As the bone collar thickens, the forms, and secondary ossification centers appear in the .

Medullary cavity, epiphyses

Explanation:

During the thickening of the bone collar, the penetration of Osteoblasts into the decomposing cartilage occurs thereby replacing it with spongy bone giving rise to a primary ossification center.

Osteoclasts then brings abiut the nreak down of the spongy bone to open up the medullary cavity. The cartilage found in the epiphyses then continue to grow increasing the bone's length.

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