This process, by which most bones form initially, begins with embryonic models of the skeletal elements made of hyaline cartilage
(1). Late in the first trimester, a bone collar develops beneath the perichondrium around the middle of the long bones' cartilage models, causing degeneration of the underlying cartilage
(2).This is followed by invasion of the degenerating cartilage by capillaries and osteoprogenitor cells from what is now the periosteum to produce a primary ossification center in the diaphysis
(3). Here osteoid is deposited by the new osteoblasts, undergoes calcification into woven bone, and is remodeled as compact bone.
(4) Around the time of birth secondary ossification centers begin to develop by a similar process in the epiphyses. During childhood the primary and secondary ossification centers gradually come to be separated only by the epiphyseal plate
(5) that provides for continued bone elongation. The two ossification centers do not merge until the epiphyseal plate disappears
(6) when full stature is achieved. Osteoblasts of the periosteum provide for growth in the bone's diameter.