114 C. M. CHILD 



affords a means of altering the relations of metabolic rate along 

 such gradients. The relation between susceptibility and meta- 

 bolic rate is briefly as follows : in concentrations or degrees which 

 kill without permitting acclimation, susceptibility varies directly 

 with metabolic rate. In low concentrations, where acclimation 

 occurs, the rate and degree of acclimation vary directly with 

 metabolic rate. Where the inhibiting agent acts only temporarily 

 the rate and degree of recovery vary directly with metabolic 

 rate. 



3. Acclimation to, and recovery from the action of the inhib- 

 iting agents used in these experiments consists in the attain- 

 ment of a higher rate of reaction, either in the presence of the 

 agent or after its action. 



4. Potassium cyanide, ethyl alcohol, ammonium hydrate, so- 

 dium hydrate, hydrochloric and acetic acid in all concentrations 

 above a certain minimum retard or inhibit development of the 

 sea urchin, but with sufficiently low concentrations, a greater or 

 less degree of acclimation may occur in the presence of the agent, 

 or a greater or less degree of recovery after temporary action. 



5. A graded difference in susceptibility to these agents exists 

 along the axes of developmental stages of the larval sea urchin 

 body. This difference in susceptibility appears either as differ- 

 ential inhibition, differential acclimation, or differential recovery 

 along the axes, according to the concentration of the agent used 

 and the period of action. 



6. These differential effects along the axes bring about char- 

 acteristic changes in form, proportions and localization of parts 

 of the larval body, which can be experimentally controlled and 

 predicted to a very considerable degree. 



7. The larval forms resulting from differential inhibition show 

 changes in form, proportions and differentiation in a certain di- 

 rection from the normal, the degree of change corresponding to 

 the degree of differential inhibition. The chief changes are de- 

 crease in size of oral lobe, which represents the apical region, 

 decrease to zero in the angle of divergence between arms, ap- 

 proach of lateral parts toward the median line and in more 

 extreme degrees fusion in the median line, the progressive oblit- 



