120 C. M. CHILD 



greater or less degree, according to concentration and metabolic 

 rate in the cells, both the processes in the head-forming cells 

 of X and the stimulation in y. In those pieces where the stimu- 

 lation of y is slight, as in the a-pieces, the inhibiting action of 

 the KCN on x appears directly as a shift downward in head- 

 frequency. In the regions where the stimulation of y is great, 

 as in the c-pieces the effect of the KCN in inhibiting this stimula- 

 tion overbalances its effect in inhibiting x, consequently the head- 

 frequency is shifted upward. 



The differences in susceptibility of the regions x and y to 

 cyanide are factors in determining these opposite effects of 

 cyanide. The relation between susceptibility to KCN and 

 various other reagents and conditions has been considered at 

 length elsewhere (Child, '13 a, '15 a, Chap. III). In general 

 the primary susceptibiHty varies directly with metaboUc rate 

 but in very low concentrations such as were used in the experi- 

 ments recorded above the higher the metaboUc rate the more 

 rapid and complete the acclimation. Moreover, the higher 

 the metabolic rate, the more rapid and complete the recovery 

 after a short period in KCN. Both acclimation in KCN or 

 other depressing agents and recovery afterward consist in a 

 gradual increase in metabolic rate. In the case of KCN, how- 

 ever, neither acchmation nor recovery is complete even with 

 concentrations as low as those used in the above experinients. 



In the piece of the planarian body the region x which under- 

 goes dedifferentiation sooner or later acquires a higher metabolic 

 rate than the region y, consequently its primary susceptibility 

 to KCN is greater than that of y, but its ability to become ac- 

 climated in KCN or to recover after it is also greater than that 

 of y. KCN decreases metabolism in both x and y but x acclimates 

 or rec'overs more readily and more completely than y. In 

 the a-pieces then, where y has httle inhibiting influence on x 

 the shift downward in head-frequency represents the direct 

 inhibiting effect of the KCN which has not been compensated 

 by acclimation or recovery. In the c-pieces, on the other hand, 

 the shift upward in head-frequency results from the fact that 

 the inhibition of stimulation of y by KCN over-balances the 



