GENERAL INTRODUCTION 21 



Barf urth 1 has defined regeneration as " the replacement of an organ- 

 ized whole from a part of the same." If the part is given by nature, 

 there is a process of physiological regeneration ; if the part is the 

 result of an artificial injury, the process is one of pathological re- 

 generation. Barfurth includes in the latter category the production 

 of a new, entire individual from a piece, as in hydra ; regeneration 

 by proliferation, as in the earthworm; and also the development of 

 pieces of an egg or of an embryo. 



Barfurth's definition of regeneration is unsatisfactory, since an egg 

 is itself a portion of an organism that makes a new whole, and this 

 sort of development is not, of course, as he himself points out, to be 

 included in the term regeneration. Nor does the use of the word 

 " replacement " save the definition, since in many cases the kind of 

 part that is lost is not replaced. The use of the word "pathological" 

 to distinguish ordinary regeneration from physiological regeneration 

 is, I think, also unfortunate, since it implies too much. There is noth- 

 ing necessarily pathological in the process, especially in such cases as 

 hydra, or as in the development of a piece of an egg where the piece 

 is transformed directly into a new organism. Furthermore, in those 

 cases in which (as in some annelids and planarians) a new head is 

 formed after or during the process of natural division, there is little 

 that suggests a pathological process ; and in this instance the regen- 

 eration takes place in the same way as after artificial section. 



Driesch, in his AnalytiscJic Theorie, states that Fraisse and Bar- 

 furth have established that during regeneration each organ produces 

 only its like. Driesch defines regeneration, therefore, as the re-awak- 

 ening of those factors that once more bring into play, by means of 

 division and growth, the elementary processes that had ceased to act 

 when the embryonic development was finished. This is regeneration 

 in the restricted sense, but Driesch also points out that this definition 

 must be enlarged, since, when a triton, for example, regenerates its 

 leg, not only does each tissue produce its like, but later a reconstruc- 

 tion and differentiation takes place, so that a leg and foot are formed, 

 and not simply a stump containing all of the typical tissues. Driesch 

 holds that regeneration should include only those cases in which a 

 proliferation of new tissue precedes the development of the new part, 

 and suggests that other terms be used for such cases as those of pieces 

 of hydra, pieces of the egg, etc., in which the change takes place in 

 the old part without proliferation of new tissue. It seems to me 

 unwise to narrow the scope of the word regeneration as Driesch pro- 

 poses, for it has neither historical usage in its favor, nor can we make 

 any fundamental distinction between cases in which proliferation 

 takes place and those in which it does not. As will be shown later, 



1 Ergebnisse der Anatomic und Entwickelungsgeschichte. 1891-1900. 



