DEMONSTRATION OF SPECIES DIFFERENTIALS 509 



largely negative results are due not so much to the lack of these species differ- 

 entials in the respective tissues, as to the difficulty experienced in extracting 

 them in a potent form, and there are indications that, by injecting organ 

 suspensions into rabbits, species-specific antibodies may be obtained. In the 

 preparation of antigens from invertebrates for the production of precipitins, 

 it is customary to make extracts from the whole animal after it has been 

 frozen, dehydrated and ground to a fine powder; however, this material con- 

 tained tissues as well as bodyfluids. 



As we have stated previously, heterotransplantation of various tissues and 

 organs shows the presence of heterodifferentials, which are not limited to one 

 kind of tissue but which are distributed throughout the body similarly to the 

 individuality differentials. But the gradations in the results of transplantations 

 are not as delicate and as definite in the case of species differentials as in the 

 case of individuality differentials. In that part of the spectrum of relation- 

 ships which represents the heterodifferentials, the analysis by means of sero- 

 logical methods is at least equal, and probably superior, to the analysis by 

 means of transplantation. In the individuality differentials, on the other hand, 

 we have to deal evidently with much more delicate and specific substances than 

 in the species differentials, and here serological analysis is the less refined 

 method. And yet, each species has its characteristic species differential. Is 

 this species differential attached to a particular substance, which is the same 

 in all organs of a species, or do different substances assume these functions 

 in different organs? There is no doubt that different substances, present in 

 different organs or tissues, may possess species differentials exhibiting the 

 specific effects. The evidence so far points strongly to the conclusion that 

 there is one chemical feature which characterizes a species and that this may 

 be attached to various substances, which are thus the bearers of the species 

 differential ; and, as a rule, the demonstration of serological differences in the 

 experiments discussed in this chapter depended upon the presence in the 

 proteins of this species differential which served as an antigen. However, it 

 seems that in some instances in which certain plant proteins were used for the 

 sensitization of animal, tissue- or substance-specific material may have called 

 forth sensitization and the subsequent anaphylactic reaction, and it is probable 

 that also in animals a combination of tissue and organ differentials may to a 

 certain extent, and with certain limitations, substitute for the real species 

 differentials. 



