CONTROL OF HEAD-FORM IN PLANARIA 125 



and so the formation of a mass of embryonic tissue from which 

 the head develops; second, the stimulation of the piece as a whole 

 follomng section, which lasts only a few hours but which, if 

 sufficiently great, retards or inhibits the reaction of the cells 

 adjoining the cut surface. 



4. The effects of cyanide in altering head-frequency are due 

 to its action on these two factors. In cases where the stimula- 

 tion following section is slight, the direct effect of cyanide on 

 the head-forming cells appears in a shift downward in head- 

 frequency, while in cases where the stimulation is great the in- 

 hibition of this stimulation by cyanide may shift head-frequency 

 upward if the concentration is not too high or the period of 

 action too long. 



5. All the facts at hand indicate that head-frequency varies 

 directly with the metabohc rate of the cells concerned in head- 

 formation and inversely as the metabohc rate of other parts of 

 the piece. It is possible to express these relations by the for- 



mula head-frequencv = where x represents the cells from 



rate y 



which the head develops and y the other parts of the piece. 



FEBRUARY, 1916. 



