CONTROL OF HEAD-FORM IN PLAN ARIA 115 



in determining the presence or absence and the type of head in 

 the different regions are either quantitatively or quaUtatively 

 different. Second, since all the different types of head may arise 

 from any level of the bod}'- and since there is every reason to 

 believe that the action of KCN is primarily quantitative, not 

 qualitative, it seems probable that quantitative rather than 

 qualitative differences in the factors concerned in head-formation 

 are concerned. Third, the fact that under natural conditions 

 the head-frequency is highest in the most anterior regions and 

 progressively lower in more posterior regions together with 

 the fact that KCN shifts head-frequency downward in the 

 anterior regions and upward in the posterior regions, suggests 

 the existence of a factor retarding or inhibiting head-formation 

 which is least effective in the niost anterior regions and becomes 

 more effective as the level becomes more posterior. If we sup- 

 pose that cyanide inhibits or retards the process of head-forma- 

 tion on the one hand and the action of the inhibiting factor on 

 the other it becomes possible to conceive how it might shift 

 head-frequency downward in one case and upward in another. 

 Fourth, the fact that short periods in KCN are less effective 

 than long in shifting head-frequency downward in anterior re- 

 gions and more effective than long in shifting it upward in the 

 posterior regions suggests alternative possibilities. A primary 

 stimulation followed by depression has been observed by some 

 authors with very low concentrations of cyanide and it might 

 be supposed that in Planaria the effect of the short periods is 

 to some extent the effect of this primary stimulation. A mo- 

 ment's consideration shows, however, that this cannot be the 

 case. The primary stimulation, if it occurs in the concentra- 

 tions used, certainly does not extend over a period of twenty-four 

 or forty-eight hours and it is evident from observation that the 

 activities of the pieces are distinctly retarded by these lengths 

 of time in KCN. Moreover, periods of less than twenty-four 

 hours have but little effect on head-frequency and periods of 

 twenty-four to forty-eight hours do not shift head-frequency 

 upward in the a-pieces. Finally, the effect of the short periods 

 differs from those of the long only in degree and there can be 



