834 



EMBRYOLOGY 



of the dental bulb, the dentin is developed in successive layers. The 

 cement is developed by successive layers upon that portion of the den- 

 tin which forms the root of the tooth. As these processes go on, the 

 tooth projects more and more, the upper part of the wall of the follicle 

 gives way, and the tooth finally appears at the surface. 



The permanent teeth are developed beneath the follicles of the tem- 

 porary, or milk-teeth. The first appearance is a prolongation, or diver- 



Fig. 240. Temporary and permanent teeth (Sappey) . 



i, i, temporary central incisors ; 2, 2, temporary lateral incisors ; 3, 3, temporary canines ; 4, 4, tem- 

 porary anterior molars; 5, 5, temporary posterior molars; 6, 6, permanent central incisors; 7, 7, per- 

 manent lateral incisors; 8, 8, permanent canines; 9, 9, permanent first bicuspids; 10, 10, permanent 

 second bicuspids; n, u, first molars. 



ticulum, from the enamel-organ of the temporary tooth, which dips more 

 deeply into the mucous structure. This becomes the enamel-organ of 

 the permanent tooth ; and the successive stages of development of the 

 dental follicles and the dental pulp progress in the same way as in the 

 temporary teeth. As the permanent teeth increase in size, they gradu- 

 ally encroach on the roots of the temporary teeth. The roots of the 

 latter are absorbed, the permanent teeth advance more and more 

 toward the surface, and the crown of each temporary tooth finally is 

 pushed out. The number of the temporary teeth is twenty, and there 

 are thirty-two permanent teeth. Thus there are three permanent teeth 



