124 Recognition of the Individual by Haemohjtic Methods 



of the animal used for the injection, but also in those of closely allied 

 species. As the degree of dilution is increased, however, the pre- 

 cipitates given with allied species gradually diminish until a point is 

 reached where the serum only gives a precipitate with the hlo(jd of 

 the species for which it has been prepared. Taking advantage of this 

 fact, the method has been used to investigate the relationship of the 

 various animal species to one another and most interesting results in 

 this direction have been obtained, notably by Xuttall and Uhlenhuth. 

 These observations have shown the relationship of man to the ape, and 

 the fact that the apes of the old world are more closely akin to man 

 than those of the new. 



These methods are extraordinarily delicate and Uhlenhuth has 

 shown that by their help it is quite possible to distinguish between 

 such closely allied bodies as the albumen of the fowl's egg and the 

 albumen of the blood of the same bird — a difference which is, of course, 

 far beyond the reach of our present chemical tests. 



It is impossible here to go into the many interesting results which 

 have been obtained by means of the pi-ecipitin reaction, but reference 

 may be made to the work of Uhlenhuth and Roemer on the protein 

 substances of the crystalline lens of the eye. These observers found 

 that the crystalline lens of all animals, right down the zoological scale 

 as far as the fishes, possesses a biologically similar protein which more- 

 over appears to have no connection with the blood proteins of the same 

 animal — a serum prepared for instance with the lens material of the 

 ox gave a precipitate equally with solutions of the lenses of the pig, 

 man, fowl, frog etc., but gave no precipitate with the blood of these 

 animals, so that the lens must therefore be regarded as regulai-ly 

 consisting of a protein foreign to the organism. 



Another well known inimuiiity reaction which is now in everyday 

 use for the practical diagnosis of typhoid and Malta fevers etc. is the 

 so-called "agglutination test" which depends upon the fact that when 

 certain bacteria (e.g. the typhoid bacillus) are injected into an animal 

 the serum of the animal so ti'eated becomes agglutinating for the 

 species of bacillus injected, that is to say when added to a suspension 

 of the bacilli, it causes these to clump together and fall to the bottom 

 of the vessel in which they are contained. The exact nature of this 

 phenomenon is not known, though various explanations have been 

 suggested; it would appear however that the agglutination is most 

 probably due to some change in the surface tension existing between 

 the bacilli and the Huid in which they are suspended. 



