XI I r 



PHYLUM CHORDATA 



559 



two the reinainiucr cells of the enainel-orfran become modified to 

 form a kind of connective tissue — the cnamcl-jiidp {en.i')lp.). 



The connective-tissue immediately surrounding the entire 

 rudiment of the tooth becomes vascular and forms a distinct 



en. 



CLTn, 



rv.pip 



La,Tn culv 



fia^ 



pap 

 dent.S 



Fig. 118:2. — Two stages in the ilevelopmeiit of the teeth of ;i Mammal (iliagrammatic .scetioiis). 

 alv. bone of alveolus ; dmt. it. dental sae ; en. m. eiiamel-nieiubrane ; eit. pip. enamel-pulp ; 

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

 below the tooth-germ ; i)ap. dental papilla, f After O. Hertwig.) 



Crt.pLfi 



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

 enamel (cn.) on the sur- 

 face of this, produced by ^^^^ 9'^ 

 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 tlie surface 

 and eventually breaks 

 through the mucous 

 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 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 



iam 



tn.m'' 



pajb' 



Fig. 11S3. — Diagrammatic section showing the develop- 

 ment of tlic milk- and permanent teeth ot Maminals. 

 alv. bone of alveolus ; dint, dentine ; ibnil. -s. dental sac ; 

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

 of milk-tooth ; cn. »(''. enamel-membrane of permanent 

 tooth ; en. pip. eiiamcl-i)ulp of milk-tooth ; rrr. dental 

 groove ; lain, dental lamina ; /'. neck connecting milk- 

 tooth with lamina ; paji. dental pajiilla of milk-tooth ; 

 ii(ip~. dental impilla of permanent tooth. (After O. 

 Hertwig.) 



