Bone forms by one of two mechanisms: en- dochondral or intramembranous ossifica- tion. Endochondral ossification is the most common form, producing the long bones; “long” refers to the longitudinal orientation of the bone. (Even the distal phalanx of the fifth toe is a long bone.) The flat bones, such as the skull and clavicle, form via intramem- branous ossification. The key distinction be- tween the two processes is that endochon- dral ossification employs a cartilage template whereas intramembranous ossification does not.

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