314 H. B. ANDERVONT 



and unitarian concepts of virus relationships and activities. While the attack on 

 these problems with established viruses has produced gratifying results, it 

 may be more advantageous to explore them through the use of viruses recently 

 recovered from the natural disease. Such viruses would not be remote 

 relatives as must be those maintained in the laboratory for many years. The 

 virus of visceral lymphomatosis may be suitable for this purpose and a 

 thorough study of the lesions through the techniques of tissue culture may 

 throw considerable light on the problem. 



Before closing this discussion of the virus of chicken visceral lymphomatosis, 

 it is essential to recall once again the many unsuccessful attempts to expose 

 viruses in both spontaneous and induced tumors of fowls (Peacock, 1957). 

 The inability to establish viruses as the causative agents of all tumors of 

 like histological structure is not limited to fowl tumors and will be mentioned 

 later in this chapter in connection with mammary cancer of mice. Hence, 

 there is no need for further discussion at this time, except to state that viruses 

 may not be responsible for the occurrence of all tumors and, to quote from 

 Rous (1936), "the future can be left to take care of it." 



B. Rous Sarcoma of Chickens 



The discovery of a virus as the etiological agent of a chicken sarcoma by 

 Rous (1910, 1911) was a basic observation which added much to knowledge of 

 tumor viruses. More than forty-five years later, this virus continues to be one 

 of major interest to cancer workers, to confront them with some of the most 

 difficult and interesting problems, and to give them encouragement in their 

 efforts to search for a similar agent in other neoplasms. The immediate 

 result of the observation was the stimulation to search for viruses in other 

 tumors of chickens; this search uncovered viral agents in a variety of fowl 

 sarcomas (Rous, 1936). The virus of Rous sarcoma, however, has remained 

 the agent of choice throughout the intervening years because it has the 

 advantage of always inducing the neoplastic state at the site of application 

 and does so in a matter of days. For these reasons, it is the most satisfactory 

 of all tumor viruses in studies designed to explore the relationship the virus 

 bears to its animal host and the cells it infects. 



Recent reviews of the Rous sarcoma virus by Harris (1953), Bryan (1955), 

 and Bryan et al. (1955) included all pertinent references to previous investiga- 

 tions and summarized the progress achieved. Hence, this discussion will 

 concern only those facets of the general problem which are now receiving 

 most attention, and, will attempt to interpret the implication of recent work. 

 It is hoped that the best procedure will be a consideration of the problems of 

 this tumor virus, which include: (1) host-virus relationships and, (2) biological 

 properties of the virus. 



