PROBLEMS CONCERNING THE TUMOR VIRUSES 313 



naturally led to the conclusion that the tumors were induced by a group, or 

 family, of related agents. During their discussion of these serological studies 

 Oberling and Guerin (1954) insert the following pertinent statements: "The 

 fowl is one of the least suitable animals for serological researches. . . . 

 Perhaps there are some natural antigens related to the sarcoma viruses which 

 the fowl absorbs and thereby immunizes itself. ... It is essential to avoid 

 regarding the appearance of the natural antibodies as indicating some 

 relationship between these viruses and the normal constituents of the cells 

 that they may infect. ... It thus seems clear that the viruses of the avian 

 sarcomas contain two types of antigens, one being virus proper, the other 

 of fowl origin." 



Despite their timely reminders of pitfalls to avoid in the interpretation of 

 serological work, Oberling and Guerin (1933) presented other data which 

 supported the idea of interrelationships in fowl tumor viruses, namely, the 

 results attending the introduction of the viruses into susceptible hosts. They 

 were the first to show that the inoculation of fowls with tissues from leukemic 

 organs or leukemic blood produced sarcomas and that these sarcomas, upon 

 passage to other fowls, produced leukemia. In some animals tumors arose at 

 sites remote from the inoculated tissue; these included a variety of tumor 

 types. Such observations present one of the most interesting problems 

 encountered with the tumor viruses. Are the different types of fowl tumors 

 dependent upon infection of cells by viruses specific for certain cells, or can a 

 single virus enter different cell types and provoke them to neoplastic pro- 

 liferation? And, when added to the possiblity of a single virus producing a 

 variety of tumors is added the well-known genetic lability of viruses and their 

 widespread distribution in nature, the potential for discovering tumor 

 viruses becomes almost limitless. 



Reasoning along these lines is perhaps why some investigators are inclined 

 to regard the virus of visceral lymphomatosis, because it is contagious, as the 

 stem virus for others included in the fowl tumor complex (Beard, 1957). 

 Regardless of whether the agent of visceral lymphomatosis is the stem virus 

 for a family of tumor viruses capable of inducing tumors in fowls, the idea of a 

 group relationship between fowl tumor viruses and of one virus eliciting a 

 variety of tumor types may some day be looked upon as an important basic 

 concept. Extension of the idea of groups of tumor viruses having a common 

 kinship in a single ancestral or stem virus leads directly to the possibility that 

 any virus capable of exciting cell proliferation could, under suitable environ- 

 mental conditions, act as a tumor virus. 



Thus, the rewarding efforts with the fowl tumor viruses, especially those 

 revealing the routes of transmission of the visceral lymphomatosis virus and 

 those suggesting a related group of viruses, lead straight to some major 

 problems concerning the tumor viruses, namely, further elucidation of the group 



