20 INTRODUCTION 



At first the proteids split up into compounds still possessing 

 many of the features of the typical proteid molecule, such as 

 albumoses and peptones, and these bodies are then further resolved 

 into simple substances, which are not aggregates of several 

 smaller molecules as are the proteids, and which can be obtained 

 in pure crystalline form. No matter which method is used we 

 find the process going through these stages, and, as before 

 mentioned, the primary crystalline products obtained are prac- 

 tically the same quantitatively as well as qualitatively. Some 

 methods, e. g., bacterial decomposition, however, lead in the 

 end to more profound or different decomposition of the cleavage 

 products into secondary substances. The similarity of the 

 results obtained in these different ways indicates that there are 

 definite lines of cleavage in the proteid molecule along which 

 separation takes place, independent of the nature of the agency 

 at work, and that the substances obtained represent, as the 

 Germans figuratively say, the " building stones " of the entire 

 molecule. A large number of such elementary constituents 

 have already been isolated and identified with certainty, 

 although there is no doubt that there remain others still undis- 

 covered. The best known of these are the following : 



1. Glycocoll, ClC-COOH. 



2. Alanin, CH 3 - CH^- COOH. 



3. Amino-valerianic acid, f ^u\ ^TT Xtr rv^rr 



^Oi 3 ) 2 = Cl v>xl LAXJU. 



4. Leucin, ( CH 3 ) 2 = CH CH 2 CH COOH. 



These four bodies are all simple amino-acids of the fatty acid 

 series, and represent typical members of the series as far as the 

 hexane derivatives. 



5. Aspartic acid, HOOC CH 2 CH COOH. 



6. Glutaminic acid, HOOC CH 2 CH 2 CH COOH. 



These two dibasic acids are also closely related to the mon- 

 atomic acids, as can be seen from their structural formula?. 



7. Phenyl-alanin, / J>-CH 2 -CH COOH. 



