On Certain Features of the Orbito-Ethmoidal Region in the Cyclostomata, Plagiostomi and Teleostomi.

IN the three above-mentioned groups of fishes the nasal capsules and nasal apertures, and the ectodermal attachment of the hypophysial pit, or cord, differ markedly in their relations to the snout and to the trabeculae, and it is the purpose of the present paper to attempt an explanation. It will be assumed throughout the discussion, that the trabecular and polar cartilages are, respectively, the pharyngeal elements of the premandibular and mandibular arches, and that the hypophysial pit is a median space left between two curved ectodermal surfaces that have been rolled, hinge-like, one upon the other (Allis, 1923, 1924 b). One of these ectodermal surfaces lies external to the ventrally and antero-ventrally presented surfaces of the embryonic brain, and the other to the anterior end of the alimentary canal, and they may be called, respectively, the frontal and visceral arch surfaces. The conditions in the Selachii will be first considered.