1. Spontaneous mammary gland tumors of inbred mice transplanted to the anterior chamber of eyes of alien strains stimulated growth of adjacent transplants of newborn lung if the tumors were autonomous.

  2. Thymus and spleen from newborn mice were similarly stimulated, newborn kidney moderately, and newborn cerebral cortex only slightly. Newborn liver gave no growth reaction of its specific cells.

  3. Embryonic, newborn, and adult lung were similarly stimulated if transplanted simultaneously with tumor. If transplanted 1 week before the tumor, only immature tissues reacted; if transplanted 1 month before, no reaction occurred in immature or mature tissue.

  4. Stimulation of growth of newborn lung occurred when the tumor and normal tissue were not in direct contact in the same anterior chamber.

  5. Freezing or separation into cells and cell clumps destroyed the stimulatory capacity of the tumor.

  6. It is suggested that the growth of normal tissues is due to the stimulation of their supporting elements, this reaction being of the same nature as that by which the tumor evokes a stromal and vascular response from host tissues.

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