There has been debate in the literature concerning whether the clavicle arises by intramembranous ossification, i.e. is a membrane bone, and whether secondary cartilage develops from its periosteal cells. A histological study of carefully staged embryos revealed that pre-clavicular mesenchyme undergoes condensation at H.H. stage 31-32, bone forms by H.H. stage 33 and that a transitory secondary cartilage appears late in H.H. stage 35, only to disappear by H.H. stage 36. Except for the transitory nature of the secondary cartilage, this histogenetic sequence is as seen in craniofacial membrane bones. Enzymic removal of the epithelium overlying clavicular mesenchyme from embryos of H.H. stages 26-34 and chorioallantoic grafting of the isolated mesenchyme, revealed an epithelial requirement for initiation of intramembranous ossification during H.H. stages 26-29, again similar to initiation of craniofacial osteogenesis. Secondary chondrogenesis was initiated neither in embryos paralysed with decamethonium iodide nor when clavicular mesenchyme (H.H. stages 29-33.5) was grafted to the chorioallantoic membranes of paralysed embryos, but did form in a small percentage (16-23%) of clavicles grafted to the membranes of mobile embryos. Failure of chondrogenesis in the former was attributed to a requirement for movement as a proximate chondrogenic stimulus and the low incidence of chondrogenesis in the latter to the stimulus provided by amniotic movements which persist in paralysed embryos. Secondary cartilage did form when clavicles were organ cultured, either submerged, or at the air-medium interface. This stands in contrast to craniofacial membrane bone such as the quadratojugal, which only forms secondary cartilage in vitro when cultured submerged. Growth of the clavicle was shown to increase 53-fold between 10 and 11 days of incubation, an increase which was diminished but not eliminated in paralysed embryos, and which correlated closely with the dramatic increase in embryonic movement which occurs between 10 and 11 days of incubation. Thus, the clavicle of the embryonic chick shares all of the features and epigenetic requirements of the craniofacial the embryonic chick shares all of the features and epigenetic requirements of of the craniofacial membrane bones, but is more dependent upon biomechanical factors for its growth.