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Show 46 MR. M. F. WOODWARD ON THE [Jan. 5, IV. The Milk-Dentition of the Rabbit (Lepus cuniculus). Although the Rabbit is so universally studied in our own laboratories and its anatomy is described in detail in so many of our practical hand-books, not one of the latter rightly describes its milk-dentition, in spite of the fact that all the details concerning it have been long ago recorded. If one examines the jaws of a Rabbit from 2-3 weeks old (Plate II. fig. 3), one finds on each side of the upper jaw 6 incisors arranged in two linear series, 3 on each side of the middle line ; the anterior tooth of each set is known to be the great permanent front incisor, while the others have been variously interpreted. F. Cuvier (3), in the first place, described them as representing the 2nd and 3rd upper incisors ; his statement has been copied by several authors (9, 15, 29), who have thus ascribed to the Rabbit at birth 3 incisors, stating that the outer one is soon lost. This determination of Cuvier's was refuted by Owen (23) as long ago as 1868 and later by Krause (17). These two observers show that the middle tooth of each series (fig. 3, di2) is in reality the 2nd milk-incisor; it is a functional tootb for the first three weeks of the animal's life, after which time it is shed, being pushed out by its successor (fig. 3, pi2). The deciduous tooth in the specimen figured is small and wedge-shaped, its crown being much worn, while its successor presents a conical unworn extremity, having only just cut the gum. This latter tooth is the one described by Cuvier as being the 3rd incisor and by others as being early lost (9, 15, 29). The probable reason that the 2nd milk-incisor and its successor are present for some time side by side, after the latter has cut the gum, may be implied in the fact that the deciduous tooth is as it were wedged in between the great anterior incisor and its own successor and is rather worn away by attrition than shed. The study of the development of these teeth shows at once that these two (fig. 3, di2, pi2) are formed from a common enamel organ, and that they possess the relations of a typical tooth of the 1st to its successor in the 2nd dentition. The fact that the permanent tooth cuts the gum posteriorly to the milk-tooth, instead of developing underneath the latter and gradually pushing .it out, goes for nothing, when we consider that the permanent tooth is typically developed on the inner side of its milk predecessor and not below it. W e see from the above that there are only 2 incisors on each side of the adult upper jaw, and no examination of even the youngest foetus in which the teeth are appearing s,hows us anv trace of a 3rd one. The deciduous premolars of the upper series are 3 in number and, as may be seen, they persist until the animal is between 3 and 4 weeks old (not, as stated by Marshall and Hurst (22), beino- shed before birth). These teeth have been long known and are figured by Owen (23); the principal point of interest about them is their possession of true fangs and their replacement by more specialized teeth which grow from persistent pulps. |