Final answer:
Bone formation during fetal development occurs through intramembranous ossification in connective tissue membranes and endochondral ossification in cartilage, responsible for flat and long bones respectively.
Step-by-step explanation:
During fetal development, bone forms in two primary patterns: intramembranous ossification, which takes place in connective tissue membranes, and endochondral ossification, which takes place in cartilage. Intramembranous ossification is responsible for the development of flat bones such as those of the skull, the mandible, and the clavicles. This process begins when mesenchymal cells within the fibrous membranes form a template of the future bone and then differentiate into osteoblasts, which produce the bone matrix and lead to the formation of compact and spongy bone directly from the mesenchymal tissue.
On the other hand, endochondral ossification is the process by which the cartilage 'model' of the bone is replaced by bone tissue. It occurs in most of the body's long bones, such as the femur and humerus, and involves the gradual replacement of cartilage by bone tissue, as the ossification centers form and expand.