120 A MANUAL OF DENTAL ANATOMY. 



shed and reproduced, so that it is less easy to find them in 

 all stages of their growth ; I believe, however, that they 

 follow a course essentially similar to that of the teeth. 



FIG. 58 0). 



It is stated by Gegenbaur that in Selachia the mucous 

 membrane of the mouth is clothed with spines of a structure- 

 similar to that of the teeth, and that these spines are often 

 limited to particular regions, extending back as far as the 

 pharynx these same regions in Ganoids and Osseous fishes, 

 being occupied by conspicuous teeth; and Hertwig has. 

 shown that the dermal spines are developed in a manner 

 precisely analogous to that described in the teeth, save that 

 the germs are even less specialised. 



In Teleostei or Osseous Fishes. In passing from 

 the consideration of the development of the tooth germs of 

 Elasmobranch to those of Osseous fishes, the first difference 

 to be noted is this : whereas in the former each tooth germ 

 was, so far as the enamel germ is concerned, derived from that 

 of the next older tooth, in the latter each enamel germ often, 

 arises independently and, as it were, de now. At all events, 



(*) Section of lower jaw of young Dog-fish, showing the continuity of 

 the dermal spines of the skin under the jaw, with the teeth which lie 

 above and over its end. 



