!)2 Basic Structure of Vertebrates 



lack true enamel or possess enamel produced by endoderm. A third 

 possibility is that enamel-organs of pharyngeal teeth consist of ecto- 

 dermal cells which have migrated from the oral epithelium back into 

 the pharyngeal endoderm, or epidermal cells which have invaded the 

 pharyngeal lining via the gill-clefts. Such migration of cells is known to 

 occur elsewhere in embryos. 



The teeth of fishes other than sharks and those of other vertebrates 

 so closely resemble sharks' teeth in the nature of their constituent ma- 

 terials, in structure, and in manner of development that there can be 

 no doubt as to their correspondence. The external skin of the verte- 

 brate may cease to produce calcareous scales, but they are retained in 

 the mouth, and especially on the jaws, and there become enlarged and 

 elaborated in various ways to serve as teeth. The teeth of terrestrial 

 vertebrates, even those of mammals, are therefore to be regarded as 

 remote derivatives of placoid scales of ancient sharklike fishes. 



Differentiation of Teeth. Comparison of teeth of vertebrates re- 

 veals marked differences in number, size, distribution, form, and de- 

 gree of permanency of teeth. If the differences in each one of these 

 particulars are arranged in a graded series, the corresponding animals 

 of that series fall into line in the following order: fishes, amphibians, 

 reptiles, mammals. Modern birds are toothless but, if ancient toothed 

 birds be included, their teeth (so far as information concerning them is 

 available) put birds into line along with reptiles. The sequence of ani- 

 mals is the same for all of the series of differences. 



The number of teeth, at its maximum in fishes — sometimes run- 

 ning into hundreds — is much reduced in amphibians, more so in rep- 

 tiles, and reaches its minimum in mammals. This reduction in number 

 is associated with restriction in distribution. In fishes, teeth are com- 

 monly widely distributed over the oral and pharyngeal surfaces. Am- 

 phibians and reptiles show a progressive tendency to restrict teeth to 

 the jaws. This tendency reaches its limit in some ancient reptiles (e.g., 

 dinosaurs) and in crocodilians and mammals, in which teeth occur only 

 on the jaws. Reduction in number of teeth is associated also with in- 

 crease in their relative size. As they acquire greater functional impor- 

 tance, they become larger, more strongly developed, and more securely 

 attached. In some fishes the small scalelike teeth are anchored merely 

 by the embedding of their basal plates in the oral membrane. Firmer 

 support is gained as the teeth become ankylosed to the bones of the 

 jaws, roof of mouth, and skeletal arches of the gills. (A theory of the 

 evolutionary origin of the superficial or "dermal" bones of the skull 

 proposes that they arose by enlargement and fusion of basal plates of 

 scales.) The fusion of teeth to the jaws may be of either the acrodont 

 or the pleurodont type (Fig. 46). In the thecodont crocodilians and 

 mammals, the teeth are lodged in sockets, but not fused to the bones. 



