The development of the human body; a manual of human embryology . ros; w, Wolffian duct.—(Keibel.) ducing a short ectodermal addition to the rectum, theend of which is the definitive anus. It will be noticed that the digestive tract thus formedconsists of three distinct portions, an anterior, short, ecto-dermal portion, an endodermal portion representing theoriginal archenteron, and a posterior short portion whichis also ectodermal. The differentiation of the tract into THE MOUTH-CAVITY. 299 its various regions and the formation of the various organsfound in relation with these may now be cons
The development of the human body; a manual of human embryology . ros; w, Wolffian duct.—(Keibel.) ducing a short ectodermal addition to the rectum, theend of which is the definitive anus. It will be noticed that the digestive tract thus formedconsists of three distinct portions, an anterior, short, ecto-dermal portion, an endodermal portion representing theoriginal archenteron, and a posterior short portion whichis also ectodermal. The differentiation of the tract into THE MOUTH-CAVITY. 299 its various regions and the formation of the various organsfound in relation with these may now be considered. The Development of the Mouth Region.—The deepeningof the oral sinus by the development of the first branchialarch and its separation into the oral and nasal cavitiesby the development of the palate have already been de-scribed (p. 103), but, for the sake of continuity in descrip-tion, the latter process may be briefly recalled. At firstthe nasal pits communicate with the oral sinus by grooveslying one on each side of the fronto-nasal process, but by. Fig. 157.—View of the Roof of the Oral Fossa of Embryo showingthe Lip-groove and the Formation of the Palate.—(His.) the union of the latter with the maxillary processes thiscommunication is partly interrupted, though the pits stillretain connection with the oral sinus behind the maxillaryprocess. At about the fifth week a downgrowth of epi-thelium into the substance of both the maxillary andfronto-nasal processes above and the mandibular processbelow, takes place and the surface of the downgrowthbecomes marked by a deepening groove (Fig. 157), whichseparates an anterior fold, the lip, from the jaw proper(Fig. 158). From the premaxillary and maxillo-pala- 300 THE DEVELOPMENT OF THE HUMAN BODY. tine portions of the upper jaw, shelf-like ridges then beginto grow backward and inward, and at about the beginningof the third month these meet in the median line to formthe palate, completing the separation of the definitive
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
