I2O REGENERATION 



in the breaking down of the tissue of the organ a substance is pro- 

 duced that excites the cells to proliferation. 



Klebs suggested that the accumulation of the leucocytes at the 

 wounded surface may act as a stimulus to growth, and that the chro- 

 matin 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 liga- 

 ture), grow larger after a time. This he believes to be due, in 

 the first instance, to the increased speed of the blood in the ves- 

 sels, 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. 



In connection with this discussion it may not be unprofitable to 

 recall that in the regeneration of the lower animals we find simpler 

 conditions in which proliferation of the cells takes place under cir- 

 cumstances where many of the factors suggested in the above discus- 

 sion are absent. In the first place we find that new growth may occur 

 without any increase in the nourishment that is brought to the organ. 

 Regeneration takes place in the entire absence of food, except so far 

 as it may be stored up in the tissues. Even in a planarian that is 

 starving and decreasing in size, proliferation of new cells will take 

 place if a part is removed. In many of the lower forms there may 

 be proportionately even a much greater proliferation than in the 

 regeneration and hypertrophy in the mammalian organs. It is true 



