140 



A. FRANKLIN SHULL 



EXPERIMENTS WITH EXTERNAL FACTORS 



Those who have hitherto worked on the hfe-cycle of Hydatina 

 have inchned strongly to one or the other of two extreme positions. 

 On the one hand are those who hold that the proportion of male- 

 producers at any time is a function of the external conditions 

 then obtaining; on the other hand are those who believe that that 

 proportion is determined by internal factors, uninfluenced by 



TABLE 33 



Showing the number of male- and female-producers in the progeny of three sister 

 individuals of Hydatina senta, one line being reared in spring water, the other 

 two in solutions of crude creatin of different strengths. 



*Remainder of family not recorded. 



external agents. Clear expression is given to the latter view by 

 Punnett ('06), who concluded that the only differences to be 

 found in the proportion of male-producers were the differences in 

 distinct pure lines, and that those differences depended on the 

 character of the gametes uniting in the resting egg from which 

 each pure line sprang. In my earlier work (Shull, '10b) it was 

 shown that this conclusion of Punnett's did not hold in certain 



