STUDIES ON THE DYNAMICS OF MORPHOGENESIS 307 



and new potentialities. And if we distinguish between 'explicit 

 potences' those resulting from a given constitution, and 'implicit 

 potences, ' those resulting from possible changes in constitution, 

 we gain nothing for if our conclusions concerning the physical 

 world are of any value, there is good reason to believe that the 

 implicit potences of all systems in nature are the same within 

 certain limits. Science can deal directly only with explicit 

 potences, and it is only by making implicit potences explicit 

 that we can actually know anything about them. A.fter such 

 knowledge is once gamed, however, it may serve as the basis for 

 prediction. 



To return to Planaria, the facts show that the morphogenic 

 reactions of a given part, depend within certain very wide limits 

 upon its physiological correlation with other parts. Planaria is 

 not then an equipotential sj^'stem in Driesch's sense. I believe 

 that investigation, unless limited by technical and other difficul- 

 ties which make exact experimentation impossible, will demon- 

 strate the same to be true for all other so-called equipotential 

 systems. I do not mean to exclude the possibility that such 

 systems may approach or become at times equipotential aggre- 

 gations — strictly speaking they cannot be called systems when in 

 this condition — but I believe that the occurrence of the process 

 of development in nature or of regulation in experiment is in 

 itself proof that the system concerned is not at the time equipo- 

 tential. I have shown above for Planaria that with sufficiently 

 extended and exact investigation of the phenomena of form regu- 

 lation the apparent equipotentiality disappears. And if the 

 piece undergoing regulation is not equipotential in its various 

 parts, what need is there for an entelechy? 



4. Preliminary analysis of the factor of size of the isolated piece 



It is evident from the above experiments that both the course 

 and the results of regulation are related in various ways to the 

 size of the piece. In the first place, a gradual change in the 

 character of the result occurs with change in the size of the piece, 

 e. g., in certain regions of the body the head becomes less and less 



