ANIMAL EXPERIMENTATION AND PROTEAL THERAPY 291 



Perhaps the most plausible explanation of this is found in the 

 suggestion that the transfused blood contained corpusclar or en- 

 zymic substances that could begin the decompounding of the 

 cancer cell, and that through such decompounding a stimulus was 

 given to the blood-forming organs of the animal that led to an 

 elaboration of the defensive mechanism that would not otherwise 

 take place. The permanent modification of the conditions in the 

 dog's system that made it resistant to future inoculation would 

 thus be attributable to activities stimulated by the cancer pro- 

 teins, acting virtually as autologou-s- antigens. 



In this view immunity thus .induced is closely comparable to 

 immunity induced by inoculation wkh autolyzed tissues. 



No doubt it has occurred to a large number of workers, that, 

 as Dr. Levin phrases it, "to induce en immunity to cancer growth 

 may indicate the way to rational treatment of cancer, since such 

 an immunity would prevent the occurrence of metastases, after 

 the primary tumor is removed." But doubtless most such 

 workers have felt, as Dr. Levin felt, that "it is self-evident that 

 the results obtained cannot be transferred as yet to human 

 pathology." 



Had it occurred to any one, however, that the active agent 

 involved was merely protein matter as such, and not enzymes 

 specifically associated with or antagonistic to the cancer cell, 

 there would have been no such embargo on transferring the re- 

 search to human pathology. 



But as the experimenters failed to gain this conception, the 

 inviting possibilities were unrealized, and the clues to a solution 

 of the problem were found in a quite different way. I have told 

 the story in some detail elsewhere and 4t need not be repeated 

 here. 



THE VALUE OF EARLY TREATMENT 



Among the most important of the observations of the animal 

 experimenters, in their possible application to the human subjects, 

 are those showing that (1) there is a marked distinction to be 

 drawn between the original susceptibility of an animal to inocula- 

 tion with cancer and the resistance to growth of the tumor in the 

 same animal at a later period ; and (2) the correlated fact that the 

 same tumor appears to develop increasing virulence as it is 

 brought through section and transplantation from the body of 

 one animal to another. 



As illustrating the first point, it is noted that a pregnant mouse 

 is very insusceptible to inoculation with a tumor transplanted 

 from another mouse, yet once the transplantation has been suc- 

 cessfully effected, the tumor grows with extraordinary vigor. 



A possible explanation of this would be that the pregnant ani- 

 mal has a system relatively well fortified against protein invasion, 



