312 



EMBRYOLOGY. 



eighth months ; then in the course of a few weeks those of the upper 

 jaw follow. The outer [lateral] incisors appear during the period 

 between the seventh and ninth months, those of the lower jaw, again, 

 somewhat earlier than those of the upper jaw. The front molars 

 usually appear at the beginning of the second year, those of the lower 

 jaw first ; then the gap thus left in the two rows of teeth is filled by 

 the eruption of the canine or eye-teeth in the middle of the second 



year. Finally, the eruption of the 

 back molars, which may be delayed 

 into the third year, takes place. 



The fundaments of the reserve teeth 

 make their appearance at the side of 

 those of the milk-teeth at an extra- 

 ordinarily early period. They also 

 take their origin from the epithelial 

 ridge. As was previously (fig. 172 

 A and B) stated, the ridge extends 

 still deeper (zl 1 ) into the underlying 

 tissue from the place where the 

 enamel-organs of the milk-teeth 

 have been differentiated from it 

 and where they remain united to 

 it by means of an epithelial cord, 

 the neck. Here in a short time 

 there again appear near the edge of 

 the ridge (fig. 174 sm 2 , zp?) flask- 

 shaped epithelial growths and dental 

 papillae, which lie on the inner 

 [median] side of the dental sacs of 

 the milk-teeth. In addition there 

 are developed at the ends of the 



epithelial ridges, in both the right and left halves of the jaw, the 

 enamel-organs of the posterior grinders (the molar teeth of the 

 permanent set), which are not subject to replacement, but are 

 formed once for all. The ossification of the second generation of 

 teeth begins a little time before birth with the first large molars, 

 and is followed in the first and second years after birth by that of 

 the incisors, canines, etc. As a result in the sixth year there are in 

 both jaws forty-eight ossified teeth, twenty milk-teeth and twenty- 

 eight permanent crowns, as well as four fundaments of wisdom 

 teeth, which are still cellular. 



Tig. 174. Diagrammatic section to show 

 the development of the milk-teeth and 

 permanent teeth in Mammals. Third 

 stage in the series of which figs. 172 

 A and B are the first and second. 



zf, Dental furrow ; zl, dental ridge ; k, 

 bony alveolus of the tooth ; h, neck, 

 by means of which the enamel-organ 

 of the milk-tooth is connected with the 

 dental ridge, zl zp, dental papilla ; 

 zp a , dental papilla of the permanent 

 tooth ; zb, dentine ; s, enamel ; am, 

 enamel-membrane ; ai 2 , enamel-mem- 

 brane of the permanent tooth; sp, 

 enamel-pulp ; se, outer epithelium of 

 the enamel-organ ; zs, dental sac. 



