74 (138) Society for Experimental Biology and Medicine. 



the factors determining a vigorous tumor growth become gradually 

 weakened. In such cases one can observe that even a long time 

 after the expansive growth of such a tumor piece has ceased, many 

 mitoses are present in the cells of the stationary or retrogressive 

 tumor. 



It is possible to diminish the virulence of tumor cells directly 

 by subjecting them to certain physical or chemical conditions. By 

 heating tumor cells up to 43 0 C. or 44 0 C. for half an hour outside 

 the body, or by leaving them before inoculation in glycerin for 1 2 

 to 24 hours, and washing them afterward in 0.85/0 sodium chlorid 

 solution, or by keeping them one or two days in n\joo KCN solu- 

 tion, before transplantation, we are able to diminish considerably 

 the energy of the succeeding tumor growth and to increase the 

 period of latency. In the author's recent tumor inoculations of a 

 salivary tumor, a similar action of glycerin in increasing the period 

 of latency was found to occur. Frequently such tumors remain 

 stationary after a short preceding period of growth. In the first 

 experiments of this kind on rat tumors, it was found that a tempera- 

 ture of 45 0 C. during half an hour kills the tumor cells. Jensen 

 found a similar sensitiveness of his mouse tumors. Sticker's 

 lymphosarcoma could be heated to 45 0 C. without being killed. 

 The power of resistance of different varieties of tumor cells varies 

 somewhat, therefore, and the means to be adopted to obtain a 

 diminished virulence in the growth of an inoculated tumor will 

 vary accordingly. In this connection it might be mentioned that 

 these facts may perhaps find a practical application, insofar as 

 pieces of tumor previously subjected to such treatment might be 

 used to procure active immunity against tumor growth. That 

 such active immunity is possible, at least in the case 01 certain 

 tumors, is especially indicated by the observations of Sticker. 



If now we wish to analyze the cause of this decrease in the rate 

 of growth of tumor cells we have to consider several possibilities. 

 It might be that the physical or chemical means employed kill 

 most of the cells, and leave only a few cells alive and able to give 

 origin to the developing tumor. Two facts speak against such an 

 interpretation. In the case of any tumor transplantation, the 

 growth starts from a relatively small number of cells, inasmuch as 

 the central part of the transplanted piece becomes necrotic. In 



