DEVELOPMENT. OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 295 



pulp. During calcification slender processes of the odontoblasts remain in minute 

 channels, or dentinal canals, forming the dentinal fibers which anastomose with 

 one another (Fig. 253). In the peripheral part of the dentine certain areas 

 apparently fail to become calcified and form the inter globular spaces. The same 

 cells that are originally differentiated from the mesenchyme probably persist 

 throughout development as the odontoblasts and produce the entire amount of 

 dentine in a tooth. Even in the fully formed tooth there is a layer of odonto- 

 blasts bearing the same relation to the dentine and pulp as in the developing 

 tooth. The chief difference between dentine formation and bone formation is 

 that in the latter the osteoblasts become enclosed to form bone cells, while in 

 the former the odontoblasts merely leave processes enclosed as the cell bodies 

 recede. 



The pulp of the tooth is of course derived from the mesenchymal tissue in 

 the interior of the dental papilla (compare Figs. 252 and 254). The blood 

 vessels and nerves grow in from the underlying connective (mesenchymal) tissue. 



At an early stage the mesenchymal tissue around the anlage of the tooth, in- 

 cluding the enamel organ, condenses to form a sort of sheath, the dental sac, 

 which is later ruptured when the tooth breaks through the gum (Fig. 254). 

 The cement is formed around the root of the tooth from the tissue of the dental 

 sac in the same manner as subperiosteal bone is formed from osteogenetic tissue 

 (p. 142). In fact, cement is true bone without Haversian systems. 



The milk teeth, which are the first to develop and the first to appear above 

 the surface, are represented by the medial incisors, lateral incisors, canines, and 

 molars, to the number of ten in the upper and ten in the lower jaw. They may 

 be indicated graphically thus: 



M. C. L.I. M.I. M.I. L.I. C. M. 



10 



20 

 10 



M. C. L.I. M.I. M.I. L.I. C. M. 



In describing the formation of the dental shelf, it was noted that the papillae 

 of the milk teeth grow into corresponding thickenings of the epithelium (p. 292). 

 The growth takes place from the side, thus leaving the edge of the shelf free to 

 grow farther toward the lingual side of the jaw. In this free edge other tooth 

 germs arise, which mark the beginnings of the permanent teeth (Fig. 252). In 

 addition to the germs that correspond in position to the milk teeth, three others 

 arise in each jaw, representing the true molars of the adult. The latter arise in a 

 part of the dental shelf which has grown toward the articulation of the jaws 

 without coming in contact with the surface epithelium. The first papilla of 

 the permanent dentition to appear is that of the first molar. It appears im- 

 mediately behind the second milk molar at a time when the milk teeth are well 



