196 



COMPARATIVE ANATOMY 



however, soon separate, one growing in labially, the other lingually. 

 The latter forms the dental ridge or lamina. As in development of a 



hair, the dental ridge is formed by cell multiplication in the stratum 



germinativum of the epidermis. 



Early in the development of the dental lamina, a series of bell-shaped 



enlargements, ten in each jaw, appear along its labial border (Fig. 148). 



These are known as enamel organs 

 since they secrete the enamel covering 

 of the crowns of the teeth. Each of 

 the twenty milk teeth has a separate 

 enamel organ, and all of them are 

 present in a 2}/^ months embryo. 

 Each enamel organ contains a mesen- 

 chymatous dental papilla, the outer 

 cells of which, the odontoblasts, 

 secrete the dentine of the tooth. 

 The remaining cells of the papilla 

 become the pulp of the tooth. As 

 development proceeds, each enamel 

 organ recedes from the dental lamina 

 with which it retains a transient con- 

 nexion by means of a "neck" or cord 

 of cells. 



^ 



~"">3 



#te 



The free edge of the dental lamina, 

 Fig. 149. — Diagrams representing losing connexion with the anlagen 



stages in the phylogenesis of the dental ^^ ^^^ ^j^j^ ^^^^. ^^^^^ ^ SeCOnd Set 

 lamina. In elasmobranchs, the placoid ' _ i- i 



scales which become teeth are lodged in of enamel organs lying on the Imgual 



a groove on the edge of the jaw. In the .^^ ^^ ^j^^ primary Set. In this 



higher animals, the groove becomes '^ ■' 



filled with indifferent cells and converted way, the anlagen of the thirty-two 



into the dent^al lamina. A primarily permanent teeth come to He embedded 



indefinite number of sets of teeth formed ^ _ ^ r i • 



in the lamina finally becomes limited in the Connective tissue of the jaws 



to two in man C, A £. and f represent ^^^ jj^ j gj^^ ^f ^j^^ primary 



stages of tooth development m reptiles; » i r 



G and H in mammals. (Redrawn from set. The permanent teeth are, how- 

 Ihie, after Boik.) &v^x, relatively slow in development, 



the third molar usually not forming in the jaw before the fifth year. 



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 mesenchymatous enamel pulp, and a layer of outer 

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

 columnar epithelial 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 multiplication of ameloblast cells 



