ON THE ORIGIN OF GIRDLES AND PAIRED FINS

Abstract

ABSTRACT Embryological and morphological data suggest that the ancestors of the gnathostomes, the pro-gnathostomes, were swimming animals with a terminal mouth and transversely situated gill-arches carrying outwardly directed gills and separate gill-covers. The body had a well-developed myotomic musculature and was provided with longitudinal median and paired ventrolateral steering keels formed by folds of the body wall (potential fin-folds). The skin contained primary scales. The exoskeletal shoulder girdle arose by fusions of such scales to form a support for the body musculature and to fulfil other requirements at the transition between head and body. The paired fins arose in connection with the muscularization of the ventro-lateral crest. Stimulated probably by an interaction between the ectoderm and the mesoderm of the crest, the ventral end of each myotome produced two dorsal and two ventral radial muscles innervated by a secondary pterygial branch of the spinal nerve of the metamere. The radial mus...