PROBLEMS CONCERNING THE TUMOR VIRUSES 353 



parotid tumors in strain C3H. These mice developed parotid tumors after 

 receiving the following materials: centrifuged or filtered extracts of induced 

 leukemias in strain C3H mice; centrifuged or filtered extracts of induced or 

 transplanted parotid tumors in C3H mice; centrifuged extracts from normal 

 embryos of strain C3H and C57BR/cd; centrifuged extracts of normal 

 lactating mammary glands from strain C3H; centrifuged extracts from 

 pooled normal organs of C3H mice used when fresh, lyophilized, and kept at 

 — 4°C. for \ to 26 months, or preserved in 50 % glycerine for 2 weeks to 2 

 years. Gross discussed the possible relationship between the leukemia and 

 parotid tumor agents and reported that only in 3 mice "among several 

 hundred" inoculated did he find both tumors and, in contrast to strain C3H, 

 strain C57BR/cd animals were resistant to the parotid tumor virus. Concern- 

 ing the development of parotid tumors in C3H mice after they had received 

 materials from other C3H mice he concluded: "This agent would therefore 

 exist in C3H mice in a masked form, usually not pathogenic for its natural 

 carrier." At this point, the writer cannot refrain from suggesting that the 

 same words could be used to explain also the appearance of leukemia in 

 Gross' strain C3H mice until either a high-leukemia strain can be freed of the 

 virus or a low-leukemia strain can be changed to a high-leukemia strain 

 through acquisition of the virus. If the use of the newborn animal is essential 

 for the production of tumors, then the role of the host must be explored 

 before an intrinsic host factor can be excluded as the most important influence 

 in tumor development. 



Gross (1956) reported the production of parotid tumors in mice following 

 administration of strain C58 leukemic extracts to different hosts. The results 

 may be briefly stated as follows: Filtrates induced leukemias but no parotid 

 tumors in C57BR/cd mice, but induced both leukemia and parotid tumors in 

 C3H hosts; centrifuged extracts of filtrates of C57BR/cd leukemias induced 

 with C58 leukemias produced leukemia and parotid tumors in C3H mice; 

 similar materials from induced tumors in C3H mice evoked leukemia and 

 parotid tumors in C3H mice; centrifuged extracts from C3H parotid tumors 

 induced with C58 leukemias produced leukemia only in C3H mice, but Gross 

 referred to previous experiments in which C3H materials had induced parotid 

 tumors in C3H mice and considered the small number of animals used 

 responsible for the negative result. In other experiments, centrifuged extracts 

 of strain C58 embryos were administered to strains C3H and C57BR/cd mice; 

 the C3H mice developed parotid tumors as did one of the C57BR/cd mice. 

 This latter tumor represented the only parotid tumor produced in a C57BR/cd 

 mouse in Gross' experience. The injection of centrifuged extracts from normal 

 organs of guinea pigs into 235 C3H mice produced parotid tumors in 4 animals. 



As stated previously, Gross (1957b) prepared a summation of his studies in 

 which he reported that filtrates from AK/n spontaneous leukemias produced 



vol. in — 23 



