1 64 SENESCENCE AND REJUVENESCENCE 



in the percentage of loss in a given time interval does occur in unin- 

 jured animals. In regenerating animals, on the other hand, the 

 loss is more rapid in earlier stages because of the use of body sub- 

 stance in the formation of new parts as well as for function. As 

 regeneration proceeds, the growth of the new parts becomes less 

 rapid and requires less material, and the loss of weight becomes 

 slightly less rapid. If these suggestions are correct, starvation in 

 Cassiopea follows essentially the same course as in Planaria and 

 is accompanied by increase in metabolic rate and some degree of 

 rejuvenescence. For the study of this aspect of starvation, how- 

 ever, the medusa is a particularly unfavorable form because the 

 volume of cellular substance is exceedingly small, as compared 

 with the volume of gelatinous material which, according to Mayer, 

 constitutes the chief source of nutrition during starvation, and since 

 this is extra-cellular, its disappearance does not alter the cellular 

 condition. For the same reason changes in chemical constitution 

 and water-content of the protoplasm, so far as they occur, are 

 inappreciable, though in an animal with so little differentiation as 

 the medusa the changes are probably not very great. There is 

 also the possibility that, as Putter believes, substances in solution 

 in the water serve as a source of nutrition to some extent. If this 

 is the case, the influence of such substances on the rate of loss of 

 weight must be greater in the later stages of starvation when the 

 animal is smaller and the absolute loss less than in the earlier 

 stages, and will therefore contribute to mask the increasing rate of 

 loss in these stages. Taking all these facts into account, it appears 

 highly probable that the changes in the cellular substance of the 

 medusae are very similar to, though probably less extensive than, 

 those in Planaria. Mayer notes that the cells decrease in size, 

 their boundaries become indistinct, and some cells die. Determina- 

 tions of the changes in susceptibility of the cellular portions of the 

 body of the medusa during starvation would be of interest. 



THE CAPACITY OF STARVED ANIMALS FOR ACCLIMATION 



In general the ability of planarians to become acclimated to 

 depressing agents or conditions varies with the rate of metabolism. 

 Young animals, for example, become much more readily and more 



