NEOPLASTIC ABNORMAL GROWTH 24I 



capable of inducing in rabbits a wholly different cytological structure, 

 that of infectious myxomatosis. The material which causes the trans- 

 formation appears to reside in the myxoma virus nucleoprotein. This 

 mutability of a virus is a striking and frequently observed property. 

 The studies of Duran-Reynals (94) on the Rous agent prove that it has 

 wholly different pathological effects with the production of acute necro- 

 tizing lesions, when injected into very young chicks, than it does when 

 inoculated into adult fowls with the resultant tumor of avian mesen- 

 chyme. Indeed, this investigator has raised the question as to whether 

 tumor production by this agent may not be a special reaction given by a 

 partly immune host. The fact should be recalled, however, that the Rous 

 agent is quite unique among viruses. It is neutralized by rabbit anti- 

 serum to normal fowl tissues and the antibodies of such serum are ad- 

 sorbed out by extracts of normal chick embryo. Furthermore, chemical 

 studies on the agent reveal a close similarity with a substance obtained 

 from normal chick embryo. The contributions of Claude (95) should be 

 consulted on this subject. 



The results reported by Rous and his associates (96) prove that the 

 cutaneous papillomas experimentally produced by the inoculation into 

 domestic rabbits of a cell-free material, the virus of Shope (97), ob- 

 tained from growths of western cottontail rabbits, frequently result in 

 the development of cancer in the involved tissue. An extensive series of 

 publications has appeared concerned with this experimental study and 

 with certain correlative data derived from immunologic studies with 

 the Brown-Pearce transplantable tumor of rabbits. The conclusion was 

 reached that both types of rabbit cancer investigated are caused by 

 "viruses" which cannot be recovered from the cancer tissue but actually 

 persist in a "masked" form. This is a most important conclusion, if 

 supported by the evidence, since it would prove that at least two and 

 possibly all forms of cancer in mammals are infectious, and, as such, to 

 be transmitted by contagion, even though no disease-producing material 

 can be demonstrated in them. 



Morphological evidence has been advanced to prove that the cancers 

 associated with transmissible papillomas in both wild and domestic rab- 

 bits arise directly from the papilloma cells. This conclusion is based prin- 

 cipally on the finding that the virus causes the differentiating epidermal 

 cells to become much larger than normal and to have much larger vesic- 

 ulated nuclei with marginated chromatin. It is stated that "all these 

 changes may be encountered individually in tar cancers (for which no 

 virus etiology is claimed) yet when found together they are highly 



