No. 642] SEROLOGICAL REACTIONS 



81 



In the process of digestion different proteins are broken 

 down into tlieir amino-acid units and these are then re- 

 built into the tissue-proteins of the living organism, each 

 tissue selecting such amino-acids as are required to re- 

 construct its own peculiar complex. That is, the architec- 

 ture of the new proteins into which the individual building 

 units are regrouped is determined by the specific con- 

 stitution of the tissue-proteins themselves. In different 

 proteins the different amino-acids may exist in very dif- 

 ferent ratios, and certain of them necessary for the meta- 

 bolic repair of protoplasm may be lacking in some, such 

 as gelatin, but the amino-acids in any particular protein 

 are constant in nature and proportion and each probably 

 has a definite position in the molecule. While kinds and 

 proportions of amino-acid units determine in large meas- 

 ure the characteristics of individual proteins, it may well 

 be that configurational differences in molecules of the 

 same chemical composition are responsible for the specifi- 

 cities of corresponding proteins in related species of 

 animals. One estimate, for instance, assigns to the serum- 

 albumin molecule alone the capability of having as many 

 as ten thousand million stereoisomers. One may perhaps 

 picture mentally, in a much simplified form, the simplest 

 protein molecule as a main chain or ring, of which tlie 

 representative links are amino-acid " nuclei." More- 

 over, to each such link 



{e.g., in simplest form, — NH.OH.CO— ) 

 H 



a side-chain, differing in constitution in different cases, is 

 attached or is attachable by replacement of a hydrogen 

 atom. 



It is, then, with such complex molecular configurations 

 that we have to do as a chemical basis for the phenomena 

 of life; and in thorn, as T have stated elsewhere,^' we have 



