Experiments designed to ascertain the reason for the cessation of growth of heart fibroblasts when they are cultivated in a plasma coagulum with embryo juice as nutrient fluid have shown that it is due, first, to the gradual removal of serum from the coagulum, and second, to an insufficient supply of embryo juice. In a medium containing embryo extract at 66 per cent concentration and serum at 8 per cent concentration, growth continued until the entire coagulum in a 3½ cm. flask was covered with tissue. The serum is needed to furnish additional nutriment, and also to prevent digestion of the coagulum.

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