168 HAROLD H. PLOUGH 



they are removed and placed in a second bottle for a second 

 period of ten days. A third brood may even be taken in many 

 cases. The results of each of these successive broods are tabu- 

 lated and compared with the results of the first broods. 



As mentioned early in this paper, the results of Bridges have 

 shown that a decrease in the amount of crossingover in the second 

 broods of second chromosome experiments occurs. The data 

 here presented confirm these findings. In table 9 the percentage 

 of crossingover in the first region — which is the only one of great 

 significance — fell off about 5 per cent in the second brood of the 

 controls. The flies which had been subjected to increased tem- 

 perature need not be considered in this case, since no effect was 

 noted when C II 1 was present. In table 7, however, second 

 brood totals are given for flies which showed a decided increase 

 in crossingover in the first broods due to heat. The percentage 

 for the shorter region — per cent II in the table — shows a falling 

 off of 7 per cent in the second broods. Though this is more than 

 the usual deviation for second broods, this decrease does not 

 bring the percentage down to that of the first brood controls, 

 therefore further evidence on the point is necessary. First and 

 second broods of both control and heat-treated flies from the 

 same Pi pairs were taken. In addition for both sets of data 

 second broods were taken from flies which were kept at the high 

 temperature rather than at 22°C. while laying the first broods. 

 The results are as follows : 



In order to make the significance of this table plain each item 

 will be discussed individually. For Nos. 1 to 4, the stock star- 

 black-curved was used. In these cases, then, only the shorter 

 region — per cent II in the table — is significant No. 1 was a 

 normal control, the females hatched at 22°, mated and laid 

 their first brood at the same temperature. No. 2, the second 

 brood after this treatment, showed the usual decrease in the 

 amount of crossingover already pointed out by Bridges. No. 3 

 differed from No. 1 only in that the female parents were hatched 

 at 31.5. These females were mated and laid the first brood at 

 22°. The values of 29.8 per cent for the black to curved region 

 is more than nine units higher than the control, due to the effect 



