792 



RADIATION BIOLOGY 



to check the mitotic time schedule by direct observations of living cells. 

 Carlson et al. (1949), using this method, showed that the 7-ray-induced 

 reduction in prometaphase-through-anaphase stages during a 7-hour 

 period following treatment was not appreciably different after dosage 

 rates of 2 and 32 r/minute at doses of 8, 32, and 64 r, but that the 32 

 r/minute rate caused a much greater reduction than the 2 r/minute rate 

 at 128 and 256 r. 



The problem becomes quite different if the dosage is spread over a long 

 period, e.g., hours or days. In such experiments the recovery factor 



1 3 5 



HOURS AFTER IRRADIATION 



Fig. 11-9. Effects of different doses of 7 rays delivered at different dosage rates on 

 mitosis in the Chortophaga neuroblast: open circle, 2 r/min; solid circle, 32 r/min (after 

 Carlson, Snyder, arid Hollaender, 1949). 



dominates the picture. A brief high-intensity treatment will be com- 

 pleted before the mitotic activity will have had time to fall off appreciably, 

 while at the end of a prolonged low-intensity treatment, radiation effect 

 and recovery processes will have reached equilibrium and the mitotic 

 activity will be at a minimum ; therefore the shape of the mitotic activity 

 curves, based on a series of counts immediately following treatment, will 

 be so different as not to be readily comparable. If the mitotic activities 

 after a certain dose delivered over a short and over a long period are com- 

 pared at corresponding times measured from the beginning or mid-point of 

 the radiation period, all the primary effects produced by the high-inten- 



