150 



NORMAL HISTOLOGY. 



the ridge. These secondary aggregations are the first indications 

 of the enamel organs of the temporary teeth. After the establish- 



FIG. 180. 



FIG. 181. 



Model of jaw of human embryo of 40 mm. : r, r, arch of 

 increased epithelium constituting dental ridge ; /, local thick- 

 enings corresponding to positions of future enamel sacs. 

 (After Rose.) 



Section of jaw of rabbit 

 embryo, showing dental ridge 

 cut across : ec , oral ectoderm ; 

 e, epithelial outgrowth corre- 

 sponding to future enamel 

 organ ; m, mesodermic tissue. 



ment of these structures the ectodermic tissue composing the dental 

 ridge atrophies and eventually disappears in the intervals between 

 the individual teeth. The enamel sacs of the permanent teeth are 



formed at a later date from the remains 

 FIG. 182. of the dental ridge, those for the three 



permanent molars being derived from a 

 special extension of the dental ridge 

 which grows independently of ectodermic 

 attachments. 



The primitive enamel organ which 

 grows from the dental ridge at first con- 

 sists of a solid cylindrical process of epi- 



VM 



tremity becomes club-shaped and slightly 



tortuous, . and later distinctly expanded 

 and flask-shaped. Coincident with these 

 changes the surrounding mesoderm be- 

 gins to exhibit proliferation and conden- 

 sation of its elements, this differentiation 

 marking the earliest stage in the forma- 

 tion of the important mesodermic dental 

 papilla, which very soon becomes a 

 conical mass of closely-aggregated meso- 

 dermic elements. 



Along with the growth of the latter 

 the now expanded end of the ectodermic plug becomes indented 



Section of jaw of rabbit embryo, 

 showing later stage of enamel organ, 

 which now exhibits differentiation 

 into outer (t) and inner (e) cells : m, 

 mesodermic tissue which at a has 

 undergone already some condensa- 

 tion ; ec, oral ectoderm. 



