472 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



were correct, it would be difficult to understand why the equilibrum between 

 antigen and agglutinin should be always balanced in such a way that no free 

 agglutinin can be demonstrated in the blood. We know that in the case of 

 toxin-antitoxin combinations such a perfect inactivation of either toxin or anti- 

 toxin is not possible. Moveover, a condition similar to that found in the case 

 of the blood groups applies also to the Forssman antigens and the corre- 

 sponding hemolysins. The blood sera of those species which belong 

 to the heterophilic guinea pig group do not contain the hemolysin re- 

 quired for this reaction, while the sera of the species belonging to the non- 

 heterophilic rabbit group do carry it. Now, in the case of the Forssman anti- 

 gen, it can be shown that only animals belonging to the rabbit group can be 

 immunized against the heterophilic antigen, while such antibodies cannot be 

 produced experimentally in the heterophilic group. There must, therefore, 

 be some mechanism which prevents the immunizing effect of antigen in the 

 latter. These findings seem to be analogous to what we observe in the case of 

 autogenous substances ; they are not able to serve as antigens. Constituents 

 of tissues are adapted in such a manner to the organism to which they belong 

 that they cannot, here, call forth the production of antibodies ; it seems, then, 

 that the possession of the same individuality (or species) differentials on the 

 part of antigen and receptive organs prevents the disequilibrium which is 

 necessary for the production of artificial immunity. It is probable that this 

 mechanism depends on the identity of the organismal differential proteins in 

 an individual or species ; in the latter, the species differential-proteins are the 

 same, and in an individual the individual differential proteins are the same. 

 Associated with and presumably superimposed upon these organismal 

 differentials are the organ and tissue differential substances, which differ 

 everywhere within the same individual. However in addition to the typical or- 

 ganismal differentials also other substances which help to maintain or play 

 a role in the autogenous or species equilibrium may be adapted to the other 

 parts of the organism in such a way that they cannot call forth the production 

 of antibodies in the body to which they belong. 



A species equilibrium can be recognized as mentioned already in a pre- 

 vious chapter in the observations of F. R. Lillie, who found that substances 

 can be extracted from the eggs of various species, which possess an agglutinat- 

 ing power for the spermatozoa of their own species, but not for those of 

 another species. Thus the egg extract of Nereis aggultinates the spermatozoa 

 of Nereis, but not the spermatozoa of Arbacia. Similarly, the extract of eggs 

 of Strongylocentrotus fransiscanus agglutinates the spermatozoa of the 

 same species, but not those of Strongylocentrotus purpuratus, though the 

 latter are agglutinated by the extracts of eggs of Strongylocentrotus pur- 

 puratus. However, such homoiogenous agglutinins cannot be demonstrated 

 in the ova of all species, as Miss Sampson has shown. Heterogenous agglu- 

 tinations may occur, but if this is the case, it is probable that the heterogenous 

 agglutinins causing them are distinct from the typical homoiogenous agglu- 

 tinins. There is another difference between the heterogenous and homoiog- 

 enous agglutinins ; the agglutination produced by the former may be irrevers- 



