ON THE DEVELOPMENT OF TEETH. 47 



from the oral epithelium ; though analogy would lead one to 

 infer the former to be the case, the small amount of avail- 

 able space and the speedy migration of the growing sac 

 render it difficult to positively decide this point. 



The sharks, in which Professor Owen was of opinion that 

 tooth- development stopped short, as it were, at the papillary 

 stage, present an instructive parallel Avith the condition of 

 things seen in either the newt or the axolotl. 



The series of dentine papillae, save that they originate from 

 a continuation of the same sheet of mucous membrane, have 

 no genetic conection with one another; on the other hand, 

 the enamel organs of the teeth of successive ages form a 

 chain the continuity of which is never broken by that sub- 

 sequent individualisation of the several tooth-germs which 

 takes place in reptiles and mammals. 



The teeth of sharks are seen, in a foetus just on the point of 

 being hatched, to be merely highly-developed dermal spines, 

 with which they are directly continuous, as there is no defi- 

 nite lip at this period, and the spinous skin is seen turning 

 in over the jaws. Hertwig has worked out, more carefully 

 than I, the development of these dermal spines, which he 

 declares to be in all respects similar to that of the teeth ; he 

 considers the excitation of greater use to be adequate to 

 account for the greater development of the dermal appendages 

 where the skin passes over the jaws. 



In the development of the shaikhs tooth we find, just as 

 in every other creature, that there is a production of an 

 enamel germ extending beyond the youngest one already 

 existing (e in fig. 7) ; that opposite to this the submucous 

 tissue becomes elevated into a dentine papilla. But there is 

 nothing like a production of free papillae, Avhich become 

 subsequently and secondarily enclosed or encapsulated ; and 

 the production of the enamel organ is in no sense secondary 

 to that of the dentine papilla. When the parts are undis- 

 turbed the flap of mucous membrane [k in fig. 7) which 

 covers in the teeth is in slight connection with the jaw and 

 teeth ; it may be very readily torn away, but it is only so 

 displaced by violence, and it is not, as was generally supposed, 

 a free flajJ, which merely fitted against the developing teeth. 



In some of the elasmobranchs a distinct coat of enamel is 

 deposited ; in others it amounts to a mere glaze, but from 

 the appearance of the enamel cells, which in other cases 

 proves to be a tolerably reliable guide, I should infer that they 

 do discharge a function, and that the outer structureless film 

 on the tooth, of Lamna for example, is to be regarded as 

 enamel. 



