234 LIFE: ITS NATURE AND ORIGIN 



At times it is demonstrable in the blood and in certain other 

 tissues, but it causes no visible changes anywhere and has no 

 untoward effect until the mouse begins to age. Then one or 

 more cancerous growths appear. This happens earlier, and the 

 tumors are more often multiple, in mice which have had many 

 litters of young and in those injected with the sex hormones 

 which stimulate mammary proliferation. From the cancers noth- 

 ing can be secured which will cause tumors on direct inoculation, 

 though they yield the 'milk factor' in quantity. The many phys- 

 ical and chemical studies made of this factor all go to show that it 

 has the attributes of a virus, and it exhibits the narrow specificity 

 of those which give rise to tumors, its presence ultimately resulting 

 in growths of a single organ and of but a single sort, the kind of 

 mammary cancer common in mice. Furthermore, antibodies 

 capable of neutralizing it are formed by the body, as in the case 

 of the tumor-producing viruses. Yet with all this it fails to cause 

 growth when injected into mammary tissue." 37 



However, many specific molecules are known to lead to the 

 development of cancers, e.g., l:2-benzanthracene and quite a 

 number of its derivatives, besides other organic substances pre- 

 viously mentioned. While these carcinogens are not themselves 

 duplicated in the cell or the organism, they give rise to auto- 

 catalysts which are duplicated, even in tissue cultures of the cells 

 whose cancerous potencies they determine. It seems more rea- 

 sonable to consider the few cases where viruses are known to 

 produce cancers as special examples under the broader view of 

 heritable cellular catalyst change, rather than to stretch the term 

 "virus" far beyond its generally accepted meaning. Nor is it neces- 

 sary to assume that "latent" viruses lurk in all plants and animal 

 cells awaiting merely for a preliminary excitation before launch- 

 ing their cancerous onslaught. The experiments of Peyton Rous 

 and W. E. Smith 39 are illuminating. Realizing that the period of 

 gestation is too short for experimental production of mouse 

 cancers in utero, they applied a mixture of the potent carcinogen 

 methylcholanthrene and Scharlach R to embryo mouse epidermis 

 inplanted in the thigh muscles of mice. The majority of the 

 tumors that appeared were progressively enlarging, invasive carci- 

 nomas. By similar technique cancerous growths were produced 

 in wide variety in lung, stomach, biliary tract, intestine, kidney, 

 ureter, urinary bladder, ovary, testis, and nerve tissue. Consid- 



