SEX-DETERMINATION IN ASPLANCHNA 245 



ture, constitutional vigor, and reaction to stimuli, the residuum 

 of apparently negative results is readily interpreted. Thus, that 

 certain high bred females should become male producers without 

 enforced starvation may follow naturally from mere accidents 

 and irregularities in feeding which the most uniform of control 

 conditions can not eliminate. And, on the other hand, that in 

 starvation experiments, a certain percentage still develop as female 

 producers, is readily explicable upon the assumption of inherent 

 variations in constitutional vitality. These individuals are sim- 

 ply not of as high potential as are the others. General experience 

 shows that sister individuals in the most normal families often 

 differ in the rate of growth and in reproductive power. 



To the foregoing results which comprise all the experiments 

 performed when working directly upon the problem of sex-de- 

 termination, we may add brief analysis of certain mass cultures 

 conducted subsequently upon the same species when working 

 upon certain problems of heredity. The reason for turning aside 

 and again repeating mass culture experiments lay in the fact that 

 we deemed ourselves in possession of stocks of the rotifer in still 

 higher potential of development. They were of the humped 

 form but at periods when this easily gave rise to the third, still 

 larger or campanulate type. Each of the mass cultures was in- 

 deed derived from humped stock descending from resting eggs 

 produced by the giant campanulates and which had been fer- 

 tilized by males so derived. A considerable number of mass 

 cultures were instituted and reared from the above stock, and 

 a number of them were treated according to the methods de- 

 scribed for producing maximum male production. Every one 

 of the attempts with this highly developed stock was successful; 

 swarms of male producers, males, and individuals with resting 

 eggs resulted. So great were these epidemics of male production 

 and resulting resting eggs that the cultures were all but swept 

 out of existence. Careful records of male production were also 

 kept of all other mass cultures at this time to note whether male 

 epidemics occurred under other conditions; but none such were 

 encountered. Likewise in all mass cultures which produced few 

 males, at least some one of the favoring factors was absent, — 



