118 A. FRANKLIN SHULL 



The life cycle of this rotifer and the substance of the previous 

 literature on the subject were given in the second paper. Experi- 

 ments were there described from which the conclusion was drawn 

 that the mode of reproduction was not directl}^ influenced by 

 starvation, nor probably by temperature differences ; but that the 

 chemical content of the water in which the rotifers lived had a 

 strong influence upon the proportion of sexual females (male- 

 producers) . 



In the present paper the nature of the substances involved is 

 more definitely stated. I shall name a few single substances 

 which have been found to influence the mode of reproduction, and 

 give the evidence for my conclusion. Among these substances 

 are several ammonium compounds, creatin, probably urea, and 

 perhaps the degree of alkalinity. Certain internal factors have 

 likewise been found to influence the proportion of sexual females, 

 and these have been variously combined in crosses. 



This work has been continued with helpful suggestions from 

 Prof. T. H. Morgan. I am also indebted to Prof. William J. Gies 

 for suggestions regarding the chemical part of the w^ork. 



PROBLEM AND METHOD 



In my earlier paper (Shull, '10b) no evidence relating to the 

 influence of temperature differences on the proportion of sexual 

 females was given, because further experiments were in progress 

 at that time. These experiments are now completed. The prin- 

 cipal problem, however, was to discover the effect of various sub- 

 stances in the water upon the proportion of sexual females; for 

 it had been found that if the rotifers were bred in a fairly concen- 

 trated solution of horse manure, sexual females might be wholly 

 prevented from appearing. Such a manure solution contains a 

 large number of substances, but it was not known which of these 

 substances affected the life cycle. 



It also seemed important to search for internal factors which 

 might likewise influence the proportion of sexual females, for some 

 students of this and other similar life cycles believe that internal 

 factors play a leading role in the production of the cycle. Punnett 



