222 



SENESCENCE AND REJUVENESCENCE 



shown elsewhere (Child, nb) that in Planaria the spatial relations 

 of parts can be altered experimentally by altering the rate of 

 metabolism in the dominant head-region. For example, a piece of 

 Planaria including any considerable portion of the postpharyngeal 

 region such as be, Fig. 86 (p. 214), when allowed to undergo recon- 

 stitution in water at room temperature, forms an animal which in 



i/ 



FIGS. 96-100. Reconstitution of similar pieces of Planaria dorotocephala under 

 different conditions, to show different positions of pharynx and lengths of prepharyn- 

 geal region: Fig. 96, reconstitution in well-aerated water at 20 C.; Figs. 97-99, 

 different degrees of reconstitution in weak solutions of narcotics; Fig. 100, reconsti- 

 tution in well-aerated water at 28 C. 



its earlier stages is like Fig. 96. The new pharynx and mouth 

 appear anterior to the middle of the piece at a certain characteristic 

 distance from the head, and in the region between the pharynx 

 and head the characteristic structure of the prepharyngeal region 

 develops. But if such pieces undergo reconstitution in weak solu- 

 tions of alcohol, ether, chloretone, or other anaesthetics, or under 



