Chimaera studies of the origin and formation of the pectoral musculature of the avian embryo.

Cervical, brachial, and thoracic somites and brachial somatopleure were transplanted from quail into 2-day chick embryos to determine their contribution to the formation of the pectoralis major muscle. The results show that the musculature of the pectoralis major and pectoralis minor is derived entirely from brachial somites (16-20) while the brachial somatopleure provides much of the connective tissue of these muscles. Migration of somitic cells into the somatopleure appears to begin before the somites are fully formed. The primordium of the pectoralis muscle forms at the base of the wing bud and extends ventro-caudally into the thoracic area to attach to the sternum.