1. A method is described for testing the growth-promoting properties of various substances on tissues in vitro.

2. When growth of cells (chick fibroblasts) has ceased in a medium of plasma and Tyrode solution, it may be restarted again by the addition of embryo extract.

3. There is a latent period of from 2 to 3 hours before a greatly increased migration of cells occurs.

4. There is a latent period of from 10 to 12 hours before cell division starts again. After that time the percentage of cells dividing increases for the next 12 hours and then declines.

5. For low concentrations of extract the growth is proportional to the concentration.

6. Cell division of neither heart fibroblasts nor of periosteal fibroblasts is inhibited by concentrations of extract higher than 15 per cent in Tyrode solution, but cultures of periosteal fibroblasts in such concentrations cause a "liquefaction" of the plasma medium. With heart fibroblasts, "liquefaction" of the medium does not readily occur, within the duration of these experiments, until the concentration of extract reaches 60 per cent.

7. The growth of two cultures in embryo extract, and embryo extract and serum respectively, has been followed for 10 consecutive days. The rate of cell division in the medium containing serum was less rapid. Cell degeneration commenced in both cultures after about 6 days, so that after about 8 or 10 days there was no further increase in the number of cells on the field. The addition of fresh fluid medium to the cultures was clearly reflected in corresponding increases in the values of the mitotic indices.

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