PHYSIOLOGICAL DOAIINAXCE 



109 



y 



the Z>-pieces it is lower, and in the c-pieccs 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- 

 peratures, 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 

 parts. In an isolated piece of the 

 planarian body (Fig. 58) the head 

 arises from the cells of the region x, 

 which are more directly affected by 

 the wound and undergo rapid 

 dedifferentiation and rejuvenescence 

 and so attain a higher metabohc 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 ;•. 

 This relation may be stated in the formula, head- 

 frequency = ?^ . This means that the higher the mcta 



Fig. 58. — Dia- 

 grammatic outline of 

 a piece of Planaria to 

 illustrate relations of 

 new apical region, .v, 

 new basal region, 

 :;, and old body 

 region, y. 



rate y 



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



