CII. XXV.] NUCLEO-PROTEIDS AND ALBUMINOIDS 403 



as they are sometimes termed, alloxuric or purine bases. The purine 

 bases are closely allied chemically to uric acid, and we shall have 

 to consider them again in relation to that substance. 



The following diagrammatic way of representing the decomposi- 

 tion of nucleo-proteid will assist the student in remembering the 

 relationships of these substances : 



Nvf LEO-PR OTEID 

 subjected to gastric digestion yields 



Proteid converted into peptone, Nuclein, which remains as an insoluble 



which goes into solution. residue. If this is dissolved in alkali 



and hydrochloric acid added, it yields 



Proteid converted into acid A precipitate consisting of nucleic 



albumin in solution. acid. If this is heated in a sealed 



tube with hydrochloric acid, it yields 

 a number of imperfectly known sub- 

 stances like thymic acid and in some 

 cases a reducing sugar. But the 

 best known and constant products 

 of its decomposition are 



Phosphoric acid. Purine bases, viz. : 



Adenine. 

 Hypoxanthine. 

 Guanine. 

 Xanthine. 



The nuclein obtained from the nuclei or heads of the spermatozoa 

 consists of nucleic acid without any proteid admixture. In fishes' 

 spermatozoa, however, the nucleic acid is united to protamine, the 

 chemical properties of which we shall be considering immediately. 



C. ALBUMINOIDS. 



The albuminoids are a group of substances which, though 

 similar to the proteid s in many particulars, differ from them in 

 certain other points. The principal members of the group are the 

 following : 



Collagen, the substance of which the white fibres of connective- 

 tissue are composed. Some observers regard it as the anhydride of 

 gelatin. In bone it is often called ossein. 



Gelatin This substance is produced by boiling collagen with 

 water. It possesses the peculiar property of setting into a jelly when 

 a solution made with hot water cools. It gives most of the proteid 

 colour tests. Most observers state, however, that it contains very little 



