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Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 453 mentary calcified tooth, which, although actually in front of the former, is nevertheless morphologically and in point of origin posterior to it. The 1st incisor of the adult is therefore the morphological 1st incisor, and in the younger of the specimens its enamel-organ only as yet is differentiated and appears as a great pear-shaped swelling of the dental lamina (enamel-ridge, " Zahn-leiste"). In an older one measuring 11-5 cm. there was present on the inner side of the enamel-organ a slight club-shaped down-growth, such as that described by Kiikenthal and others in Bidelphys as representing the permanent or replacing tooth (" Ersatzzahn " ) . The morphological 2nd incisor is a very minute calcified rudimentary (or rather vestigial) tooth measuring '25 m m . by *14 m m . ; its enamel-organ (fig. 2, i1) is only slightly developed and apparently, although possessing an enamel-epithelium, does not secrete any enamel; it shows a distinct but small downgrowth on its inner side (iJ) obviously representing the ventral continuation of the dental lamina and the undeveloped permanent tooth of Kiikenthal, Rose, and Leche, and according to the interpretations of these observers this small calcified tooth must be a vestigial milk-tooth. The tooth itself consists of dentine and possesses an irregular contour and a variable pulp-cavity ; the relation of the odontoblasts suggest that they are concerned more in absorption than in formation of tooth-substance. The dental lamina, after giving rise to these two teeth, disappears for a few sections, but soon reappears and swells out again to form the 2nd functional tooth, which I at first took to be the 3rd incisor. However, on investigating carefully the relation of the extremely reduced 2nd rudimentary tooth (fig. 3, i3), it was obvious that the enamel-organ of the latter was connected with the g u m slightly anterior and external to that of the former, although, owing to the large size and swollen nature of the enamel-organ of the 2nd functional incisor, part of this tooth appears in sections anterior to the rudimentary one; but the only important point to be ascertained in determining the morphological relation of teeth is not which tooth is anterior to the other, but which enamel-organ arises in front of the other from the dental lamina. This 2nd rudimentary tooth is then the morphological 3rd incisor, the 2nd functional one being the 4th. The 3rd incisor is, like the 2nd, a vestigial structure only present in the embryo and although possessed of an enamel-organ it has no enamel; its body, consisting of dentine, is very irregular and is obviously being reabsorbed. It is smaller and still more vestigial even than the 2nd incisor and shows no trace of any downgrowth from its enamel-organ, but from its resemblance to the 2nd incisor I conclude that it also belongs to the 1st dentition. The remaining incisors are very interesting and present such striking relationships that they call for a very careful examination, especially in the light of Rose's statement regarding the 3rd adult incisor. PROC. ZOOL. Soc-1893, No. XXXI. 31 |
