1. The average duration of mitosis in the Yoshida ascites tumor is 51 minutes. The duration of the interphase is approximately 17½ hours. Thus, during the first 5 days of growth, when mitotic rate is high, the cell population should double every 18½ hours.

  2. Total-body x-radiation with 425 r will produce complete inhibition of mitosis in 1 hour which is sustained for another 3 hours.

  3. There is no inhibition of those cells which are already dividing when irradiation occurs. These go on to completion of mitosis. It is emphasized that they constitute less than 4 per cent of the total cell population and are the only mitoses which are ever directly exposed to ionizing radiations.

  4. Of greater importance is the effect of radiation on the 96 per cent of cells treated while in the resting stage. Mitosis is inhibited in at least 64 per cent of the entire cell population. The important point is not that some interphase cells might be more liable to mitotic inhibition than others, but that with moderate dosages most of the cells radiated while in interphase have mitosis delayed.

  5. There is slow recovery from mitotic inhibition. The preradiation mitotic rate is regained about 20 hours after radiation.

  6. The biological significance of mitotic inhibition on tissue growth still remains to be clarified.


This investigation was conducted during the tenure of Postdoctoral Fellowships granted by the National Cancer Institute, U.S. Public Health Service, and by the Damon Runyon Memorial Fund.

This is publication No. 883 of the Cancer Commission of Harvard University.

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