230 C. p. RHOADS 



likely to be the intermediate metabolites as they are to be the original 

 materials. Dobriner and Rhoads (65), Boyland (66), Jones (67), and 

 Weigert (68) have all reported on studies of the metabolism of these 

 substances. The very great insolubility of the original materials and the 

 data which attest to the formation of highly labile intermediate prod- 

 ucts in the course of conversion support the view that the attempts to 

 cause mutations may not have subjected the cells studied to exactly the 

 compound which is active in the mammalian tissue. 



The studies of Earle (125) are particularly important as they con- 

 cern the alterations undergone by cells in the course of becoming malig- 

 nant. He cultured mouse fibroblasts in the usual heterologous serum- 

 chick embryo extract mixture. Subcultures were exposed for different 

 periods to varying concentrations of methyl-cholanthrene, and the cells 

 were inoculated into mice of the strain from which they had been 

 derived. The cultures assumed characteristic growth characteristics and 

 certain of them, after a rather narrow range of brief exposure to the 

 carcinogen, were found to have neoplastic properties. This was also 

 true of the mouse fibroblasts cultured for longer periods without methyl- 

 cholanthrene. 



The conclusion from this work is unavoidably that mouse fibroblasts 

 grown for long periods under very abnormal conditions, particularly 

 with a carcinogen, become sarcoma cells. Further details of the environ- 

 mental circumstances to which the cells are exposed and the composition 

 of the abnormal cultures will be anticipated. 



As far as is known, no studies acceptable to the geneticist have been 

 made on cells of the mutation-producing eflFect of the azo-carcinogens 

 of the type of dimethylamino-azobenzene. In the case of these com- 

 pounds much information is at hand from the studies of Kensler (69) 

 and his associates, as well as from the Wisconsin school (70), con- 

 cerning the pathways followed in the breakdown of the parent sub- 

 stance and the biological activities of the various derivatives. For 

 example, dimethylamino-azobenzene is wholly inactive in inhibiting the 

 metabolism of the tissue cells which have been studied manometrically 

 in acute experiments. Certain derivatives, however, are exceedingly 

 toxic. Suggestive evidence is at hand, furthermore, that other deriva- 

 tives, not hitherto isolated, may also be biologically active. It is not 

 sufficient to conclude that because the juxtaposition of a carcinogen, 

 such as methyl-cholanthrene, and a cell susceptible to mutation, such as 

 Neurospora, fails to give mutation, the chemical is incapable of doing 

 so. It must be shown, before this conclusion is reached, that the cell 



