Mitosis in normal and regenerating planarians

The rate of mitosis in planarians was measured by two methods. First, by tabulating the proportion of nuclei displaying metaphase mitotic figures after varying lengths of time in colchicine. Second, by measuring the total DNA in a colony of planarians as a function of time. The first method gave a rate of mitosis by non‐regenerating Dugesia dorotocephala of 1.4 × 10−4 divisions/cell hour. This rate remains constant for the first 24 hours following decapitation but increases to 2.7 × 10−4 divisions/cell hour by 54 hours. The rate of mitosis in the blastema was no larger than that of the mesenchymal tissues of the older portions. Measurements of the total DNA versus time gave an estimate of the average rate of cell division for spontaneously fissioning planarians of 5.8 × 10−4 divisions/cell hour which, in view of the tendency of the colchicine method to underestimate the true rate, is in satisfactory agreement. The implications of these results for mechanisms of regeneration are discussed.