1. A study has been made of the mitosis rate and of the diurnal cycles of male mice during each of the first 20 months of life. The mice used belonged to the Kreyberg's white label and the Strong's CBA strains. Most of the observations were made on the ear epidermis, but some attention was also given to other tissues.

2. It was discovered that, when judged from the point of view of mitotic activity, the life of a male mouse consists of four ages. During the immature age the animals are still growing and their mitosis rate is generally high, although the ear epidermis provides an exception to this rule. During the mature age which lasts from about the 3rd to the 12th month the mitosis rate is lowered. During the middle age which follows the mitosis rate increases, but in senility it is again reduced.

3. Coincident with these changes in the mitosis rate are changes in the spontaneous bodily activity. The mice are most active during immaturity and maturity. In middle age their activity is reduced by about half, and in senility they spend almost the whole time resting. Particularly in the Strong's CBA mice there are also changes in the timing of the diurnal cycle of spontaneous bodily activity, and these are immediately mirrored by changes in the timing of the diurnal cycle of mitotic activity so that throughout life a general inverse relationship between bodily activity and mitotic activity is maintained.

4. In middle-aged Strong's CBA males the daily rest period extends almost without interruption from 06.00 to 18.00 hr. However, the most active cell division develops only at the beginning of this period, and it is evident that in prolonged sleep a lack of some vital factor develops. It is shown that subcutaneous injections of starch overcome this lack in sleeping mice and result almost immediately in the redevelopment of a high mitosis rate. Thus it would appear that sugar is the vital factor involved, and that the sugar content of the tissues is quickly used up during high mitotic activity.

5. These results are discussed particularly in relation to the problem of carcinogenesis.

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