CELL DIVISION, MORPHOLOGY, VIABILITY 791 



effects produced by the high dosage-rate irradiation would have had less 

 time for recovery than all except the last effects produced by the low 

 dosage-rate treatment. An optimum effectiveness would then be 

 expected for the dosage rate for which a minimum of recovery had 

 occurred at 80 minutes after the end of treatment. 



A more laborious but at the same time more informative procedure is to 

 make counts at intervals following irradiation. Comparison can then be 

 made of the rates of fall and rise, as well as the minimums of mitotic 

 activity at certain times after the beginning, mid-point, or end of the 

 irradiation period. 



Using a dose of 100 r delivered at rates of 9.3, 29.8, and 103 r/minute, 

 Lasnitski (1946) determined that depression of mitotic activity was 

 approximately the same 80 minutes after treatment for all three dosage 

 rates. Recovery occurred more rapidly after 103 r/minute than after 

 29.8 r/minute and more rapidly after 29.8 r/minute than after 9.3 r/min- 

 ute. After 2500 r, however, no mitotic cells were present within 24 hours 

 if the dose was delivered at 101 r/minute, 0.4 per cent of the cells were in 

 mitosis at 24 hours if the rate was 29.8 r/minute, and about 1 per cent if 

 the rate was 9.3 r/minute. (The per cent of mitotic cells in the control 

 cultures at any one time during the same period ranged from 2.6 to 8.0.) 

 Counts of the mid-mitotic cells in hanging-drop preparations of grass- 

 hopper neuroblasts give comparable results. Curves based on twenty 

 successive counts of ten to fourteen embryos made at 22-minute intervals 

 beginning 20 minutes after the mid-point of the radiation period were not 

 significantly different for doses of 8 and 64 r at high (32 r/min) and low 

 (2 r/min) dosage-rate y radiation (Carlson et at., 1949). After doses of 

 128 and 256 r the rate of fall in mitotic activity and the length of the 

 cessation period are comparable for both the 2 and 32 r/minute dosage 

 rates, but the return of the mitotic count to normal is much slower after 

 the 32 than after 2 r/minute treatment (Fig. 11-9). 



Another method of comparing the efficiencies of different dosage rates 

 in reducing the mitotic activity of a tissue is to compare the totals of the 

 mitotic counts obtained at uniform intervals over a certain period follow- 

 ing treatment. The time interval at which counts are made may be 

 fixed arbitrarily, but, if the interval taken represents the average time 

 required for the cells to pass through the mitotic stage or stages to be 

 counted, the total of the average counts will approximate closely the 

 number of cells passing through the stages counted and therefore through 

 mitosis during the period under consideration. This method presup- 

 poses, however, that the stages counted are not prolonged to such an 

 extent by either of the treatments that the same cell is counted twice; 

 otherwise a high count and, therefore, a false indication of increased 

 mitotic activity will result when mitotic retardation actually occurs. 

 This procedure can be used with confidence, then, only when it is possible 



