CHAPTER VII 



THE ROLE OF NUTRITION IN SENESCENCE AND REJUVENES- 

 CENCE IN PLANARIA 



REDUCTION BY STARVATION IN Plauaria 



The various species of Planar ia are capable of living for months 

 without food from external sources. During such periods of 

 starvation, however, they undergo reduction in size, many cells 

 degenerate, and some organs may completely disappear. Various 

 investigators, among them F. R. Lillie, 'oo; Schultz, '04; Stoppen- 

 brink, '05, have considered one phase or another of this process of 

 reduction, and Lillie and Schultz particularly have called attention 

 to the fact that in its proportions and chief morphological charac- 

 teristics the animal reduced by starvation resembles the young 

 animal and have pointed out that the changes which occur during 

 reduction indicate that the process of development is reversible. 

 In an earlier chapter (p. 57) I have suggested that it is preferable 

 to use the term " regressibility " rather than reversibility for such 

 changes, since the occurrence of reduction or dedifferentiation in an 

 organism does not necessarily imply a reversal of the reactions con- 

 cerned in progressive development. Only from the morphological 

 viewpoint are we justified in speaking of a reversal of development. 



The reduction in size of Plauaria during starvation is unques- 

 tionably due to the re-entrance of its structural material into 

 metabolism as a source of energy. Schultz finds that reduction 

 in Plauaria is due to the disappearance of whole cells and organs 

 rather than to decrease in size of the cells in general. This is un- 

 doubtedly true to a large extent, but my own unpublished observa- 

 tions indicate that some decrease in size does occur in at least many 

 cells in the starving planarian, and other authors who have investi- 

 gated the cellular changes in animals during starvation have reached 

 similar conclusions.' 



'The following references constitute a partial bibliography of the subject: 

 Kasanzefifj '01, and Wallengren, '02, found marked reduction in the size of Paramecium 

 during hunger. Citron, '02, observed decrease in size of ectoderm cells in a coclcntcratc 



'3D 



