PHYSIOLOGICAL DOMINANCE 115 



reaction is less rapid than that of x (see Fig. 31), because 

 they represent a lower level in the gradient and their 

 relation to the region y is different from that of x. The 

 rate of development and completeness of the new basal 

 end varies directly with the metabolic rate in y; any 

 conditions which decrease the metabolic rate in y 

 decrease the development of the basal end, and vice 

 versa. We may say then that tail-frequency =^H but 

 that under the usual conditions, when a gradient is 

 already present in the piece, rate z is so low that it 

 becomes negligible, and the formula becomes tail- 

 frequency = rate y. This holds true as long as a new 

 zooid does not arise in this basal region. If a new zooid 

 does arise there in consequence of physiological isolation, 

 as is often the case in headless pieces, then the lower 

 the rate in y, the more rapid the development of this 

 posterior zooid. In headless pieces the large size of the 

 posterior outgrowth (cf. Figs. 52, 53, with Fig. 50, p. 103) 

 is due to the fact that this region is not physiologically 

 the basal end of the individual but a second individual. 1 

 The development of the basal region is then depend- 

 ent upon the presence and influence of more apical 

 regions, while the development of the head occurs 

 independently of other parts, so far as it is not inhibited 

 by them. The relation between the major axial gradient 

 and these differences of behavior in different regions is 

 evident. The process of reconstitution of a new indi- 

 vidual from a headless piece in Planaria is a process of 

 development beginning at two different levels, first, at 

 the apical end of the piece with the formation of a 



1 Child, " Studies on the Dynamics of Morphogenesis. Ill," Jour. 

 ofExp.Zool.,Xl, ign. 



