TEETH 



313 



tough, hairless integument, and in Ornithorhynchus somewhat 

 resemble the beak of a Bird. 



The chief organs of the oral cavity are the teeth, the glands, and 

 the tongue. 



Teeth. 



The teeth are developed quite independently of the endo- 

 skeleton, and both ectoderm and mesoderm take part in their 

 formation. The first traces of the teeth are seen primarily in 

 the form of superficial papillae of the mucous membrane; but 

 secondarily, owing to want of space, the epithelium of the mouth 

 grows inwards so as to give rise to a dental lamina which becomes 



ZC 



FIG. 228. DIAGRAM OF THE DEVELOPMENT OF A TOOTH. 



By, connective tissue follicle or sac surrounding the tooth ; DS, dentine ; 

 EM, epithelium of mouth ; Ma, enamel epithelium ; O, odontoblasts ; SK, 

 dental lamina ; ZK, dental papilla. 



FIG. 229. SEMIDIAGRAMMATIC FIGURE OF A LONGITUDINAL SECTION THROUGH 



A TOOTH. 



PH 1 , aperture of the pulp-cavity (PH) ; ZB, dentine : ZC, cement ; ZS, enamel. 



enlarged dis tally at certain points to form the so-called enamel- 

 organs. These, as they grow deeper into the mesoderm, become 

 bell-shaped, and enclose modified masses of connective tissue, the 

 dental papillae ; the upper cells of the papillae, i.e. those next to 

 the enamel-organ, are known as odontoblasts (Fig. 228). The 

 epithelial and connective tissue germs come into the closest rela- 



