CONCLUSIONS FROM EXPERIMENTS 179 



by starvation increases the rate of metabolism and so the capacity 

 for growth and development. From an advanced physiological 

 age it is possible to bring the animals back practically to the begin- 

 ning of post-embryonic life by forcing them to use up and eliminate 

 the substance which they have accumulated during post-embryonic 

 growth and development. Here no reproductive process, asexual 

 or sexual, is involved, but, to return to the analogy between the 

 organism and the flowing stream, the metabolic current is forced 

 to erode its channel instead of depositing material along its course. 



These experiments leave no basis for the contention that the 

 organism or the cell cannot become young after it has once 

 undergone senescence, and that the only source of youth is an 

 undifferentiated germ plasm. The planarian reduced by starva- 

 tion consists entirely or almost entirely of cells which formed 

 functional differentiated parts of the original, physiologically and 

 morphologically old animal, but after renewed feeding it is younger 

 in every respect and in all parts of the body, so far as can be deter- 

 mined, than before starvation, and is again capable of growth and 

 senescence. In short, these experiments demonstrate that the 

 differentiated somatic cells can return to a physiological condition 

 which at least approaches that of embryonic or undifferentiated 

 cells, and there is no reason for believing that a hypothetical 

 parcel of germ plasm in the nucleus of these cells is in any way 

 responsible for this regression. The results of these physiological 

 experiments are in complete agreement with the conclusions reached 

 by E. Schultz ('04, '08), on the basis of morphological data. 



The few experiments on the influence of the kind of nutrition 

 upon the course of the life cycle indicate clearly that the course and 

 results of senescence may differ widely with the character of the 

 food. The experiments do not throw any light on the question of 

 the factors concerned in the differences produced, but with more 

 complete control of the kind of nutrition more definite results on 

 this point will doubtless be possible. Even these experiments 

 show, however, that the age cycle in these lower animals is by no 

 means independent of nutritional factors. Perhaps the most 

 important point is that with certain foods a progressive senescence 

 from generation to generation occurs, while with other foods 



