LARVAL DEVELOPMENT IN THE SEA URCHIN 67 



From the metabolic point of view, -acclimation consists in the 

 gradual increase in rate of reaction in the presence of the inhib- 

 iting agent, and the inverse relation between capacity for accli- 

 mation and metabolic rate means that, the higher the metabolic 

 rate, the greater the capacity for gradual increase of rate after 

 the first inhibiting action. 



Recovery after the temporary action of an inhibiting agent 

 consists, like acclimation, in the attainment of a higher metabolic 

 rate, but the rate attained is much higher than in acclimation 

 because the inhibiting agent is completely removed. Where the 

 action of the inhibiting agent has not gone so far that recovery 

 is impossible, the rapidity and degree of recovery like acclima- 

 tion vary directly with metabolic rate; the higher the metabolic 

 rate the more rapid and complete the recovery. 



If these conclusions are correct and if axial gradients are pres- 

 ent in organisms and are actual effective factors in development, 

 these relations between susceptibility and metabolic rate afford 

 a basis for the control and modification of development in two 

 opposite directions. First, by the use of concentrations of an 

 inhibiting agent sufficiently high to prevent acclimation to the 

 reagent, or concentrations and periods of action sufficient to 

 prevent recovery after temporary subjection, it should be pos- 

 sible to inhibit the regions of higher rate of reaction in an axial 

 gradient to a greater extent than those of lower rate. If the 

 rate of reaction in the apico-basal axial gradient decreases from 

 the apical region basally, the apical region should be most in- 

 hibited, the basal least under such conditions. Second, by the 

 use of low concentrations which permit some degree of acclima- 

 tion, or by temporary action of concentrations whose effect is 

 readily reversible, it should be possible to obtain a greater de- 

 gree of acclimation or more rapid and complete recovery in the 

 region of high rate, i.e., the apical region, and so to inhibit 

 its development to a lesser extent than that of the regions of 

 lower rate. In short, it should be possible to produce, in the one 

 case, a gradient or gradients in inhibition of development, and, 

 in the other, gradients in acclimation or recovery. In fact, by 

 determining proper concentrations and times of exposure, it 



