Child, Driescli's harmonic equipotential systems in form-regulation. (517 



den Begriff des harmonisch-äquipotentiellen Systems ja gerade kon- 

 struiert für solche Restitutionserscheinungen, die nicht Regene- 

 rationen sind" (Dries ch, 1908, p. 410). 



The facts of the case are then these. Driesch has included 

 Planaria in his list of harmonic equipotential systems, but has 

 practically ignored the part which true regeneration plays in regu- 

 lation in this form. Cerianthus, which is certainly as much a har- 

 monic equipotential system as Planaria, so far as method of regu- 

 lation is concerned, Driesch regards at one point as such a system, 

 while elsewhere he takes the opposite view because regeneration 

 occurs in Cerianthus. I am quite willing to admit that I find 

 myself unable to arrive at Driesch's real meaning with regard to 

 this point. 



I note also that Moszkowski (1907) regards the actinian body 

 as a harmonic equipotential system. His conclusions are based on 

 a study of Actinia aequina and Actinoloha dianthus in which resti- 

 tution occurs in essentially the same manner as in Cerianthus. 



It is beyond the scope of the present paper to discuss the 

 essential differences between harmonic equipotential and complex 

 equipotential systems, though I believe it can readily be shown 

 that what Driesch regards as the essential differences are either 

 hypothetical or purely differences of degree. In fact, if I under- 

 stand Driesch's latest definition of the harmonic equipotential 

 system, its actual existence seems to me somewhat doubtful. In 

 this definition (Driesch, 1908, p. 414) it is stated that „die zu 

 diesem Resultate (i. e., the whole) führenden Einzelleistungen auf 

 die Einzelelemente (Zellen) des Fragments, so wie es da ist, 

 verteilt werden". It is the clause „so wie es da ist" of whose 

 meaning I am uncertain. If it means that no changes which play 

 a part in determining the morphogenetic result are brought about 

 by the act of isolation, then I believe that the harmonic equi- 

 potential system does not exist in nature, for the data of experi- 

 ment indicate that the changes in the piece resulting from its 

 isolation are, at least in many cases and probably in all, very im- 

 portant factors in the following localization and differentiation. If 

 the clause means that localization is accomplished in such cases 

 without the formation of new cells or new material, then certainly 

 Planaria is not such a system, and in Tubularia cell division 

 occurs to a considerable extent before visible differentiation. As 

 regards the other adult forms the data ore not conclusive, but iso- 

 lated blastomeres (e. g. in the sea-urchin) evidently become wholes 

 without the formation of new cells, and the question as to whether 

 new material is formed in these cases is at present idle. 



