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A strain of fibroblasts, obtained from the heart of a chick embryo on January 17, 1912, has completed the 10th year of its life in vitro. On January 17, 1922~ our incubators contained about 60 cultures which represented the 1860th generation of the connective tissue cells. The purpose of this paper is to describe the actual condition of the strain, to discuss its meaning, and to summarize the findings. 1. Present Condition of tke Strain.--The growth of the tissue fragments is as rapid today as during the past years. Each fragment generally doubles its volume in 48 hours. The cultures (Fig. 1) have not modified their appearance. 1-4 They are composed of typical fibroblasts which grow in a dense tissue (Fig. 2) at the periphery of which individual cells may be observed and photographed (Figs. 3 and 4). Many cells divide mitotically (Figs. 5 to 8). 2. Unlimited Proliferation and Potential Immortality ofFibroblasts. N The fact that fibroblasts have been kept in active condition for 10 years has an important significance. I t demonstrates that tissue cells living in vitro transform the foodstuffs of their medium into protoplasm, and that their mass increases effectively. Some experimenters believed that ceils cultivated outside of the organism did not use the substances contained in the medium, 5 and that the mass of the tissue did not increase. This opinion applies to tissues cultivated in Locke

A TEN YEAR OLD STRAIN OF FIBROBLASTS