374 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



again the principal factors which may enter into the transplantation of normal 

 tissues and of tumors. There is (1) the relation between the individuality dif- 

 ferentials or species differentials of host and transplant. This, as we have 

 seen, plays a role in cancers as well as in normal tissues, numerous data 

 confirming such a conclusion. (2) Variable factors relating to the mode of 

 inoculation or transplantation may greatly influence the number of successful 

 transplantations ; these are non-genetic in nature. To mention an example : 

 the usual figures for takes relate to experiments in which the transplant is 

 placed into the subcutaneous tissue. Intracutaneous or intraperitoneal in- 

 oculation may alter these figures considerably. As we have seen, transplanta- 

 tion into the brain, and especially into the anterior chamber of the eye, may 

 make possible a tumor growth, which would not take place after subcutaneous 

 transplantation. And, as various investigators have shown, even heterogenous 

 cancers may grow in the chorio-allantoic membrane or in the anterior cham- 

 ber of the eye. Likewise, the amount of material inoculated is of importance. 

 In many instances, transplantation of a larger quantity increases the number 

 of successful transplantations. But if the quantity exceeds a certain optimum, 

 the number of definite takes may decrease again in certain cases, because 

 the added tissue may increase the amount of material which may serve as 

 antigen and call forth the production of injurious immune substances. Also, 

 in the case of mouse leukemia the quantity of injected leukemic cells helps to 

 determine the result. While, as Furth has shown, a single leukemic cell may 

 be able to transfer the new formation to another host, on the whole, prospects 

 of a successful transplantation are greater and this tumor-like condition de- 

 velops and kills the inoculated mouse more rapidly if the number of inoculated 

 cells is greater. Also experimentally it is possible to diminish the virulence or 

 growth energy of tumor cells by the application of various physical and 

 chemical agents. (3) Certain extraneous, non-genetic factors which alter the 

 susceptibility or the power of resistance of the host tissue to transplanted 

 material. Application of X-rays, injection of colloidal dyes or other material, 

 may diminish the resistance of the host, presumably by affecting the reticulo- 

 endothelial system. There is good reason for assuming that also without the 

 use of these experimental means, differences exist in different individuals 

 and strains in the intensity of the reaction against transplants bearing a dif- 

 ferent organismal differential. (4) Differences in the resistance of different 

 tissues, of which the tumors are composed, to injurious influences which may 

 prevail in the host. These differences we have found in normal tissues, such as 

 thyroid and cartilage, and there is evidence that they exist also in tumors. Thus 

 the malignant chondroma of a mouse observed by Ehrlich could readily be 

 transplanted into other mice, irrespective of their genetic constitution. The 

 tumor grew slowly but was able to resist unfavorable conditions. When a 

 piece of cartilage becomes permanently endowed with greater growth energy 

 and thus assumes the characteristics of a tumor, it still retains some of the 

 essential characteristics of cartilage, such as its relatively great resistance to 

 the action of injurious body fluids and antagonistic cells. On the other hand, 

 if thyroid becomes endowed with great growth potentiality, it likewise still 



