LIFE CYCLE OF HYDATINA SENTA 123 



The results of both repetitions agree with that of the preceding 

 experiment, in yielding more male-producers at the lower tem- 

 perature. It seemed important to test whether the same would 

 occur with other pure lines and at different times. The next experi- 

 ment was performed after all the preceding ones were concluded, 

 and with rotifers that were probably not in any way related to those 

 previously used. ' 



Experiment XVIII. From an old stock jar in the laboratory, 

 which had been stocked with Hydatina about two years and a half 

 before the opening of this experiment and to which none had been 

 added since, a rotifer was isolated in January, 1910. Of its daugh- 

 ters, two became the parents of the two lines in table 20, one of 

 which was reared at room temperature (20° to 22° C), the other 

 in an ice-chest at 7° to 11.5° C. 



Here the difference is in the opposite direction; the higher pro- 

 portion of male-producers is produced at the higher temperature. 

 The meaning of these seemingly opposite results is discussed else- 

 where. In connection with that discussion, it is important to note 

 the sudden rise in the proportion of male-producers at room tem- 

 perature at the end of January, and the corresponding smaller 

 rise at the lower temperature early in February. 



Influence of various undetermined constituents of feces on the 

 percentage of male-producers 



After it had been determined that a solution of horse manure 

 could wholly prevent the appearance of male-producers, the imme- 

 diate problem was to discover what constituent or constituents of 

 the feces had this effect. Two methods of investigation were prac- 

 ticable. Substances which were known to be present in horse 

 manure, or in feces in general, could be directly tested by experi- 

 ment ; or the solution of manure could be treated in such a way as 

 to remove from it substances having given properties, after which 

 the effect of the substances removed, or of those that remained, or 

 of both, could be determined. By the latter method, the number 

 of substances to be tried by the former method might be consider- 

 ably reduced. It is this indirect method which is the subject of 



