The morphologic changes which follow the extirpation of one kidney, and which result in a compensatory hypertrophy of the remaining organ, have been even more extensively studied than the functional disturbances which obtain under such conditions. The following study adds, therefore, but little to our knowledge of the changes involved, but is of interest chiefly in that it correlates these changes with the functional disturbances described in the first section of this paper, and also in that it makes clear the reason for certain discrepancies which are found when the weight of the kidney is taken as a measure of the amount of functioning tissue present in it. As examples of earlier morphologic work two studies may be briefly cited which summarize our knowledge of the subject. Lorenz1found that the increase in the size of the remaining kidney was chiefly due to an increase in the volume of the