PHYSIOLOGICAL DOMINANCE 109 



the 6-pieces it is lower, and in the c-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 6-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 g^ammatic outline of 



a piece of Planana to 



planarian body (Fig. 58) the head iii ustr ate relations of 

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

 which are more directly affected by new basal re s ion > 



. 111 j z an " old body 



the wound and undergo rapid region y 

 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 metabolic 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 = jj^- . This means that the higher the meta- 

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



