226 CLAUDE W. MITCHELL 



In brief, the results of the work previously done upon the 

 problem of sex-determination in rotifers is as follows: The well 

 known work of Maupas upon Hydatina senta led him to the 

 conclusion that a rise in temperature results in an increase of 

 male production. Following Maupas, Nussbaum found the fac- 

 tor of temperature negative, while he attributed to starvation 

 alone the fact of male production. Punnett discarded the fac- 

 tors of both temperature and starvation, finding, as he thought, 

 that high or low male production ran in given lines or strains, 

 which latter fact he attributed to an internal factor, 'ay go tic 

 constitution,' Whitney again concludes that sex-determination 

 is controlled by external conditions, male production being fa- 

 vored by the presence of some substance in solution, the nature 

 of which is undetermined, but the absence of which predisposed 

 to the production of parthenogenetic females only. Shull at 

 first also maintained that sex is determined by external condi- 

 tions — lack of certain chemical matter in solution — though later, 

 finding that different strains of the same species produce varying 

 numbers of males even under identical conditions, he returns to 

 the assumption of some internal factor inherent in lines or races. 

 In progeny of a cross between different lines of high and low male- 

 producers he finds the potentiality of male production increased in 

 some instances and lowered in others. In a later work he returns 

 to the assumption of 'genotypic constitution' as the determining 

 factor, which is in essence a return to the position of Punnett. 

 The relation of such internal factors to the influence of external 

 conditions remains quite unexplained. It seems plain, therefore, 

 that additional work is much needed before final conclusions can 

 be reached. 



Before proceeding further it is necessary to justify briefly the 

 use of the phrase 'sex-determination' in its application to the 

 phenomena we are studying. Shull and writers in allied fields 

 have maintained that the phenomona in question are not really 

 matters of sex-determination as such, but represent transitions 

 from parthenogenetic reproduction on the one hand to sexual 

 reproduction on the other. Viewed broadly this contention at 

 first seems natural; but under analysis we fail to agree with it. 



