How bones form, how they grow longer, and how a broken bone heals.
The big idea: bone never appears out of nothing. It is always built on top of something else. In intramembranous ossification, bone is built directly in a membrane of mesenchyme. In endochondral ossification, bone replaces a cartilage model. Fracture healing reuses the endochondral plan: clot, then cartilage, then bone, then remodeling.