28 Mr. J. C. Mottram and .Dr. S. Russ. Susceptibility and 



Part III. — G-eneral Discussion and Conclusions. 



Preparatory to a brief discussion on the trend of our observations, we give 

 below some of the main facts which have been ascertained : — 



1. Jensen's rat sarcoma almost invariably grows when inoculated into rats. 



2. Once having been inoculated, the rats, in over 90 per cent, of the cases, 

 are immune to a second inoculation. The different phases of growth which the 

 tumours exhibit are intimately bound up with the varying degree of immunity 

 set up by the animal. 



3. When a rat is immune to the inoculation of the sarcoma cells, the 

 spleen of the animal generally shows a high content of lymphocytes and 

 plasma cells. 



4. Mixture of the spleen with the sarcoma cells before inoculation causes a 

 retardation of growth in the resulting tumour ; this is more marked with the 

 spleen of an immune animal than with that of a normal one. 



5. Sarcoma cells may remain as long as three days in an immune rat, and 

 then be successfully re-inoculated ; this period corresponds with the interval 

 required for the accumulation of lymphocytes around the graft. 



6. The essential difference in the processes initiated on introducing 

 sarcoma cells into normal and immune rats consists in a marked accumula- 

 tion of lymphocytes around the graft in the immune rat. 



7. By damaging the sarcoma cells by irradiation, the subsequent reaction 

 of the normal animal resembles that of the immune one. 



8. By damaging the rat, the accumulation of lymphocytes is delayed and 

 growth of the sarcoma occurs. 



9. Eats may be made immune by inoculation of sarcoma cells which have 

 previously been exposed to the /3- and 7-rays from radium. 



10. By exposure to X-rays, an immune rat may be converted into a 

 tumour-bearing animal. 



The majority of normal rats are susceptible; in a few cases only, small 

 nodules follow inoculation. In all such cases, when microscopical examina- 

 tion of the early stages of grafts has been made, some prohferation of the 

 sarcoma cells has been observed. The complete inhibition of proliferation, 

 which occurs in immune animals, has not been observed in normal animals. 

 A complete natural immunity, comparable with acquired immunity, has not 

 been met with. 



The microscopical appearances of a graft whose growth is being controlled 

 at an early stage is similar to what is seen in disappearing tumours ; what is 

 termed natural immunity appears to be an immunity acquired during the 

 regression of a small nodule. 



