XIII 



PHYLUM CHORDATA 



559 



two the remaining 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. 



en,. pip 



La,m' a,lv 



Lam 



FIG. 1182. Two stages in the development of the teeth of a Mammal (diagrammatic sections). 

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

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

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



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

 into the papilla. 



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

 llSSjdent) 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 d eveloped. 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 

 enamel-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 a minute opening leading into the pulp-cavity (Fig. 1181, 

 ///- V) ; but in some there are no roots, the pulp-cavity being 



M M 2 



FIG. 1183. Diagrammatic section showing the develop- 

 ment of the milk- and permanent teeth of Mammals. 

 alv. bone of alveolus ; dent, dentine ; dent. &. dental sac ; 

 en. layer of enamel ; en. m. outer layer of enamel-organ 

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

 tooth ; en. pip. enamel-pulp of milk-tooth ; (jr. dental 

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

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

 pap 2 , dental papilla of permanent tooth. (After O. 

 Hert 



