JOHN H. NORTHROP 



317 



cooling for ten generations without any noticeable change in the 

 upper temperature limit. A second culture was kept continuously 

 at 28° for fifteen generations. In this case also there has been no 

 noticeable change in the temperature limit; i.e., the organisms are 

 still unable to grow for more than one generation at a continuous 

 temperature of 29° or over. 



TABLE n. 



Efect of Placing Cultures, Raised at 30°, at 22°C. 



Days. 



1, 50 to 100 imagos from normal aseptic culture generation No. 89 (raised at 20°) 



placed at 30°C. 



2. Many eggs laid. 



4. Larv£e developing. Parent imagos removed. 



8. Imagos of the new generation (Fi) placed at 22° and put back at 30° after 

 varying time intervals as stated below. 



EXPERIMENTAL. 



Temperature Control. — The cultures were kept in water jacketed incubators 

 regulated as described in another paper. ^ 



II Food.— All cultures were fed on a sterilized suspension of yeast in water. The 

 excess water was absorbed by cotton added to the flask, as described below. 

 ri Method of Transferring Cultures, Etc. — The insects were kept in 1 liter fiat 

 bottom Florence flasks having a side tube fused on as in a Pasteur flask. This 

 side tube was closed with a rubber tube and glass plug, and the neck of the flask 

 plugged with cotton. About 25 cc. of a thick suspension of yeast in water were 



9 Northrop, J. H., /. Gen. Physiol., 1919-20, ii, 309. 



