Final answer:
The bones mentioned are formed by intramembranous ossification, where undifferentiated connective tissue directly develops into bone.
Step-by-step explanation:
The flat bones of the skull, most of the facial bones, mandible, and the medial part of the clavicle are formed through a process known as intramembranous ossification. This process involves the direct development of compact and spongy bone from sheets of mesenchymal (undifferentiated) connective tissue. It begins in utero during fetal development and continues into adolescence, helping the skull and shoulders to deform during birth and allowing for the continued growth and expansion of the skull as the brain enlarges.