INTRODUCTORY. TEETH. 7 



are produced; finally however definite roots are formed and 

 growth ceases. 



The teeth of any animal may be homodont, that is, all 

 having the same general character, or heterodont, that is, 

 having different forms adapted to different functions. The 

 dentition is heterodont in a few reptiles and the majority of 

 mammals. 



SUCCESSION OF TEETH. In most fishes, and many amphibians 

 and reptiles the teeth can be renewed indefinitely. In sharks, 

 for example, numerous rows of reserve teeth are to be seen 

 folded back behind those in use (see fig. 15). The majority of 

 mammals have only two sets of teeth, and are said to be diphy- 

 odont ; some have only a single series (monophyodont). 



DEVELOPMENT OF TEETH. A brief sketch of the method in 

 which development of teeth takes place in the higher verte- 

 brates may here be given. Along the surface of the jaws a 

 thickening of the epiblastic epithelium takes place, giving rise 

 to a ridge, which sinks inwards into the tissue of the jaw, 

 and it is known as the primary enamel organ. At the points 

 where teeth are to be developed special ingrowths of this 

 primary enamel organ take place, and into each there projects 

 a vascular dental papilla from the surrounding mesoblast of 

 the jaw. Each ingrowth of the enamel organ forms an 

 enamel cap, which gradually embraces the dental papilla, 

 and at the same time appears to be pushed on one side, owing 

 to the growth not being uniform. The external layer of 

 the dental papilla is composed of long nucleated cells, the 

 odontoblasts, and it is by these that the dentine is formed. 

 Similarly the internal layer of the enamel organ is formed of 

 columnar enamel cells, which give rise to the enamel. The 

 mesoblastic cells surrounding the base of the tooth give rise 

 to the cement. 



Bone is in many cases exoskeletal, but it will be most 

 conveniently described with the endoskeleton. 



