Immunity of Rats towards Jensen's Rat Sarcoma. 11 



the nature of the processes operative in immune animals it seemed advisable 

 to see vi'hether by a large inoculation the immune condition could be over- 

 come. For this purpose 50 immune rats were given six inoculations of 

 0"05 c.c. of sarcoma cells in the subcutaneous tissues of the flanks and 

 abdomen. In the majority of these small nodules could be felt for 12 to 

 14 days ; in only two cases did they attain sufficient size to be measured. 



Twenty immune rats were splenectomised and were then given six or 

 eight inoculations of 0'05 c.c. of tumour. In several cases persistent 

 nodules resulted : in one case a small tumour grew, but subsequently 

 regressed ; in another case six large tumours resulted in a single animal. 



(c) Exposure to X-Rays. — When an animal such as a rat is exposed 

 in toto to X-rays, a moderate degree of irradiation is sufficient to produce 

 changes in its blood which can easily be recognised. The white cells are 

 more affected by these rays than are the red cells, and of the white cells 

 the lymphocytes are especially vulnerable ; by prolonged irradiation the 

 circulating blood may be temporarily rid of practically all its lymphocytes. 



The work which has been described shows that the lymphocytes play some 

 part in the processes by which immunity to the sarcoma cells is main- 

 tained in the rat, and it was thought that their relative importance would 

 be shown by the following procedure : 



A rat, the immunity of which to sarcoma was proved, was exposed to 

 X-rays for a period sufficient to cause the lymphocytes to drop to a few per 

 cent, of their previous numbers, and an inoculation of sarcoma cells was then 

 made. 



The experimental arrangements were as follows : — A rat, proved to be 

 immune, was placed in a small wooden box with a thin lid and exposed to 

 X-rays from a Coolidge tube, the anode of the tube being 27 cm. distant. 

 The radiation selected for this purpose had a wave-length ranging from 

 about 2-6 to 4-2 x IQ-^ cm. 



It was produced under the following conditions : the heating current in 

 the Coolidge tube was 4'3 amperes, and the equivalent spark-gap was 6 cm. 

 between spheres 5 cm. in diameter. The quantity of radiation was 

 measured by allowing the beam of X-rays to enter a small gold leaf electro- 

 scope placed 225 cm. from the anode. The readings of the instrument were 

 used as a check on the constancy of the radiation from the Coolidge tube, 

 and the instrument was itself checked by the reading produced by the 

 7-ray8 from a standard quantity of radium placed at a fixed distance from 

 the electroscope. 



There is no generally accepted method by which the quantity of X-radia- 

 tion may be specified. Some idea, however, of the quantity of radiation 



