At birth, a baby's skeleton has 270 bones and upon reaching adulthood will have 206. The bones of the skull and the bones of the limbs are both formed by ossification. Ossification is the process of bone tissue slowly developing from and replacing membranous tissue or cartilage. There are two distinct types of ossification: intermembranous and endochondral.
Early in fetal development, a baby's skull is comprised of membranes and near the end of the first trimester, these membranes begin to ossify; therefore, the process responsible for skull formation is intermembranous ossification. This process continues into adulthood until the bones of the skull are fused and fixed (except for the mandible).
For the bones of the limbs to develop, ossification occurs, but by an entirely different process. During endochondral ossification blood vessels enter the cartilage and ossification begins in the center of the cartilage. So the bone develops from the middle outward. The nature of this process is what allows for bone growth to continue in length and width until completed at various stages in adulthood.