32 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



advisable to transplant a larger number of tissues simultaneously, such as 

 thyroid, xiphoid cartilage with associated tissues, ovary and striated muscle. 

 A combination of this kind makes possible an accurate appraisal of the degree 

 of compatibility in the large majority of cases. As a general rule, applying to 

 all experiments of this kind, it is necessary to carry out the operations in a 

 sterile manner, whenever this can be done, and to inflict as little injury as 

 possible on the tissues. ■ 



4. In the evaluation of the results of the experiments various accidental 

 complicating factors must be considered. Very important in this connection is 

 infection with bacteria, which may occur notwithstanding the measures which 

 may have been taken to avoid such an accident. In the majority of instances it is 

 easy to recognize the effects of bacteria, with which the tissues were con- 

 taminated during the process of transplantation. The presence of localized 

 masses of polymorphonuclear leucocytes around or in the graft indicates 

 strongly the presence of extraneous microorganisms. However, in some ex- 

 periments it may be difficult to decide whether the leucocytes may not have 

 been attracted by sterile necrotic tissues. This difficulty is encountered espe- 

 cially in the mouse, where an infection may more readily take place, owing to 

 the small size of these animals. Here we may find, in or around the transplants, 

 either more scattered leucocytes or small accumulations of these cells, especial- 

 ly around the fat cells ; these changes seem to be, on the average, more wide- 

 spread and more intense if host and donor of the transplants are not nearly 

 related, whereas, they usually remain localized when the degree of incom- 

 patibility between the individuality differentials of host and transplant is only 

 slight; in cases of heterogenous transplantation, polymorphonuclear leucocytes 

 are quite commonly attracted. There are certain other conditions when doubt 

 may arise as to the significance of certain changes which have taken place in 

 the transplant, as for instance, in case of injury of the transplanted tissues. 

 Thus injury to fat tissue surrounding the xiphoid cartilage, caused by pressure 

 of the forceps during the process of grafting, may lead to localized necrosis of 

 fat tissue and cartilage ; subsequently, the necrotic fat tissue may be replaced 

 by fibrous tissue, and around the necrotic cartilage a plate of new perichondrial 

 cartilage may form. 



Another difficulty may be encountered when the thyroid of the guinea pig 

 is transplanted. If this organ is surrounded by much fat tissue, the latter may 

 prevent the ready ingrowth of capillaries from the host into the graft and the 

 transplant may become necrotic over a smaller or wider area. Such a result is 

 obtained especially if well-nourished, older animals, in which considerable 

 amounts of fat tissue surround the thyroid, are used as donors and it will be 

 necessary to guard against this complication. Further obstacles to a correct 

 interpretation of the experiments may be due to the presence of adventitious 

 factors, which may accelerate, intensify or retard the lymphocytic reaction. 

 While the essential factor that determines the intensity of this reaction is the 

 degree of incompatibility existing between the individuality differentials of 

 host and transplant, certain tissues are more prone to call forth a strong reac- 

 tion than others. Thus in guinea pig and rat, thyroid tissue is usually more 



