794 RADIATION BIOLOGY 



only the reduced opportunity for interaction of effects, but also the 

 mitotic stages of the cells at the time of treatment and at observation and 

 the relation of these to the radiosensitivity of the stages involved. 



Spear (1932) compared the effects of 2 3^- and 5-minute continuous 

 exposures of fibroblast cultures to a given radium source with two 2}4~ 

 minute exposures, separated in the one case by 80 minutes and in the 

 other by 160 minutes. In the former, the second dose of y rays was 

 delivered at the depth of depression of mitotic activity, which was 

 lowered still further. The mitotic activity was thus lowered to a greater 

 extent and for a longer time after the two 2^-minute exposures than 

 after the continuous 5-minute exposure. In the latter experiment the 

 second fraction was given at the time the mitotic activity had returned 

 to normal, and the curve representing the resulting change in mitotic 

 count closely resembled the first one. These experiments suggest that 

 there is an optimum dose that is more effective per r than other doses in 

 reducing mitotic activity and that the 2^-minute exposure represents a 

 dose nearer to the optimum than the 5-minute exposure. This is sub- 

 stantiated by a comparison of the 2)4- and 5-minute continuous exposure 

 curves. The former shows a minimum mitotic count only about 7 per 

 cent lower and a return of mitotic activity to normal only about 20 

 minutes later than the latter. 



Temperature. It appears that the temperature at which cells are 

 irradiated has little or no effect on their subsequent mitotic activity 

 (Gorman S. Hill, unpublished). Grasshopper embryos were given 8 r of 

 X rays while at temperatures of 2, 18, and 38°C, maintained subse- 

 quently at 38°C, and fixed at 44, 88, 132, 176, and 220 minutes. There 

 were no significant differences in the number of neuroblasts in prometa- 

 phase, metaphase, and anaphase at any of these time intervals for the 

 three temperatures tested. 



The times after treatment at which the mitotic activity will reach a 

 minimum and then will begin to rise during recovery are functions of the 

 temperature at which the cells are maintained after irradiation. Main- 

 tenance of the cells at a low temperature after irradiation prolongs the 

 period of mitotic decrease and delays the return of mitotic activity. In 

 the Allium root tip, the time of minimum mitotic activity after 150 r is 

 about 10 hours at 24° and about 24 hours at 16°C (Darlington and 

 La Cour, 1945). In the grasshopper neuroblast, the low point of mitotic 

 activity is reached at about 66 minutes after 32 r of y rays at 38°C 

 (Carlson et al., 1949) and at about 100 minutes after 31 r of X rays at 

 26°C (Carlson, 1942) ; the time intervals between treatment and resump- 

 tion of mitotic activity are about 165 and 195 minutes at 38 and 26°C, 

 respectively. 



Low temperature following X irradiation also delays both the fall in 

 the mitotic count immediately after treatment and its rise during recov- 

 ery. Walter G. Adams (unpublished) has studied the effect of low 



