330 THE DIGESTIVE SYSTEM 



a b 



FIG. 310. FROM A SECTION OF A HUMAN TOOTH WHICH HAD BEEN GROUND TO 



EXTREME THINNESS. 



a, dentin; b, granular layer of Tomes; c, cementum. Photo. X 140. 



DEVELOPMENT OP THE TEETH 



The teeth arise in part from the ectodermal epithelium of the oral 

 cavity and in part from the mesenchyma of the alveolar processes. In 

 the seventh week of fetal life there appears upon the surface of the 

 maxillary ridges a thickening of the epithelium which grows into the 

 subjacent mesenchyma in the form of a longitudinal plate or shelf, 

 the labiodental strand, whose position is indicated superficially by a 

 dental groove which indents the epithelial surface of the primitive gum. 

 The labiodental strand divides at its deeper border into a nearly vertical 

 continuation, the labial lamina, which subsequently becomes hollow to 

 form the labiogingival groove ; and a horizontal inwardly directed shelf, 

 the dental lamina, which forms the earliest" anlage of the enamel, the 

 enamel organ. 



At the beginning of the third month the dental shelf shows upon 

 its deep margin a series of shallow inverted cups, one for each of the 

 temporary teeth, produced by an up-pushing of a corresponding series 

 of cone-shaped areas of condensing mesenchyma at the site of each 

 tooth germ. Each mesenchymal thickening forms the anlage of a dental 

 papilla. That portion of the dental shelf which spreads out laterally 

 to cover the dental papilla of each tooth forms its enamel germ, from 

 which the dental enamel is eventually produced. Further development 

 of the enamel germ and dental papilla causes the former to surround the 

 papilla like a cap. Figures 311 and 312 show four stages in the early 

 .development of a tooth. 



