2OO 



C. M. CHILD. 



isolated pieces of Planaria dorotocephala brings about rejuvenes- 

 cence to a greater or less extent, according to the size of the piece,, 

 the smaller piece giving rise to an animal which is physiologically 

 younger than that produced by a larger piece. In that species 



starvation may also be a factor 



in rejuvenescence. Some experi- 

 ments on the effect of starva- 

 tion on Planaria velata will be 

 described in another paper. At 

 present it need only be said that the 

 result is the same in both species. 

 In my earlier paper on senes- 

 cence the conclusion was reached 

 that senescence results from the 

 accumulation of structural prod- 

 ucts of metabolism which con- 

 stitute in one way or another 

 obstacles to the chemical reac- 

 tions. The processes of differentia- 

 tion and growth undoubtedly ope- 

 rate also in another way not 

 considered in the earlier paper, 

 to bring about a decrease in the 

 rate of metabolism per unit of 

 weight or volume. What we are 

 accustomed to call the undiffer- 

 entiated or embryonic cell repre- 

 sents the general metabolic sub- 

 stratum of the organism. Differ- 

 entiation consists in the formation and accumulation of certain 

 substances in the cell, some of which constitute more or less 

 permanent structural features. At least certain of the substances 

 composing these structural features are relatively stable under 

 the usual physiological conditions and while certain chemical 

 changes may occur in them, they are not broken down and elimi- 

 nated to so great an extent as certain other substances. This 

 relative stability must, in fact, be the basis of their persistence 

 as elements of structure. The accumulation of these structural. 



12 



