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Show 560 MR. M. F. WOODWARD O N [May 5 The discovery by Leche (9) of the extremely variable nature of the upper deciduous canine (Plate X X I V . fig. 9, dc.) forms the kev to his conclusions, for this enabled him to perceive that the milk set were in part undergoing reduction, and to formulate the belief that the knob-like labial growth of the dental lamina which he found in connection with 1^3 was the last trace of the enamel-organ of the milk predecessor of that tooth: from this he concluded that the milk predecessors to the following teeth r--. 1 ^- had been r ° i. 2, c, pm. 1 entirely suppressed. The conclusions may seem very bold, but their correctness is proved beyond a doubt by the two stages which I have been fortunate enough to obtain. Taking as a starting-point the upper canine, I find that in m y younger stage the enamel-orgau of the permanent canine (pc.) is in a very backward condition, whereas labially a small tooth is developing (fig. 1, dc.; see also Leche, Taf. iv. figs. 41 -50) ; from the condition of this latter structure it is possible that it might develop into a small functional milk-tooth (fig. 9, dc.), and from its position and general relationship it is obviously the milk-canine. In the older stage we note (fig. 1 a, pc.) that the enamel-organ of the permanent canine is more developed, and that attached to the labial side of the neck of this structure, i. e. the dental lamina, is a slight outgrowth, indenting which is a small irregular calcification (dc.) ; this is in the position of the germ of the deciduous canine of the younger stage, and evidently represents that tooth in a more advanced condition, i. e. as regards calcification, but at the same time retrograded, for it is so small and irregular that it could not become a functional tooth and probably would not even cut the gum. Thus we see that the canine may vary from a functional tooth (Leche) to a minute irregular calcification of no physiological importance (cf. figs. 9 & 1 a, dc.). Considerable doubt has been expressed at one time or another concerning the exact homology of the first upper maxillary tooth of Erinaceus, its form in the permanent series being so unlike that of a typical canine, for the reason that it possesses indications of two fangs; moreover, it is apparently situated a considerable distance behind the premaxillo-maxillary suture. If, however, a young skull be examined (fig. 9), we find that the deciduous canine, when present as a conspicuous tooth, bears but a single fang and is situated close to the suture, as also is the developing permanent canine, the apparent change in position of the latter tooth being due to the forward extension of the maxilla, growing so as to embrace the premaxilla both labially and on its palatal border (fig. 9 a); thus the external premaxillo-maxillary suture in the adult is apparently situated far in front of the canine tooth. The true position of this tooth can be ascertained even in an adult skull if the palatal aspect of the latter be examined, then the canine is seen to be situated almost within the true suture and certainly not far behind it. |
