NEOPLASTIC ABNORMAL GROWTH 253 



the milk factor dies out by the third passage through animals not gen- 

 erating it, the likelihood is real that this factor also is gene controlled 

 and indeed has certain similarities to the Kappa factor described for 

 Paramecia by Sonneborn. 



Huseby and Bittner (123) provide confirmation for the results just 

 described in their studies of the architecture of the mammary glands in 

 the same strains of animals. Comparisons were made between the three 

 groups each lacking one of the essential factors for the development of 

 breast cancer and with mice possessing all three factors. Precancerous 

 nodules of alveolar hyperplasia were found to occur only in animals 

 with a high incidence of cancer irrespective of which of the essential 

 factors was lacking. It is concluded that "the same three factors that 

 are etiologically important for the development of mammary cancer are 

 necessary for the development of precancerous alveolar hyperplasia." 



If a virus is to be defined as an agent which is continually propagated 

 by living cells genetically qualified to manufacture it and which is regu- 

 larly associated with characteristic properties of those cells, the milk 

 factor and other substances found in tumors certainly qualify. If, how- 

 ever, a virus is to be defined as a parasite entirely foreign to its host 

 tissue, it must meet additional requirements. It should be capable of 

 causing disease in several species, it should be more independent of 

 genetic factors in its pathogenicity, it should be antigenic in the species 

 which is its natural host, it should be capable of being propagated in 

 tissue culture or on the chorio-allantoic membrane of the chick embryo, 

 and the disease caused by it should be contagious. It is clear that some 

 tumor cells contain substances which are in size and composition like 

 viruses and also like genes. One, and the only one clearly etiologic for 

 cancer in mammals, is dependent on genes for its continued production 

 and is equally dependent on genes for its ability to produce its charac- 

 teristic effects. It is also apparent from the studies of Claude (124) that 

 the protoplasm of mammalian cells, unaffected by viruses or any dis- 

 order, contains complexes of protein of similar size and composition. 

 Immunologic studies on these particles would be of great interest, and 

 particularly desirable would be the demonstration of their control by 

 genes or alteration of their composition by substances which are known 

 to change the structure and function of genes, or, particularly, the abil- 

 ity of these particles to alter genes. 



There can be no doubt that cells contain substances which affect other 

 cells and can be propagated in turn by the recipients. Ample evidence 

 for the existence of substances of this type has been advanced from 



