PLATE V. 



Fig. 6. — Transverse section of upper jaw of a newt. 



To the right of the working tooth, which is tipped by a double point of 

 enamel, are two younger tooth-sacs ; beyond the second or youngest of 

 these latter is the epithelial process (enamel germ) of a third, and beyond 

 this again (/) the forming enamel germ of a fourth. Thus, in a single sec- 

 tion we have a perfected tooth and four stages in the formation of teeth 

 exemplified. 



Fig. 7. — Section of lower jaw of a young dog-fish. 



Two only of the teeth seen in the section were exposed and in use j those 

 lower down would be covered in by the fold of mucous membrane (k) 

 (which in the section has been accidentally torn away and displaced to the 

 right). The enamel organs form a continuous chain, each new one being 

 derived from its immediate predecessor. 



Fig. 8. — Section of lower jaw of a mackerel. 



The developing teeth are lodged in a groove in the thin edge of the jaw ; 

 the perfected tooth is fixed in place by a sort of bony scaffolding, thrown 

 out from the sides and bottom of this groove. 



Fig. 9. — Two tooth-sacs of the common eel. 



Fig. 10. — Tooth-sac of an eel more highly magnified. 



The large cells of the enamel organ are confined to the region where the 

 enamel cap is being formed ; below this the enamel-cells are dwindled and 

 insignificant. 



Fig. 11. — Tooth-sac of a newt. 



This is entirely cellular, so that under pressure it readily breaks up, 

 nothing but cells remaining. The enamel organ consists principally of the 

 enamel-cells (internal epithelium), the external epithelium being only feebly 

 developed, and the stellate reticulum being wholly absent. 



