142 



CHORDATE ANATOMY 



ment proceeds, each enamel organ recedes from the dental lamina with 

 which it retains a transient connexion by means of a "neck" or cord of 

 cells. 



The free edge of the dental lamina, losing connexion with the anlagen 

 of the milk teeth, forms a second set of enamel organs lying on the lingual 

 side of the primary set. In this way, the anlagen of the thirty-two 

 permanent teeth come to lie embedded in the connective tissue of the jaws 

 on the hngual side of the primary set. The permanent teeth are, however, 

 relatively slow in development, the third molar usually not forming in the 

 jaw before the fifth year. 



blood-vessel 

 \ in pulp 



dentine 



enamel 



blood-vessel in 

 mesenchyme 



odontoblast ' dentinal Tbme's ameloblast outer epithelium 



fiber process layer of enamel organ 



Fig. 130. — Projection drawing of small segment of developing incisor from 130 mm. 

 pig embryo to show formation of enamel and dentine. X350. (From Patten's 

 "Embryology of the Pig.") 



Soon after the enamel organs emerge from the dental lamina, they 

 become differentiated into three layers, an inner ameloblast layer which 

 secretes the enamel, a mesenchyme-like enamel pulp, and a layer of outer 

 enamel cells. The ameloblast cells which line the enamel organs are 

 columnar epitheHal cells derived directly from the stratum germinativum 

 of the epidermis. Viewed from the inner surface, each ameloblast cell is 

 hexagonal and each secretes a simple hexagonal prism of enamel. As 

 the enamel increases in thickness, the multiphcation of ameloblast cells 

 results in an increase in the number of enamel prisms. The twisting 

 and curvature of the prisms in the developed tooth are a consequence of 

 the torsion of the ameloblast layer during active secretion. While the 

 enamel grows by addition from the outside, the dentine increases in 

 thickness from within. Consequently as the tooth is formed the amelo- 



