I9O WILLIAM C. YOUNG AND HAROLD H. PLOUGH. 



Fertile Cultures. Sterile Cultures. 



31 9 X 25 d" 13 2 



25 9 X 31 d" 6 5 



In spite of the excess in the number of fertile cultures from 

 females this result was taken to indicate that "neither the eggs 

 nor sperm are injured by exposure to 31 C." Cytological exam- 

 ination showed that apparently normal eggs and sperm were 

 present in flies sterilized by heat, and copulation was observed 

 between these flies at the high temperature. Plough and Strauss 

 thus summed up the matter: 'The reason for the failure to 

 produce offspring after the first generation seems to lie either 

 in a failure of the sperm to reach the eggs, that is unsuc- 

 cessful copulation or in the failure of the fertilization process 

 itself." 



Further investigation of the question was delayed until the 

 winter of 1924-5 when one of us Young was making a series 

 of tests designed to determine whether strains of flies showing 

 differing degrees of toleration of high temperature could be 

 isolated from a single mass culture of a wild stock. The results 

 of this series of tests will be reported elsewhere, but the experi- 

 ments offered an excellent opportunity to settle the question 

 raised above. 



An examination of the Plough and Strauss data quoted above 

 shows only that eggs and sperm are not injured by exposure to 31 

 degrees in every case. Sterile cultures did occur, and a com- 

 parison of the relative numbers suggests, in addition, that males 

 are more likely to be sterilized than females. Since the 31 degree 

 flies used were not actually tested and shown to be sterile at 31 

 degrees, but were taken from susceptible stocks (i.e., stocks which 

 ordinarily failed to reproduce at 31 degrees), it is possible that 

 some of the flies which gave offspring in crosses might have 

 been fertile even if they had been mated inter se at 31 degrees. 

 For these reasons we returned to the question of the possible 

 injury to the germ cells of the flies by high temperature, and made 

 more extended breeding tests, and more careful cytological 

 examination of the gonads from flies sterile at 31 degrees. Most 

 of the breeding tests were made by Young, while the remainder 

 together with the cytological study were made by Plough. 



