STUDIES ON THE DYNAMICS OF MORPHOGENESIS 271 



last ten years and to be presented in this and several following 

 papers is merely preliminary to more extended investigation 

 along various lines, some of which is going on at the present time. 



Before turning to the experimental data it is necessary to point 

 out their bearing upon certain questions more directly connected 

 with the special field of form regulation. It is an interesting 

 fact that in spite of the extensive literature on the phenomena 

 of form regulation in Planaria manj^ features which demonstrate 

 the close relation between the processes and results on the one 

 hand and various factors, internal and external on the other, 

 have received little or no attention. No species of Planaria 

 which has thus far been subjected to experiment is at any given 

 time in its development actually an equipotential system, yet the 

 apparent equipotentiality of parts seems often to be regarded as 

 the chief result of earlier work upon this genus. In the interest 

 aroused by the remarkable capacity of planarians as well as various 

 other simple forms for reconstitution of wholes from parts, but 

 little attention has been paid to the factors which limit this capac- 

 ity. If our investigation and analysis of such factors had beea 

 more extensive and rigid in the past it would never have been 

 possible to regard either Planaria or other simple organisms as 

 equipotential systems. As a matter of fact the limiting and deter- 

 mining factors are perhaps of even greater interest and importance 

 than the great capacity for regulation, for it is these factors 

 which enable us to learn something of the character and relations 

 of the processes involved. The existence of such limits of regu- 

 lation is totally ignored by Driesch in his definition of the equi- 

 potential system. It is certainly not true, either for Planaria 

 or for any other form that any part is capable of producing any 

 other part, or that any part is capable of producing a whole. 

 Only pieces of a certain character possess such capacities, and we 

 are already able to determine some of the factors which go to 

 make up that character. 



Moreover, as will appear below, a 'whole ' is not something with 

 clearly defined characteristics, but is rather a very vague notion. 

 Actually our brief examinations of the results of form regulation 

 are totally inadequate to determine whether they are 'wholes' 



