540 A. FRANKLIN SHULL 



between periods of many males, should attempt to alter male- 

 production by introdut3ing something into the culture five or six 

 daj^s before the beginning of the period of males was due, he could 

 use any one of a dozen agents and obtain the same result, namely, 

 an increase of male-production a few days later. 



Notwithstanding the lack of controls, Whitney's conclusion 

 that the use of green organisms for food in these several species 

 of rotifer increases male-production is probably correct; it is un- 

 doubtedly correct in the case of Hydatina, as Whitney ('14) sat- 

 isfactorily showed in an earlier paper and as I have confirmed in 

 my experiments. The chief question is how^ much of this effect 

 of green organisms is due to nutritive differences, how much to 

 other agents associated with the green organisms. 



One of Whitney's experiments bears upon this point, but the 

 results are not easily understood. That experiment is repre- 

 sented by his table 5 (Whitney, '17). Information on which one 

 could base a judgment of the significance of this experiment is 

 not given. The cultures were apparently of some size, all of 

 them containing green organisms. These cultures had been in 

 existence from five to thirty days, presumably in the sunlight 

 (either direct or diffuse) , before the beginning of the experiment. 

 Some of them were then excluded from the sunlight, bu^ for how 

 long a time is not stated. If the duration of the darkness was not 

 prolonged, it is to be expected that the average oxygen content 

 would be appreciably greater than in a culture which had never 

 been kept in the light. The experiments described in table 4 

 of this paper, test No. 5, indicate that oxygen accumulated in 

 photosynthesis is only very gradually lost. It is not clear, then, 

 that the oxygen content of the cultures in the darkness was as 

 much low^r than that of the sunlight cultures as Whitney sup- 

 posed. It is almost certain that, unless they were excluded 

 from the sunlight for a considerable period, these dark cultures 

 contained appreciably more oxygen than a culture never kept 

 in the sunlight would contain. 



It is a pertinent question, therefore, whether the smaller 

 amount of oxygen in the cultures in darkness was as effective as 

 the larger quantity in the sunlight. Some of Whitney's cultures 



