SEX-DETERMINATION IN ASPLANCHNA 253 



selves become the mothers of many young, which, through the 

 slightest check in nutrition, become male producers. We think 

 that any one with this idea clearly in mind will find ShuU's table 

 in close accord with our hypothesis. Without wishing to de- 

 preciate the value of Shull's work we may here point out the 

 sharpness of the difference between our viewpoint and his. He 

 interprets the table of which we have spoken in terms of five- 

 day periods and is plainly seeking for some factor which may 

 exert a general influence upon male production of the whole 

 series, regardless of rhythm or special known nutritive condition. 

 Our results lead us to find the cause of male production in the 

 rhythm of the series and in definite factors working upon single 

 individuals of every generation. 



In conclusion we wish to point out, further, that in the entire 

 range of experiment upon Hydatina senta no investigator, with 

 possible exception of Nussbaum, has developed a definite method 

 by which the rate of male production can be increased at will or 

 even definitely controlled at any point other than by its reduction 

 to zero. Second, attention may be called to the general fact 

 that all investigators either used uniform feeding or at least laid 

 little stress upon qualitative food change. Third, we may well 

 point out the probability of the correctness of the negative or 

 inhibiting factors in regard to male production as demonstrated 

 by Shull and Whitney. There may very probably be many such 

 factors. If male production depends primarily upon a high state 

 of effective nutrition it would seem natural that any inhibitory 

 or deleterious substance should limit or prevent its occurrence. 



In thus rehabilitating the all but discarded factor of nutrition 

 as a sex-determinant, and as the effective sex-determinant, in 

 Asplanchna amphora, we do not wish to be understood as passing 

 adverse judgment upon the claims that other and diverse modes 

 occur elsewhere in the animal kingdom. But as far as rotifers 

 are concerned certain experiments conducted in our laboratories 

 upon other species, together with sufficient correlated observa- 

 tions in nature, lead us to the probable though tentative conclu- 

 sion that qualitative and quantitative changes in nutrition will 

 be found the universal sex-controlling factors in this group. 



