The mean cell diameter, the scatter of cells, and the steepness of frequency distribution curves were estimated in individual patients and the results arranged in age groups. The mean cell diameter increases immediately after birth, decreases from the 2d week until the end of the 1st year and increases again from the 2d year onwards. The differences found were statistically significant. The reduction of the cell scatter from the 2d week to the 3d month which was also found to be significant, agrees with the normal reduction of anisocytosis in the neo-natal period. Frequency curves are steepest during the period immediately before changes in size, as shown by the Price-Jones curves, which are due to a disturbance of the equilibrium of cell production and cell destruction.