14 SENESCENCE AND REJUVENESCENCE 



as a scientific problem. And since it was evident that chemical 

 reactions play a very large part in hfe processes it became essen- 

 tially a chemical problem. From this time on our knowledge of 

 the chemistry of organisms increased rapidly, and certain investi- 

 gators have been so sanguine as to believe that we were on the 

 threshold of the synthesis in the laboratory of living matter. Dur- 

 ing the same period the visible structural basis of life was being 

 studied under the microscope. In 1837-39 the cell theory was 

 formulated by Schleiden and Schwann, and in the half-century 

 following the problems of cellular and protoplasmic structure 

 claimed the attention of biologists to a large extent. 



These investigations soon made it evident that life is closely 

 associated in some way with the substances which we call proteids. 

 These are found in all organisms and so far as we know nowhere 

 else. Excepting water, they are the chief constituent of the visible 

 substance characteristic of organisms, i.e., protoplasm. It was also 

 demonstrated that life is associated with a complex of chemical 

 activities. Certain substances are taken up by the organism and 

 others are eliminated. Between ingestion and elimination a com- 

 plex series of chemical reactions was found to occur, and the whole 

 process was called metabolism. 



The conception of the metabolic process and its relation to 

 protoplasm, which was most widely accepted during this period of 

 chemical and morphological investigation in the latter half of the 

 nineteenth century, was that metabolism consisted fundamentally 

 of two parts. Of these one, the anabolic or assimilative process, 

 was in its essential features the recombination and synthesis of the 

 nutritive substances into extremely complex proteid molecules 

 which constituted the "living substance." These proteid mole- 

 cules were regarded as highly labile chemically, or "explosive," so 

 that they were able to respond to stimulation of various kinds by 

 decomposition and the very rapid liberation of energy. The various 

 steps in the decomposition of these living proteid molecules consti- 

 tuted the process of katabolism or dissimilation. Investigation 

 showed that the molecular weight of the proteids was in general 

 very high, and this was believed to indicate very great complexity 

 of the molecules. The highly unstable or labile character of the 



