244 CONTINUITY OF THE GERM-PIASM AS THE [IV. 



cannot attain to complete parthenogenesis when, as in Fol's 

 supernumerary sperm-nuclei, their powers of assimilation are 

 insufficient to enable them to reach the requisite size. As 

 before stated, the cell-body does not force the nucleus to divide, 

 but vice versa. It would, moreover, be quite erroneous to sup- 

 pose that parthenogenetic eggs must contain a larger amount of 

 nutritive material in order to facilitate the growth of the nucleus. 

 The parthenogenetic eggs of certain Daphnidae [Bythotrephes, 

 Polyphemits) are very much smaller than the winter-eggs, which 

 require fertilization, in the same species. It is also an error 

 for Strasburger to conclude that ' it has been established with 

 certainty that favourable conditions of nutrition cause partheno- 

 genetic development in Daphnidae, while unfavourable con- 

 ditions cause the formation of eggs requiring fertilization.' It 

 is true that Carl Diising ^ in his notable work upon the origin 

 of sex, has attempted, in a most ingenious manner, to prove, 

 from my observations and experiments on the reproduction of 

 Daphnidae, 'that winter or summer-eggs are formed according 

 to the nutritive condition of the ovary.' I do not, however, be- 

 lieve that he has succeeded in this attempt, and at all events it 

 is quite clear that the validity of such conclusions is not fully 

 established. I have observed that the maturing eggs break up 

 in the ovaries and are absorbed in those Daphnidae (Sida) 

 which are starved because sufficient food cannot be provided 

 in captivity. Hence such animals live, as it were, at the ex- 

 pense of their descendants ; but it would be quite erroneous 

 to conclude with Diising, from the similarity which such dis- 

 appearing egg-follicles bear to the groups of germ-cells which 

 normally break up in the formation of winter-eggs, that with a 

 less degree of starvation winter-eggs would have been formed. 

 Diising further quotes my incidental remark that the formation 

 of resting-eggs in Daphnia has been especially frequent in 

 aquaria ' which had been for some time neglected, and in which 

 it was found that a great increase in the number of individuals 

 had taken place.' He is entirely wrong in concluding that 

 there was any want of food in these neglected aquaria ; and if 

 I had foreseen that such conclusions would have been drawn, 

 I might have easily guarded against them by adding that in 



^ Carl Busing, ' Die Regulirung des Geschlechtsverhaltnisses.' Jena, 

 1884. 



