542 



ZOOLOGY 



SECT. 



the enamel-organ, the cells (odontoblasts) become arranged into a 

 layer having the appearance of an epithelium the dentine-forming 

 layer. The cells of the enamel-organ form two layers, of which 

 that in contact with the dental papilla assumes the character of a 



en 



lam. 



la,m' 



fiaf 



en,. pip 



dent.s 



Fia. 1194. 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. Hertwig.) 



layer of long cylindrical cells the enamel-membrane (en. m.) ; the 

 more superficial layer consists of cubical cells. Between the 

 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 rudi- 

 ment of the tooth becomes 

 vascular and forms a dis- 

 tinct investment the den- 

 tal sac (dent, s.) ; from 

 this blood-vessels extend 

 into the papilla. 



Ossification begins by 

 the formation of a cap of 

 dentine (Fig. 1195, dent.), 

 produced by the dentine- 

 forming cells, and of a 

 layer of enamel (en.) on 

 the surface of this, pro- 

 duced by the cells of the 

 enamel membrane. To 



FIG. 1195. Diagrammatic section showing the develop- 

 ment of the milk- and permanent teeth of Mam- 

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

 dental sac ; en. layer of enamel ; en. m. enamel-mem- 

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



permanent tooth ; en'plp. enamel-pulp of milk-tooth; these additional layers are 



gr. dental groove: lam. dental lamina; n. neck jj j i--\ 1-1 



connecting milk-tooth with lamina; pap. dental added Until the Crown ot 



^t^'^J&gS? *"*** the tooth becomes fully 



developed. The substance 



of the dental papilla gives rise to the pulp. As the tooth elongates, 

 it projects on the surface and eventually breaks through the mucous 

 membrane of the gum, the remains of the enamel-organ becoming 

 thrown oft. The cement is formed by the ossification of the con- 

 nective-tissue of the dental sac. 



