Principles and practice of operative dentistry . embryonic tissues. With the higher powers of the microscope itbecame possible to delve deeper into the mysteries of the evolution of thedental tissues than these old masters had been permitted to do. The re-sults of these investigations have proved conclusively that the enamel-organs had their origin in the epithelial tissues, and hence were formedfrom the same tissue elements as the hair, the nails, and the epitheliallining of the glandular structures of the skin and the mucous membrane,—namely, epithelial cells. EVOLUTION OF EPITHELIAL TISSUE.
Principles and practice of operative dentistry . embryonic tissues. With the higher powers of the microscope itbecame possible to delve deeper into the mysteries of the evolution of thedental tissues than these old masters had been permitted to do. The re-sults of these investigations have proved conclusively that the enamel-organs had their origin in the epithelial tissues, and hence were formedfrom the same tissue elements as the hair, the nails, and the epitheliallining of the glandular structures of the skin and the mucous membrane,—namely, epithelial cells. EVOLUTION OF EPITHELIAL TISSUE. In order that the student may approach the subject of the evolution ofthe teeth with a clear understanding of the tissue elements which enterinto their structure, it will be advisable to present in brief review themorphology and the character of epithelial tissue in general. In all animals which are developed from an ovum (Fig. 84), as soon asimpregnation has taken place and the proper conditions of incubation are 25 26 OPERATITE Division of mammal ovum (half diagrammatic).1, the yolk divided into two globules (cells) withnuclei; 2, quadrupled ; 3, a large number of nucle-ated cells; 4, a, b, isolated cells. establislied,—and this is best studied in tlie egg of the barn fowl,—thereoccurs a rapid proliferation of embryonic cells in the germiTial spot ordisk of Pander. This disk is at first composed of a germinal cell, -^^^- °^- which by the process of karyokine-sis—indirect division or segmenta-tion of the cell—produces a rapidmultiplication of these elementalcells (Fig. 85, 1, 2, 3, 4). These embryonic cells soon ar-range themselves into two layers,known as the epiderm or epiblast,and the liypoderm or hypoblast, theepiblast forming the upper layerand the hypoblast the lower (). Later a third stratum of cellsis developed between the epiblastand the hypoblast, which is knownas the mesoderm or mesoblast ( and 88). From these three layers all of
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectdentist, bookyear1920
