XI I J 



PHYLUM CHORDATA 



527 



cells of the enamel organ become modified to form a kind of 

 connective tissue the enamel pulp (en. pip.). 



The connective tissue immediately surrounding the entire 

 rudiment of the tooth becomes vascular and forms a distinct 



en. 



Lam 



FK;. 1113. Two stages in the development of the teeth of a Mammal (diagrammatic sections). 

 f<li-. bone of alveolus ; dent. s. dental sac; en. m. enamel membrane; en. pip. enamel pulp ; 

 fir. dental groove ; lam. dental lamina ; lam', part of dental lamina which grows downwards 

 below the tooth-germ ; pap. dental papilla. (After O. Hertwig.) 



investment the dental sac (dent, s.) ; from this blood-vessels extend 

 .into the papilla. 



Ossification begins by the formation of a cap of dentine (Fig. 

 1114, dent.) produced by the dentine-forming cells, and of a layer of 

 enamel (en.) on the sur- 

 face of this produced by 

 the cells of the enamel- 

 membrane. To these 

 additional layers are 

 added until the crown 

 of the tooth becomes 

 fully developed. The 

 substance of the den- 

 tal papilla gives rise 

 to the pulp. As the 

 tooth elongates it pro- 

 jects on the surface 

 and eventually breaks 

 through the mucous 

 membrane of the gum, 

 the remains of the en- 

 amel organ becoming 

 thrown off. The cement Hertwig.) 



is formed by the ossi- 

 fication of the connective tissue surrounding the tooth-papilla. 



In the teeth of most Mammals distinct roots are formed, each with 



FIG. 1114. Diagrammatic section showing the develop- 

 ment of the milk and permanent teeth of Mammals. 

 alv. bone ,of alveolus ; dent, dentine ; dent. s. dental 

 sac ; en. layer of enamel ; en. m. enamel membrane of 

 milk tooth ; en. m 2 . enamel membrane of permanent 

 tooth ; en. pip. enamel pulp of milk tooth ; gr. dental 

 groove ; lam. dental lamina ; n. neck connecting milk 

 tooth with lamina ; pap. dental papilla of milk tooth ; 

 pap%. dental papilla of permanent tooth. (After O. 



a minute opening leading into the pulp-cavity (Fig. 



but in some there are no roots, the pulp-cavity being open below 



