SEX-DETERMINATION IN ASPLANCHNA 251 



food there followed in the fifty-ninth generation the transition 

 to the humped type and also to male production. In our w-©rk 

 at this time, however, the duplication of Whitney's experiment was 

 followed by a series of experiments which completely eliminated 

 the possibility, for Asplanchna, of the truth of Whitney's assump- 

 tion, namely, that dissolved substances in the culture medium 

 determine the sudden advent of male production. The fact of 

 nutritive change remains the one residual causal factor. 



The work of Shull upon the general topic of the life cycle and 

 the sexual phase of H. senta is so extensive that a detailed review 

 of it here is impossible. Certain of his essential conclusions 

 however require mention and comment. In several of his earlier 

 articles he develops at length the hypothesis that substances dis- 

 solved in culture medium influence male production; yet the in- 

 fluence ascribed to these substances is, curiously enough, the re- 

 verse of that which Whitney ascribes to his hypothetical matter 

 in solution. He deems that various dissolved substances act as 

 sex inhibitors, predisposing to exclusive partheno genetic female 

 production. 



Shull-' in his later work again finds much evidence of lines or 

 races possessing somewhat definite capacities for male produc- 

 tion. ^° He has also carried out interesting experiments in the 

 matter of crossing these diverse lines, the results of such crosses 

 leading sometimes to an increase and sometimes to a decrease 

 in the amount of male production. But no farther facts as to 

 the cause of male production were discovered. Shull noted a 

 frequent if not general decrease in the amount of male production 

 throughout his long continued series. He thinks this gradual ap- 

 proach toward pure parthenogenesis to be "independent of both 

 genotypic constitution and the immediate external environment." 

 In light of our experiment it is natural to raise the question 

 whether this gradual cessation of male production may not have 

 been the consequence of the unchanged and unmixed diet em- 

 ployed? Experiments with more varied feeding of Hydatina 

 could hardly fail to be of interest. 



3 Jour. Exp. ZooL, vol. 12, no. 2, 1912. 



'*> He attributes these diverse races to fundamental differences in 'genotypic 

 constitution.' 



