The results in these experiments seem to justify us in concluding that the growth of mouse tumors in rats is very definitely promoted by mouse-rat parabiosis, the percentage of " takes" being markedly increased, the rate of growth accelerated, and the duration of active growth extended to at least seventeen days.

These findings are in accord with Ehrlich's hypothesis, although, on account of the complexity of the factors involved in such experiments, other explanations may be advanced.

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