ASEXUAL CYCLE OF PLAXARIA VELATA. 



199 



polarities arise very readily in the small pieces which result from 

 fragmentation, probably because the pieces are so short t'hat the 



original axial gradient (Child, 'i3c) is practically eliminated 

 and chance differences in the rate of metabolism in different parts 

 of the piece are sufficient to establish new polarities. 



VII. CONCLUSION. 



In Planaria velata the individual very evidently undergoes a 

 process of senescence as it grows and either experimental or 

 natural asexual reproduction brings about rejuvenescence. More- 

 over, the animal apparently returns to essentially the same 

 physiological stage with each generation, for the species is able 

 to persist without sexual reproduction and, as a following paper 

 w r ill show, numerous asexual generations have been bred in the 

 laboratory without any indication of senescence of the stock. 



I have shown elsewhere (Child, 'lib) that the regulation of 



