20 SENESCENCE AND REJUVENESCENCE 



chemically. In short, the organism is a physico-chemical system 

 of a certain kind. 



One point should perhaps be emphasized. The importance of 

 the proteids for life is no less according to this theory than on the 

 assumption of the labile proteid molecule. But the proteids are 

 physical as well as purely chemical factors in the result. We know 

 also that metabolism is not simply a process of building up and 

 breaking down of proteids, and that the proteids of the protoplasm 

 are only one of the products of the reaction-complex and may or 

 may not play an important chemical role after their formation. 

 Since the investigations of recent years point more and more clearly 

 to some such physico-chemical conception of the organism as 

 this as the only satisfactory working hypothesis, it is necessary 

 to consider certain features of the organism in the light of 

 this conception. 



THE COLLOID SUBSTRATUM OF THE ORGANISM 



The classical investigations of Kossel and Emil Fischer have 

 established a firm foundation for the belief that the complexity of 

 the proteid molecule is not as great as was formerly believed. The 

 proteids are apparently built up from certain relatively simple 

 chemical compounds, the amino-acids and their derivatives, to- 

 gether with certain other substances, and the proteid molecule, 

 though very large, apparently consists essentially of a nimiber of 

 these components linked together. Of course such a constitution 

 affords the possibility of a very great variety of chemical reactions, 

 but it does not afford a basis for the assumption of extreme labUity 

 in the proteids of the living organism. On the contrary the results 

 of chemical as well as of morphological investigation indicate that 

 at least many of the proteids are relatively stable in the living 

 organism as well as in the test-tube. 



The proteids exist in the colloid condition. Graham ('6i) 

 distinguished two groups of substances, the colloids and crystal- 

 loids, and although we now know that no sharp distinction exists 

 between the two groups and that any substance may, at least 

 theoretically, exist in the colloid condition, certain substances 

 usually appear as colloids and others as crystalloids. In general 



