The appendicular skeleton consists of the pectoral and pelvic girdles
and bones of the upper and lower limbs. The bones of the appendicular skeleton develop from mesenchyme derived from the somatic mesoderm and not from the somites. Mesenchymal differentiation occurs in the upper limbs first and proceeds in a proximodistal direction in each limb. Mesenchymal models are replaced by cartilaginous models during the 6th week of development. Primary centers of ossification begin to appear in the 8th week of development and are seen in almost all bones of the extremities by the 12th week. All secondary centers of ossification (except that of the lower end of femur) appear after birth. Fusion of the primary and secondary centers occurs mostly in the late adolescence. One exception to the general pattern of development of the appendicular bones is the clavicle. This bone initially develops by intramembranous method of osteogenesis but later develops growth cartilages at both ends and the adult bone is mostly endo-chondral in development.
Development of Joints
Development of the joints (articulations) starts in the 6th week of development and advances quickly, so that, structurally, the developing joints come to resemble the adult joints by the end of the 8th week. The joints develop from mesenchyme which becomes condensed between two developing bones and is called interzonal mesenchyme.