NUTRITION IN SENESCENCE AND REJUVENESCENCE 167 



both susceptibility and capacity for acclimation undergo a gradual 

 decrease as the animal once more becomes physiologically older. 

 After at most a few feedings, then, the reduced animal is indistin- 

 guishable from the young animal in nature, and, as regards sus- 

 ceptibility, carbon-dioxide production, and capacity for acclimation, 

 is capable of undergoing senescence again. That a real rejuvenes- 

 cence has occurred during starvation cannot be doubted. 



PARTIAL STARVATION IN RELATION TO SENESCENCE 



The asexual life history of Planaria velata was described in 

 chap, vi and it was pointed out that in this species the decrease in 

 rate of metabolism characteristic of the period of growth, differen- 

 tiation, and senescence apparently leads automatically to fragmen- 

 tation of the body and so to the reconstitution from the fragments of 

 small, physiologically young animals, which repeat the life history. 



If this process of fragmentation is associated with senescence 

 and if starvation and reduction bring about rejuvenescence, it 

 should be possible, not only to prevent the occurrence of fragmen- 

 tation, but to keep the animals indefinitely at a certain age by 

 giving them a quantity of food just sufficient to prevent reduction 

 but not sufficient to permit growth. This experiment has been 

 performed with a stock of these animals. During almost three 

 years they have been fed at intervals varying from two or three 

 days to two or three weeks, the feeding being regulated according 

 to the condition of the animals. If growth occurred, the intervals 

 between feedings were increased, and if the animals decreased in 

 size they were fed with greater frequency. If some animals showed 

 more growth or reduction than others, they were isolated and the 

 feedings regulated as required until all were again of approximately 

 the same size. During the early stages of the experiment growth 

 was twice allowed to proceed too far, and a few of the larger worms 

 of the stock underwent some fragmentation. 



During the three years of the experiment the animals have been 

 kept at lengths varying from four to seven millimeters. In all 

 this time no fragmentation has occurred except in the two cases 

 mentioned above, when growth was allowed to go too far. The 

 animals are still in good condition and show the activity of young 



