6 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



to the greatest advantage. While the serological tests are especially useful 

 in the analysis of the differentials of groups of animals, such as species, 

 genera, orders and classes, transplantation experiments are best suited for 

 the analysis of the differences between individuals as expressed in their indi- 

 viduality differentials. The study of transplantation among more primitive 

 organisms may contribute to our knowledge of the phylogenetic development 

 of the organismal differentials, and experiments in hybridization as well as 

 in transplantation of embryonal tissues may aid in the analysis of the onto- 

 genetic development of the organismal differentials. 



We are concerned principally with the study of that type of organismal 

 differential which we have designated as the individuality differential, and here 

 the basic experiment is the following: Various organs or tissues are trans- 

 planted from one animal, e.g., a guinea pig, into two other guinea pigs not 

 directly related to the first guinea pig from which the tissues were taken ; this 

 is called homoiotransplantation. It is seen that the reactions of the hosts of 

 the multiple grafts toward the latter differ in accordance with the degree of 

 genetic relationship between host and donor, but the host behaves in approxi- 

 mately the same way toward the various tissues from the same donor. In one 

 animal the reactions are severe to all the tissues, in the other one they may be 

 very light. These reactions consist in the activity of the lymphocytes, the con- 

 nective tissue cells and blood vessels of the host towards the grafts ; in addi- 

 tion, tissues, especially the more sensitive ones, are also influenced by the 

 degree of their compatibility with certain constituents of the blood of the 

 host, and the degree of this sensitiveness again depends upon the genetic 

 relationship between host and transplant. In general, tissues are injured by 

 the bodyfluids of a strange host, and in some species this injurious action 

 plays a greater role than in others. However, in all the species which we have 

 studied so far it is the lymphocytes which sense or recognize the finest degrees 

 of similarity or difference in the constitution of the individuality differentials 

 between host and transplant. The distinctive reaction of the connective tissue 

 cells becomes noticeable if there is a slightly greater difference between these 

 differentials. The statement that all the tissues from the same donor elicit 

 the same intensity of reaction on the part of the same host is true in a relative, 

 but not in an absolute, sense. Different tissues have an unequal power to call 

 forth these reactions ; thus, for instance, thyroid gland usually induces a 

 stronger reaction than cartilage and perichondrium. This is evidently due to 

 the fact that a certain substance responsible for the reaction, the individuality 

 differential, is given off in sufficient quantities more readily by thyroid than 

 by cartilage, which latter has a more inert metabolism. However, notwith- 

 standing these differences between different tissues and organs, in all of them 

 the genetic relationship between host and transplant determines the intensity 

 of the reaction of the host against the individuality differentials of the trans- 

 plant. 



There is a second type of experiment which brings out the meaning of the 

 individuality differential. This introduces variations in the relationship be- 

 tween host and transplant which are expressed by the terms : auto-, syn- 



