138 INDIVIDUALITY IN ORGANISMS 



In Planaria also the positions and space relations 

 of parts along an axis and the range of dominance 

 can be altered and controlled by means of conditions 

 which alter metabolic rate/ At ordinary room tempera- 

 tures in well-aerated water the isolated postpharyngeal 

 region of Planaria (Fig. 65) forms a new individual Uke 

 that in Fig. 66. The new mouth and pharynx form 

 near the middle of the piece at a certain distance from 

 the new head, and the region in front of the pharynx 

 undergoes the internal changes which make it over into 

 the prepharyngeal region of the new individual. If, how- 

 ever, the rate of metabolism in such a piece is decreased 

 by means of dilute narcotics, by the presence of carbon 

 dioxide and metabolic products in the water, or by other 

 means, the head develops slowly, is small and usually 

 abnormal, and the lower the metabolic rate during 

 development the nearer to the head the mouth and 

 pharynx arise and the less the length of the new pharyn- 

 geal region. Fig. 67 shows the effect of a slight decrease, 

 Fig. 68 of a greater, and Fig. 69 of a still greater decrease 

 in metabolic rate during reconstitution. The length 

 of the region undergoing reconstitutional change is less 

 in Fig. 67 than in Fig. 66, still less in Fig. 68, and in 

 Fig. 69 practically no changes occur below the level of 

 the very rudimentary head. 



Reconstitution of similar pieces with a very high 

 metabolic rate (at high temperature) results in forms 

 like Fig. 70, in which the pharynx and mouth arise at a 



' Child, "Physiological Isolation of Parts and Fission in Planaria," 

 Archiv fur Entwickelungsmechanik, XXX (Festband fiir Roux), II. Teil, 

 1910; "Studies on the Dynamics of Morphogenesis, etc. Ill," Jour, 

 of ExPer. ZooL, XI, iqii. 



