The development of the human body; a manual of human embryology . ning the origin ofthe mesenchyme from which the mammalian skull is de-veloped, yet, since there is probably a continuation for-ward into the cranial region of the series of mesodermicsomites, it is suppos-able that these furnishthe mesenchyme forthe skull, just as themore posterior somitesfurnish that for thevertebra?. In the earliest stagesthe human skull is rep-resented by a continu-ous mass of mesen-chyme which investsthe anterior portion ofthe notochord and ex-tends forward beyondits extremity into thenasal region, forming a
The development of the human body; a manual of human embryology . ning the origin ofthe mesenchyme from which the mammalian skull is de-veloped, yet, since there is probably a continuation for-ward into the cranial region of the series of mesodermicsomites, it is suppos-able that these furnishthe mesenchyme forthe skull, just as themore posterior somitesfurnish that for thevertebra?. In the earliest stagesthe human skull is rep-resented by a continu-ous mass of mesen-chyme which investsthe anterior portion ofthe notochord and ex-tends forward beyondits extremity into thenasal region, forming a core for the fronto-nasal process (see p. 104). From eachside of this basal mass a wing projects dorsally to enclosethe anterior portion of the medullary canal which willlater become the cerebral part of the central nervous sys-tem. No indications of a segmental origin are to befound in this mesenchyme; as stated, it is a continuousmass, and this is likewise true of the cartilage which laterdevelops in it. The chondrification occurs first along the median line. Fig. 98.—Reconstruction op the Chon-drocranium of an embryo of 14 mm. as, AHsphenoid; 60, basioccipital; bs,basisphenoid; eo, exoccipital; m,Meckels cartilage; as, orbitosphenoid;p, periotic; ps, presphenoid; so, sellaturcica; s, supraoccipital.—(Levi.) 192 THE DEVELOPMENT OF THE HUMAN BODY. in what will be the occipital and sphenoidal regions of theskull (Pig. 98) and thence gradually extends forward intothe ethmoidal region and to a certain extent dorsally at thesides and behind into the regions later occupied by thewings of the sphenoid (as and os) and the squamous portionof the occipital (s). No cartilage develops, however,in the rest of the sides or in the roof of the skull, but themesenchyme of these regions becomes converted into adense membrane of connective tissue. While the chondri-fication is proceeding in the regions mentioned, the mesen-chyme which encloses the internal ear becomes convertedinto cartila
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
