LARVAL DEVELOPMENT IN THE SEA URCHIN 93 



As regards general character and direction of modification of 

 form, no specific differences appear in the action of the different 

 agents used, but the limits of differential modification along any 

 particular axis differ somewhat with different agents, because the 

 relations between differential inhibition, acclimation and recov- 

 ery are somewhat different for different agents, presumably be- 

 cause they act on the living protoplasmic system to different 

 degrees or in different ways. With KCN, for example, all de- 

 grees of differential inhibition are readily produced by either 

 continuous or temporary action, but KCN is so highly toxic to 

 living protoplasm, and its effects are so persistent that, even in 

 low concentrations, the degree of acclimation during the period 

 of development is much less than with alcohol and acids, and, 

 even in recovery after temporary action, if the concentration of 

 KCN is high enough to produce any marked degree of differen- 

 tial inhibition, this is not usually entirely compensated by dif- 

 ferential recovery. In other words, the differences in meta- 

 bolic rate along the axial gradients are more completely and more 

 permanently levelled down and obliterated by KCN than by 

 alcohol and acids. KCN then is the most satisfactory agent 

 among those used for producing axial differential inhibitions of 

 development, free from, or but little complicated by differential 

 acclimation or by differential recovery. 



Next to KCN in this respect stands NH 4 OH. While it is 

 much less powerful as a protoplasmic poison than KCN, it is 

 much more effective in producing differential inhibition than 

 differential acclimation, because acclimation occurs so slowly 

 that development is either completely arrested by differential 

 inhibition, or, in low concentrations, is completed before any 

 great degree of differential acclimation occurs. 



NaOH acts distinctly as an inhibiting agent on these marine 

 forms, though higher concentrations than of NH 4 OH are neces- 

 sary to produce a given differential effect. Acclimation occurs 

 more readily than to NH 4 OH and consequently greater differ- 

 ential effects of acclimation on the form and proportions are 

 possible with this agent than with either KCN or NH 4 OH. 



