The growth rate of Lewis lung carcinoma, either as primary subcutaneous tumors or as spontaneous lung metastases, decreases with increasing age or mass. The length of the median cell cycle of the primary tumors increased from 17.6 hr at Day 5 to 25.9 hr at Day 21 with a concomitant increase in experimental doubling time from 2.5 to 9.6 days. The length of the median cell cycle of the metastatic lung nodules was 15.2 hr in the mice bearing 21‐day primary tumors. Two mathematical models were used to analyse the per cent labeled mitoses data and the results are given for comparison. The labeling index of the primary tumors decreased in a nearly linear manner with time postimplant and the percentage of cells labeled in the metastatic nodules was higher than in the primary tumors when measured in the same hosts.

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