THE DIGESTIVE TRACT. 



FIG. 183. 



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 impression of the dental papilla 

 upon the overlying enamel organ 

 is probably not to be attributed 

 to mechanical obstruction op- 

 posed to the advancing ecto- 

 dermic tissue, but has its cause 

 in more deeply lying laws of ex- 

 pansion along lines of unequal 

 growth. As the invagination 

 of the enamel organ progresses, 

 more and more of the dental 

 papilla becomes covered, until 

 about two-thirds of the meso- 

 dermjc cone are embraced within 

 the sides of the ectodermic cap. 

 The enamel organ itself under- 

 goes a differentiation into three 

 distinct layers : the outer layer, 

 directly continuous for a long 

 time with the ectodermic cells 

 of the oral cavity, is composed 

 of one or two layers of low 

 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. 



Section of jaw of cat embryo ; the dental papilla 

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

 densed mesoderm, whose summit is enveloped by 

 the invaginated enamel organ (e) ; ec, oral epi- 

 thelium, still attached by the atrophic isthmus (a) 

 with the enamel organ, whose outer (6), middle 

 (c), and inner (ef) layers are differentiated ; e', 

 beginning of enamel organ for permanent tooth. 



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, bone. 



distinctly columnar, constituting the inner layer of the enamel organ 

 containing the beautiful enamel cells. The outer and inner layers 



