THE ARTIFICIAL PRODUCTION OF THE PAR- 

 THENOGENETIC AND SEXUAL PHASES OF 

 THE LIFE CYCLE OF HYDATINA SENTA 



A. FRANKLIN SHULL 

 Columbia University 



The causes that determine the transition from the 

 parthenogenetic to the sexual mode of reproduction in 

 the rotifer 11 ijdat'nm so/fa have been the subject of a 

 number of investigations, which have led to contradic- 

 tory conclusions. Maupas was led to attribute the ap- 

 pearance of sexual forms to changes of temperature, 

 Nussbaum to starvation. Punnett found that neither 

 temperature changes nor starvation had any effect, but 

 obtained what seemed to be distinct strains, each pro- 

 ducing a fairly constant proportion of sexual forms, re- 

 gardless of external conditions. One strain produced 

 many sexual females (about 40 per cent.), another few 

 sexual females (2 per cent), and a third none at all. 

 His conclusions regarding temperature and starvation 

 were supported by Whitney, but the latter worker ob- 

 tained no evidence of strains, finding that so-called 

 strains of one type might break up into those of any 

 other type within a single parthenogenetic series. 



I believe I have evidence which may explain the diffi- 

 culties hitherto encountered, evidence that goes far 

 towards bringing tlx 1 previous contradictory conclusions 

 under a single point of view. I shall attempt to show 

 in this brief presentation that the presence of substances 

 in the water, other than food, exerts a strong influence 

 on the inauguration of the sexual phase. 



In this paper I shall speak of the male-producers as 

 sexual females. The identity of the two has long been 

 suspected from certain numerical relations, but more 



