MALE-PRODUCTION IN HYDATINA 



539 



TABLE 9 



Showing the number of eggs laid at the surf ace film of water and at the bottom of the 

 dish by two lines of rotifers, one from Nebraska, the other from, New Jersey, when 

 kept in air and in a 60 per cent oxygen atmosphere. The excess of oxygen causes 

 more eggs to be laid at the bottom, but it zs questionable which line is most affected 

 this way, since their behavior is different under the same conditions 



DISCUSSION 



A\Tiile this paper was in preparation an article by Whitney 

 ('17) on a similar subject, the relative effectiveness of oxygen 

 and food as sex determiners, was published. These two papers, 

 however, in no sense duplicate, since they approach the problem 

 from different angles and with different criteria of judgment. 



A considerable part of Whitney's paper is devoted to experi- 

 ments to show that green organisms as food increase the pro- 

 duction of males in other rotifers than Hydatina. It is to be 

 regretted that many of these experiments were performed with 

 mass cultures without controls. If these rotifers are like Hyda- 

 tina in producing males in 'epidemics,' the lack of controls is 

 most unfortunate. It is well known to every student of Hyda- 

 tina that, especially in lines that produce only a moderate pro- 

 portion of males, these males often appear in well-defined 'waves,' 

 and it has been shown that these epidemics not infrequently 

 have a rather regular periodicity (Shull, '15b). If, in an experi- 

 ment without control, a line which regularly produces numerous 

 males once a month, the experimenter, ignorant of the interval 



