352 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



not on homoiotransplants. However, the primary degeneration of the more 

 central parts of a homoiotransplant is due to the more unfavorable condition 

 of these areas, involving a deficiency of oxygen and possibly also of other 

 foodstuffs during the period directly following transplantation; the central 

 portions of the tumor may therefore be more accessible to the action of the 

 injurious homoiotoxins, while in the peripheral parts these toxins are not 

 strong enough to destroy tissues which live under relatively favorable condi- 

 tions. On the other hand, the more active heterotoxins, especially the immune 

 heterotoxins, may accomplish a direct injury also of the peripheral parts. 



The reaction against homoiogenous tumor transplants depends, at least in 

 part, on the development of an active acquired immunity in the host ; whereas, 

 in the case of normal homoiogenous tissue transplants, the injurious effect 

 seems to be due largely to the action of primary homoiotoxins and to the 

 direct response on the part of the host tissues. Similarly, while in the case 

 of heterotransplanted normal tissues the preformed heterotoxins and the 

 activity of the cells of the host play the principal role, and immune hetero- 

 toxins seem to enter into the reaction only secondarily, in the case of hetero- 

 transplanted tumors the effect of active immunization can be more readily 

 demonstrated. Through a previous inoculation with normal tissues from 

 the heterogenous species to which the tumor belongs, immunization of the 

 host can be accomplished and the destruction of the tumor transplant can 

 be much accelerated. While under these circumstances the action of the im- 

 mune heterotoxins is the most important agency that causes the rapid destruc- 

 tion of the transplant, an intensified reaction on the part of the lymphocytes 

 may play a part here, as well as after homoiotransplantation of tumors into 

 actively immunized animals ; it is by means of this accumulation of cells 

 around the graft, rather than by a lack of ingrowth of connective tissue and 

 blood vessels from the host into the transplant, that the incompatibility 

 between the differentials of the host and transplant may become manifest. 



We may then conclude that in the case of tumors, as well as of normal 

 tissues, it is primarily the primary, performed heterodifferentials which call 

 forth the reaction of he host tissue against the transplant, and that it is these 

 heterotoxins which injure the transplanted tumor. Secondarily, such hetero- 

 differentials may act as antigens and call forth the production of immune 

 heterotoxins, which are especially effective in the case of tumor transplants; 

 associated with this process may be an intensification of the cellular reaction. 

 Tumors are the descendents of normal tissues; they have retained the or- 

 ganismal differentials of the latter in all essential respects ,and they call forth, 

 therefore, the same primary reaction in the hosts. But some changes take 

 place in the normal tissues during their transformation into tumors and it is 

 in consequence of these changes that tumor tissues differ in certain respects 

 from normal tissues in their transplantability and in the reactions they call 

 forth in the host. 



That, however, notwithstanding these modifications the organismal differ- 

 entials of the tumors play a significant role in transplantation, is also made 

 evident by the fact that in order to accomplish an immunization against a 



