342 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



noted in the case of normal homoiotransplanted tissues; but added to these 

 primary reactions are secondary immune reactions, which are much less 

 evident in the case of normal tissues than of tumors. However, at the time 

 when these observations were made, very closely inbred strains approaching 

 homozygosity were not yet available; if transplantations are carried out in 

 such almost homozygous strains, the differences in the individuality differen- 

 tials between host and transplant may be much reduced, or almost entirely 

 eliminated, and if there are still some differences to be found in the behavior 

 of spontaneous and transplanted tumors under these conditions, these must 

 essentially be due to changes which took place in the tumor cells in the course 

 of transplantation. Also, normal tissues grow very much better after trans- 

 plantation into individuals belonging to the same inbred strain than into those 

 belonging to different strains, and this fact again proves the similarity of 

 the role of the individuality differentials in the behavior of tumors and of 

 normal tissues after transplantation. 



We see, then, that even when tumors grow well in homoiogenous animals, 

 differences which exist in the constitution of analogous tissues in different 

 individual hosts assert themselves ; but this fact was appreciated only after 

 it had been shown that the relative readiness with which auto- and homoio- 

 transplantation can be carried out in the case of tumors is the same as in the 

 case of normal tissues. The first systematic investigations concerning such 

 differences in the behavior of tumors after auto- and homoiotransplantation 

 were made in 1901 and 1902, when we studied for this purpose a mammary 

 adenoma of the rat, and subsequently, with S. Leopold, a mixed tumor of 

 the breast in a dog. After autotransplantation the tumors — their epithelial 

 as well as their connective tissue constituents — remained alive, while after 

 homoiotransplantation they died. As to the rate of growth, the autotrans- 

 plants showed the slow rate of the original tumors; but if, under the influ- 

 ence of pregnancy, the original tumor grew more rapidly, the autotransplants 

 likewise assumed a rapid growth, which ceased after the conclusion of preg- 

 nancy. We drew, then, the conclusion that the composition of the body- 

 fluids in the individual in which the tumor originated differs in some respects 

 from that in other individuals of the same species, and that in the former it 

 is much more favorable for the life and growth of transplanted cells. This 

 conception we have applied to tissue transplantation in general and as far as 

 this conception holds good we have considered tumor transplantation merely 

 as a special kind of tissue transplantation. We would now attribute these 

 individual differences in the composition of the bodyfluids to the primary 

 differences in the individuality differentials which are present in the cells of 

 these animals, and these cellular differences are associated with secondary 

 differences in the constitution of the bodyfluids. From such individual spe- 

 cific substances we distinguished growth substances of an intrinsic character, 

 inherent in the tumor cells, and representing the essential stimulus to tumor 

 growth, and lastly, extraneous growth substances, especially certain hor- 

 mones, such as those given off by ovarian structures, and other similar sub- 

 stances, which were able to influence tumor growth as well as the growth 



