FIELDS AND GRADIENTS IN NORMAL ONTOGENY 335 



Fig. 155 



Differential susceptibility in the early development of sea-urchins {Arbacia). 

 A, B, Normal pluteus. C, D, Differential inhibition by dilute KCN; C, applied 

 throughout development; D, applied for a short period. Inhibition of apical 

 region (oral lobe) with (in D) correlative increase in basal regions and consequent 

 parallel-armed condition. E-G, Diffefential acclimatisation in very dilute 

 solutions. The apical regions become relatively very large, with consequent 

 wide-angled condition of the arms. (From Child, Physiological Foundations of 

 Behavior, New York, 1924.) 



