CH. V.] 



DEVELOPMENT OF TEETH. 



77 



quantity of hyaline 

 cement substance. In 

 the deeper part of the 

 enamel, between the 

 prisms, are often small 

 lacuna;, which have the 

 processes or fibrils lying 

 in the dentinal tubes in 

 connection with them 



(fig- 97. ) 



Crusta Petrosa. 



The crusta petrosa, or 

 cement (fig. 95, e, d), is 

 composed of true bone, 

 and in it are lacunae (/) 

 and canaliculi (g\ which 

 sometimes communicate 

 with the outer finely 

 branched ends of the 

 dentinal tubules, and 

 generally with the inter- 

 globular spaces. Its 

 laminae are bolted to- 

 gether by perforating 

 fibres like those of 

 ordinary bone (Shar- 

 pey's fibres). Cement 

 differs from ordinary 

 bone in possessing no 

 Haversian canals, or, 

 if at all, only in the 

 thickest part. Such 

 canals ;ire more often 

 met with in teeth with 

 the cement hypertro- 

 phied than in the normal 

 tooth. 



Fig. 08. Section of the upper jaw of a foetal sheep. 

 A. i, common enamel germ dipping down into 

 the mucous membrane ; 2, palatine process of 

 jaw ; 3, Rete Malpighi. B. Section similar to A, 

 but passing through one of the special enamel 

 germs . here becoming flask-shaped ; c, c', epithe- 

 lium of mouth ; /, neck ; /', body of special 

 enamel germ. C. A later stage ; c, outline of 

 epithelium of gum ; /, neck of enamel germ ; 

 / , enamel organ ; />, papilla ; , dental sac form- 

 ing; fp, the enamel germ of permanent tooth; 

 m, bone of jaw ; v, vessels cut across. (Waldeyer 

 and Kulliker.) 



Development of the Teeth. 



The first step in the development of the teeth consists in a 

 downward growth (fig. 98, A, i) from the deeper layer of stratified 

 epithelium of the mucous membrane of the mouth, which first 



