TEETH. 19 



and birds, they may be entirely absent. In the mammals fleshy 

 lips moved by muscles first occur, and even here they are lacking 

 in monotremes and cetaceans. In turtles and birds the edges of 

 the jaws, and to a greater or less extent the roof of the mouth, 

 is covered with a cornified epithelium forming the so-called beak, 

 and the same is true of the adult monotremes. The surface of 

 this may be thrown into folds for the purpose of crushing the 

 food, but these structures are not to be compared with true teeth. 

 Teeth. In the formation of teeth two layers, ectoderm and 

 mesenchyme, are concerned. The epithelium lining the mouth 

 becomes inpushed into the deeper layers, where teeth are to be 

 formed (Fig. 21). In the lower vertebrates there is a separate 

 inpushing for each tooth, but in the mammals there is a con- 

 tinuous ingrowth, the dental ridge. In other 

 respects the features of development are essen- 

 tially the same in all. The ingrowth is to be 

 regarded, morphologically, as vesicular ; and the 

 deeper wall of the vesicle becomes pushed in- 

 side the other, so that there results a two- 

 walled cup, the cavity of which becomes filled 



with mesenchyme. The cells of the inner layer 



FIG. 21. Tooth 

 become columnar and form the enamel organ ; germ of Ambiysto- 



the immigrant mesenchyme cells constitute a ma. d, derma ; e, 

 dental papilla, the external cells of which are e P idermis ; ' en ~ 



amel organ ; /, den- 



known as odontoblasts, from their power of tal pap ju a ' 

 secreting a bone-like substance, the dentine 

 or ivory of the tooth. The inner surface of the enamel organ 

 likewise secretes a cup of denser substance, enamel, upon the 

 outer face of the dentine. 



By growth of the deeper portions (dentine) the enamel cov- 

 ered tip or crown of the tooth is forced up through the epithelial 

 layers so that it comes into position for use. The deeper por- 

 tion or root contains a central or pulp cavity, in which are remains 

 of the mesenchyme, together with nerves and blood-vessels, these 

 together forming the pulp. In the mammals the root is cov- 

 ered by a second coat, the cement, formed by the surrounding 

 tissues (Fig. 22). 



In the lower forms the process of tooth formation may con- 



