A scanning electron microscopic study of the development of the shoulder, visceral arches, and the region ventral to the cervical somites of the chick embryo

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Abstract
Scanning electron microscopy (SEM) was used to investigate the development of the shoulder in the chick embryo. Initially the wing grows on an axis perpendicular to the dorso‐ventral and cranial‐caudal axes of the embryo, but soon begins to grow in a ventral and partially caudal direction. The change in axis of outgrowth occurs while the shoulder forms at the cranial proximal portion of the wing. Analysis of SEM observations, together with an analysis of serially sectioned embryos and photographs of live embryos in ovo has demonstrated that the shoulder continues to grow out on an axis perpendicular to the dorso‐ventral axis of the embryo, while the caudal and distal portions of the wing grow ventrally. The change in axis of outgrowth seems to be due to (1) the formation of the viscera under the wing, (2) the closing of the membrana reuniens to form a continuous sheet covering the viscera under the wing, (3) caudal movement of the duct of Cuvier and the cranial margin of the pleural coelom, and (4) ventral movement of the lateral body fold caudal to the wing. Although the visceral arches undergo major morphogenetic changes during this period, the visceral arches do not appear to have an influence on shoulder development.