FIG. 183. 



ec. 



THE DIGESTIVE TRACT. , ,. j 



or invaginated to form an epithelial cap, which embraces the meso- 

 dermic dental papilla, and, from its future important function, is 

 known as the enamel organ. The enamel organ, however, sub- 

 serves another important pur- 

 pose, since its inner and outer 

 layers grow towards the base 

 of the dentinal papilla and 

 invest the young mesoder- 

 mic tissue with a two-layered 

 envelope, the epithelial 

 sheath. This structure plays 

 an important role in deter- 

 mining the form of the future 

 tooth, since the young dentine 

 is deposited along its inner sur- 

 face. Vestiges of this sheath ap- 

 pear as isolated epithelial nests. 

 The enamel organ itself under- 

 goes a differentiation into three 

 distinct layers : the outer layer, 



directly COntinUOUS for a long densed mesoderm, whose summit is enveloped by 



time with the Prtodermir Cells the invaginated enamel organ (^) ; ec, oral epi- 



' WltJ thelium, still attached by the atrophic isthmus (a) 



Of the Oral Cavity, is COmpOSed with the enamel organ, whose outer (A), middle 



Of One Or tWO layers Of low (0, and inner (^) layers are differentiated ;^, 



* beginning of enamel organ for permanent tooth. 



columnar or polyhedral cells ; at 



the point where they are reflected to form the inner, invaginated 



part of the original epithelial sac, the cells become elongated and 



FIG. 184. 



c/. 



Section of jaw of cat embryo ; the dental papilla 

 is seen as a projecting conical mass (/) of con- 



Section of jaw of cat embryo with four developing teeth slightly farther advanced than in the pre. 

 ceding stage : ec, oral epithelium ; a, dental groove ; e, enamel organ ; /, /, dental papillae ; m, 

 mesodermic tissue ; b, b t bone. 



distinctly columnar, constituting the inner layer of the enamel organ 

 containing the beautiful enamel cells. The outer and inner layers 



