HETEROTRANSPLANTATION 127 



tilage in cases of homoiogenous and even inter-racial transplantation, is lacking 

 in this type of heterotransplantation. There was seen only one instance of 

 heterotransplanted cartilage in which at an early period a slight attempt at 

 regeneration was apparently observed ; but in this instance the interpretation 

 was not certain. These findings indicate that even in cases in which struc- 

 tural appearances indicate the survival of the cartilage and perichondrium, 

 these tissues are functionally and metabolically no longer normal or com- 

 parable to the corresponding homoiotransplanted tissues. Similarly, the nu- 

 clear multiplication, which is a sign of an abortive regeneration in injured 

 areas of homoiotransplanted striated muscle fibers, is lacking in heterotrans- 

 plantations ; here, again, there was one possible exception observed at an early 

 period following exchange of tissues between rat and mouse, and in this 

 instance, likewise, the interpretation was doubtful. 



Of special interest in these experiments were some differences which we 

 observed between reciprocal transplantations in mouse and rat. The number 

 of polymorphonuclear leucocytes, as a rule, was greater in the rat to mouse 

 transplants than in the mouse to rat transplants ; this corresponds to the fre- 

 quent appearance of leucocytes in the mouse also in many cases of homoio- 

 transplantation, especially in the fat tissue. On the other hand, in the mouse to 

 rat transplants the fibrous tissue formation was more advanced, which may 

 perhaps be due to the greater reactivity of connective tissue cells in the rat 

 than in the mouse, which we had noticed likewise in homoiotransplantations. 

 Similarly, the lymphocytic reaction was stronger in the rat than in the mouse ; 

 again this corresponds to findings in homoiotransplants. In the circumference 

 of these heterotransplants the lymphocytic infiltration was in some instances 

 even more pronounced than around homoiotransplants, although the invasion 

 of the heterotransplant itself by lymphocytes was usually less than that found 

 in many homoio- and even in some syngenesiotransplants. 



Heterotransplantations from Peromyscus maniculatus to mice of strain C57 

 and the reciprocal transplantations. Although the number of experiments we 

 could carry out in this series was much more limited than in other series of 

 heterotransplantations, nevertheless the results were concordant and we are 

 therefore able to draw some additional conclusions regarding heterotrans- 

 plantations. The difference in reactions against the grafts taking place in 

 reciprocal transplantations was evident again in these experiments. The 

 results are of special interest also on account of the somewhat diminished 

 severity, in certain cases, of the heteroreactions. In transplantations from 

 C57 to Peromyscus there was almost complete destruction of the transplanted 

 thyroid, the cartilage with adjoining fat tissue, and the striated muscle; this 

 was noted as early as 8 days and was observed thereafter up to 20 days 

 following the operation. Parts of cartilage were occasionally found preserved, 

 and after 12 days even a mitosis was seen in a peripheral cartilage cell. Lym- 

 phocytes as well as scattered polymorphonuclear leucocytes were noted fre- 

 quently in these transplants, and sometimes there were, instead of scattered 

 cells, collections or even masses of leucocytes. The fat tissue was largely or 

 entirely replaced by fibrous tissue. The grades were 1 throughout this series. 



