522 A. FRANKLIN SHULL 



experiment referred to above, in which 60 per . cent oxygen pro- 

 duced no effect, was performed at a time when the rotifers were 

 going through one of their well-known 'epidemics' of male-pro- 

 duction (Shull, '15 b), whereas the experiment with 40 per cent 

 oxygen came at a period when male-production in the control line 

 was low. No opportunity was found to test these two possible 

 explanations, owing to the loss of the stock cultures of rotifers, 

 and the experiments were published while this question was still 

 unanswered. 



The important discovery by Whitney ('14) that rotifers fed 

 upon a green organism, Chlamydomonas, produced many more 

 male-producing offspring than those fed upon other material, 

 raised new questions. Whitney took no account of certain 

 initial chemical differences between the contrasted lines nor of 

 the fact that green organisms yield oxygen as a by-prv)duct of 

 photosynthesis. Shull and Ladoff pointed out that although 

 nutrition might on a priori grounds be expected to have the effect 

 which Whitney ascribed to it, the other obvious factors should 

 be ehminated before attributing the residual effect to nutrition. 

 It remained a question, therefore, how much of the increase in 

 male-production which Whitney observed upon feeding with green 

 organisms was due to food, how much to .oxygen or other factors. 



In the experiments described in this paper an answer to these 

 questions is found. The initial chemical differences referred to 

 above have been eliminated. The effects of oxygen and green 

 organisms have been measured and compared. In addition, the 

 effects of these two agents, which increase male-production, are 

 compared with two of the principal agents known to decrease 

 male-produ c tion . 



EXPERIMENTS 



Effect of sixty per cent oxygen 



To show whether saturation with a mixture of air and oxygen 

 of which 60 per cent was oxygen caused any change in the amount 

 of male-production, the following experiments were performed : 



Experiment 1. On each of the days named in table 1 three 

 female rotifers were placed in each of two dishes. In one was 



