A transplantable sarcoma of the fowl, known as Chicken Tumor XVIII, in our series, succeeds better in chickens of an alien breed (Plymouth Rock) than in those of the variety in which it originated (brown Leghorn). This is not due to gross physical differences in the two breeds but to some more subtle factor and one which perhaps acts by influencing the agent causing the tumor. It would seem that Chicken Tumor XVIII, as it occurred in nature, was an instance of a disease appearing spontaneously in an animal of relatively insusceptible variety.

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