LARVAL DEVELOPMENT IN THE SEA URCHIN 95 



terion of the toxicity of the different agents. The more toxic 

 the inhibiting agent, the less rapidly and completely are its dif- 

 ferential inhibiting effects reversed, either in its presence or 

 after its action. 



As regards the relation between the stage of development at 

 which the inhibiting agent begins to act and the concentra- 

 tion or period of action required for a particular effect, it is 

 evident* that, since susceptibility increases very greatly during 

 early development up to the gastrula stage (Child, '15 b, pp. 

 412-418), earlier stages require higher concentrations or longer 

 periods than later stages for the production of a given differen- 

 tial effect. Moreover, since the later the stage at which action 

 begins the less time there is for differential acclimation or re- 

 covery, it is evident that the more extreme degrees of differential 

 acclimation and recovery are most readily produced by action 

 on the earlier stages. Where the action of the agent begins at 

 the beginning of development and continues throughout, the 

 result, of course, depends merely on the concentration and toxic- 

 ity of the agent, and the same is true for action beginning at 

 any particular stage, though, as already noted, the later the 

 stage of development up to the gastrula, the lower the concen- 

 tration necessary to produce a particular effect. 



As regards concentration, it should be noted that in the case 

 of KCN, when the period of action is only a few hours, the 

 physiological effect does not increase proportionally to the con- 

 centration above a certain limit. For periods of four or five hours 

 or less, for example, the physiological effect of KCM m/100 is 

 not very much greater than that of m/1000, but with lower con- 

 centrations and with longer periods of action, the effect is much 

 more nearly proportional to the concentration. No attempt 

 has been made to determine the factors concerned in these rela- 

 tions between concentration and physiological effect of KCN 

 solutions, but there are several possible factors, viz., increase in 

 alkalinity, decrease in dissociation, with increase in concentra- 

 tion, the time necessary for penetration and perhaps others. 

 Whether similar relations exist for very high concentrations of 

 other agents used, has not been determined. These are ques- 



