DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 323 



along the edges of the processes that bound the slit-like entrance to the mouth. 

 When the lip groove appears (Fig. 178), the epithelial thickening comes to lie 

 along the edge of the jaw, or in other words, along the edge of the gums. It 

 then grows into the mesenchymal tissue (mesoderm) of the jaw obliquely toward 

 the lingual surface to form the dental shelf. A little later the dental groove 

 appears on the edge of the jaw, along the line where the ingrowth of epithelium 

 took place. 



Epithelium of mouth cavity 



Inner 

 enamel cells 



Dental papilla -J-H 



Neck of 

 enamel organ 



Germ of 

 permanent tooth 



M Bone of 

 lower jaw 



FIG. 290. Section of developing tooth from a 3^ months human foetus. Szymonowicz. 



Note the portion of the original dental shelf connecting the developing tooth with the 



epithelium of the mouth cavity. 



The dental shelf is at first of uniform thickness, but in a short time five 

 enlargements appear in it in each upper and lower jaw, indicating the begin- 

 nings of the milk teeth. When the embryo reaches a length of 40 mm. (an age of 

 eleven to twelve weeks) the mesenchymal tissue on one side of these enlargements 

 (above and to the inner side in the upper jaw, below and to the inner side in the 

 lower jaw) becomes condensed and pushes its way into the epithelium. Each of 

 these mesenchymal ingrowths is a dental papilla. Thus at this stage the anlage 

 of each tooth is a mass of epithelium fitting cap-like over a mesenchymal papilla. 

 The epithelium is the forerunner of the enamel organ; the papilla is destined to 

 give rise to the dentine and pulp. The anlagen are connected with one another 



