4b CHARLES S, TOMES. 



If one were desirous of representing diagrammatically the 

 development of the teeth of a shark and a newt, one would 

 draw almost the same thing, save only that in the former the 

 several tooth-sacs are somewhat less individualised. 



Hertwig entertains peculiar views as to the homologies of 

 the parts of placoid scales and sharks' teeth, regarding their 

 basal portions as ceinentum ; his paper has been in my hands 

 so short a time that I am not prepared to express an opinion 

 upon this matter, but if we admit HertAvig's views as to 

 the basal part of a selachian tooth, we must probably extend 

 the term cementum to a large number of structures never 

 hitherto ranked as dental tissues at all. 



Passing from the sharks and rays to the osseous fish, I 

 believe that a broad distinction may be laid down. In all 

 mammals, reptiles, batrachia (the frog?), and elasmobranch 

 fishes, successional teeth are derived, though the medium of 

 their enamel organs, from their predecessors. In all osseous 

 fish which I have examined, successional teeth appear to be 

 produced de novo, i. e. from new inflections of the oral 

 epithelium (see fig. 8, representing a young tooth- sac of a 

 mackerel). 



With this exception the process is the same ; there is the 

 same formation of the enamel germ from the oral epithelium, 

 which is transformed into an enamel organ over the 

 uprising dental pulp ; the latter becoming always calicfied, 

 the former sometimes dwindling away without any enamel 

 formation taking place. 



There are many fish {e.g. eels) which have spear-like ter- 

 minal points of enamel, but no enamel on the sides of the 

 teeth ; in the tooth-germ the enamel organ is^ nevertheless, co- 

 extensive with the dentine pulp, though its internal epithelium 

 or enamel cells are only largely developed over the point where 

 enamel is to be formed, and are small and dwarfed over the 

 rest of the tooth (see fig. 10). 



From what has already been said it will be seen that 

 there is considerable uniformity in the manner in which 

 tooth-germs are developed, and that it is quite impossible 

 to maintain the generalisations that the teeth of fish and 

 reptiles typify any certain transitory stages in the develop- 

 ment of mammalian teeth, &c. And it will also be seen that 

 Goodsir's original error of observation, small though it was, 

 becomes vastly magnified when his views are stretched 

 to make them apply, for instance, to the common snake 

 (see fig. 4). 



Although, morphologically, as Professor Huxley reminds 

 me, an inflection of epithelium is not a very different thing 



