INTRODUCTION 37 



Loeb has made an intensive study of the reactions of the host to 

 the implanted graft. The destruction of the graft is due to two processes : 

 a development of connective tissue throughout the tissue and consequent 

 "strangulation" of it and an invasion of lymphocytes. Leucocytic invasion 

 plays a minor role. A close study of the engrafted tissue showed that the 

 lymphocytes acted as primary destructive agents and not merely as scav- 

 engers. The line of demarcation between the invaded and the destroyed 

 areas was sharp ; there was no transitional zone of degenerating tissue 

 such as would have been found if the lymphocytic invasion were secondary. 

 Loeb ascribes both the lymphocytic infiltration and the connective-tissue 

 cell proliferation, which occurs in the graft, as due primarily to the 

 changes in the metabolism of the invading cells rather than to any hostile 

 reaction of the host. The altered metabolism gives rise to toxins which 

 attract lymphocytes arid change the activity of the connective-tissue cells. 

 The more closely related are the host and donor the less active is the 

 formation of these toxins. As a secondary reaction the proteins of the 

 invading cells may act as antigens and stimulate the production of anti- 

 bodies upon the part of the host. These antibodies may then serve to 

 accelerate the metabolic changes in the invading cells and thus contribute 

 to the final destruction of the graft. In view of the important part played 

 by lymphocytic infiltration and the well-known destructive effect of X-rays 

 or radium upon lymphocytes, it is possible that the irradiation of the graft 

 might add to the percentage of successful "takes." 



According to Halsted, grafts ordinarily do not survive unless there 

 is a preexistent physiologic deficiency of the gland in question. 



To secure nutrition adequate to keep the graft alive until it can 

 become vascularized is difficult. To accomplish this the tissue is often 

 cut into thin slices and inserted into some highly vascular organ such as 

 the spleen. The sex glands seem to lend themselves especially well to 

 this type of experimentation. Wheelon and Shipley have submitted 

 graphic evidence of the functional integrity of testicular transplants. 

 They found that castration in dogs resulted in a material decrease in 

 sympathetic irritability as manifested by the vasomotor response to intra- 

 venous injections of nicotine, which is a selective sympathetic stimulant. 

 Later, a considerable period after testicular tissue was implanted in the 

 experimental animals, the reaction to nicotine was materially augmented. 



In physiological experimentation some method of creating a clean-cut 

 excess of hormone production is greatly to be desired, as producing the 

 equivalent of a functional overactivity of the various endocrin organs. 

 Multiple grafting as a means to this end has as yet not been successful. 

 Possibly in the future some method of augmenting the tolerance of the 

 host for transplanted tissue may be evolved and lead to valuable results. 

 Up to the present time organ grafting, as an experimental method, has 

 been of value chiefly as a check upon extirpation experiments. The 



