8 J. AVILLIAM BUCHANAN 



Rate X 

 of the expression p ■;r^. This is merely a brief statement of 



the fact that the cells at X give rise to a head if their metabolic 



rate is high enough in relation to that of Y and that the region 



Y may retard or prevent head formation in X if its rate of 



metabolism is high enough in relation to that of X. 



It was noted above that head frequency can be either increased 



or decreased experimentally. This has been accomplished by 



Rate X 

 altering the value ^ :^ by a differential change of Rate X and 



Rate Y and thus altering the normal head frequency. Using 

 KNC, which is known to reduce the rate of oxidative reactions 

 in many organisms (Child, '19 a; Hyman, '19 a), Child was able 

 by the use of appropriate concentrations to control head fre- 

 quency to some extent in pieces of a certain length (Child, '16). 

 In pieces of the same length from the extreme anterior regions 

 of a given number of worms of the same size, e.g., A in figure 3, 

 subjected immediately after section during the period of stimu- 

 lation of Rate Y to certain concentrations of KNC, the head 

 frequency was decreased. The result is attributed to the direct 

 effect of the KNC on the cells of the X region; the Y region is 

 known to be not greatly stimulated by section in such anterior 

 pieces, and this stimulation has been foimd insufficient to in- 

 hibit head formation to any marked extent (Child, '13 a; Child 

 and Robbins, '20; Hyman, '19 a). Consequently, head fre- 

 quency in these pieces depends chiefly on Rate X. Therefore, 



Rate X 

 in such experiments the value ^ , y is decreased by KNC in 



A pieces by reason of its direct inhibition of Rate X. In pieces 

 of the same length from more posterior regions, e.g., C in figure 

 3, subjected to the same concentrations for the same length of 

 time after section, the head frequency was increased. This 

 result is attributed to the reduction or prevention of stimulation 

 of section in the Y region by the depressing action of the KNC. 

 The X region thus obtains the degree of independence necessary 

 for the formation of a more or less complete head. As stated 

 previously, the stimulation of the Y region is greatest at lower 



