296 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



complex harmony is the teeth. Next to the teeth, foot 

 structure is most important in classification. 



Mammalian Teeth. — 1. O rigin and Development. — In 

 speaking of the skeleton we said (page 224) that teeth 

 do not belong to the true internal skeleton, but are an 



FIG. 180.— 1, 2, 3, 4, s, 6, 7, showing successive stages in the development 

 of a human incisor : tp, tooth pulp ; ts, tooth sac. 



epidermal structure, or, more specifically, an epithelial, 

 a gum structure. The evidence of this is found in their 

 embryonic development. The following figures (Fig. 



