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 cornification 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 canbe 
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 
enamelorgan. 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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