2. Maximum growth of the tissue, as measured by dry weight, with the formation of collagen protein, based on the estimation of hydroxyproline, was obtained in periods of up to 6 days.

3. Appreciable amounts of protein-bound hydroxyproline were formed during the first 24 hour growth period, but electron microscopy of portions of the same cultures failed to demonstrate the presence of any typical collagen fibrils.

4. The subsequent formation of many characteristic collagen fibrils was not associated with a significant rise in the mean hydroxyproline content of the tissue.

5. The cytoplasmic granules of the osteoblasts stained intensely with the P.A.S. technique when the collagen fibrils were being formed.

6. It is suggested that collagen-forming cells synthesise and secrete a hydroxyproline-rich precursor of protein or large peptide nature, which subsequently becomes directly transformed into typical collagen fibrils.