320 H. B. ANDERVONT 



previously, such observations suggest that variations of the virus within its 

 natural host can be the result of either quantitative or qualitative changes. 

 Variations of the virus in heterologous hosts have received considerable 

 attention and have proved interesting. Duran-Reynals (1953) performed the 

 pioneer work in this field and obtained best evidence of virus variation when 

 the duck was used as the heterologous host. The virus was administered to 

 day-old ducklings and they developed two types of lesions. The first arose 

 within 30 days, could not be propagated in ducks but, upon reinoculation into 

 chickens, produced the typical Rous sarcoma. The second appeared several 

 months after inoculation, could not be transferred directly back to chickens, 

 but produced a new tumor type in ducks. Duran-Reynals interpreted the 

 results as showing that the second type of duck lesion was, in reality, 

 induced by a variant of the Rous virus. Some duck variants could, by suitable 

 procedures, be reversed to produce tumours in chickens and some of these 

 tumors differed from the original Rous sarcoma. 



This brief account does not include all the interesting observations 

 accompanying heteroinoculation of the Rous sarcoma virus, but it is hoped 

 that sufficient information is presented to show that this virus may, with 

 suitable experimental procedures, produce tumors in foreign hosts. Duran- 

 Reynals used large amounts of inocula in his efforts to infect other species 

 and, although no experimental evidence is available, it is reasonably safe to 

 assume that large amounts of virus were introduced. It would be of interest 

 to know whether the highly potent material procured by Groupe et al. (1957b) 

 is capable of infecting adult ducks. 



When the interesting work with variants of the Rous virus is considered in 

 retrospect, it is clear that this virus possesses properties similar to those of 

 other viruses. Fluid from an innocuous fever blister, when introduced into 

 the corneal epithelium of the rabbit, can initiate a reaction which terminates 

 in a fatal encephalitis. Newly hatched chicks are more susceptible than full- 

 grown chickens to cutaneous, corneal, or intracranial inoculation with the 

 virus of vaccinia, and suckling mice are susceptible to Coxsackie viruses when 

 adults are almost completely resistant (Andervont, 1957). By this time, the 

 reader may be aware that the purpose of this chapter is to show that the 

 tumor viruses, in common with other viruses, display a wide range of 

 biological properties and that the major problem concerning the tumor 

 viruses is not to reveal wherein they resemble or differ from other viruses 

 but to emphasize the importance of ascertaining why the tumor viruses 

 incite unlimited cell proliferation and how many tumors are caused by 

 viruses. 



The remaining properties of the Rous virus have a direct bearing upon the 

 relationship between it and the cell and are, therefore, more closely concerned 

 with the tumor process. 



