308 H. B. ANDERVONT 



only transmissible to individuals of contemporary generations, but are 

 transferred to successive generations. 



The tumor viruses are unique in that their activity or growth is adjusted to 

 the host cell in such a manner that they modify the cell without destroying it. 

 They are capable of inciting pronounced alterations in their host cells which 

 enable the cells to ignore or to overcome the normal processes of control. 

 And some tumor viruses, in contrast to the slow response to chemical 

 carcinogens, produce this permanent change within a few days. 



To those interested in the biological approach to the cancer problem the 

 virus theory has always been of considerable interest and it has never lacked 

 the vigorous support of a few investigators. 



Extensive reviews on the subject have been published within the past five 

 years. These reviews (Duran-Reynals, 1953; Oberling and Guerin, 1954; 

 Dmochowski, 1957) and other publications (Kous, 1936, 1946; Andrewes, 

 1950, 1953; Stanley, 1957; Annals New York Academy of Sciences, 1952, 

 1957; Texas Reports on Biology and Medicine, 1957) set forth clearly support- 

 ing evidence for the idea that viruses could be etiological agents of cancer and 

 the progress attained in the studies of known tumor viruses. In view of the 

 excellence and inclusiveness of these presentations it is unnecessary at this 

 time to prepare another comprehensive review. Instead, a few known tumor 

 viruses have been selected for a discussion of their properties and to present 

 briefly the major contributions and problems with which they are associated. 



II. Problems Reviewed 



A. Visceral Lymphomatosis of Chickens 



The virus of visceral lymphomatosis deserves first mention because fowl 

 leukemia was the first tumor shown to be caused by a virus (Ellerman and 

 Bang, 1908). Years of research (Engelbreth-Holm, 1942) revealed that the 

 etiological factor consisted of a group of viruses, each of which elicited a 

 characteristic syndrome (Foulds, 1934). The virus of visceral lymphomatosis 

 serves here as a representative member of the group because much is known 

 of its biological properties and its routes of transmission. 



Knowledge of the relationships between the virus and its hosts was 

 acquired through years of patient investigation and is of considerable 

 importance to the entire field of cancer research (Burmester, 1957). Chicks 

 were found to be far more susceptible than adult chickens. Burmester and 

 Denington (1947) used 1 -day-old chicks in their successful transmission 

 experiments with cell-free extracts of the naturally occurring disease. 

 Burmester (1952) inoculated 5 age groups of chickens with a filtrate from a 

 transplantable strain of visceral lymphomatosis and in doses proportional to 



