PROBLEMS CONCERNING THE TUMOR VIRUSES 319 



According to Harris (1953), Carr (1942) injected the Rous virus into a 

 relatively nonsusceptible line of fowls and observed regression of tumors, but 

 as long as a year later tumors occurred at the injection site. Bryan (1957) 

 reported the occurrence of tumors resembling Rous sarcomas at sites remote 

 from the site of application as long as 90 days after the chickens had received 

 the virus. The ability of this infectious tumor virus to persist, presumably in 

 cells, for long periods of time without provoking overt tumors suggests that 

 unknown factors exert some control over the cancer process. The use of 

 inbred hosts in studies of host-virus relationships would assist in the identi- 

 fication of these factors. 



2. Biological Properties of the Virus 



It is impossible to draw a clear distinction between the properties of the 

 Rous virus and the reactions between it and its host, but, for the sake of 

 convenience, such properties as variation and adaptation, masking, multi- 

 plication and assay, as well as the problems of purification, are best discussed 

 in relation to the virus itself. 



a. Variation and Adaptation. Variation and adaptation of the virus have 

 been discussed adequately by Harris (1953), Oberling and Guerin (1954), and 

 Dmochowski (1957). Here, such changes in the Rous virus are considered as 

 they occur in the chicken, its natural host, and in other species, its heterolo- 

 gous hosts. 



Variation, or deviation from the normal sequence of events within the 

 natural host, is a common trait in viruses, and such variation usually occurs 

 when the virus gains access to a new type of host cell, acquires a greater 

 degree of activity, or is introduced into a host of high susceptibility. It would 

 be surprising if the Rous virus, during the many years of animal passages in 

 laboratories throughout the world, had not shown variations in activity. 

 Older workers will recall the common practice, now forgotten, of adding a 

 trace of kaolin or kieselguhr to a filtrate to facilitate virus activity, and the 

 difficulties of performing an intravenous injection without producing a 

 subcutaneous tumor at the site of needle puncture. In a recent paper, 

 Groupe and Rauscher (1957a) described the growth of the virus in chick 

 brain and, while the presence of Rous sarcoma cells beneath the meninges 

 could have accounted for an increase in virus, there were excellent reasons 

 for assuming that the virus was propagated in the brain tissue. In either 

 event, the authors concluded: "the growth curve of Rous sarcoma virus in 

 chick brain paralleled that of many non-neoplastic animal viruses." It will be 

 recalled that the same investigators with Bryan (1957) found that further 

 passages in chick brains produced a highly active virus which, when intro- 

 duced intracerebrally, produced typical "hemorrhagic disease." As stated 



