CONTROL OF HEAD FORMATION IN PLANARTA 39 



Second, decreases in head frequency in A pieces are general, 

 and the longer the time of exposure or the higher the concentra- 

 tion, the greater the decrease. In A pieces the stimulation of 

 .the Y region by section is slight and it evidently has little effect 

 in inhibiting head formation, since head frequency is nearly 

 100 per cent in the A pieces of large animals. Disregarding 

 this slight stimulation, largely or completely prevented by the 

 anesthetic, there remains to be considered the condition of the 

 cells of the X region. If we grant that chloretone has an inhibi- 

 tory effect on developmental processes, it follows that the de- 

 creases in head frequency in A pieces result from the direct inhibi- 

 tion of the processes of dedifferentiation and development of 

 the X region by the chloretone solution. 



Third, decreases in head frequency occur in B pieces when 

 the period of exposure is relatively long or the concentration 

 very high, especially in the former case. In other experiments 

 the head frequency of the B pieces is not altered or is increased. 

 These effects, intermediate between the effects on A and C pieces, 

 are to be expected, since B pieces are obviously intermediate 

 anatomically and physiologically between A and C pieces. 



The results obtained with chloroform, ether, and chloral hy- 

 drate support the conclusions drawn from the chloretone data. 

 Chloroform produces more striking increases in head frequency 

 in the C pieces and less striking decreases in the A pieces than 

 chloretone. This is true to an even greater degree in the ex- 

 periments with ether. The reasons suggested for these differ- 

 ences in the effects of anesthetics will appear farther on in this 

 paper. I have been able to control head frequency to a limited 

 extent with ethyl alcohol, but have been unable to obtain definite 

 and uniform data showing that solutions of ethyl alcohol inhibit 

 the stimulation of section. These results, when considered in 

 the light of the very definite relations established between the 

 inhibition of the increased oxidations attendant upon section 

 and the changes in head frequency with the other anesthetics 

 used, suggest that the factors, whatever they may be, that 

 complicate the effects of the alcohol on the oxygen consumption 

 of the pieces immediately after section are not greatly concerned 



