1. An improved method of organ culture, employing embryonic chick tibiae, demonstrated consistent patterns of osteogenesis. With all other factors constant, the amount of bone and collagen formed or destroyed could be controlled solely by altering the amount of oxygen available to the explants from the gaseous phase of the culture.

2. Maximum osteogenesis was observed with an oxygen concentration of 35 per cent. Collagen-fiber formation and osteogenesis were suppressed by lowering the oxygen concentration to 5 per cent and, to a lesser degree, by raising it to 95 per cent. High oxygen concentrations also appeared to cause osteoclasia.

3. Cartilage matrix and cells near the cut ends of explants also demonstrated significant alterations under various oxygen concentrations. High oxygen concentrations (95 per cent) caused extensive chondroclasia. Intermediate oxygen concentrations (20 to 65 per cent) produced numerous large collagen fibers in and about the lacunae of degenerating chondrocytes, while other chondrocytes, undergoing alterations of morphology, came to resemble osteoblasts. Metachromasia of cartilage matrix was lost. Low (5 per cent) and high (95 per cent) oxygen concentrations did not produce similar collagen fiber production, alterations of chondrocyte morphology, or loss of matrix metachromasia.