INTRODUCTION 23 



acter. The tissues of the intestines and various other organs then build up 

 from these split products new complex proteins which possess the specific 

 species character of the new host. Hamburger (1903) assumed that special 

 chemical groups characterize the different proteins of a certain species and 

 that these groups are common to the various tissues of the same organism. 

 Similar were the conceptions of Obermayer and Pick, who at this time had 

 begun their investigations into the chemical factors which determine the 

 species-specificity of proteins. Hamburger extended these ideas also to in- 

 dividuals and he held that the proteins of each individual have a chemical 

 characteristic in common, which differs from that of every other individual 

 belonging to the same species. Subsequently it was found that in addition to 

 the proteins, also carbohydrates and lipoids, as well as other simpler sub- 

 stances, if they are combined with foreign proteins (Landsteiner) can serve 

 as antigens which give rise to specific immune bodies, and that these non- 

 protein substances as such may interact in a specific way with these antibodies. 

 It had thus become possible to differentiate by means of immune reactions 

 between substances characteristic of different species and thus to obtain tests 

 for species differentials ; also, in a few cases, to differentiate by these means 

 even between different individuals belonging to the same species (Ehrlich and 

 Morgenroth, Todd). 



Furthermore, a distinction can be made in this way, not only between species 

 differentials but also between the characteristic constituents of different 

 organs and tissues within the same organism (organ differentials) ; and it was 

 found that, as a rule, it is necessary to combine the organ differentials of one 

 species with a strange species differential in order to produce organ-specific, 

 immune substances ; these combined antigens are an expression of the intimate 

 relations which exist also embryologically and genetically between these two 

 types of differentials or their precursors. In certain protein substances both 

 species-specific and organ-, tissue-, or substance-specific groups may be 

 present. 



It has been mentioned above that observations made in the grafting of 

 tissues led to the conclusion that constitutional differences may exist between 

 individuals of the same species ; we have referred already to the work of 

 Oilier, who in the second half of the last century found differences between 

 the readiness with which periosteum of one animal can be transplanted to the 

 same individual and to other individuals. Similar differences were noted also 

 in the transplantation of other organs, above all, in the skin-grafting which 

 Thiersch perfected ; according to Schoene, Thiersch had suggested biochemical 

 characteristics as the cause of these differences. Somewhat later, Knauer 

 found that also in the case of ovaries, autotransplantation was more successful 

 than homoiotransplantation. Between 1901 and 1907 we carried out, in rats 

 and dogs, several series of transplantations of mammary gland adenoma, 

 which were intermediate in character between normal and cancerous tissues, 

 and found that while autotransplanted pieces continued to live and in some 

 cases proliferated, homoiotransplanted pieces died. 



We recognized three factors as responsible for this result: (1) the exist- 



