DIGESTIVE ORGANS. 209 



There are two types of teeth, much alike in function, but differing 

 markedly in structure and development and without genetic relation- 

 ships. The typical vertebrate teeth are comparable to placoid scales; 

 they arise as a calcareous secretion at the junction of ectoderm and 

 mesenchyme and are a product of both layers. The other type contains 

 purely cuticular teeth, formed by a cornincation of the epithelium and 

 have their analogues in many invertebrates. 



True Teeth. The ability to form scales is characteristic of the 

 skin of many vertebrates. The primitive type of these scales is the 

 placoid (p. 40), consisting of a basal portion of dentine capped with 

 enamel and the apex projecting through the integument as a spine. 

 When invaginated to form the stomodeum the skin retains this capacity 

 of forming hard structures and hence any portion of the stomodeal 

 walls may secrete scale-like plates. In fact, in the teeth of some 

 elasmobranchs (Raia, Mustelus, Trygon, etc.) the placoid scale can be 

 recognized with scarcely a modification. In the ichthyopsida teeth 

 may form anywhere in the oral cavity where there are skeletal parts 

 cartilage or bone to support them. Thus they may occur, not only 

 on the margins of the jaws, but on vomers, palatines and parasphenoid, 

 and in some teleosts on the tongue, where they are attached to the 

 hyoid. In the amniotes -(some squamata excepted) teeth occur only 

 on the margins of the jaws. Teeth are lacking, here and there, in 

 various families of vertebrates as well as from all turtles and living 

 birds, but some extinct birds had teeth. In the embryos of both 

 chelonians and aves the dental ridge is formed (vide infra), but it soon 

 completely disappears. 



In the development of a tooth, as of a placoid scale, there is 

 first a thickening of the ectoderm, the basal layer of which pushes into 

 the cutis, and at the same time the mesenchyme cells of the latter layer 

 multiply beneath the centre of the ectodermal ingrowth, pushing it 

 outward, so that the basal layer forms a cup with the opening toward 

 the deeper tissues (fig. 210). The mesenchyme within the cup forms 

 the dental papilla, while the ectoderm cells lining the cup form the 

 enamel organ. With farther development the outer cells of the papilla 

 are converted into odontoblasts, so-called from their function of form- 

 ing a bone-like substance, the dentine or ivory of the tooth. This, 

 in accordance with the method of its formation by secretion from the ends 

 of the odontoblasts, has a prismatic structure. The basal surface of 

 the enamel organ secretes a denser substance, the enamel, which lies like 

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