HETEROTRANSPLANTATION 117 



The connective-tissue reaction around the heterotransplants is in general 

 very strong; there is a tendency for the connective tissue soon to become 

 transformed into fibrous-hyaline tissue, which latter encapsulates the trans- 

 plant and may surround some of its constituent parts and injure it through 

 the exertion of mechanical pressure; but in certain heterotransplants, such 

 as those of thyroid or kidney, where the tissues early become entirely necrotic, 

 the ingrowth of the connective tissue cells was at first less marked than it is in 

 those homoiotransplants where the reactions are severe ; it seems that a heter- 

 ogenous tissue, which is either completely necrotic or is near death, and in 

 which the metabolism is therefore very weak or wholly suspended, may 

 attract fibroblasts less actively than a more energetically metabolizing tissue 

 which is giving off homoiotoxins. 



In syngenesiotransplants and in some homoiotransplants large and dense 

 masses of lymphocytes may invade and destroy the grafted tissue independ- 

 ently of a preceding activity of the connective tissue. Such a condition we do 

 not find in heterotransplants. Here, lymphocytes may invade the graft usually 

 only in association with the connective tissue of the host. This invasion of 

 lymphocytes may, in the course of time, be quite marked; it may, however, 

 remain slight or be lacking altogether if the heterotransplant becomes entirely 

 necrotic at an early date, as often occurs when thyroid or kidney is trans- 

 planted ; but even in these cases a considerable lymphocytic infiltration 

 may later take place in the fibrous capsule surrounding the graft, or in the 

 fibrous tissue adjoining it, or sometimes also in the fibrous-hyaline tissue that 

 has replaced the graft, where it may exceed, in density, the infiltration found 

 in the majority of homoiotransplants. 



As stated, the appearance of larger numbers of polymorphonuclear leuco- 

 cytes distinguishes heterogenous transplants from homoio- and syngenesio- 

 transplants; these cells accumulate in and around the capsule, they may 

 pentrate into the transplant and be found around or in the necrotic tissue; 

 they may either be scattered or may form small accumulations, or even dense 

 masses, in certain areas. Necrotic material seems to be their chief point of 

 attraction. In and around homoiotransplants, on the other hand, leucocytes as 

 a rule are noted only in the first three days following the operation when 

 necrosis and changes in the circulation and in the permeability of vessels may 

 be responsible for their appearance; they occur in these, and even in syn- 

 genesiotransplants of the mouse, more frequently in places where much fibrous 

 tissue has been produced, and, above all, in fibrous tissue that has invaded and 

 replaced fat tissue. The possibility exists that in the mouse we may have to 

 deal with bacterial infection in those homoiotransplants in which leucocytes 

 appear in larger numbers', and this raises the question as to whether also 

 in heterotransplants the accumulation of polymorphonuclear leucocytes may 

 not at least in part be due to contamination with bacteria. The presence of 

 bacteria and their responsibility for the accumulation of polymorphonuclear 

 leucocytes is suggested particularly also by the development, in some instances, 

 of localized, abscess-like masses of these cells in or around the heterotrans- 

 plants. Furthermore, the fact that if a piece of mammalian tissue is trans- 



