In an earlier paper (21), reasons were given for the desirability of having strains of human malignant cells comparable to the cultures of transplantable animal tumors. Following a discussion of the technic and the selection of suitable specimens, three tumors were briefly described from which pure strains of cells were isolated and kept growing for from three to nine months. Detailed description of the strains and demonstration of the fact that they constituted the specific elements of the

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... rs were reserved for the present article. DESCRIPTION OF THE CULTURES In all of the cultures three main types of cells were encountered: (1) connective-tissue cells; (2) cells of the reticulo-endothelial system; ( 3 ) those which were considered to be the specific malignant cells. (1) Connective-tissue cells of the type usually termed " fibroblasts " were the first to appear unless liquefaction of the fibrin clot by other cell types prevented their growth. The fibroblasts, often showing evidence of mitotic division, were rather large spindle or somewhat stellate cells with elongated nuclei and long, stiff, tapering processes. Their characteristics need not be discussed here. Strains of human fibroblasts of various origin have been carried in several laboratories, including our own ( Fig. 1A and 2A ). Their properties have been carefully studied by Chlopin ( 3 ) in cultures of human embryonic mesenchyma. They usually overgrow any other type of cell present, but in our cultures they disappeared completely, usually in less than two weeks. (2) The reticulo-endothelial system was represented by the " macrophages." These appeared as highly motile cells of ameboid shape containing large numbers of fat droplets and ingested material, and were numerous in early cultures (Fig. 2C, 7A ). In the case of the two intracranial tumors these cells were very much like the macrophages which are a regular constituent of cultures of embryonic brain. It has been well established that there they represent the " third element," Hortega's microglia, and correspond to the histiocytes of other organs. The subject was reviewed recently by v. lThis work was made possible by a grant from the Ella Sachs Plotz Foundation for the