84 C. M. CHILD 



'16 c), and the minor axes are less effective as factors in locali- 

 zation and differentiation, or become effective later than the 

 major axis. This differential obliteration of axial gradients is 

 one of the most significant results of these experiments, and the 

 forms in which only the apico-basal gradient remains as an 

 effective factor in development approach in certain respects the 

 more primitive types of echinoderm larvae. 



THE FORMS RESULTING FROM DIFFERENTIAL RECOVERY 



In recovery after temporary action of an inhibiting agent, a 

 distinction must be made between the general and the differential 

 effect. Where the effect of the inhibiting agent has not been so 

 great that recovery is impossible, all parts of the body undergo 

 an increase in metabolic rate after return to water. This is the 

 general effect. The differential effect appears in the differ- 

 ences in rate and degree of recovery along the axes and is like 

 the differential effect in acclimation; i.e., the higher the metabolic 

 rate, the more rapid and complete the recovery, but since in the 

 developmental stages of the sea urchin recovery, like acclimation, 

 follows and is superimposed on the differential effects of inhibi- 

 tion, the forms resulting from differential recovery, like those 

 resulting from differential acclimation, may show various com- 

 binations of differential inhibition and differential recovery. 

 Since recovery involves not merely the differential changes in 

 metabolic rate, but a great increase in rate in all parts after 

 removal of the inhibiting agent, development may proceed 

 farther, if the temporary inhibiting action be not too great, 

 than where the agent is present throughout. This difference is 

 responsible for certain interesting features of recovery which 

 will be considered below. 



Unfortunately the data concerning recovery are not complete. 

 The close of the breeding season of Arbacia put an end to the 

 experiments when only KCN and alcohol had been used in this 

 way, but, since the results with these agents are similar to the 

 results of differential acclimation, there is every reason to believe 

 that the same would be true for alkalies and acids. 



