Studies on Regulation. IF. 99 



food indicates that the stimulus which brings about the regen- 

 eration is sufficiently powerful to deprive the old portions of ma- 

 terial. There is little doubt that this difference indicates a dif- 

 ference in metabolic activity between the old and the growing 

 regions. If this conclusion be correct it follows that in the pres- 

 ence of nutrition the new parts will grow more rapidly than the 

 old. Moreover, we find that in the absence of food growth of 

 the new tissue may cease long before the amount of tissue removed 

 has been replaced. We must conclude therefore either that the 

 stimulus to regeneration decreases as regeneration proceeds, or 

 that the old portions give up material less rapidly as the process 

 continues (Child, '03d). That there is an actual difference in 

 quality between the new and old tissue is clearly shown by ob- 

 servations which I have made repeatedly, viz., that in regenerat- 

 ing pieces kept without food until death occurs from starvation, 

 infection or other causes, the old parts usually disintegrate before 

 the new. In many cases I have seen the old tissue disintegrate al- 

 most completely in pieces of Leptoplana, while the new tissue re- 

 mained alive and apparently healthy for a considerable time after- 

 ward. Moreover, the nev/ parts in regenerating specimens show a 

 greater degree of muscular and other functional activity than do 

 the old parts. 



For reasons which I hope to state in full at some future time, I 

 believe that this difference is at least in part dependent upon 

 functional conditions, viz., that it concerns the use or ac- 

 tivity of the parts. In the earlier stages of regeneration 

 other factors are very probably concerned in greater or less 

 degree. The presence of a cut surface places the cells adjoining 

 it under conditions widely different from those existing before 

 the cut was made. The equilibrium in physical conditions is de- 

 stroyed by the removal of the part and the absence of pressure 

 from other parts on one side may itself be sufficient to bring about 

 a migration or growth of tissue outward from the cut surface. It 

 is extremely difficult in such cases to determine how much of the 

 " new tissue " is the result of migration and how much of actual 

 proliferation. But if the attempt is made by the animal to use 

 the ' new tissue " thus formed in the manner characteristic of 



