1906.] Resistance of Mice to the Growth of Cancer. 177 



B. (1) Experiment 62, ii K. Thirteen days after the inoculation recorded above, 15 of 

 those mice which had presented no tumours were re-inoculated in the left axilla, with 

 tumour of transplantation 61, Series ii X. No tumours developed, while 15 normal 

 animals inoculated with the same tumour presented 13 tumours after 10 days (87 per 

 cent.). 



(2) Experiment 62, ii L. The remaining 15 mice of Experiment 61, ii M, were 

 inoculated in the left axilla on the same day with a tumour of transplantation 61, 

 Series ii J. No tumours developed, while 15 normal animals inoculated with the same 

 tumour all presented tumours after 10 days (100 per cent.). 



The protected animals were further inoculated unsuccessfully, and were then used to 

 test the properties of the blood. 



Protection similar to that afforded against Jensen's tumour by its 

 ■spontaneous absorption is also manifested by animals in which other 

 spontaneous tumours of different histology have grown for a time, and the 

 tumours been absorbed. Of the tumours on which such observations have 

 been made, XIX was an adeno-carcinoma with cystic spaces and widespread 

 haemorrhages ; XXVII another adeno-carcinoma tending to become alveolar, 

 i.e., approaching the histological type of Jensen's tumour, and XXXVII 

 a malignant adenoma with little tendency to form solid alveoli, while 

 XXXVIII was also an adeno-carcinoma with small acini interspersed with 

 solid alveolar portions. 



The animals in which tumours have disappeared after exposure to radium 

 •are refractory in the same way and to a similar degree as those in which 

 absorption has occurred spontaneously. This is only what was to be expected 

 in view of the probable identity between the process of absorption in the 

 two cases, as described in a previous paper. 



The general conclusion can be drawn that recovery from one form of 

 tumour of the mamma protects against subsequent inoculation with other 

 forms. When protection is absolute in the case of the tumour recovered 

 from, it is not necessarily also absolute for other tumours. There is thus 

 a suggestion of a specificity in the protection. The mice which have 

 recovered from experimental cancer have undergone a change conferring on 

 them a degree of protection as high as that presented by the most refractory 

 of normal animals. This alteration is not confined to the tissues in the 

 immediate neighbourhood of the spontaneously absorbed tumour ; it must 

 have become general by means of the body fluids. 



(4) Protection may be Induced or, when Naturally Present, Enhanced 

 by the Inoculation of Tumour Material when no Growth 

 Eesults. 



We have already pointed out that the animals which have been unsuccess- 

 fully inoculated with small fragments of a tumour giving a low or moderate 



