TUMOR GROWTH 433 



against tumors retained distinctive features and there was the expectation and 

 hope that a study of immunity against transplanted tumors might lead to the 

 discovery of methods of immunization also against spontaneous tumors, which 

 would prevent their development or cause the retrogression of tumors which 

 had already developed. Similar views were held, also, by subsequent in- 

 vestigators, Tyzzer, Woglom, Uhlenhuth, Chambers and Scott, Caspari, 

 Lewin and Lumsden. 



We approached the problem of tumor transplantation essentially from the 

 point of view of the experimental analysis of tissue growth, and from the 

 beginning we emphasized the parallelism between the behavior of tumors and 

 normal tissues after transplantation. The favorable results of autotransplanta- 

 tion were attributed to the similarity in the constitution of host and spon- 

 taneous tumor, and the reactions in homoiotransplantation, and the still 

 stronger reactions in heterotransplantation, were correspondingly interpreted 

 as due to the relative strangeness of the constitution of host and transplanted 

 tumor. In this sense we explained also the experimentally produced variations 

 in growth energy of tumor cells which we had observed under various condi- 

 tions, and the conclusion was drawn by us that the potential immortality of 

 tumor cells which the serial transplantation of tumors had revealed was not 

 peculiar to tumors, but was shared by the majority of normal tissues, at least 

 by all of those that could give origin to tumors. A comparison of the struc- 

 tural changes of normal tissues and tumors after transplantation and of 

 cellular reactions taking place around them revealed additional similarities, 

 and it was possible to distinguish between the constitutional factors, which 

 would permit the tumor cells to live in the host, and the increased proliferative 

 tendency inherent in the tumor cells, which enabled them to grow after 

 transplantation. A distinction was made also between transplantability and 

 the factors determining the growth energy of tumors and at the same time 

 the analysis of the constitutional factors underlying these conditions was 

 further developed. There was noticeable a great similarity in the behavior of 

 normal tissues and tumors after auto-, homoio- and heterotransplantation. 



These points of view were extended by Peyton Rous, who (1910) compared 

 the immunity against embryonal tissues and against tumor tissues, when both 

 tissues grew side by side in the same host. He found that immunization against 

 embryonal tissue, and that against tumor tissue, took a similar course. A few 

 years later (1916) we further compared the reactions of the host against 

 transplanted normal and tumor tissue, and we observed in both instances a 

 parallel reaction of the lymphocytes and connective tissue of the host against 

 the transplant. Thus there was formed gradually on the basis of these com- 

 parative studies of tissue and tumor transplantation, the concept of organismal 

 differentials as the essential factors underlying both of these processes. 



A definite divergence existed, therefore, between these two tendencies in the 

 development of cancer research, and especially in the study of the transplanta- 

 tion of tumors ; in the one, the immunity against tumors was the central prob- 

 lem, in the other, it was the comparative behavior of normal and tumor tissues. 

 However, this distinction was not quite so complete as it might appear. Thus, 



