of the Tooth- Rudiments in Rodents. 311 



lower jaw between the incisor and the molars, in which 

 distinct vestiges of a dental fold can never be detected. 



Now as regards the incisors of the premaxilla in rabbit- 

 embryos of about 1 centim. in cephalic length {cf. fig. 13): con- 

 trary to the statements of Cliabry and Poucliet, the rudimentary 

 tooth in the premaxilla is also already completely developed 

 in embryos measuring 1 centim. in cephalic length, and about 

 2 centim. in all. As is shown in fig. 13, it is situated upon 

 the epithelium of the oral cavity close behind the mur plon- 

 gemit, which is still but little developed, in such a manner that 

 a special connective cord between its enamel-organ and the 

 oral epithelium cannot be distinguished. The enamel-organ 

 is longer than in the lower jaw ; the longitudinal axis appears 

 to be directed obliquely upwards and outwards. At the 

 upper end, displaced a little towards the front, we find the 

 very narrow aperture, surrounded by a swollen margin, which 

 leads into the interior of the pulp-cavity. The solid dentine 

 tooth is developed just as beautifully as in the lower jaw. 

 Owing to the absence of a special connective cord, the rounded 

 tip of the very long tooth is separated from the surface of the 

 epithelium of the oral cavity only by a few layers of cells. 

 In the upper jaw also the enamel-organ of the rudimentary 

 tooth is in direct continuity with that of the large incisor. 

 In the model it is easy to recognize the peculiar conditions 

 ■which are presented by the attachment of the first large 

 incisor to the epithelium of the oral cavity, — conditions which 

 can hardly be understood from tlie examination of sections, 

 and to which it is only with difficulty that justice can be done 

 in description also (in the figure, too, they can be seen but 

 imperfectly). For behind the spot at which tiie enamel-organ 

 of the large incisor is attached, the roof of the oral cavity 

 bulges out downwards in the form of a step. (A similar step 

 follows further back at the spot at which the enamel-organ of 

 i. 2 is attached.) Moreover, the connective cord of the 

 enamel-organ of the large incisor has a peculiar shape ; for it 

 consists of two plates, one placed transversely, the front of 

 which, like the greater portion of the enamel-organ itself of 

 {. 1, is fused with the enamel-organ of the rudimentary tooth, 

 and another in the shape of a sagittal plate, attached at right 

 angles to the inner edge of the former, and prolonged back- 

 wards as far as the enamel-organ of i. 2. In the case of the 

 latter tooth the same conditions are repeated. We can also 

 express the facts by saying that the connective cords, which 

 unite the enamel-organs of i. 1 and i. 2 with the epithelium 

 of the oral cavity, are hollowed out from behind and some- 

 what from inside. The connective cord of the first incisor is 



