The development and the origin of the pectoral and pelvic girdles have been studied in bird embryos by homo- or heterotopic transplantations of somitic and somatopleural mesoderm.

Experiments consisted in implanting a piece of somitic or of somitic and somatopleural mesoderm obtained from a 2- to 2·5-day quail or chick embryo into a chick host of equal age.

Results showed that the scapula derives from the somitic mesoderm, while clavicle, coracoid, sternum and pelvic girdle originate from the somatopleural mesoderm.

The longitudinal span of the territories of the various skeletal elements, expressed as the number of the corresponding somites, was found to be as follows: scapula 15–24, clavicle 10–15, coracoid 15–17, sternum 12–26, and pelvic girdle 26–32.

It was also demonstrated that both somitic and somatopleural mesoderm are regionalized as early as 2 days of incubation, prior to somitic segmentation, with respect to their ability to give rise to the skeletal elements of the girdles.

These results were compared to those acquired previously concerning the other morphogenetic potentialities (vertebrae, ribs, limb musculature, dorsal plumage) of the para-axial and lateral mesoderm.

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