TUMOR GROWTH 435 



in fixing the number of genes for this purpose, he assumed that these factors 

 were specific determinants of tumor immunity, and that they did not apply to 

 tissues in general. 



These differences in the theories of various investigators are shown more 

 clearly in additional investigations. Jensen (1908-1909) compared tumors 

 growing after transplantation into other individuals with metastases of spon- 

 taneous tumors, an interpretation subsequently expressed by various other 

 authors. He believed, furthermore, that if a change in diet can affect trans- 

 plantability of tumors — as it apparently did in Haaland's experiments — it 

 might equally influence metastasis formation. No distinction is recognized 

 here between the conditions in auto- and in homoiotransplantation. On the 

 other hand, Bashford, Murray and Cramer made a sharp distinction between 

 the conditions that cause the formation of a spontaneous tumor and those 

 determining the growth of a tumor once it has formed ; they were led to this 

 distinction by the observation that an animal unsuccessfully inoculated with 

 a transplantable homoiogenous tumor, subsequently could develop a spon- 

 taneous tumor. They did not, however, distinguish in these cases between the 

 development of a spontaneous tumor possessing the same or almost the same 

 individuality differentials as the host and the growth of homoiogenous (trans- 

 planted) tumors, in which the individuality differentials of tumor and host 

 differ; this can be seen from their statement that they observed — evidently 

 contrary to their expectations — that animals affected by spontaneous cancers 

 are not greatly more susceptible to inoculation with cancerous tissue than are 

 normal animals. They believed that spontaneous tumors which do not grow 

 in other animals of the same species not affected by cancerous growth, rarely 

 grow when transplanted to other parts of the animal's own body, and not 

 at all in other animals bearing spontaneous tumors ; there was no need, there- 

 fore, for any subsidiary assumption as to the importance of a constitutional 

 condition inherent in the normal cells of the animal in which the tumor origi- 

 nated and in the fully developed tumor cells for the growth of spontaneous 

 cancer after transplantation. What these authors called "individuality" of 

 tumors was not the chemical constitution of the tumor cells as determined by 

 genetic factors; identity of individuality did not mean identity in chemical 

 composition of cells due to genetic factors, but it was considered to be the 

 result of identity of the sum total of changes which had taken place in the 

 tumor cells, in consequence of past experiences in the life of the organism. 

 According to this conception, every individual mouse was therefore different 

 from all the others, and this difference would increase with the increasing 

 length of life of the animal. This point of view is expressed in a paper by 

 Bashford, Murray and Cramer on the resistance of mice to the growth of 

 cancer (1907). 



Bashford and his collaborators attributed differences in transplantability 

 of tumors to factors inherent in the host as well as to variable factors which 

 distinguish different tumors. Among the latter they also recognized the sig- 

 nificance of the growth energy of the tumor, and they insisted especially on 

 the great significance of growth rhythms in tumor cells which occur spon- 



