In vivo studies of Ehrlich ascites tumor cells during the first 5 days of growth in peritoneal cavities of mice consisted of the following: 1. Determination of growth curves by direct enumeration of cells. 2. Estimation of the duration of each phase of the mitotic cycle based on incidence of cells in different phases. 3. Radioautographic studies to determine the proportion of cells in different phases of the mitotic cycle that incorporate tritiated thymidine during a single brief exposure to this precursor of DNA. 4. Estimation of the rate of incorporation of tritiated thymidine at different times during the period of DNA synthesis by comparison of mean grain counts over nuclei in radioautographs at different times following exposure to tritiated thymidine. The assumptions underlying these experiments and our observations concerning the duration of the period of DNA synthesis and its relation to the mitotic cycle are discussed. It is concluded that DNA synthesis is continuous, occupying a period of 8.5 hours during the interphase and that the average rate of synthesis is approximately constant
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