An account of the enamel cord in setonix brachyurus (marsupialia) and on the presence of an enamel knot in trichosurus vulpecula

Abstract

The disposition of the enamel cord was studied in a dated series of pouchyoung of Setonix sectioned transversely and stained with Masson's trichrome. In the upper jaw the enamel cord was associated with the incisal edges of the three functional incisors, the crest of the ridge of the first functional premolar and the paracone of the deciduous premolar. In the lower jaw the enamel cord was associated with the crest of the ridge of the first functional premolar, the protoconid of the deciduous premolar, the protoconid, metaconid and crest of the protolophid of the first molar as the cord passed transversely across the surface of the crown, and the metaconid of the second molar. Two exceptions to the normal disposition of the enamel cord, which generally runs antero-posteriorly in relation to the primary ridge, were observed in the lower jaw. In the first molar the enamel cord passed transversely across the crown in relation to the protolophid, and in the second molar the cord was associated with the anterolingual metaconid. The antero-posterior dimension of the enamel cord in certain enamel organs, especially the upper second incisor, was such that the structure should more correctly be called an enamel septum. It would seem that the enamel septum has been derived from the enamel cord. The alternative explanation is that the enamel cord and the enamel septum are entirely different structures. In the upper second incisor an enamel septum was present 22 days after the appearance of the enamel cord. The significance of the enamel cord is discussed. An enamel cord was not encountered in every tooth germ in Setonix and indicates either that the cord is sometimes so transitory as to pass unobserved or that the cord is not essential during tooth ontogeny. An enamel knot was observed in the upper second molar of Trichosurus.