124 C. M. CHILD 



to attain it A\dth the degree of uniformity necessary for experi- 

 mental test. It ^\ill probably be possible to make the test in 

 certain other forms, but I have not yet had opportunity to do 

 so. But if intrinsic differences exist in the cells of different 

 levels of the body they can be only quantitative in their effect 

 in this species, for heads of all types may arise from any level 

 of the body under proper conditions, i.e., the cells of all levels 

 are intrinsically capable of giving rise to any type of head and 

 the particular type of head in any case is determined by the re- 

 lations between x and y, which, as I have shown, can be altered 

 and controlled experimentally. In following papers other meth- 

 ods of control of head-frequency will be considered. 



In conclusion, attention may be called to one other point, 

 viz., that the various types of head resulting from the action 

 of KCN are the same as those determined by the differences in 

 size of piece and region of body. There is no indication of 

 any specific action of KCN or any other agent or condition on 

 head-formation and this fact will become increasingly evident 

 in following papers. 



V. SUMMARY 



1. By means of low concentrations of potassium cyanide it 

 is possible to alter and control experimentally the character of 

 the head formed and the frequency of head-formation in pieces 

 of Planaria dorotocephala. In general, cyanide shifts the head- 

 frequency downward in pieces representing the anterior third 

 of the first zooid while it has little effect on the middle third, 

 and if the concentration is not too high, or the period of action 

 too long, it shifts the head-frequency upward in the posterior 

 third. 



2. Short periods in cyanide are more effective than long in 

 shifting the head-frequency upward in posterior pieces and less 

 effective than long in shifting it downward in anterior pieces. 



3. The facts indicate that two factors are chiefly concerned 

 in determining whether a head shall develop on a piece or not. 

 These factors are: first, the reaction of the cells adjoining the 

 cut surface, a process of dedifferentiation division and growth 



