SEX-DETERMINATION IN ASPLANCHNA 247 



At the same time we would lay much emphasis upon the ne- 

 cessity of refined feeding technique with whatever species work 

 is done. Few if any rotifers feed exclusively upon one type of 

 food, and it probably does not suffice, in experimenting with 

 them, to employ a single food organism even though such nutri- 

 tion be capable of maintaining growth and reproduction. 



We would next point out that in one way or another, all previous 

 workers — ^Nussbaum, Punnett, Whitney, Shull — -have found that 

 certain strains or races of Hydatina were more predisposed to 

 male production than others. Nussbaum did not himself thus 

 interpret his results; but it has been pointed out none the less 

 that the cultures with which he worked certainly differed in this 

 respect. It has been further shown by Whitney that these 

 'strains' which thus differ in their male producing capacity may 

 be more or less altered in this fundamental respect by external 

 conditions. No one, however, seems to have definitely associated 

 the conditions which thus modify the male producing capacity of 

 a strain directly, as a cause, with male production. Now, with 

 Asplanchna amphora we have pointed out that these male and 

 non-male producing strains not only exist but are characterized 

 by marked morphological differences and we have further pointed 

 out that these strains are also produced by nutritive change. 

 Is it not probable that a like causal relation obtains between the 

 less marked though different strains of Hydatina senta? 



Considering more definitely the results of individual investi- 

 gators, we turn first to those of Nussbaum. After negativing 

 the hypothesis of temperature as a sex-determinant, he arrived, 

 through a long series of experiments, at very definitely formulated 

 ideas that starvation was the positive factor inducing male pro- 

 duction. These general conclusions are so important and definite 

 that they may best be stated in his own words: 



Bei Hydatina senta bestimmt wahrend einer gewissen Entwicklungs- 

 phase die Ernahrung das Geschlecht des ganzen Geleges eines jeden 

 jungfraulichen Weibchen. Wird das auskriechende Weibchen bis zur 

 Reifung seines ersten Eies gut ernahrt, so legt es nur weibliche Eier, 

 wird es bis zur Geschlechtsreife mangelhaft ernahrt, so legt es nur 

 mannliche Eier. Vor und nach dieser Periode hat die Ernahrung auf das 

 Geschlecht keinen Einfluss. 



