The cultivation in vitro of mouse tissues derived from normal organs from 86 spontaneous epithelial tumors of the mammary gland, and from 27 Ehrlich carcinomas, has been undertaken, together with a study of the properties of the various cell types.

1. The tissues liquefied fibrin from mouse and rat plasma more readily than fibrin from chicken plasma. Clots made of chicken plasma alone, if thoroughly washed, did not inhibit the migration of the cells. Normal and tumor tissues liquefied fibrin from the mouse, rat, and chicken more actively than Ehrlich carcinoma did.

2. Mouse epitheliums, both normal and malignant, showed greater activity than connective tissue cells from the same origin and were not overgrown by the latter.

3. Mouse epithelium was more active in rat serum than in mouse or chicken serum and in embryonic juice from chickens, mice, and rats. None of these fluids, however, supported cell proliferation indefinitely except in the case of Ehrlich carcinoma.

4. These results indicate that mouse tissues possess nutritional requirements which are different from those of fibroblasts and epithelial cells of other animals. Nutritive media that suffice for prolonged cultivation of the normal and malignant tissues of the rat and the fowl, and also of Ehrlich carcinoma, are not suitable for the cultivation of adult mouse epithelium derived from normal organs or from spontaneous mammary gland tumors.

5. Rat serum supported the life of spontaneous tumors for a limited period of time only, whereas it enabled the Ehrlich carcinoma to proliferate indefinitely. Normal organs and spontaneous tumors were not capable of invading normal tissues as Ehrlich carcinoma did.

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