I6O C. M. CHILD. 



the susceptibility and activity of different levels. The region of 

 greatest activity is most inhibited and less active regions are less 

 inhibited. In the apico-basal or antero-posterior axis for ex- 

 ample, the degree of inhibition 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. Micro- 

 cephaly, for example, is a characteristic result of differential in- 

 hibition along the polar axis. In differential acceleration the 

 alterations of proportion are in the opposite direction and mega- 

 cephalic 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 

 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 sym- 

 metry gradients. In differential inhibition in bilateral forms, 

 for example, median regions are more inhibited, while in differ- 

 ential acceleration they are more accelerated than lateral, and 

 in differential acclimation and recovery median regions are finally 

 less inhibited than lateral. 



These various modifications are, with respect to their more 

 general features, non-specific as regards agents, and all except 

 the differential accelerations which require the action of ac- 

 celerating agents can be produced in some degree by a large num- 

 ber, probably by all agents which inhibit general protoplasmic 

 activity and which in lower concentrations or intensities permit 

 at least some degree of acclimation or recovery. Moreover, 

 modifications of the same general type with respect to a par- 

 ticular axial gradient can be produced in widely different organ- 

 isms, e.g., flatworms, echinoderms, fishes, frogs. In many re- 

 spects these definite developmental mdifications constitute the 

 strongest evidence for the relation between susceptibility and 

 the rate of fundamental activities of living protoplasm. 1 And 



1 Data on the control and modification of development through differential 

 susceptibility have appeared as follows: Child, 'ua, 'i6d, 'i?d; Bellamy, '19; 

 and further data on hydrozoa, echinoderms and amphibia are still unpublished. 



