. The development of the chick; an introduction to embryology . )reparationin which the distinctly nervouselements are stained black, andthe epithelial and germinalcells are seen only very indis-tinctly. The stained elementsare the neuroblasts, and it willbe observed that they form alayer roughly intermediate inposition between the marginalvelum and the nuclei of theepithelial cells. They areusually regarded as derivedfrom germinal cells that havemigrated from their centralposition outwards; but it is m/3.~ m/4i. hIbU Fig. 139.—Transverse section through the sjiinal cord and ganghon of a chick
. The development of the chick; an introduction to embryology . )reparationin which the distinctly nervouselements are stained black, andthe epithelial and germinalcells are seen only very indis-tinctly. The stained elementsare the neuroblasts, and it willbe observed that they form alayer roughly intermediate inposition between the marginalvelum and the nuclei of theepithelial cells. They areusually regarded as derivedfrom germinal cells that havemigrated from their centralposition outwards; but it is m/3.~ m/4i. hIbU Fig. 139.—Transverse section through the sjiinal cord and ganghon of a chick about the end of the third day; prepared by the method of Golgi. (After Ramon y Cajal.) C, Cones of growth. Nbl. 1, 2, .3, 4, Neuroblasts of the lateral wall (1 and 2); of the spinal ganglion (3); of the ventral horn (motor neuroblasts) (4). possible that some of them may have been derived from epithelialcells. However this may be in such an early stage, it is certainthat the neuroblasts formed later are derived from germinal will be observed that each neurol)last consists of a cell-body and a ])rocess ending in an enlargement. The processarises as an outgrowth of the cell-body, and forms the axis cylin-der or axone of a nerve-fiber; the terminal enlargement is knownas the cone of growth, because the growth processes l)y whichthe axone increases in length are presumably located here. Itmay be stated as an invariable rule that each axone process of amedullary neuroljlast arises as an outgrowth,
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