PHYSIOLOGICAL DOMINANCE 113 



compared with y overbalances the absolute decrease in 

 rate produced by the narcotic. In pieces like a, where y 

 is only slightly stimulated by section and rate x is so 

 much higher than rate y that the head-frequency is 

 very high, the effect of narcotics is to decrease head- 

 frequency, because in such cases dominance is not 

 reversed and only the direct inhibiting effect of the 

 narcotic on the region x appears. 



Head-frequency may be increased in all pieces by 

 inducing them to move about.' The apical end, of 

 course, precedes in such movement, the cells of the region 

 X are subjected to more excitation than in a piece which 

 is not moving, and the higher metabolic rate of x results 

 in increased head-frequency. 



In Planaria maculata and certain other species the 

 degree of subordination of basal regions of the body is not 

 as great as in P. dorotocephala; consequently the increase 

 in metabolic rate after section in pieces from this region is 

 less than in P. dorotocephala, and in these species the 

 head-frequency of such pieces is almost or quite as great 

 as that in more apical pieces. Various other differences 

 in the reconstitutional process in different species of 

 planarians only serve to confirm the conclusions reached 

 in the case of P. dorotocephala. 



These and many other facts have forced me to the 

 conclusion that the head which appears in the recon- 

 stitution of a piece is not physiologically a part of the 

 piece and is not formed by the piece, but develops, so to 

 speak, in spite of it. Only when the metabolic rate 

 of the cells at x is high enough to make them essentially 



^ Child, "Experimental Control of Morphogenesis in the Regulation 

 of Planaria,^' Biol. Bull., XX, 191 1. 



