Klebs suggested that the accumulation of the leucocytes at the wounded surface may act as a stimulus to growth, and that the chromatin of their nuclei might be absorbed by the cells of the tissue, and combining with the nuclei of these cells bring about the new growth. But Ziegler points out that we now know that although the leucocytes are dissolved and absorbed over the wounded surface, no process of absorption, of the sort postulated by Klebs, takes place. Ziegler thinks that Nothnagel is wrong in supposing that an increase in the blood supply, bringing with it an increase in the nourishment, can account for the hypertrophy of the kidney. On the contrary he believes that the growth is the result of an increase in the function of the organ due to the increase of the chemical substance, urea, that is brought to the secreting cells. The muscles of the body also hypertrophy as a result of their activity and not as a result of the additional blood supply. In connection with these problems of hypertrophy it may be pointed out that, under certain conditions, blood vessels may enlarge and their walls become thickened. To cite a single example, Nothnagel found that if the femoral artery of the rabbit is tied, the blood vessels, that come off immediately above the ligature, and which have already, through their subdivisions, connections in the muscles with other branches of the same femoral artery (that come off below the ligature), grow larger after a time. This he believes to be due, in the first instance, to the increased speed of the blood in the vessels, and thereby the bringing to these arteries of an increased food supply. Other writers have given different interpretations. Ziegler himself believes that several factors may be capable of bringing about the result. He thinks it improbable that the increase in the food supply can alone be the cause, and thinks it much more probable that the increased work that the vessels must perform while carrying more blood will account for the enlargement.