790 RADIATION BIOLOGY 



case. Using dosage rates of 120-960 r/minute, Henshaw and Francis 

 (1936) found that X irradiation of the sperm was more effective than X 

 irradiation of the egg in producing mitotic delay in the z} r gote. On the 

 other hand, a dose given at 7800 r/minute produces greater mitotic delay 

 when administered to the egg than to the sperm (Henshaw, 1940b; Hen- 

 shaw and Cohen, 1940). Apparently the initial effect of irradiation is 

 greater in eggs than in sperms, but, owing to recovery during the treatment 

 period, the residual effect at the end of treatment will be less in the egg 

 than in the sperm, if the treatment period is prolonged. 



Accurate comparisons of the effects of different dosage rates on tissues, 

 however, in which the results are based on counts of cells in different 

 mitotic stages at certain intervals of time following treatment, introduce 

 special difficulties. We may consider separately the problems presented 

 by relatively short and relatively long exposures. 



If the times of irradiation are short, i.e., occupying only a small part of 

 the period during which the number of mitoses is falling and therefore a 

 small portion of the duration of one mitotic cycle, the minimum levels of 

 mitotic activity reached following treatment can be compared to deter- 

 mine the effectiveness. In such experiments, where a single count of 

 certain mitotic stages following irradiation is used as a measure of effec- 

 tiveness, the results will be different depending on whether the mitotic 

 count is made a certain number of minutes after the beginning, the mid- 

 point, or the end of the treatment period. Probably timing from the mid- 

 point of the treatment period is the most reliable procedure, when com- 

 parisons of the effects of different exposure times are to be made, unless 

 the time of the low-intensity treatment is greatly prolonged. The results 

 of the 7-ray studies on dosage-rate effect by Canti and Spear (1927) and 

 Spear and Grimmett (1933) on chick fibroblast in vitro are based on 

 counts of the numbers of cells in mitosis in treated cultures fixed 80 min- 

 utes after the end of the irradiation period and expressed as the percentage 

 of the number of mitoses in the controls. Both studies indicated that at 

 dosage rates of approximately 20 r/minute and higher, the dose required 

 to produce a given reduction in mitosis — to 50 per cent of that in the 

 controls in the former study and to 40 per cent in the latter — was inde- 

 pendent of the dosage rate. At rates below 20 r/minute, however, Canti 

 and Spear found that the dose increased successively with successively 

 lower dosage rates, while Spear and Grimmett's more detailed study in 

 the 4-20 r/minute region shows a fall in dosage from 17.2 to 8.7 r/minute 

 and a rise from 8.7 to 4.3 r/minute. This apparent demonstration of an 

 optimum dosage rate below or above which the mitotic depression is less 

 might be due to the fact that all their results were based on counts made 

 on cultures fixed 80 minutes after the end of the treatment period, while 

 the suppression of mitosis by irradiation is initiated at the beginning of 

 irradiation. At the time the counts were made, therefore, all the primary 



