of the Tooth- Rudiments in Rodents. 315 



septum and tlie hard palate), a characteristic arrangement of 

 the connective-tissue elements is still evident ; by this, how- 

 ever, it is in no way intended to im])ly that the epithelial 

 fold is not already present before this stage when the palatal 

 cleft is open, and subsequently when the palatal suture has 

 disappeared. My series furnish me with sufficient proofs of 

 both cases. I merely wished to characterize the stage at 

 which the fold appeared to me to be most developed. The 

 epithelial fold at its anterior commencement is very shallow 

 and inconspicuous, as it is also reproduced in model 1, and is 

 connected with the enamel-organ of i. 2. In the region of 

 Stenson's canal it is sometimes so inconsiderable that it is 

 with difficulty detected. Behind this it speedily increases in 

 height, and attains its greatest elevation at the transition into 

 the rudiments of the molars. In tlie sections it appears as an 

 epithelial cord, which proceeds from the epithelium of the 

 oral cavity, inclined somewhat obliquely inwards, and is 

 buried in the mesoderm. In the neighbourhood of the oral 

 epithelium it appears narrow, having a breadth of only two 

 very low rows of cells; the sunken end is thickened into a 

 button-shape, owing partly to the increased height of the 

 peripheral cells and partly to the intercalation of several cells 

 in its interior. The form and size naturally vary very con- 

 siderably according to the direction of the section and other 

 accidental circumstances. Fig. 4 gives a representation of 

 the epithelial fold in a series in which it appears particularly 

 broad. In the other series it looks decidedly more slender 

 and longer. In the section figured (tig. 4) the length of the 

 epithelial depression amounts to about 100 /x, the breadth of 

 the thick end to about 60 (jl, and that of the narrowed neck to 

 30 ^l. In the series from which the model shown in fig. 13 

 is constructed, the height of the epithelial fold at about the 

 middle of its length amounts to nearly 120 //., the breadth at 

 the thickened end only to something over 20 yu-, and at the 

 narrowed neck only to about lO //.. 



Even in an embryo measuring 11 millim. in cephalic lengtii 

 and 34 millim. in all, but much more distinctly and nmch 

 more extensively in an older one of 17 millim. and 44 millim. 

 respectively, the dental fold in the diastema exhibits a remark- 

 able modification. At first only for a limited space, but in 

 the older embryo almost throughout its entire length, the neck 

 of the epithelial fold appears broken through by the mesoderm. 

 The appearances in this connexion are precisely similar to 

 those seen in other places, where a strand of epithelium is 

 displaced by the adjacent connective tissue. The breach 

 sometimes atfects only the portion beneath the button-shaped 



