1. chondrocytes at center of cartilage enlarge, chondrocytes die as matrix calcifies. 2. newly made osteoblasts cover the cartilage in a thin layer of bone. 3. blood vessels penetrate cartilage, osteoblasts for spongy bone, center is primary center of ossification. 4. bone enlarges, osteoclasts for marrow cavity, new cartilage forms. 5. blood vessels are in epiphysis, are filled with spongy bone, new ossification centers form at ends of bone.