DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 325 



of least resistance in which the tooth grows, and it may convey nourishment 

 to the enamel cells, the enamel organ being non-vascular. 



The Dentine and Pulp. At first the dental papilla is simply a condensation 

 of mesenchyme, but later it is converted into a sort of connective tissue pene- 

 trated by blood vessels and nerves (Fig. 292). The cells nearest the enamel 

 organ become columnar and arranged in a single layer, with the nuclei 

 toward their inner ends. The outer ends are blunt, while the inner ends are 



Epith. of mouth cavity 



Outer) 



> enamel cells 



Dental sac 



Bone of jaw 



Blood vessel 



Enamel pulp 

 (remnant) 



Papilla 

 FIG. 292. Longitudinal section of a developing tooth of a new-born puppy. Bonnet. 



continued as slender processes that extend into the pulp and probably fuse 

 with other cell processes. These columnar cells are the odontoblasts, under the 

 influence of which the lime salts of the dentine are deposited, and which are com- 

 parable with the osteoblasts in developing bone. 



Toward the end of the fourth month the odontoblasts form a membrane- 

 like structure, the membrana preformativa, between themselves and the enamel. 

 This membrane is first converted into dentine by the deposition of lime salts, 

 after which the process of calcification progresses from the enamel toward the 



