ANIMAL TUMOURS 



The first is that where the tumour-promoting property can be 

 transplanted only in or by whole cells. This condition arises in 

 mammals both spontaneously and artificially. It can be induced by 

 radium, ultra-violet and X-radiation, and also by the action of 

 specific chemical agents. These carcinogens, apart from the sterols, 

 produce their effects on particular tissues. Thus p-dimethyl- 

 aminoazobenzene (or butter-yellow for short) acts only on the 

 livers of rats or mice, and dibenzanthracene only on the skin of mice. 

 Moreover, some carcinogens act with some strains of mice and not 

 with others. Evidently, therefore, the change is a specific reaction 

 between the carcinogen and some protein or protein-system. And 

 in respect of this reaction cells differ as between tissues and as between 

 races, just as they do with regard to spontaneous tumours. The 

 carcinogen is a mutafacient agent, a highly specific one. The new 

 particles produced in this way must be self-propagating, since the 

 growth they induce is characteristic and unlimited and it persists 

 when the carcinogen is removed. As particles, however, they have 

 not the capacity of invasion. They spread by cell division and 

 they are inherited by cell-lineage: they are transmitted only by 

 transplantation, that is by grafts. Their spreading through the body 

 is due simply to the migration of whole cells. In all these respects 

 the spontaneous tumours of man and other mammals agree with the 

 induced tumours. All of them have obviously arisen in the stocks 

 in which they were found. 



The second order of transmission is found chiefly in fowls. Several 

 types of spontaneous tumours in fowls can be passed on by injection 

 of filtrable particles of known properties. Thus the Rous Sarcoma 

 particles are known to be about 70 m/x in diameter, and to contain 

 a specific antigen in addition to a normal fowl antigen. These 

 particles have the properties of invasion of cells, although not of 

 infection. They attack only damaged cells and they can be trans- 

 mitted only artificially. They must, therefore, again have arisen 

 where they were first found. 



The third order of transmission is found in a type of breast cancer 

 of the mouse, which is conditioned in its occurrence, as we might 

 expect, by hormonal activity and by race, that is by nuclear geno- 

 type. Transmission is by the milk of the mother or foster-mother. 

 It follows the milk line, not, like a plasmagene, the egg-line. Again 



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