. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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.
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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. â 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,â 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 ^ ^ , â¢^ â ' Fig. 21. Tooth become columnar and form the enamel organ; germ of Arnbiysto- the immigrant mesenchyme cells constitute a ma. d, derma; e, dental papilla, the external cells of which are ep'^^mis; "â en- amel organ; p, den- known as odontoblasts, from their power of j^j papina. 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 c
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