506 



MAMMALIA. 



germs are formed as buds from the outer (labial) side and near 

 the free end of this structure (Fig. 266, .4). The first formed of 

 these buds give rise to the milk teeth. As soon as the rudiments 

 of these are established the free edge (zl) of the tooth -band 

 grows deeper into the subepithelial mucosa and gives rise, on its 

 labial side as before, to a second series of buds : these are the 

 rudiments of the permanent teeth (Fig. 266, B, C). Inasmuch 

 as the deeper part of the tooth -band becomes inclined inwards 

 towards the middle line the first formed buds and the tooth- 

 germs developed from them lie on the labial (outer) side of the 

 buds formed later. Each of the epithelial buds formed from 

 the outer lamina of the tooth-band gives rise to the enamel organ 

 (se, sp, sm) of a tooth and ultimately loses its connection with 



c 



FIG. 266. A, B, C, Three stages in the development of a milk and permanent tooth of a 

 mammal in diagrammatic section (after O. Hertwig). h neck by which the enamel organ 

 of the milk tooth is connected with the tooth-band ; k bone of dental alveolus ; s enamel ; 

 se, sm inner and outer epithelium of enamel organ ; sm 2 inner epithelium of enamel organ 

 of permanent tooth ; sp enamel pulp ; zf dental furrow ; zl tooth-band ; zl' free deep end 

 of tooth-band, from which the permanent tooth will be developed ; zp dental papilla ; 

 zs dental sac ; zp 2 - dental papilla of the permanent tooth ; zb dentine. 



the tooth-band. The conversion of a bud into an enamel organ 

 is associated with the formation of a dentine-papilla or dentine- 

 germ (zp) which gives rise to the dentine of the tooth and the 

 cement of the root. The process is as follows : the bud becomes 

 swollen at its end, and then bell-shaped, the concavity of the 

 bell being directed inwards, i.e. away from the lining of the 

 mouth. The subepithelial mesoblastic tissue in the concavity 

 of the bell is the dentine -papilla. We thus get a bell-shaped 

 enamel organ (Fig. 266, C) consisting of ingrown buccal epithe- 

 lium, connected by a narrow stalk (h) with the tooth-band and 



