The developmental origin of the sternal component of the ribs and the conditions for the development of the rib cage have been elucidated in bird embryos. Experiments consisted in homo- or heterospecific transplantations of 2-day-old quail or chick embryo somitic mesoderm into chick hosts of the same age in ortho- or heterotopic position along the cephalo-caudal axis.
Results show that vertebral halves, ribs (not only their vertebral segment, but also their sternal component when they possess one), the trunk and intercostal muscles, as well as at least part of the scapula originate exclusively from the somitic material, while the sternum, ventral muscles and the other parts of the pectoral girdle are derived from the somatopleural mesoderm.
The development of the rib basket is subjected to following rules:
— only the somitic mesoderm of the prospective thoracic region (somites 19–26) is able togive rise to ribs.
— only the somitic mesoderm of the posterior thoracic region (somites 22–26) is able to develop ribs with a sternal component.
— the vertebral component of ribs can develop outside the thoracic region.
— contrariwise, the sternal component can form only in the vicinity of the sternal anlage, i.e within about three somites in front and rear of the normal limits of the thoracic region.
It is concluded that the somitic mesoderm is already regionalized at a stage slightly preceding its metamerization and that the somatopleural territory of the sternum plays a morphogenetic role in the development of the sternal component of ribs, although it does not make a cellular contribution to their construction.