TEETH 155 



This evidence for the identity of teeth and placoid scales is not invali- 

 dated by the fact that the endodermal lining of the pharynx of some 

 elasmobranchs is also beset with placoid scales like those of the external 

 skin. This simply shows that the potentiahties of the germ-layers are 

 greater than was formerly supposed. The hning of the pharynx has also 

 taste-buds, although sense organs usually develop from the ectoderm. 

 The presence of scales and of taste-buds in the pharynx has led some 

 morphologists on a priori grounds to assume that ectoderm has invaded 

 mouth, pharynx, and even the esophagus. There is no question that 

 the hning of the pharynx is primarily endodermal, and no one has ever 

 been able to discover evidence that this primary hning degenerates and 



Fig. 141. — Comparison in development and structure between a placoid scale and a 

 tooth, a, b, and c represent the scale; d, e, and / the tooth. In all the figures 

 the stratum corneum is dotted, the stratum germinativum is represented by a layer of 

 large cells with nuclei; and the cutis is presented as composed of fibers with scattered 

 cells. X, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity. 

 (From Wilder's "History of the Human Body," Henry Holt & Co.) 



is secondarily replaced by ectoderm. Until such evidence is presented, 

 we have no alternative to the conclusion that the endodermal hning 

 persists and produces both taste-buds and enamel. Whatever the origin 

 of the enamel of these pharyngeal scales, there is no doubt that the 

 enamel of the teeth, like that of dermal scales, is of ectodermal origin. 



Originally, in vertebrates, the teeth were for seizing and holding 

 prey. Grinding and cutting teeth, tusks, and fangs, are all modifications 

 of the primitive mouth trap. 



The number of these holding teeth is indefinite in elasmobranchs, 

 which may have as many as one hundred. They are not attached to 

 the jaws, but merely imbedded in the skin of the mouth. They are all 

 about alike; and when one is lost, another moves forward into its place. 



