DYNAMICS OF MORPHOGENESIS 149 



be presented which has to do with the physiological resistance of 

 different levels of the body to certain agents and the relation 

 between this resistance and the rate of reaction. We shall see 

 that the axial gradient of resistance to anesthetics, KCN, etc., is 

 similar to that of regulatory morphogenesis. In both cases the 

 same processes are involved : in the one case we examine the mor- 

 phological records of the processes, in the other we compare the 

 processes and the results of the two methods confirm each other. 



The experiments recorded in Section C show, however, that 

 the axial gradient changes more or less abruptly between the 

 posterior end of the first and the anterior end of the second zooid. 

 As a matter of fact each zooid possesses a gradient of its own, 

 though in the posterior zooid where the development is not ad- 

 vanced the individual gradients are much less sharply distinguish- 

 able from each other than in the case of the first and second zooids. 



As regards the different effects of the KCN in the three series, 

 nos. 432, 512 and 494 given in the tables in Section C, it is im- 

 possible to go into details until further data concerning the nature 

 of the axial gradient and the dynamics of regulation are presented. 

 It may be said, however, that all these differences are readily 

 interpreted on the basis of the following facts which, as will be 

 shown, are all well established : first, the axial gradient is a gradient 

 in the rate of the dynamic processes and primarily of the oxida- 

 tions, and the head region is dominant because it is the region of 

 highest rate. Second, the physical isolation of a part increases 

 temporarily its rate of reaction in direct proportion to the degree 

 of its subordination to more anterior regions, in other words, the 

 direct effect of isolation in accelerating the rate of reaction in a 

 piece increases with increasing distance from the original head 

 region. This holds only within the limits of a single zooid: the 

 anterior region of each zooid is one in which physical isolation 

 produces little or no acceleration of the rate of reaction and this is 

 so because these regions are relatively independent of others^ i.e., 

 are dominant. Third, the formation of a new head in a headless 

 piece is not in any sense a restitution of a missing part, but the 

 formation of a new individual head first. The new head forms 

 from the cells which are directly affected by the wound and the 



