| OCR Text |
Show 1896.] MAMMALIAN DENTITION. 561 The Upper Incisors. 1.1 & i. 2 are present as functional teeth both in the milk and replacing dentition, but the latter (pi. 1 & pi. 2) develop late, and in my sections are only indicated by well-marked lingual growths of the dental lamina. Pi-2 is the most variable in the different species, and in the younger stage examined no signs of it are yet visible. 13 as a functional tooth is known only in the adult dentition. Leche refers this to the replacing series, because he finds a bud-shaped labial outgrowth of the dental lamina related to the enamel-organ of this tooth. In both m y specimens I find a calcified structure connected with this labial growth (fig. 2, di.3); this in the younger stage is a distinctly cup-shaped dentinal body, while in the older specimen (fig. 2 a) the condition is more like that figured by Leche (Taf. vii. fig. 52, Jd. 3), save there is a small calcification indenting his Jd. 3 from behind. A comparison of this labial calcification (di. 3) with the reduced dc. (fig. 1 a) shows that these two structures evidently belong to the same order, i. e. are reduced teeth of the milk series, the incisior being more vestigial. This confirms Leche's view that the adult 13 belongs to the replacing series, and is the true pi-3. The Lower Incisors. The first enlargement of the dental lamina in the lower jaw is situated in front of the enamel-organ of the first functional incisor; it is a very conspicuous structure in the younger stage, being slightly bell-shaped and possessed of a small labial outgrowth (Plate XXIII. fig. 3 ) ; this evidently corresponds with what Leche believes to be a remnant of the true Ul (see Taf. ii. figs. 13 & 14), which is here possibly represented both in the milk and permanent series. The second incisor, i. e. the anterior functional one, is a very large and highly differentiated tooth in both specimens, and exhibits a strong lingual growth of the dental lamina, which eventually forms the enamel-organ of the successor, this tooth 12 being well developed in both dentitions. The posterior functional incisor 173 is very backward in its development and variable, for it is larger in the younger of the two specimens examined. A long, narrow, cord-iike (in section) band of cells grows out from the neck of the enamel-organ of this tooth on its labial side (fig. 4, dI73), being sometimes swollen at its free end and slightly indented ; this evidently represents the last trace of an earlier dentition, and from a comparison with (1L3 in the older stage, one is justified in concluding that it represents the enamel-organof di.3 undergoing suppression. Between i. 3 and c. the dental lamina is very strongly developed, and suggests the possible presence of the last trace of one of the missing Marsupial incisors. The lower canine is represented by a bell-shaped enamel-organ attached to the buccal epithelium by a well-marked neck of dental |
