DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 325 



by intermediate portions of the dental shelf, and with the surface by the 

 original ingrowth of epithelium. 



THE ENAMEL. The epithelial cells nearest the dental papilla become high 

 columnar in shape, forming a single layer. Those in the interior of the mass 

 become separated and changed into irregular, stellate, anastomosing cells, with 

 a fluid intercellular substance, constituting the enamel pulp. Those farthest 

 from the papilla become flattened (Fig. 290; compare with Fig. 291). Calcifi- 

 cation begins in the basal ends of the columnar cells, or in the ends next the 



Enamel 



Dentine | Enamel prisms 



Odontoblasts 



Papilla 



Outer 



Inner . 



Enamel pulp 



Basal memb. 



of enamel 

 ' cells 



- <- v."^V. i < ' -* 

 FIG. 291. Section through the border of a developing tooth of a new-born puppy. Bonnet. 



papilla, and in the intercellular substance, and gradually progresses throughout 

 the cells, the latter at the same time becoming much more elongated. Thus the 

 cells are transformed into enamel prisms which are held together by the calci- 

 fied intercellular substance (Fig. 291). 



The formation of enamel begins in the milk teeth toward the end of the 

 fourth month and probably continues until the teeth break through the gums. 

 The enamel organ at first surrounds the entire developing tooth except where 

 the papilla joins the underlying mesenchymal tissue (Fig. 290). Later the 

 deeper part of the organ disappears as such, and the enamel is formed only on 

 that part of the tooth which eventually becomes the crown. The enamel pulp 

 increases in amount for a time, but subsequently disappears as the tooth grows 

 into it (Fig. 292). Its function is not fully understood. It may serve as a line 



