The 'Oral Placode' in Certain Selachii and its Bearing on the Origin of the Vertebrate Mouth

Abstract

IN the course of a study of the early development of the sensory placodes of the head region in some cartilaginous fishes from the Red Sea, namely, Carcharinus melanopterus (Quoy and Gaimard), Rhynchobatus djiddensis (Bloch and Scheider) and Rhinobatus halaui (Muller and Henle), it was found that in the embryos of length 5–10 mm., the epidermis is generally thickened on either side of the roof of the mouth invagination and in the neighbourhood of the hypophysis (see drawing). This ectodermal thickening is of the same structure as the epibranchial placodes which, in these embryonic stages, are situated at the dorsal edges of the corresponding branchial slits. In view of its particular situation over the mouth, it should be regarded as homonymous with these epibranchial placodes and as belonging to them. The structure in question should then have the same relation with the oral slit as the epibranchial placodes have with the spiracle and the following gill-slits. The term 'oral placode' was suggested for this structure, which is thus interpreted as providing an interesting confirmation and adding further evidence to Dohrn's1 well-known and much discussed2 theory considering the mouth of vertebrates as derived from a pair of anterior coalesced gill-slits.