NUTRITION IN SENESCENCE AND REJUVENESCENCE 173 



ently too far advanced in senescence to recover, and undergoes 

 complete death and disintegration or gradual degeneration and 

 resorption. That it serves as a source of nutrition for the portion 

 which remains active is indicated by the fact that the reduction 

 in size of this portion is much less rapid than in starving normal 

 animals. Nevertheless, it is evident that the supply of food 

 in the involution mass is not adequate to prevent the occurrence of 

 reduction sooner or later, and since the animal during resorption 

 of the posterior region is without pharynx or mouth, it cannot take 

 food in the usual way; consequently as the source of supply in the 

 involution mass gradually fails, the anterior region gradually 

 starves and undergoes reduction. But when a certain stage of 

 reduction is reached, the new posterior end and pharynx develop 

 at the expense of other regions, and the process of rejuvenescence 

 is completed. In these cases, then, senescence leads to death in 

 certain parts of the body while other parts remain alive and undergo 

 rejuvenescence by starvation, reduction, and reconstitution. 



The question of the conditions concerned in the localization of 

 death in the posterior region of the body requires some considera- 

 tion. The facts indicate that fragmentation is usually inhibited 

 by certain internal conditions and that, as the rate of metabolism 

 decreases during senescence, the lower limit for the continued 

 existence of differentiated structure is finally reached and passed 

 in the posterior region, and the processes of involution or disinte- 

 gration begin. The earthworm diet has been repeatedly used with 

 animals of different stocks and the results are always essentially 

 the same. Continued feeding in successive generations of the same 

 <,tock has not thus far brought about any further changes, and the 

 animals which do not die show no indications of progressive senes- 

 ( ence in successive generations. 



Another diet used consists of the bodies of fresh-water mussels. 

 The portions used for food are chiefly the reproductive organs and 

 the digestive gland, and the animals apparently eat chiefly the 

 reproductive cells. 



In the first generation the effect of this diet is to decrease the 

 frequency of fragmentation. In most animals the involution of 

 ihe posterior region occurs, as in Figs. 58-61, but very commonly 



