SEX-DETERMINATION IN ASPLANCHNA 237 



cent of these starved mothers became male producers (table 4) . 

 This percentage is seen to be nearly three times that in the pre- 

 ceding experiment. Of the four special controls one only pro- 

 duced males, which is close to the normal average, 21 per cent, 

 in the general well fed humped series which were the additional 

 controls. The result of starvation in these isolation cultures thus 

 bears out to a considerable extent the conclusions suggested by 

 the mass cultures. By repetition of such experiments it would 

 seem that evidence might be accumulated to establish the general 

 fact that a check in nutrition, or sudden starvation tends to 

 produce an unusual proportion of males. 



In the course of our experiments, however, we preferred to 

 check this conclusion in the attempt to solve «the more delicate 

 problems offered for instance, by the individual cases in which 

 our control cultures produced males without apparent preceding 

 starvation and again, by the lesser portion of the starved cul- 

 tures wliich did not show male production. In short we hoped 

 if possible to achieve a complete explanation of sex-determina- 

 tion. It seemed probable that this must be sought not only in 

 food and starvation but also in the matter of rhythm which, as 

 we have pointed out, involves profound physiological and mor- 

 phological differences as well as differences in rate of reproduction. 



To do this required that individuals chosen for experiment 

 should be isolated from pedigree lines at known stages in their 

 physiological rhythm, that is when reproduction is at its 

 maximum and minimum. 



EFFECT OF T.OW FEEDING AND STARVATION UPON INDIVIDUALS 

 CHOSEN FROM SERIES DURING PERIODS OF DEPRESSION 



Thus to demonstrate the scarcity of males produced by indi- 

 viduals chosen at periods of rhythmic depression and bred under 

 unfavorable nutritive conditions, which tend toward low potential, 

 twenty-five humped individuals were chosen from families dur- 

 ing known periods of depression. These were isolated in cul- 

 tures containing no food, but after twenty-four hours food was 

 added in small quantities. The young were in every case females. 



