JOHN H. NORTHROP 



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either 25 or 30° than they do at 27.5°. This secondary decrease in 

 the rate at temperatures above 27.5° was compared with a similar 

 decrease in the rate of enzyme action, and was ascribed to a similar 

 cause; namely, injury and subsequent slowing up of the growth 

 processes. 



In all these experiments the eggs whose development was studied 

 were produced by images which had been raised at temperatures of 

 from 15-20°C. If there was any hereditary adaptation to higher 

 temperature it would be expected that flies which had developed near 

 the upper temperature limit would be able to produce eggs slightly 



TABLE I. 



Ejffect of Temperature at which Parent Generation Develops on Upper Temperature 

 Limit for Development of the Succeeding Generation. 



more resistant to temperature {i.e. able to develop at a slightly higher 

 temperature) than flies which had developed at a lower temperature. 

 In order to test this assumption, cultures of imagos which had 

 developed at 20 and 32° respectively were placed in incubators at 

 29, 32, and 33°, The development of the eggs produced by these 

 imagos was then followed. The results are summarized in Table I. 

 It will be seen that those imagos which had developed at 20° and 

 were then transferred to a temperature of 29 or 32° were able to pro- 

 duce eggs capable of developing into imagos at that temperature. 

 The eggs produced at 33°, however, do not develop beyond the pupal 

 stage. The imagos which had developed at 32° are unable to pro- 

 duce eggs capable of development into imagos at temperatures higher 

 than 29°. The effect of raising Drosophila at high temperatures, 



