The development of the human body; a manual of human embryology . sed (Fig. 42), the cells which lie alongthe line of transition between the lips of the groove andthe general ectoderm form a distinct ridge readily recog-nized in sections andtermed the neural ridge(Fig. 213, A). When thelips of the groove fusetogether the cells of thecrest unite to form awedge-shaped mass, completing the closureof the canal (Fig. 213,B), and later proliferateso as to extend outwardover the surface of thecanal (Fig. 213, C).Since this proliferationis most active in the re-gions of the crest whichcorrespond to th
The development of the human body; a manual of human embryology . sed (Fig. 42), the cells which lie alongthe line of transition between the lips of the groove andthe general ectoderm form a distinct ridge readily recog-nized in sections andtermed the neural ridge(Fig. 213, A). When thelips of the groove fusetogether the cells of thecrest unite to form awedge-shaped mass, completing the closureof the canal (Fig. 213,B), and later proliferateso as to extend outwardover the surface of thecanal (Fig. 213, C).Since this proliferationis most active in the re-gions of the crest whichcorrespond to the meso-dermic somites there isformed a series of cellmasses, arranged seg-mentally and situated in the mesenchyme at the sidesof the medullary canal (Fig. 200). These cell-masses rep-resent the posterior root ganglia, and certain of their con-stituent cells, which may also be termed neuroblasts, earlyassume a fusiform shape and send out a process from eachextremity. One of these processes, the axis-cylinder,grows inward toward the medullary canal and penetrates. Fig. 213.—Three Sections Throughthe Medullary Canal op an Em-bryo OP mm.—{von Lenhossek.) 39§ THE DEVELOPMENT OF THE HUMAN BODY. its marginal velum, and, after a longer or shorter course inthis zone, enters the mantle layer and comes into contactwith the dendrites of some of the central neuroblasts. Theother process extends peripherally and is to be regarded asan extremely elongated dendrite. The processes from thecells of each ganglion aggregate to form a nerve, thatformed by the axis-cylinders being the posterior root of aspinal nerve, while that formed by the dendrites soonunites with the ventral nerve-root of the correspondingsegment to form the main stem of a spinal nerve.
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
