CONTROL OF HEAD FORMATION IN PLANARIA 31 



inhibition of the stimulation of the Y region is therefore more 

 or less completely excluded in these experiments and any changes 

 in head frequency that result must be due chiefly to direct effects 

 on the X cells. 



The possible direct effects of dilute solutions of anesthetics 

 on the dedifferentiative and redifferentiative processes of the 

 X cells include, 1) possible stimulation of these processes; 2) 

 direct inhibition of these processes; 3) different degrees of accli- 

 mation of the X cells of the A, B, and C pieces to the inhibitory 

 effect of the anesthetics (see foot-note, p. 3). 



In event of direct inhibition of the X cells by the anesthetics 

 followed by incomplete acclimation, we should expect decreases 

 in head frequency. The decreases should be less in A pieces 

 than in C pieces because, 1) the X cells of A pieces have a higher 

 rate of metabolism than that of the X cells of C pieces and accli- 

 mate more completely to the inhibitory effect of the anesthetic; 

 2) the stimulation of the Y region in A pieces is slight and has 

 httle effect in inhibiting head formation, for the A pieces of 

 control series show head frequencies nearly 100 per cent normal. 

 The decreases in head frequency in the C pieces should be very 

 much greater than in A pieces because, 1) the X cells in C pieces 

 have a lower rate of metabolism than that of the X cells of A 

 pieces and they acclimate less completely to the inhibitory effect 

 of the anesthetic; 2) the stimulation of the Y region by section 

 is greater in C pieces than in A pieces and its inhibitory effect 

 on head formation is added to the direct inhibition of the X 

 cells by the anesthetic. The decreases in B pieces should be 

 intermediate between those in A and C pieces. 



These expectations are realized in the results obtained in solu- 

 tions of mol. 1/3000 to mol. 1/4000 chloroform, mol. 1/800 

 chloral hydrate, and mol. 1/300 ether. In such solutions there 

 are distinct decreases in head frequency, least in A pieces and 

 greatest in C pieces, and the results in B pieces are intermediate 

 between A and C pieces. 



In extremely dilute solutions of chloretone, mol. 1/35000, 

 and chloral hydrate, mol. 1/8000, and in mol. 1/20 ethyl alco- 

 hol, increases in head frequency occur in the B and C pieces. 



THK JOURNAL OF EXPERIMENTAL ZO <LOGY, VOL. 31, NO. 1 



