242 C. P. RHOADS 



characteristic of the action of the virus" (98). This differentiation by 

 cytological criteria could now conceivably be strengthened by evidence 

 derived from studies employing tracer elements. 



Irrespective of whether virus can be recovered from the papillomas 

 or not, an antibody capable of fixing complement, in the presence of a 

 saline extract of the papilloma as an antigen, appears in the blood of 

 rabbits bearing the growths, and it increases in titer as the growths en- 

 large. Similarly, extracts of papillomas which yield no virus will, when 

 injected intra-peritoneally into normal rabbits, evoke specific antiviral 

 antibody, which fixes complement in the presence of the original antigen 

 and only that material, and, when mixed with virus, reduces or removes 

 (neutralizes) that agent. Not only are the papillomas antigenically ac- 

 tive, but the transplanted cancers are also, even though no virus can be 

 demonstrated in them. The virus-neutralizing antibody which circulates 

 in the blood of rabbits carrying the virus-induced papillomas has no 

 effect upon the course of the lesions however, or upon the growth of the 

 cancers, a fact which is ascribed to a protection of the virus by its host 

 cells from the action of the antibody. 



The absence of virus from the papillomas and cancers of domestic 

 rabbits is notable, as is its lack in the cancers of the cottontails, although 

 it is abundant in the papillomas of that species. Kidd (99) sees two pos- 

 sible explanations of these findings : that the virus had disappeared from 

 the papilloma when the cancer developed and did not exist in the cancer, 

 or that in both the tissues virus was present but was masked, i.e. could 

 not be demonstrated, though actually present. To prove that the second 

 explanation was the correct one, a search was instituted for a virus- 

 neutralizing substance or inhibitor in the cancer. Table II of Kidd's 

 paper (99) presents the evidence that such an inhibitor was found. Ex- 

 tracts of glycerinated cancer tissue were mixed with papilloma virus 

 and inoculated into domestic rabbits. The controls were of virus mixed 

 with Tyrode solution. No controls with benign papilloma tissue were 

 used, since it was assumed that they could not contain an inhibitor be- 

 cause virus could be demonstrated in them, although the assumption 

 that the cancer did not contain a virus was not accepted. Of fourteen 

 tests with the virus cancer tissue mixtures, virus activity was found in 

 thirteen, but delay in papilloma growth as compared with the Tyrode 

 control was found in eleven. It should be noted that a second control re- 

 ported, with Tyrode and virus, showed no more activity than was seen 

 in eleven of the fourteen tests presented to prove the presence of an 

 inhibitor (100). 



