GENERAL INTRODUCTION 2$ 



its axes reversed as compared with the old part. As an example of 

 this may be cited the development of an aboral head on the pos- 

 terior end of a piece of the stem of Tubularia (Fig. 15, B), or the 

 development of a tail at the anterior end of a posterior piece of an 

 earthworm (Fig. 2). 



The term " physiological regeneration " I shall use in the ordinary 

 sense to include such changes as the moulting and replacement of 

 the feathers of birds, the replacement of teeth, etc., changes that 

 are a part of the life-cycle of the individual. In some cases it can 

 be shown that these processes are closely related to ordinary re- 

 generation, as when a feather pulled out is formed anew without 

 waiting for the next moulting period, and formed presumably out of 

 the same rudiment that would have made the new feather in the 

 ordinary moulting process. 



It is sometimes convenient to contrast the process of physiological 

 regeneration with all other kinds. The use of the term " pathological 

 regeneration " for the latter seems to me, as has been said, unsatisfac- 

 tory. The two terms proposed by Delage, 1 viz. " regular regeneration " 

 and "accidental regeneration," have certain advantages, although 

 there is nothing accidental, or at least occasional, in regard to the pro- 

 cess itself, as it is entirely regular, although it may only occur after 

 an accident to the animal. The term " regular regeneration " is, I 

 think, more satisfactory than " physiological regeneration," but the 

 latter has the advantage that it has come into current use. For what 

 is known as pathological or accidental regeneration, I propose the 

 term " restorative regeneration," and I shall continue to use the 

 term "physiological regeneration" as generally understood. 



1 Delage, K La Structure du Protoplasma, etc., '95. 



