THE MOUTH, ORAL OR BUCCAL CAVITY 



1215 



dermic layers, the ectoderm and mesoderm. From the former the enamel is developed; from 

 the latter the dentinal pulp, dentine, cementum, and peridental membrane. It is customary to 

 view the development of the permanent and temporary teeth as separate studies. 



The earliest evidence of tooth formation in the human embryo is observed about the sixth 

 week. The epithelium covering the embryonic jaws is seen to rise as a ridge along the summit 

 of each jaw. This ridge is the maxillary rampart of Kblliker and Waldeyer. A transverse 

 section through the jaw will show the elevation to be due to a linear and outlined activity of the 

 germinal epithelial layer; a corresponding epithelial growth is seen to sink as a band into the 

 mesodermal tissue beneath. This band is called the dental lamina or dental shelf. The local 



Epithelium of 



oral cavity 

 Basal layer 



Enamel pulp 

 Superficial cells 

 of enamel organ 



Epithelium of 

 ....^ ora i cavity 



:' ''-.. | .;..-,. Enamel pulp 



;';,'; '; . '''... ' 'f '.,' '::' Dentinal papilla 



FIG. 929. Anlage of the enamel germ con- 

 nected with the oral epithelium by the 

 enamel ledge. X 110. 



FIG. 930. First trace of the dentinal papilla. X 110. 



Epithelium of 



oral cavity 

 2K Basal layer 



Superficial cells of 

 enamel organ 



Enamel pulp- 

 Enamel cells 



papilla 



FIG. 931. Advanced stage with larger papilla and 

 differentiating enamel pulp. X 110. 



Epithelium of 



~~~ oral cavity 

 r Basal layer 



Part of enamel ledge 

 of a permanent tooth 



Enamel germ of 

 permanent tooth 



Enamel pulp 



Dentinal papilla 



Odontoblasts 



Enamel cells 



FIG. 932. Budding from the enamel ledge of 

 the anlage of the enamel germ, which later goes 

 to form the enamel of a permanent tooth; at the 

 periphery of the papilla the odontoblasts are be- 

 ginning to differentiate. X 40. 



FIGS. 929 to 932. Four stages in the development of a tooth in a sheep embryo (from the lower jaw). 



(From Bohm and Davidoff.) 



cell activity continues, and in its descent the band appears to meet with a resistance which causes 

 a flattening of its extremity into a continuous lamina. From the outer or labial surface of the 

 shelf epithelial buds are given off successively, ten in number, one for each temporary tooth; 

 the earliest (central incisors) appear about the seventh to the eighth week. 



The growth of each bud continues, and each expands into a flask-like form, the walls covered 

 by a layer of germinal cells, its interior composed of stellate cells. The bulb is now seen to flatten 

 upon its deep surface, as though it had met with an outlined resistance from the mesodermal 



