166 HAROLD H. PLOUGH 



giving the percentages of crossingover for the second region — 

 curved to speck — simply emphasizes the conclusion drawn from 

 the data on the star to black region in table 7, namely, that for 

 factors between which there is a high percentage of crossingover, 

 any actual increase in single crossingover is compensated by a 

 similar increase in double crossingover, with the result that no 

 increase at all can be read from the table. The results shown by 

 the second brood counts will be discussed below. 



V. THE EXTENT OF THE TEMPERATURE EFFECT 

 Increase in percentage of crossingover not inherited 



If, then, the fact has been established that heat and cold 

 applied to female flies while they are going through their de- 

 velopment from egg to adult cause an increase in the percentage 

 of crossingover among the first ten day broods of their offspring, 

 then we can carry the analysis further. It is, of course, a common 

 place of work with Drosophila that no crossingover takes place 

 in the germ cells of males. Several tests established the fact 

 that this condition is not altered when the males are subjected 

 to the high temperatures. Males so treated show exactly the 

 same genetic differences as do males which have developed at 

 room temperature. The next question which suggests itself 

 is: What is the extent of the temperature effect as produced 

 on the female germ cells? Such a question must be resolved into 

 two parts: 1) Does the temperature produce any effect on the 

 offspring themselves? 2) Does the temperature affect the 

 entire output of eggs of any one female or simply the brood of 

 the first ten days? 



Since the offspring forming the first ten day brood are normal 

 in appearance, the first of the questions proposed means simply, 

 Is the increased percentage of crossingover induced by heat or 

 cold inherited? Counts from a very small number of cultures 

 showed that this question should be answered in the negative. 

 Female offspring of females which had been treated with high 

 temperature behaved genetically exactly as do females hatched 

 from control cultures. Counts of two cultures will make this 



