ORGAN (TISSUE) DIFFERENTIALS 549 



these species. They may call forth the production of a number of different 

 immune substances, which differ from one another by the possession of 

 different species differentials, and the antibodies corresponding to the species 

 differential of the antigen used for immunization predominate over the asso- 

 ciated antibodies in their combining power with the antigen. 



The experiments with egg yolk suggest that certain substances which serve 

 as reserve material, or are of a paraplastic nature, may undergo in the course 

 of evolution chemical changes which more or less correspond to the systematic 

 relationship of the species in which these substances are found, but that these 

 chemical transformations may be independent of and may follow a some- 

 what different course from those which concern the primary, typical organis- 

 mal differentials. 



