III. IMPERFECT VIRUSES 157 



cells. The structure shares its fate, therefore, with the organism in 

 which it has developed. The structure has to cease to exist when the 

 organism perishes. 



The writer designates such structures as imperfect viruses. 

 Nevertheless, it is impossible to draw definitely a line of demarkation 

 between imperfect and perfect viruses. For example, as already re- 

 ferred to, natural infections are said to be possible with such tumours 

 as fowl leucosis and plant wound-tumour. Moreover, a mutant strain 

 of rabbit fibromatosis has been shown to cause acute inflammatory 

 lesions similar to those of ordinary virus diseases instead of the usual 

 proliferative change (65). Rivers (53) claimed that myxomatosis of 

 rabbits, which has the remarkable relationship to this fibromatosis, 

 mediates between tumours and ordinary virus diseases. The disease 

 is transmissible to normal rabbits naturally by mosquitoes. Labora- 

 tory transmission of fibromas in rabbits by means of fleas and mosqui- 

 toes is also possible {66). Duran-Reynals (67) has made an investigation 

 with a duck variant of Rous sarcoma and found that lesions induced 

 in the central nervous system of ducklings by the virus is very simi- 

 lar to that observed in the infection by ordinary viruses. 



