1. The effect of X-rays is divisible into two periods, one showing the immediate effect, the second period being one of recovery.

2. The immediate effect includes gradual cessation of mitosis, and the production of abnormalities in those cells which were in the process of division or were about to divide at the time of irradiation.

3. The length of time during which no cells enter into mitosis depends on the size of the dose.

4. During the recovery period some cells again enter into mitosis but their division is usually of an abnormal type.

5. Irradiation acts on cells in the interphase by interfering with some physiological process. These cells are then unable to enter into mitosis until the effect of irradiation has passed off.

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