THE RANGE OF DOMINANCE 137 



that a stem as well as a hydranth may be formed. 

 According to Driesch this adaptation is not determined 

 physico-chemically, but by the principle which he 

 calls entelechy and which as he believes controls develop- 

 ment. Unfortunately for Driesch's view this "adapta- 

 tion" does not occur in all cases, and is very incomplete, 

 for, as I have pointed out (pp. 96-99), x these short 

 pieces often give rise to hydranths or apical parts of 

 hydranths without stems or basal parts. The experi- 

 mental evidence indicates that the shorter hydranths in 

 short pieces are merely hydranths which are partially 

 inhibited by other regions of the piece, just as the head of 

 Planaria may be partially inhibited by other regions of 

 the piece. As in Planaria, short pieces, particularly those 

 from the more basal regions of the body, are more stimu- 

 lated by section, and their metabolic rate is therefore 

 higher throughout than that of longer or more apical 

 pieces. Under these conditions the gradient arising at 

 the cut end is much less effective in determining the devel- 

 opment of a new structure, the hydranth, than it is when 

 the general metabolic rate is lower. Figuratively speak- 

 ing the new gradient is partially obliterated by the gen- 

 eral high metabolic rate in the piece. Consequently its 

 length is less and the length of the hydranth determined 

 by it is correspondingly less than in longer pieces, and 

 development is also retarded. A piece of given length 

 may produce a single short hydranth and stem, or a 

 longer hydranth without stem, or biaxial hydranths, or 

 apical portions, and all these differences in behavior are 

 determined by simple differences in the gradient relations. 



1 See also Child, "An Analysis of Form Regulation in Tubularia, 

 Regulation in Short Pieces," Archiv fur Entwkkelungsmechanik, XXIV, 

 1907. 



