1. A pure strain of fibroblasts has been isolated from Sarcoma 10 of the Crocker Foundation. After about 16 months of life in vitro, the malignancy of the strain is as great as that of the original tumor.

2. The strain has been compared with a strain of normal rat fibroblasts. The malignant cells are generally larger, coarser, and more refringent than normal cells. They possess all the morphological characteristics of fibroblasts. They do not show any abnormalities and never degenerate and die. They are to all appearances healthy cells. Their mode of locomotion is identical with that of normal fibroblasts. Their colonies are larger, but the architecture is similar.

3. The residual activity of both cell types, the duration of their life, and their rate of growth in a nutrient medium are almost identical.

4. The sarcomatous fibroblasts liquefy a rat plasma coagulum while normal fibroblasts do not. They turn phenol red golden yellow whereas, under the same conditions, normal cells turn it pinkish orange.

5. Sarcomatous and normal fibroblasts of the rat multiply to an unlimited degree in chick embryo juice. They live for only a short time in rat serum and chick serum. Calf liver digest will suffice for an unlimited proliferation of sarcoma fibroblasts, but fails to support the life of normal fibroblasts for very long.

6. The presence of bone marrow greatly increases the rate of growth of sarcomatous fibroblasts cultivated in rat serum, while it only slightly affects that of the normal cells. The unlimited growth of the sarcomatous tissue in animals to which it is transplanted may be attributed to the presence of macrophages, which are a normal constituent of the tumor, and possibly are a necessary factor of its growth in vivo.

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