Growth of cartilage model Mesenchyme - differentiates into osteoprogenitors, osteoblasts . . . Nutrient artery stimulates bone formation in periosteal region Osteoblasts form spongy bone . . . Spongy bone grows outward . . . Formation of primary ossification center spongy bone replaces cartilage interior Periosteum developed . . . . Osteoclasts form marrow cavity . . . .
Bone grows outward in two directions . . . .