192 



WILLIAM C. YOUNG AND HAROLD H. PLOUGH. 



TABLE I. 



TESTS RUN AT 31 DEGREES. 



becomes still more significant when the figures are examined in 

 comparison with the corresponding controls. Where these were 

 fertile, all of the females were fertile, yet one half of the males 

 were sterile. Apparently the males of a culture begin to be 

 sterilized by heat, before there is any effect on the females 

 whatever. When the controls were sterile, in only one out of 

 twelve cultures did the males show fertility, while about half of 

 the females are affected and half are still fertile. It would 

 appear that high temperature causes at least partial sterilization 

 of the males at a point where the females are largely unaffected. 

 Eventually at least some of the females are sterilized. It was 

 noticeable that in the strains showing a high degree of tolerance 

 for 31 degrees, the males only showed sterility. 



In order to establish these facts beyond question it seemed 

 wise to run a more extended series of matings to normal stock 

 using cultures each of which was actually known to be sterile 

 at high temperature. A series of cultures of flies of different 

 strains was placed in the incubator at 32 degrees. This is 

 sufficiently high to cause sterility in nearly all strains after the 

 first generation, and in every one of the cases tabulated this was 

 the result. Each fly was then mated to another of the opposite 

 sex which had been hatched at the optimum temperature of 24 

 degrees, and all were replaced at 32 degrees. The results of 

 these tests are given in Table II. 



The totals show that from cultures which gave no offspring at 

 high temperature, 96 per cent, of the males were actually sterile, 

 while only 50 per cent, of the females were affected. More 

 detailed analysis makes plain the fact already demonstrated by 



