| OCR Text |
Show 562 MR. M. E. WOODWARD ON [May 5, lamina; growing out from this labially is in both stages a small bud-like mass of cells (tig. 5, do7), in one case swollen at its free end, close to which is a small irregular calcification similar hi appearance to the often vestigial dc.5 but smaller in size: this is obviously the vanishing milk-tooth dc", the canine of the adult belonging to the replacing series. The Premolars. In the upper jaw there are three premolars, which Leche believes to be the 2nd, 3rd, and 4th respectively. It is true there is a slight gap between the canine and the anterior of these premolars, but there is a more conspicuous one between the two posterior teeth, and in this latter gap the dental lamina has a slight tendency to become specialized and enlarged (Plate XXIII. tig. 6 6 ) ; but it is perhaps hardly large enough to be regarded as a tooth anlage, and, further, we know that when suppression affects the premolar series in the Placentalia, the 1st tooth of that series generally suffers suppression earliest. In the lower jaw there are only two premolars, but between them is a long stretch of dental lamina, which exhibits a most distinct development from its adamantine face; this growth is slightly indented by a specialized mass of mesoblast (fig. 7 b), the whole structure presenting a great similarity to a developing tooth : this, I believe, represents the last trace of a suppressed tooth, corresponding with the middle premolar above. The two posterior upper, and the posterior lower, premolars are present as functional teeth in both dentitions ; but the middle upper one is very variable, and is often wanting in some adult skulls, while in E. micropus and E. pictus it is very minute. The anterior premolar, above and below, in both my stages, exhibits a large enamel-organ, bell-shaped in the older specimen; attached to the dental lamina forming the necks of these structures, on the labial side, is in each case a mass of epitheloid cells (figs. 8 and 8 a, dpm. 2) ; the free ends of these buds are swollen and flattened: closely applied to these is, in each case, an irregular calcification, resembling the most reduced stage of dc.; these are evidently reduced milk-premolars, the anterior functional premolars being then, as Leche supposed, replacing teeth. With regard to the last premolar, its milk representative resembles a molar in form, thus differing markedly from its successor; a feature so characteristic of the 4th premolar of other Placentalia, that I think we may be quite safe in homologizing these two teeth with one another. If we examine the mutual relations of the 4th premolar and its successor during their development, we find that the replacing tooth, PP'"-4, originates almost entirely in front of its supposed milk predecessor from the dental lamina between dpm. 3 and dpm.4; the enamel-organ of ppm.4 being more conspicuous in the sections in front and in the anterior region of dpm. 4 than in its posterior |
