EKUPTION OF THE TEETH. 381 



by a progressive development, while the spongy tissue proportion- 

 ately diminishes, and at last entirely disappears, together with the 

 epithelial cells ; when the development of enamel is completed. 



Thus the membrana prasformativa (p. 379) has an epithelial layer 

 upon both of its surfaces ; the dentine being developed from the 

 inner one, while the enamel is formed from the outer one; the ves- 

 sels affording the plasma being on the distal side of the epithelial 

 layer, from the membrana praaformativa, in both cases. 



3. The cementum is believed by Kolliker to be formed by the 

 "portions of the dental sac lying between the pulp and the enamel- 

 organ." We consider that the sac merely embraces the enamel- 

 organ, and isolates it from the dental pulp ; while the latter affords 

 the epithelial layer whence the dentine is developed. Another source 

 of the cementum must therefore be sought, and none is more pro- 

 bable than the periosteum of the alveoli. After the body of the 

 tooth is completed, the pulp elongates towards the bottom of the 

 alveolus, and thus the fang is developed from it, the body at the 

 same time penetrating through the gum in the opposite direction. 

 Kolliker states that the sac elongates at the same time with the 

 pulp, and thinks that its inner surface affords a blastema whence the 

 cementum is formed and deposited on the outer surface of the den- 

 tine of the fang. 



It is difficult to account for the development of Nasmyth's jpem- 

 brane. On the supposition, however, that the membrana praeforma- 

 tiva originally lines the whole of the enamel-cavity, it may be 

 produced by calcification of the membrane, gluing together and 

 protecting the outer ends of the prisms of the enamel. 



The permanent teeth are also developed in the manner just de- 

 scribed, from the secondary pulps mentioned on page 376, 1. 



The two sets of teeth appear at the following ages : 



I. MILK-TEETH. 



Four central incisors (the lower first), 7th month. 

 Lateral incisors (lower first), 7th to 10th " 

 Anterior molars, 12th to 13th " 



Canine teeth, 14th to 20th " 



Posterior molars, 18th to 36th " 



