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



latter often attain to considerable size ; tumours measuring as much as 

 6 sq. cm. have subsequently been observed to disappear. 



2. The Coiidition of Immunity. 



(a) General. — Eats are rarely found to be immune to a first inoculation of 

 the sarcoma cells. The animals in which the resulting tumours spontaneously 

 regress are almost invariably resistant to a second inoculation ; when tested 

 two or three months later, they are generally in a similar state. Our studies 

 upon the immune condition have been largely made upon rats which at one 

 time have borne tumours, and their resistant nature proved by the failure of 

 a second inoculation. 



The wide variations observed in the behaviour of the inoculated sarcoma 

 cells are not due to differences in the inoculated material. A progressively 

 growing tumour may be removed from an animal, made into a uniform 

 emulsion and inoculated into a batch of, say, 20 normal rats of about 

 100 grm. weight each. It may be fairly confidently stated that the subse- 

 quent tumours will furnish examples of all four types of growth which have 

 been described ; this being so, it is rational to attribute the varying fate of 

 the sarcoma cells to the defensive mechanism which the rats are able to 

 bring to bear against these cells. This defensive methanism is one which 

 may eventually overpower the growing sarcoma cells, causing their complete 

 disappearance {vide type 4) ; such animals are immune to subsequent inocu- 

 lation, and the sarcoma cells of tumours in the course of disappearing are 

 rarely transplantable with success {vide fig. 2). The oscillating tumours 

 indicate a condition in which the growth of the cells is just balanced by the 

 defensive mechanism ; animals bearing such tumours are generally found to 

 be immune to a subsequent inoculation. The transplantation of oscillating 

 tumours gives a moderate number of growing tumours {vide fig. 3). 



With still smaller degrees of defensive power on the part of the animal 

 there appear retarded tumours, and, lastly, progressive tumours. In these 

 last named, the defensive mechanism is not always entirely absent, for 

 re-inoculation of the animals does not invariably result in growing tumours. 

 Kecourse is naturally had to tumours of type 1 or 2 for the continued 

 propagation of the tumour. 



(b) Production of Immunity. — If sarcoma cells are exposed to the yS-rays 

 from radium under suitable conditions of exposure (1), it is found that when 

 the irradiated cells are inoculated into normal rats they do not develop into 

 tumours. If the degree of irradiation is not too prolonged, the anjmals are 

 frequently found to be immune to a fresh inoculation of normal sarcoma 

 cells. 



