Final answer:
The human fetus develops bones mainly through intramembranous ossification, which forms flat bones like the skull, and endochondral ossification, which creates the template for long bones. Both processes begin with a cartilage template and continue to develop into childhood with growth plates allowing for further lengthening of long bones.
Step-by-step explanation:
The human fetus develops bones through two main processes: intramembranous ossification and endochondral ossification. In intramembranous ossification, embryonic mesenchymal tissue directly transforms into bone, primarily forming the bones of the skull, which continues to grow post-birth as the fontanelles transform into sutures. In contrast, endochondral ossification involves a cartilage blueprint, where mesenchymal cells first differentiate into cartilage cells that form a hyaline cartilage template of the bone to be replaced by bone tissue over time. This method is responsible for forming the long bones of the body as well as the base and facial bones of the skull.
Ossification in the human fetus begins at the primary ossification centers and progresses to secondary ossification centers in the ends of the bones. Cartilage remains at the growth plates of long bones to allow for lengthening throughout childhood. The flat bones of the face, which ossify via intramembranous ossification, are the last to reach maturity at the end of adolescence.