1. Blood vessels penetrate the perichondrium in center of diaphysis and stimulate osteoprogenitor cells of internal layer of perichondrium to enlarge and become osteoblasts 2. Osteoblasts secrete enzyme, encouraging Ca salts to secrete matrix 3. Osteoblasts form thin layer of calcified bone (bony collar) 4. Bony collar and newly calcified matrix restrict nutrient flow to chondrocytes, so they die opening cavities 5. In cavities, osteoblasts form new spongy bone tissue 6. Osteoclasts digest out more cavity, forming marrow cavity for diaphysis