306 



EMBRYOLOGY. 



of the dermis, which comes from the embryonic mesenchyme, small 

 papillae composed of numerous cells (fig. 169 zp), and these penetrate 

 into the thick overlying epidermis. The latter also undergoes 

 changes on its part, which are directed toward the formation of the 

 tooth ; for those of its cells which immediately cover the papilla 

 grow out into very long cylindrical forms, and produce an organ the 

 function of which is to secrete enamel, the so-called enamel-membrane 

 (fig. 169 sm). By means of further growth the whole fundament 



sm tp 



Fig 169.- Very young fundament of a dermal tooth (a placoid scale) of a Selachian embryo. 

 zp, Dental papilla ; sm, enamel-membrane. 



next assumes a form which corresponds to the future hard structure 

 (fig. 170). 



Then the process of ossification begins. There is secreted by the 

 most superficial cells of the papilla (0), the odontoUasl-layer (mem- 

 brana eboris), a thin layer of dentine (zb), which rests upon the 

 papilla like a cap. At the same time the enamel-membrane (sm) 

 begins its secretive activity, and coats the outer surface of the 

 dentinal cap (zb) with a firm, thin layer of enamel (s). The body of 

 the tooth is developed and becomes ever firmer and larger by the 

 subsequent continual deposition of new layers on the first-formed 

 ones, on the dentinal cap new dentine from within through the 

 activity of the odontoblasts ; on the coating of enamel new layers of 

 enamel from without, through the action of the enamel-membrane. 

 Thus the structure projects more and- more above the level of the 



