64 FUNCTION OF POLAR CELLS. 



vigorous as possible. Sexual differentiation (even in hermaphrodites) 

 is clearly very inimical to the production of the maximum number of 

 individuals. There can be little doubt that the ovum is potentially capable 

 of developing by itself into a fresh individual, and therefore, unless the 

 absence of sexual differentiation was very injurious to the vigour of the 

 progeny, parthenogenesis would most certainly be a very constant occur- 

 rence ; and, on the analogy of the arrangements in plants to prevent self- 

 fertilization, we might expect to 6nd some contrivance both in animals and in 

 plants to prevent the ovum developing by itself without fertilization. If 

 my view about the polar cells is correct, the formation of these bodies 

 functions as such a contrivance. 



Reproduction by budding or fission has probably arisen as a means of 

 increasing the number of individuals produced, so that the co-existence of 

 asexual with sexual reproduction is to be looked on as a kind of compromise 

 for the lo^s 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 (Apifs, 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, Ceci- 

 domyia, Gall Insects (Neuroterus, etc.), Daphnia 1 . 



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 2 , but there are instances in the vegetable king- 

 dom where so-called parthenogenesis appears to be capable of recurring for 

 an indefinite period. One of the best instances appears to be that of 

 Coalebogyne, 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 partheno- 

 genetic, for the embryos contained in the seed of a flower which has 

 certainly not been fertilised, may have been formed, not by the development 

 of the ovum, but by budding from the surrounding tissue of the ovule. 



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. 



1 Mr J. A. Osborne has recently shewn (Nature, Sept. 4, 1879), that the eggs of a 

 Beetle (Gastrophysa raphaui) may occasionally develope, up to a certain point at any 

 rate, without the male influence. 



2 Dicyema, which is an apparent exception, has not yet been certainly shewn to 

 develope true ova. If its germs are true ova it forms an exception to the above rule. 



