224 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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 con- 

 tains purely cuticular teeth, formed by a cornification of the epithe- 

 lium and have their analogues in many invertebrates. 



True Teeth. — The abiUty to form scales is characteristic of the skin 

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

 placoid (p. 44), 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-Uke plates. In fact, in the teeth of 

 some elasmobranchs {Rata, 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, an indication of their descent from 

 toothed ancestors. 



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 

 corium 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. 234). 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 forming a bone-Uke 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 (fig. 

 237). The basal surface of the enamel organ secretes a denser sub- 

 stance, the enamel, which Hes like a cap, firmly united to the top and 



