Anatomy and embryology of the Wing Musculature of the domestic fowl (gallus)

Abstract

The anatomy of the musculature of the wing and pectoral girdle of the domestic fowl is redescribed. The embryonic development of the musculature has been studied, and the homologies of several muscles of the avian and reptilian shoulder regions have been elucidated. As in other tetrapods which have been studied, the musculature of the fore limb of the fowl first appears in the embryo as a pair of opposed masses of condensed mesenchyme, one ventral and the other dorsal, forming in the limb bud independently of the somites. These premuscle masses are lobed, and their lobation enables the identification of the primordia of various muscle groups at a very early stage of development, before the individual muscles have differentiated. The definitive muscles are formed by a process of splitting or cleavage of the premuscle masses. The cleavages take place in an orderly sequence, which closely resembles that which has been described in the lizard Lacerta, the turtle Chrysemys, and the opossum Didelphys. It is concluded that the resemblances in the lobate outlines of the premuscle masses and in the pattern of cleavages in these vertebrates of different taxonomic groups are a reflection of the phylogenetic history of the limb muscles. Thus the homologies of muscles in tetrapods widely separated from one another taxonomically can be determined more reliably by comparing their embryology than by comparing adults in which the relationships of the muscles are obscured by adaptive changes. Myological nomenclature is somewhat confused, owing to the difficulty of determining homologies by studies confined to adult animals. A revision of the names of avian wing muscles has therefore been carried out with the aim of stabilizing the nomenclature for birds; in addition, some preliminary steps towards a uniform nomenclature generally applicable to the muscles of all tetrapods have been essayed, so far as comparative embryological data permit conclusions to be drawn.