn (_Nature_, Sept. 4, 1879), that the eggs of a Beetle (Gastrophysa raphani) may occasionally develop, up to a certain point at any rate, without the male influence. On my hypothesis the possibility of parthenogenesis, or at any rate its frequency, in Arthropoda and Rotifera is possibly due to the absence of polar cells. In the case of all animals, so far as is known to me, fertilization of the ovum occasionally occurs[35], but there are instances in the vegetable kingdom where so-called parthenogenesis appears to be capable of recurring for an indefinite period. One of the best instances appears to be that of Coelebogyne, an introduced exotic Euphorbiaceous plant which regularly produces fertile seeds although a male flower never appears. The recent researches of Strasburger have however shewn that in Coelebogyne and other parthenogenetic flowering plants, embryos are formed by the _budding_ and subsequent development of cells belonging to the ovule. This being the case, it is impossible to assert of these plants that they are really parthenogenetic, for the embryos contained in the seed of a flower which has certainly not been fertilized, may have been formed, _not by the development of the ovum_, but by budding from the surrounding tissue of the ovule. [35] Dicyema, which is an apparent exception, has not yet been certainly shewn to develop true ova. If its germs are true ova it forms an exception to the above rule. The above view with reference to the nature of the polar bodies is not to be regarded as forming more than an hypothesis. _Impregnation of the Ovum._ A far greater amount of certainty has been attained as to the effects of impregnation than as to the changes of the germinal vesicle which precede this, and there appears, moreover, to be a greater uniformity in the series of resulting phenomena.