EXISTING ORGANISMS— ANATOMY 83 



semblance to the much smaller placoid scales in form. Unlike the 

 teeth of higher vertebrates, these are superficially attached by a 

 broad base. Their structure is exactly similar to that of the 

 placoid scales, consisting of an inner core of dentine surroimding 

 a mesodermal papilla and covered by a hard enamel layer. In all 

 respects, these teeth are so like the placoid scales that the transition 

 of development is almost obvious. They are larger, but since they 

 are found in a region where transition from skin to buccal epithe- 

 lium occurs, it is conceivable that even the slight roughness caused 

 by placoid scales might be useful in holding prey and that useful- 

 ness might account for the greater development of the teeth. 



The teeth of higher vertebrates differ in l^eing deeply seated 

 in sockets in the bones but when we remember that the bones 

 which support them are not derived from primitive cartilages like 

 those of the elasmobranch jaws but are developed from the dermal 

 layer, this relationship is not surprising. During development 

 the teeth are differentiated from portions of the ectoderm which 

 grow into the underlying tissues (Fig. 57). 



In structure teeth consist, like the placoid scales and teeth of 

 elasmobranchs, of an inner layer of dentine surrounding a meso- 

 dermal core, the pulp, and an outer enamel layer which is thickest 

 on the exposed points. Cement, a substance which covers the 

 root and is conspicuous in the teeth of some animals is added by 

 outer mesench>Tnal tissue. The two principal parts of the tooth 

 have exactly the same origin as the primitive scales and teeth of 

 the sharks, for the enamel is epidermal and the dentine, dermal. 

 Teeth vary greatly in form according to the habits of animals. 

 Some are pointed for grasping and tearing, others are sharp edged 

 for cutting, and still others have broad rough surfaces for grinding. 

 All three types are present in the human mouth. Rodents have 

 highly developed chisel-like incisors and some snakes have fangs 

 for injecting poison. Such animals as the anteaters have no use 

 for teeth, and they are accordingly reduced or absent. 



Scales, Feathers, and Hair. The scales of fishes above the elas- 

 mobranchs have no enamel covering and so are wholly dermal in 

 origin; nor are they Uke the scales of reptiles and birds, which 

 consist of epidermal folds, cornified to some extent and nourished 

 by dermal papillae. The feathers of birds are of exactly the same 

 origin as the scales of this group and the reptiles, though they are 

 much more complex in structure. Scales in the mauunals are 



