Mitotic Effects of Prolonged Irradiation with Low-Intensity Gamma Rays on the Chortophaga Neuroblast

1. Prolonged treatment of Chortophaga neuroblasts with low-intensity γ radiation reduces mitotic activity much less than a comparable dose given at high intensity. 2. Treatment for 6 days at 3.4 r/hour reduced the mid-mitotic count to about 40% of normal for a period of about 7 hours following treatment. (The same dose administered in a few minutes would have reduced the mid-mitotic count to zero for about 5 hours beginning within an hour of the end of treatment.) 3. Continuous irradiation at 0.80 r/hour (1) failed to produce a significant mid-mitotic effect at the end of two days, (2) produced a significant increase in the number of mid-mitoses through the second counting period at the end of four days, and (3) produced a significant increase in the number of mid-mitoses through the second counting period after 6 days of treatment. 4. The radiation-induced increase in the number of mid-mitoses immediately following four and six days of treatment is shown to result, not from a stimulating effect of the radiation on mitotic activity, but from the simultaneous progression into mid-mitosis of neuroblasts that have accumulated in late prophase as a result of the inhibiting effect of the radiation on this stage.