; AGAMIC REPRODUCTION AND REJUVENESCENCE 127 



younger than the latter. Since the results of these experiments 

 are in all respects essentially identical with those obtained with 

 pieces artificially isolated by section, it is unnecessary to present 

 them in detailed form. 



In the process of fission the separated posterior zooid undergoes 

 much more extensive reorganization than the anterior zooid. In 

 an animal of medium size fission usually occurs at about the level 

 indicated by the line jf in Fig. 32. The posterior piece b (Fig. 32) 

 is much smaller than the anterior a, and it develops a new head and 

 a new pharynx, and extensive changes in the alimentary tract 

 occur in the formation of the prepharyngeal region. Moreover, 

 it cannot take food until the new mouth and pharynx have reached 

 a certain stage of development, consequently the energy for develop- 

 ment is derived from its own tissues and it undergoes more or 

 less reduction during the process. In Fig. 33 the animal developed 

 from the posterior fission-piece is drawn to the same scale as Fig. 32. 

 This animal is physiologically much younger than the parent from 

 which it came. Its susceptibility is much higher and it is capable 

 of more rapid growth than the original animal. 



In the anterior fission-piece (a, Fig. 32), on the other hand, the 

 original head and the mouth and pharynx persist, the only out- 

 growth of new tissue formed is at the posterior end, and the only 

 other change in form is the growth of the postpharyngeal at the 

 expense of the prepharyngeal region, in consequence of which the 

 pharynx seems to migrate forward (Fig. 34) . When food is present, 

 this piece may feed and increase in size during the whole process 

 of reconstitution, but even when it is not fed, the degree of reduction 

 during reconstitution is slight, because the developing regions have 

 a relatively large mass to draw upon as a source of energy. The 

 relation which was shown in the preceding chapter to exist between 

 the size of the piece, the amount of reconstitutional change, and the 

 amount of increase in susceptibility would lead us to expect that 

 the increase in susceptibility resulting from the reconstitutional 

 changes in the anterior fission-piece would be much less than in the 

 posterior piece, and this is in fact the case. 



The increase in susceptibility in the posterior piece is the same 

 as that in artificially isolated pieces of the same size. In Planaria 



