OCR Text |
Show 5d6 MR. M. E, WOOUWARD ON [Mayo, probably would not cut the gum, although at times it may do so (Thomas, 23). There is a complete replacement of the lower incisors, but the successors develop at very different times, pi. i and pTTB maturmcr long before pT~2. Consequently in m y foetus no sign of the future pi. 2 was visible, although the enamel-organs of pTfl and pTTwere just recognizable. As a matter of fact pi. 2 is, I believe, the last permanent ante-molar tooth to cut the gum, appearing soon after the eruption of ppm. 3. The Canines. The milk-canines possess single roots, and are but slightly larger than the incisors (fig. 13) ; whereas the permanent canines are very large teeth, with pointed crowns, and each provided with two fangs. The Premolars. p^ are said to be present in one dentition only. If this be the case these teeth would appear to belong to the milk-dentition, for there is present on the lingual sides of their enamel-organs well-marked continuations of the dental lamina, precisely similar in their relationship to that seen by the side of dc., which gives rise to the enamel-organ of pc., only the inner ends of the former are perhaps a trifle less swollen. It is just possible that these structures may give rise to the enamel-organs of successors at a late period, for the first premolar of the adult skull appears to be a slightly stouter tooth than that of the young animal. The lingually situated dental lamina in both the upper and lower jaws gets smaller and more irregular behind pm. 1, but soon becomes definitely swollen, and forms the commencement of an enamel-organ (figs. 11 & 12, ppm.2); this, from its position and backward condition, is evidently that of a successional tooth, viz., ppm. 2. This identification is rendered more certain by finding on the labial side of this structure a small calcified tooth (dpm. 2), devoid of enamel, but possessing a much reduced enamel-organ'. In the case of the upper tooth this reduced enamel-organ is attached to the gum close to, but independent of, the swollen, lingually-situated dental lamina above referred to (ppm. 2). In the lower jaw, however, the enamel-organ of the vestigial tooth (tig. 12, dpm. 2) is apparently attached to the corresponding lingual swelling of the dental lamina, thus exhibiting the normal relationships of a milk and a replacing tooth. The deciduous 2nd premolar is then reduced and early lost, while its successor is somewhat precociously developed (cf. tig. 11, 1 In a preliminary note, read before the British Association, 1895 (27). I stated that there were traces of five premolars in Gymnura: this is not the case ; the error arose through a misinterpretation of a curious development of the pulp of this tooth, dpm. 2 (see Pl. X X I V . fig. 11, p), which was mistaken for a successor. |