PHYSIOLOGICAL DOMINANCE 109 



the ^-pieces it is lower, and in the ^-pieces lowest of 

 all. If such a series of pieces is placed for a few hours 

 after cutting in a low concentration of cyanide, alcohol, 

 etc., the head-frequency in the a-pieces is considerably 

 lower than in water, that in the Z)-pieces slightly lower 

 or about the same as in water, while that of the c-pieces 

 is higher than in water. This result is characteristic, 

 but the actual percentages can be altered by differences 

 in concentration of the reagents, tem- 

 perature, and many other factors. 



Although at first glance these re- 

 sults appear hopelessly confusing, 

 they depend upon a very simple rela- 

 tion between that region of the piece 

 which gives rise to the head and other Fig. 58.— Dia- 

 parts. In an isolated piece of the grammatic outline of 



. . \ 1^, piece of Planaria to 



planarian body (Fig. 58) the head niustrate relations of 

 arises from the cells of the region x, new apical region, x, 

 which are more directly affected by ^^^^ ^asal region, 



- Ill . 1 2, and old body 



the wound and undergo rapid j-ggion 

 dedifferentiation and rejuvenescence 

 and so attain a higher metabolic rate than cells farther 

 away from the cut surface and begin soon after 

 section to divide and grow rapidly. If these cells give 

 rise to a head, the region y undergoes more or less 

 transformation to form the body of the new individual. 

 I have found that the head-frequency varies directly 

 with the metabohc rate in x, the head-forming region, 

 and inversely with the metabolic rate in the region y. 

 This relation may be stated in the formula, head- 

 frequency = ^^^ . This means that the higher the meta- 

 bolic rate in x, the more likely the piece is to give rise 



