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Show 454 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2 The enamel-organ of the 2nd functional incisor (morphological 4th) soon loses its connection with the gum, and the dental lamina from which it is developed does not again obtain any connection with the g u m in the incisor region. The enamel-organ of this tooth is too young to show any trace of the successional tooth, although the examination of other forms (M. giganteus and brachyurus) show that it is undoubtedly to be referred to the 1st dentition. The dental lamina, after giving rise to this tooth, is continued back as a slightly thickened lamina (club-shaped iu section) into that backward prolongation of the premaxilla which is situated internally to the maxilla (fig. 29) and in which the 3rd incisor of the adult is developed; in this region it becomes once more swollen (fig. 4, i°) to give rise to the latter tooth, which is, however, developed very late in life and therefore hardly shows at this early stage. Halfway between these two teeth and attached to the external (adamantine) face of the dental lamina is situated a very perfectly formed small, pointed tooth; this is the 3rd rudimentary tooth above referred to (fig. 1, i5). In section this tooth is seen to be well developed, possessing the most highly differentiated enamel-organ, which secretes a thick layer of enamel, internal to which is a well-developed mass of dentine surrounding the pulp-cavity. The entire tooth measured -4 m m . long by '25 m m . broad. Its general structure, however, suggests that it is fully formed, and its ultimate fate is probably that of the two anterior vestigial teeth, viz., it is reabsorbed. A very important question n o w arises, D o these two teeth belong to the 1st or the 2nd dentition, and also what is their numerical position amongst the incisors ? In attempting to solve this point it will be well to bear in mind Rose's suggestion, viz., that the 3rd functional incisor of the adult (ie) belongs to the second dentition and is not preceded by a deciduous tooth. Eig. 4 represents a wax model reconstructed from the sections by means of a camera lucida. A n examination of this shows that the enamel-organ of this small calcified tooth (i3) is situated external to the 2nd and 3rd functional incisors (i4 and i6) and connected with the dental lamina by a long neck; its external position certainly suggests that of a milk-tooth, but on the other hand it is no more nearly related to the 3rd than to the 2nd functional incisor, being situated just about halfway between the two, so that supposing it were a milk-tooth it would be nearly impossible to say to which tooth it is related. Further, if it belonged. to the 1st dentition its enamel-organ should be attached to the dental lamina nearer to the point of origin of the latter from the g u m ; whereas in the specimen under consideration these three teeth (i\ i5, i°) all arise from the dental lamina at the same level, which suggests that they belong to the same series and not to two dentitions distinct from one au other in point of time, for in that case the one belonging to the 1st dentition would arise from the dental lamina superficially to the corresponding tooth in the 2nd dentition. |
