PHYLUM OHOHDATA 



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 

 rudiriient of the tooth becomes vascular and forms a distinct 



CCUTTty 



la.m' 



cLerit.s 



Fig. 1182. — Two stages in the development of the teeth of a Mammal (diagrammatio sections). 

 ttZv. bone of alveolus ; dznt. s. dental sac ; en. -ni. enamel-membrane ; en. pl-p. enamel-pulp ; 

 gr. dental groove ; lam. dental lamina ; lav}!, 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. 



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

 118S,denf.) produced by the dentine-forming cells, and of a layer of 

 enamel {en.) on the sur- 

 face of this, produced by ^^^^^ y^ 

 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 



e.n.m, 

 en.filfi 



Lam, 



Cn.TTt"- 



-fiafi^ 



through the mucous 



Fig. 1183. — Diagrammatic section showing the develop- 

 ment of the milk- and permanent teeth of Mammals. 

 alv. bone of alveolus ; dent, dentine ; dent. 5. dental sac ; 

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

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

 tooth ; en. xyl'p. enamel-pulp of milk-tooth ; fir. dental 

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

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

 ))aj)2. dental papilla of permanent tooth. (After O. 

 iiertwig.) 



membrane of the gum, 

 the remains of the 

 enamel-organ becoming 

 thrown off. The cement 

 is formed by the ossi- 

 fication of the connective-tissue surrounding the tooth-papilla. 

 In the teeth of most Mammals distinct roots are ibruied, each 

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

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



M M 2 



