NUTRITION IN SENESCENCE AND REJUVENESCENCE 169 



In these experiments with partial feeding the susceptibility 

 does not of course remain the same at all times. Each feeding is 

 followed by a distinct decrease in susceptibility, and later, as the 

 animals begin to starve, the susceptibility increases again. Thus 

 the life of such animals actually consists of alternating periods of 

 senescence and rejuvenescence. But if the intervals between 

 feedings are sufficient, the changes in the two opposite directions 

 balance each other and the mean physiological condition remains 

 the same. 



THE CHARACTER OP NUTRITION IN RELATION TO THE AGE CYCLE 



Up to the present time the problem of the relation between the 

 character of nutrition and the life cycle has received comparatively 

 little attention, although it is evident from the results already 

 obtained that an interesting and important field of investigation 

 is open here. In the attempt to find a suitable food for the breeding 

 of Planaria velata in the laboratory it was soon observed that the 

 size attained before the animals ceased to feed, the character of 

 fragmentation, and even its occurrence and the physiological con- 

 dition of the small animals which develop from the encysted frag- 

 ments, were all dependent to some extent upon the character of 

 nutrition. In these experiments the food did not in all cases con- 

 sist of single tissues or organs, so that it is not possible to correlate 

 the effects produced with the characteristics of particular tissues 

 and still less with particular chemical constitution. There is no 

 doubt, however, that this species constitutes favorable material 

 for nutrition experiments of this kind, such, for example, as Guder- 

 natsch ('12, '14) and Romeis ('13, '14) have carried out on the tad- 

 pole, using various tissues and organs, including thyroid, thymus, 

 adrenals, etc., as nutritive material. 



Only certain important points in the feeding experiments on 

 P. velata need be mentioned here. When the animals are fed beef 

 liver the life cycle approaches more closely to that of animals in 

 nature than with any other food thus far used, but cessation of 

 feeding and fragmentation occur at a smaller size than in nature. 

 The liver-fed animals also differ from animals in nature in not 

 losing their pigment before fragmentation and in encysting rather 



