CONTROL OF HEAD-FORM IN PLANARIA 121 



direct uncompensated inhibiting effect on x which appears only 

 in the lesser upward shift and the lower frequency of normal 

 heads in the longer periods. In the 6-pieces the two effects of 

 the KCN are almost balanced, so that little change in head- 

 frequenc}^ occurs. From this point of view it is easy to account 

 for the difference in effect between short and long periods in 

 KCN. The short period of a day or two is as effective as a 

 much longer period in inhibiting the stimulation of y but has 

 much less effect upon the gradual changes in x than the long 

 period because only the earUer stages of these changes occur 

 during the short period and these are retarded by the KCN so 

 that most of the change in x occurs after removal to water and 

 recovery and is therefore much less inhibited than when it occurs 

 in KCN. 



In the a-pieces where the stimulation of y is shght and hrs but 

 httle effect in inhibiting head-formation the short period in 

 KCN is much less effective than the long in shifting head- 

 frequency downward, because recovery in x after the short 

 period is much more complete than the acclimation of x in the 

 cyanide. The difference in head-frequency between the control 

 and the long period a-pieces in cyanide represents the metabolic 

 difference between standard natural conditions and acclimation 

 of the X region to the cyanide. 



In the c-pieces, on the other hand, where the stimulation of 

 y is sufficiently great to inhibit head-formation in a large per- 

 centage of cases, the short period is just as effective as the long 

 in inhibiting this stimulation and at the same time inhibits 

 the change in x to a less degree than the long. Consequently 

 in the c-pieces the short period in KCN produces the gre^.test 

 shift upward in head-frequency, while with the long period the 

 effect on the x region plays a larger part. This effect on x 

 is evident in table II c. III c, and table 6, V c, where the head- 

 frequency in general is shifted upward as compared with the 

 control, while the frequency of normal heads remains the same 

 as or lower than in the control and much lower than in the 

 short period lots. 



