104 



SENESCENCE AND REJUVENESCENCE 



V 



FIGS. 14-17. Reconstitution of 

 pieces of Planar ia dorotocephala: Fig. 

 14, body-outline indicating levels of 

 section; Figs. 15-17, three stages in 

 the reconstitution of an isolated piece. 



size. Various details of the pro- 

 cess differ according to the size of 

 the piece, the level of the body 

 from which it is taken, the physio- 

 logical condition of the animal, 

 and the environmental conditions, 

 and a limit of size exists which 

 also varies with all these factors; 

 pieces below this limit of size do 

 not reproduce complete normal 

 animals. The influence of these 

 various factors is evident chiefly 

 in the character of the head, 

 which may range from the normal 

 through a series of teratological 

 forms with a headless condition 

 as the extreme term of the series 

 (Child, '116, 'nc; see also Figs. 

 20-23, pp. 111-12). In other 

 species of planarians the process 

 of reconstitution is in general 

 much the same, but with differ- 

 ences in details and in the relation 

 to the various factors mentioned 

 above. 



The process of reconstitution 

 in these cases differs somewhat 

 from the replacement of a missing 

 part in higher animals. The 

 isolated piece of Planaria does not 

 replace the missing parts in their 

 original condition and size, but 

 develops merely a new head and 

 posterior end and then undergoes 

 an extensive reorganization into 

 a new individual of small size, 

 the size being dependent upon the 



