Un libro di testo di istologia normale: tra cui un conto dello sviluppo dei tessuti e degli organi . ntalridge atrofizza ed eventualmente scompare negli intervalli trai singoli denti. Le sacche di smalto dei denti permanenti sono formate in una data successiva dal remainsof la cresta dentale, quelli per il threepermanent molari essendo derivata da aspecial estensione del dental ridgewhich cresce indipendentemente ectodermicattachments. Il primitivo organo dello smalto whichgrows dalla cresta dentale in un primo momento con-sists di un solido processo di cilindrica di epi-thelial tessuto; presto, tuttavia, l'ex-
1313 x 1903 px | 22,2 x 32,2 cm | 8,8 x 12,7 inches | 150dpi
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Textbook of normal histology: including an account of the development of the tissues and of the organs . ntalridge atrophies and eventually disappears in the intervals betweenthe individual teeth. The enamel sacs of the permanent teeth are formed at a later date from the remainsof the dental ridge, those for the threepermanent molars being derived from aspecial extension of the dental ridgewhich grows independently of ectodermicattachments. The primitive enamel organ whichgrows from the dental ridge at first con-sists of a solid cylindrical process of epi-thelial tissue; soon, however, the ex-tremity becomes club-shaped and slightlytortuous, and later distinctly expandedand flask-shaped. Coincident with thesechanges the surrounding mesoderm be-gins to exhibit proliferation and conden-sation of its elements, this differentiationmarking the earliest stage in the forma-tion of the important mesodermic dentalpapilla, which very soon becomes aconical mass of closely-aggregated meso-dermic elements. Along with the growth of the latterthe now expanded end of the ectodermic plug becomes indented. Section of jaw of rabbit embryo, showing later stage of enamel organ, which now exhibits differentiationinto outer (b) and inner (e) cells : m, mesodermic tissue which at a hasundergone already some condensa-tion ; ec, oral ectoderm. THE DIGESTIVE TRACT. 151 or invaginated to form an epithelial cap, which embraces the meso-dermic dental papilla, and, from its future important function, isknown as the enamel organ. The impression of the dental papillaupon the overlying enamel organis probably not to be attributedto mechanical obstruction op-posed to the advancing ecto-dermic tissue, but has its causein more deeply lying laws of ex-pansion along lines of unequalgrowth. As the invaginationof the enamel organ progresses, more and more of the dentalpapilla becomes covered, untilabout two-thirds of the meso-dermic cone are embraced withinthe sides of the ectodermic cap.The enamel organ itsel