2O8 BIOLOGICAL LECTURES. 



hypothesis or as a possible point of view, and not as an elabo- 

 rated theory of regeneration. In fact, I think it would be a mis- 

 take at the present time to attempt to construct a final theory, 

 for, if I have been in the least successful in this discussion of 

 the problem, I hope to have made it clear that the process of 

 regeneration involves many factors. It is obvious that until we 

 have analyzed the problem into its component parts it would 

 be ridiculous to attempt to formulate a theory of regeneration. 

 It is not difficult to show that there are in reality many factors 

 in the process quite different in kind. For instance, the clos- 

 ing in of the exposed surface seems to be due to some sort of 

 cytotropism in the cells as well as to other factors ; the produc- 

 tion of new cells from the old ones must be due to another set 

 of processes, as well as their migration out over the exposed 

 end ; the form of the piece and its size are also factors ; the 

 specific character of the cells derived from the old ones is still 

 another problem ; and finally, if my analysis is sound, the sub- 

 sequent molecular arrangement taking place throughout the 

 new part is one of the most important changes that take place. 

 I have laid emphasis only on this latter characteristic because 

 it seems to me that it is just this change that comes nearer to 

 what we mean by regeneration than any of the others that I 

 have named ; they all enter into the problem, however, and 

 none of them can be neglected, but to find a theory to account 

 for them all at once would be, I think, extravagant to attempt 

 and probably disastrous in its results, 



