CH. V.] 



DEVELOPMENT OF THE TEETH 



59 



prism, but this view has been disproved by recent research. The 

 layer of keratin-like material is outside the bodies of the cells, although 

 a process of each adamantoblast extends into it as a tapering fibre 

 (process of Tomes), and it is usually produced simultaneously with 

 the first layer of uncalcified dentine: when it undergoes calcifica- 

 tion, the first layer of enamel is complete. The adamantoblasts 

 then repeat the process, first causing a deposition of keratin-like 

 material, and this in turn is calcified, and so on. During the forma- 

 tion of layer after layer of enamel, the adamantoblasts retire. By 

 the time the enamel is approaching completion the other layers 

 of the enamel organ have almost disappeared, and they entirely 





FIG. 74. Highly magnified view of a piece of the enamel organ in a kitten's canine, d, Superficial 

 layer of dentine, e, Newly formed enamel stained black by osmic acid. T, Tomes' processes from 

 the adamantoblasts, ad. ; str. int., stratum intermedium of the enamel organ, p, Branched cells of 

 the enamel pulp. (After Rose.) 



disappear when the tooth emerges through the gum. But for some 

 little time there is a somewhat more persistent membrane covering 

 the crown ; this is Nasmyth's membrane, or the enamel cuticle ; this 

 is the last- formed keratinous layer of enamel which has remained 

 uncalcified. 



As with the dentine, the formation of enamel appears first on the 

 apex of each cusp. 



The cement or crusta petrosa is formed from the internal tissue of 

 the tooth sac, the structure and function of which are identical with 

 those of the osteogenetic layer of the periosteum ; or, in other 

 words, ossification in membrane occurs in it. 



The outer layer or portion of the membrane of the tooth sac forms 

 the dental periosteum. 



