42 SENESCENCE AND REJUVENESCENCE 



This is true, not only as regards the different structures in a cell, 

 but also as regards different cells of an organism, and the cells of 

 different organisms. Many of the more or less evanescent struc- 

 tural appearances in protoplasm are perhaps nothing more than 

 visible indications of differences in the aggregations of the colloid. 

 The more highly aggregated portions, which form more or less 

 dense colloid gels, appear as more or less definite structures, the less 

 aggregated portions as indefinitely granular, alveolar, or fluid. But 

 even in such cases the denser portions of the protoplasm are probably 

 for the time being less subject to chemical change than the more 

 fluid portions because of their physical condition. It is evident, 

 however, that many of the more permanent structural features 

 result from the accumulation in the cell of specific substances 

 which possess a relatively high degree of physiological stability 

 under the existing conditions. But there is little doubt that in at 

 least most organic structures which are not mere inclosures in the 

 protoplasm or extra- cellular secretions a greater or less degree of 

 chemical breakdown, of degradation of the structural substance, is 

 more or less constantly occurring while life continues. In some 

 cases this may be very slight in amount or may involve only certain 

 components, in others it may involve the whole structural basis 

 of the organ or organism. When the conditions are such that the 

 new material added exceeds in amount that undergoing breakdown, 

 growth occurs, but when the rate of breakdown exceeds that of 

 accumulation, reduction is the result. 



According to the theory of the labile proteid molecule, func- 

 tional activity results primarily from the breakdown of the struc- 

 tural substratum itself, or at least of its proteid constituents. 

 But if the substratum consists of comparatively stable by-products 

 of metabolism, as the facts seem to indicate, then it is clear that 

 the energy of functional activity must ordinarily come chiefly from 

 other sources, i.e., from the breakdown of other substances which 

 do not constitute an essential structural part of the protoplasm. 

 Under the usual conditions the structural substratum is probably 

 to a large extent a field in which the reactions occur rather than 

 the reacting substance or substances, but in the absence of other 

 nutritive substances, i.e., in starvation, it may itself become the 



