III. IMPERFECT VIRUSES 155 



tumour inoculum, it depends to an even greater extent upon factors 

 within the primary tumours as yet unknown. This may indicate that 

 the structure of tumour-cell protoplasm varies with conditions under 

 which the tumour grows, and that if it becomes stronger, the structure 

 will remain intact to a greater extent in the emulsion, meanwhile 

 the assimilase action itself becoming more effective, resulting in the 

 increase in the number of metastases. If the structure became still 

 stronger, it would be retained even in the filtrable particles; thus the 

 virus would become demonstrable. 



Gye and his associates (61) (62) have carried out a series of inter- 

 esting experiments to demonstrate the virus etiology of animal cancers 

 which cannot be transmitted with the filtrates. They have made ex- 

 tensive use of freezing and drying technics and have indicated the 

 successful transplantation of cancer tissues after treatments which 

 are considered sufficient to kill all the cells. Thus, normal embryonic 

 tissue could not be successively transplanted, and no normal cells 

 could be found by histological examinations after an hour's exposure 

 to —75° C, and normal embryonic tissue was also killed when treated 

 with glycerol and stored at —40° C, while fine emulsions of frozen 

 tumour tissue made with distilled water, 40 per cent glycerol or 40 

 per cent glucose, caused cancer. Three mouse sarcomas, one induced 

 chemically and the other spontaneously, were successively propagated 

 with tissues dried completely after freezing, some mouse sarcomas 

 even retained their activity after drying without preliminary freezing. 



Hirschberg and Rusch (63), however, held the opinion that freezing 

 technics alone are not suitable for the demonstration of virus etiology 

 of the animal cancer, since there are a number of evidences for the 

 survival of normal cells after freezing. Nevertheless, they acknowl- 

 edged that the introduction of drying after freezing appears to provide 

 a stronger evidence against the presence of intact cells. 



According to the writer's opinion, it may be not impossible that 

 the assimilase action can be retained in the tissue emulsion after the 

 above stated treatments whether the tissue be normal or cancerous. 

 Since such a treatment as freezing or drying may be less destructive 

 upon the structure capable of acting as assimilase than the filtration 

 procedure, even when the demonstration of the infectivity in the fil- 

 trate failed, tissue emulsions would sometimes stand the treatment 

 and retain their infectivity. There are, however, a vast number of 

 evidences that the structure of cancer cells are, as a rule, not so 

 strong as to be capable of transmitting their structure in the form of 

 protoplasm particles. If Shope papilloma is transplanted to domestic 

 rabbits, not only do papillomas develop readily, but a considerable 

 proportion of them undergo cancerous transformation, and at the same 



