Final answer:
The flat bones of the face are the last to complete intramembranous ossification, a process that forms bone directly from mesenchymal tissue without a cartilage model.
Step-by-step explanation:
The last bones to undergo intramembranous ossification are the flat bones of the face. This type of ossification is a process where bone develops directly from sheets of mesenchymal connective tissue. Cranial bones, most notably the flat bones of the face, as well as the clavicles (collarbones), are formed through this process. Intramembranous ossification begins in utero during fetal development and continues into adolescence, with the flat bones of the face reaching their adult size at the end of adolescent growth. Unlike endochondral ossification, which forms bone from cartilage models, intramembranous ossification results in the formation of bone without a prior cartilage stage.