164 A. FRANKLIN SHULL 



In the light of these experiments on rotifers, it is obvious, it 

 seems to me, that the assumed influence of temperature and nutri- 

 tion upon the life cycle of the aphids and daphnians must be re- 

 examined, to discover whether some of the phenomena observed in 

 those groups and attributed to the two factors named, may not 

 rather be due to the action of chemical substances. 



SUMMARY 



At an average temperature of about 20° C and 24°. 5 C, respec- 

 tively, two pure lines of rotifers yielded practically the same pro- 

 portion of male-producers. 



At an average temperature of 10° C. the rotifers yielded in 

 several cases a decidedly higher proportion of male -producers than 

 at 20° C, but in one instance a lower proportion. This may be 

 taken to indicate that the influence of temperature is indirect. 



A solution of horse-manure may wholly prevent male-producers 

 from appearing. Boiling such a manure solution does not dimi- 

 nish its effect, neither does drying and redissolving it. 



The substance in manure solution which prevents male-pro- 

 ducers from appearing is apparently not soluble in ether nor abso- 

 lute alcohol. 



The brown colored substance of manure solutions probably 

 does not alTect the proportion of male-producers. 



A higher degree of alkalinity seems to result sometimes in fewer 

 male-producers than does a lower degree of alkalinity; but the 

 differences obtained were slight and the results were not uniform. 



A solution of urea tends to reduce the proportion of male-pro- 

 ducers. Other substances having the same effect are ammonium 

 hydroxid and three ammonium salts, the chlorid, the carbonate, 

 and the nitrate. 



Beef extract and creatin solutions greatly reduce the proportion 

 of male-producers. 



Two pure lines obtained from widely separated localities, and 

 hence not related to one another, were found to yield a constantly 

 d fferent proportion of male-producers, even though external con- 

 ditions were the same for both. 



