THE DEVELOPMENT OF THE TEETH. 115 



formation of enamel by its own conversion into a calcined 

 tissue, for I have shown that it is to be found in the germs 

 of teeth which have no enamel ; in fact, in all tooth germs 

 whatever. 



That part of the tooth germ destined to become dentine 

 is often called the dentine papilla, having acquired this 

 name from its papilliform shape ; and in a certain sense it 

 is true that the enamel organ is the epithelium of the den- 

 tine papilla. Yet, although not absolutely untrue, such an 

 expression might mislead by implying that the enamel 

 organ is a secondary development, whereas its appearance 

 is contemporaneous with, if not antecedent to, that of the 

 dentine germ. The most general account that I am able 

 to give of the process is, that the deeper layer of the oral 

 epithelium sends down into the subjacent tissue a process, 

 the shape and structure of which is, in most animals, dis- 

 tinguishable and characteristic before the dentine germ has 

 taken any definite form. This process enlarges at its end, 

 and, as seen in section, becomes divaricated, so that it bears 

 some resemblance to an inverted letter Y ; or it might more 

 truthfully be compared to a bell jar with a handle ; this 

 constitutes the early stage of an enamel germ (see Fig. 64), 

 while beneath it in the mucous tissue, the dentine germ 

 assumes its papilliform shape. The details of the process 

 varying in different creatures, I will at once proceed to the 

 description of the development of teeth in the various 

 groups. 



In Elasmobraiich Fishes. If a transverse section 

 through the jaw of a dog-fish (Scyllium canicula) be ex- 

 amined, we shall find that the forming teeth lie upon the 

 inside of the semi-ossified jaw-bones, the youngest being at 

 the bottom (Fig. 57) ; progressing upwards, each tooth is- 

 more fully calcified till, on passing over the border of the 

 jaw, we come to those teeth whose period of greatest useful- 

 ness is passed and which are about to be cast off in the 



i 2 



