STUDIES ON THE DYNAMICS OF MORPHOGENESIS 305 



metabolism, upon the character and degree of physiological corre- 

 lation between parts and it is also limited, as will appear later by 

 external conditions, though these limits are not constant, but 

 vary with the internal factors. For example, larger pieces from 

 a given region will produce wholes at a lower temperature than 

 smaller pieces. Certainly the facts in regard to Planar ia as we 

 know them at present afford no basis for the assumption of an 

 entelechy. 



3. The 'potences' of parts 



It is sufficiently evident from the above experiments that the 

 regulatory processes in the cells at a given level of the body 

 differ in degree and kind, according to their relations with other 

 cells. If for example, we isolate the second fourth of the body 

 (ei, fig. 17), we find that in well nourished worms it produces a 

 head at its anterior end and this head is commonly normal 

 (table 1, series 201, no. 2). If, however, we cut it transversely 

 in half, both of its halves produce heads only rarely and these 

 heads show abnormal eyes (table 1, series 198, nos. 3 and 4). 

 Again, if we cut the original quarter into fourths, none of these 

 produces a new head (table 1, series 197, nos. 5-8). Evidently 

 the reaction of the cells at the anterior end of the original piece 

 is dependent, at least as regards its intensity, not merely upon 

 these cells alone, but upon the cells of all levels of the p'ece,for we 

 find that with the successive removal of cells from the posterior 

 end of the piece, the abilityof thecells at the enteriorend to form a 

 head decreases and disappears. In a later paper I shall show that 

 the rate or intensity of the dynamic processes is an important 

 factor in head formations. 



What shall we say then concerning the potences of these cells 

 at the anterior end of the piece? We must of course admit that 

 they are potentially capable of forming a head, but the experi- 

 ments demonstrate that such an assertion means nothing very 

 different from the statement that an acid is potentially capable 

 of forming a salt. In both cases we mean merely that a certain 

 reaction is possible under certain conditions. But the mechan- 



