154 A. FKANKLIN SHULL 



So far, however, the chemical agents of which we have definite 

 evidence, if added to spring water, all decrease, instead of increase, 

 the proportion of male-producers. I have therefore preferred to 

 assume, until more is known of the external agents, that the re- 

 sponse of the rotifers to internal conditions may also be modified 

 by temperature. 



More effective than temperature in modifiying the proportion 

 of male-producers, are certain chemical substances, as a glance 

 at tables 27 to 33 will show. Creatin has a remarkable effect 

 in reducing the proportion of male-producers. Beef extract has 

 an equal effect, but it is a mixture of many substances. Ammon- 

 ium hydroxid and three ammonium salts, the chlorid, the car- 

 bonate, and the nitrate have a distinct effect, but not as marked 

 as creatin or beef extract. Urea may almost certainly be put in 

 the same class. All these substances tend to reduce the propor- 

 tion of male-producers. Unlike temperature differences, the effect 

 produced by these substances was in every case of the same sign. 

 It is to be noted, however, that all of the experiments with a given 

 substance were performed with a single pure line, whereas those 

 with temperature included experiments with two distinct lines. 

 This may or may not make a difference in the result. 



To the above substances which have an undoubted influence 

 may be added perhaps the degree of alkalinity. In several cases 

 a greater alkalinity seemed to produce fewer male-producers, but 

 the effects were slight and the results were not uniform. 



Whitney ('10) in a recent communication is in substantial 

 agreement with the author, in that he finds chemical substances 

 responsible for considerable effects upon the life cycle. He has 

 experimented with the manure solution used by me, and from 

 these experiments, together with certain observations, he con- 

 cludes that certain substances in the manure solution affect the 

 proportion of male-producers. But in the details of the conclu- 

 sion we differ. In my two former papers (Shull, '10a and '10b) 

 I had expressed the conviction that certain substances present 

 in the manure solution prevented the male-producers from appear- 

 ing. Whitney believes that a certain substance may be present 

 which causes male-producers to appear, and that it is only when 



