The development of the human body; a manual of human embryology . n the median line by a deep groove, the floor ofwhich is the somewhat thickened floor-plate. About thefourth month there appears in the roof-plate a transversegroove into which the surrounding mesenchyme dips, and,as the groove deepens in later stages, the mesenchyme con-tinued within it becomes converted into blood-vessels,forming the chorioid plexus of the fourth ventricle, astructure which, as may be seen from its development,does not lie within the cavity of the ventricle, but is 4o8 THE DEVELOPMENT OF THE HUMAN BODY. separa
The development of the human body; a manual of human embryology . n the median line by a deep groove, the floor ofwhich is the somewhat thickened floor-plate. About thefourth month there appears in the roof-plate a transversegroove into which the surrounding mesenchyme dips, and,as the groove deepens in later stages, the mesenchyme con-tinued within it becomes converted into blood-vessels,forming the chorioid plexus of the fourth ventricle, astructure which, as may be seen from its development,does not lie within the cavity of the ventricle, but is 4o8 THE DEVELOPMENT OF THE HUMAN BODY. separated from it by the portion of the roof-plate whichforms the floor of the groove. In embryos of about 9 mm. the differentiation of thedorsal and ventral zones into ependymal and mantlelayers is clearly visible (Fig. 218), and in the ventral zonethe marginal velum is also well developed. Where thefibers from the sensory ganglion of the vagus nerve enterthe dorsal zone an oval area (Fig. 218, fs) is to be seenwhich is evidently comparable to the oval bundle of the. Fig. 218.—Transverse Section through the Medulla Oblongata of an Embryo of , Dorsal zone; fp, floor-plate; js, fasciculus solitarius; /, lip; rp. roof-plate ; vz, ventral zone; X and XII, tenth and twelfth nerves.—{His.) cord and consequently with the column of Burdach. Itgives rise to the solitary fasciculus of adult anatomy, andin embryos of 11 to 13 mm. it becomes covered in by thefusion of the reflected lip of the dorsal zone with the sidesof the myelencephalon, this fusion, at the same time,drawing the margins of the roof-plate ventrally to form asecondary lip (Fig. 219). Soon after this a remarkable mi-gration ventrally of neuroblasts of the dorsal zone rapidly in number the migrating cells pass on THE MYELENCEPHALON. 409 either side of the solitary fasciculus toward the territory ofthe ventral zone, and, passing ventrally to the ventralportion of the mantle layer, into which fibers
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
