64 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(1) Origin of the Enamel.— The enamel is formed by the 

 epiblast of the stomodaeum. At the sixth week the epiblast 

 within the labial margin grows downwards so that a narrow 

 semicircular invagination of epithelium is formed. To the plate 

 of epiblast thus infolded the name of dental shelf is given ; its 

 position is marked superficially by an epithelial crest — the dental 



lower lip 



labio-dental groom 



position of oral membrane 

 -epiblast 

 -tongue 



i dental shelf (epiblast) 



mandib arch 



Fio. 



r enamel bud of 

 { perm, incisor 



_j enamel bud of 

 1 milk incisor 



r dental papilla 

 1 (meso-blast) 



: 8, Section through the lip and mandible of a foetus in the third month, 



showing the down-growth of the Dental Shelf. 



ridge (Fig. 48). From its ingrowing or deep margin ten epi- 

 thelial buds arise, both in the upper and lower jaw. Each of 

 these twenty enamel buds or organs produces the enamel to cover 

 the crown of a milk tooth. Each bud grows downwards and 

 inwards from the surface and comes against a condensed formation 

 in the mesoblast of the jaw — the dental papilla. On the papilla 

 the enamel bud becomes partly invaginated, the papilla coming to 

 lie within the invagination (Figs. 48 and 49). The epithelium 

 covering the papilla becomes a layer of columnar enamel-pro- 

 ducing cells or ameloblasts. The basal part of the ameloblasts 

 are converted gradually into enamel, or to put it somewhat 

 differently, they form and deposit enamel in their bases and thus 

 produce a coating for the dental papilla. Each ameloblast is 

 gradually converted into an enamel fibre, but their more super- 

 ficial parts are never so converted but persists as the cuticular 

 membrane which covers the enamel at birth and is soon after- 

 wards worn off. The enamel of the milk teeth is completely 



