PHYSIOLOGICAL BASIS OF PATTERN 37 



region of greatest activity is most inhibited and less 

 active regions are less inhibited. In the apico-basal or 

 anteroposterior axis, for example, the degree of inhibi- 

 tion is greatest in the apical or head region and decreases 

 basipetally or posteriorly, consequently the positions 

 and proportions of parts are altered in a definite way, 

 the apical or head region being relatively smaller 

 and the basal or posterior regions relatively larger 

 than in the normal form. Microcephaly, for example, 

 is a characteristic result of differential inhibition along 

 the polar axis. In differential acceleration the altera- 

 tions of proportion are in the opposite direction, and 

 megacephalic forms result. In differential acclimation 

 and recovery growth or development is first inhibited 

 to some extent, but the more active levels of a gradient 

 acclimate or recover more readily and more completely 

 than the less active, so that in these cases also growth or 

 development is finally relatively more rapid or greater in 

 amount apically or anteriorly than basally or posteriorly. 



Similar modifications also appear with respect to 

 the symmetry gradients. In differential inhibition in 

 bilateral forms, for example, median regions are more 

 inhibited, while in differential acceleration they are 

 more accelerated than lateral, and in differential accli- 

 mation and recovery median regions are finally less 

 inhibited than lateral. 



These various modifications are, as regards their 

 more general features, nonspecific as regards agents, and 

 all except the differential accelerations which require 

 the action of accelerating agents can be produced in 

 some degree by a large number, probably by all agents 

 which inhibit general protoplasmic activity and which 



