120 A. FRANKLIN SHULL 



('06) was inclined, in fact, to believe that external agents have no 

 influence on the cycle of Hydatina, and that many of the phenom- 

 ena observed by him were attributable to internal differences in 

 distinct pure lines. It was shown in my earlier paper, cited above, 

 that many phenomena are not due to internal differences; but 

 it was left an open question whether internal differences might 

 not explain other results. If distinct pure lines could be found, 

 it was proposed to test the nature of their internal differences by 

 appropriate crosses. 



The method of conducting the experiments has been in all essen- 

 tial respects the same as that described in the previous article. 



EXPERIMENTS WITH EXTERNAL FACTORS 



Influence of temperature on the percentage of male-producers 



Experiment XV. On October 6, 1909, a female was taken from 

 the pure line recorded in Experiment I of my former paper, which 

 was being reared at room temperature (18° to 23° C), and was 

 placed in a dish in a closed box kept on or near a large water 

 bath. The temperature in this box was noted frequently, and 

 found to vary between 23° and 26° C. A higher temperature could 

 not be maintained without such great mortality that the experi- 

 ment would have small value. This pure line, and the control 

 derived from a sister individual and reared at 18° to 23° C, are 

 recorded in table 17. The difference of temperature here used, 

 which averaged 4° or 5° C, had practically no effect upon the 

 proportion of male-producers. 



Experiment XVI. Simultaneously with the beginning of the 

 preceding experiment, a line of rotifers was removed to an ice- 

 chest, where the temperature was found to vary between 7° and 

 14° C, nearly always between 9° and 11° C. The same control 

 was used as in the preceding experiment, the three lines being 

 derived from sisters. Table 18 gives the data. 



The line bred at the lower temperature yielded a very much 

 higher proportion of male-producers. This result is opposed to 

 that of Maupas ('91), who found that the higher temperature 

 resulted in the greater proportion of male-producers. Because of 



