I36 P- B. SIVICKIS. 



it may be increased by the same concentration of cyanide or anes- 

 thetic. On the basis of these and many other facts concerning the 

 physiology of reconstitution a theory of head frequency has been 

 advanced (Child, '14a, '14^ '16) which is confirmed by later work 

 (Buchanan, '22). This theory, which is considered more fully 

 below (p. 56), maintains that head frequency in any particular 

 case is determined primarily by the relation between two opposing 

 factors : the one the rate of metabolism in the cells at the cut sur- 

 face which form the head, the other the rate of metabolism in 

 other parts of the piece. The higher the first in relation to the 

 second, the higher the head frequency; the higher the second in 

 relation to the first, the lower the head frequency. The differential 

 susceptibility of the cells at the cut surface and the other parts of 

 the piece makes it possible through differential inhibition, differ- 

 ential acclimation, and recovery (Child, '20b, '21) to alter the 

 relation between the two factors in both directions. 



The correlative factor retarding or inhibiting head formation is 

 apparently in part or wholly a matter of nervous stimulation of 

 the cells not directly concerned in head formation. It is most 

 effective during the first few hours after section, when increased 

 C0 2 production, oxygen consumption and susceptibility all show 

 that the pieces are stimulated. In P. dorotocephala this stimula- 

 tion is inhibited and head frequency increased by anesthetics such 

 as ether and chloroform used during a few hours following section 

 (Buchanan, '22), but a day or two later such anesthetics in the 

 same concentrations bring about no increase (Buchanan, unpub- 

 lished). 



Concerning the experiments on P. lata, it may be noted, first, 

 that the two factors are concerned in this species, and, second, that 

 apparently nervous stimulation is less effective in decreasing and 

 nervous inhibition in increasing head frequency than in P. doroto- 

 cephala. Head frequency can be altered in both directions in 

 P. lata, but so far as experiments have gone, it appears that general 

 protoplasmic depressants, such as acids, are highly effective, while 

 anesthetics in the stricter sense have little or no effect. These 

 facts are in accord with observations on the living animal, which 

 indicate that the nervous organization is considerably lower than 

 in P. dorotocephala. 



