LARVAL DEVELOPMENT IN THE SEA URCHIN 75 



The forms shown in figures 6, 7, 12 to 16, and 19 to 21, are 

 the final stages of development attained in the different degrees 

 of inhibition. This does not mean that the animals die at these 

 stages, but simply that no further development occurs, although 

 life and more or less movement may continue for days. It is 

 evident that the various degrees of inhibition are likewise pro- 

 gressive steps in the elimination of the axial relations as effective 

 factors in development and differentiation. If the axes are fun- 

 damentally metabolic gradients this is easy to understand, for, 

 since susceptibility varies directly with metabolic rate, the de- 

 crease in rate is greater in regions of higher than in regions of 

 lower rate, and the metabolic gradient is therefore more or less 

 completely obliterated by leveling down according to the degree 

 of inhibition. When this obliteration by leveling down pro- 

 ceeds to a certain point, the metabolic gradient ceases to be an 

 effective factor in development and differentiation, and the ani- 

 mal becomes physiologically anaxiate, and, therefore, develop- 

 ment and localized differentiation cease, although life may con- 

 tinue for a long time. 



THE FORMS RESULTING FROM DIFFERENTIAL ACCLIMATION 



Differential acclimation to an inhibiting agent is of course pre- 

 ceded by a greater or less degree of differential inhibition which, 

 however, is always less than where the action of the agent is 

 sufficient to prevent acclimation. The effect of differential accli- 

 mation on the course of development and on the form of the 

 body, differs in degree, according to the degree of inhibiting 

 action and the rapidity and degree of acclimation, but the char- 

 acteristic feature of the morphological modifications resulting 

 from differential acclimation is that they are opposite in direc- 

 tion to those resulting from differential inhibition. The regions 

 most inhibited in the one case show the greatest relative ac- 

 celeration and over-development in the other. 



Of course where acclimation is slight in degree or occurs 

 slowly, differential acclimation may only partially obliterate the 

 differential effects of inhibition, so that the resulting form merely 

 shows a less extreme degree of differential inhibition than it 



