116 C. M. CHILD 



no doubt that the action of KCN in the long periods is inhibitory 

 for the process of reconstitution is greatly retarded and the 

 motor activity of the pieces is greatly decreased. The effect of 

 the short periods as compared with the long cannot then be the 

 result of a primary stimulation, but must be in general inhibitory 

 like that of the long. 



The second possibiUty is that the short periods act chiefly 

 on the factor inhibiting head-formation which must then be a 

 factor acting rapidly following section of the piece, while the 

 long periods act on the more gradual processes in the cells which 

 give rise to the head, i.e., on the process of head-formation itself. 

 As will appear below, the facts support this hypothesis of a 

 differential inhibition as the basis of the effect of cyanide on head- 

 frequency. 



These considerations bring us back to the conclusions reached 

 in the seventh and eighth papers of this series (Child, '14 a, 

 '14 b). In the first of these two papers it was shown that pieces 

 are stimulated by section, the degree of stimulation differing 

 according to size of the piece and region of the body from which 

 it was taken. This relation between degree of stimulation, 

 size of piece and region of body is briefly as follows : short pieces 

 from any level are more stimulated than long, posterior pieces 

 within the first zooid are more stimulated than anterior, and. the 

 degree of stimulation of posterior pieces as compared with that 

 of anterior pieces increases as their length decreases. In pieces 

 of one-sixth or less of the total body-length the posterior pieces 

 in the anterior zooid are so much more stimulated than the 

 anterior pieces that their rate of metabolism after section is 

 usually actually higher than that of anterior pieces and remains 

 so for several hours, although before section the posterior region 

 of the first zooid has a much lower rate than the anterior region. 



In the eighth it was shown that the determination whether a 

 head shall form or not on a piece occurs \vithin a few hours 

 after section and during the period of stimulation of the piece 

 and it was further pointed out that head-frequency shifts down- 

 ward or decreases directly as the degree of stimulation of the 

 piece by section increases. These facts suggest that the stimu- 



