Endochondral ossification is best exemplified in the long bones. 1) Mesenchymal cells aggregate and differentiate into chondroblasts which produce a model of hyaline cartilage that precedes each bone. At about 8 weeks into development cells in the mid-region begin to differentiate into osteoblasts which produce a thin layer of bone around the cartilage model. In long bones a bone collar develops around what will become the shaft (diaphysis).