DEVELOPMENT OF THE NERVE-CELLS.
The neuroblasts, from which the nerve-cells arise, are developed in the innermost layer of the medullary tube, bordering the central canal. Thence they migrate outward through the inner layer and localize within a dorso-ventrally expanding region, that is bounded medially by the inner layer and laterally by the marginal zone. On examining a cross-section of the medullary tube of a four-weeks human embryo (Fig. 102), the cleft like central canal is seen in the middle, bordered by the inner plate, outside of which lies the stratum of neuroblasts, broad ventrally and thinner dorsally. Follow ing His, we call this stratum the mantle layer. Peripheralward, the latter joins the marginal zone —the Randschleier of His.
The neuroblasts are pear shaped cells, with oval nuclei, which send out a peripherally di rected process that bears at its end a characteristic thickening, the of Cajal. This process is nothing less than the
later nerve-fibre. While the rapidly growing fibres endeavor to reach their objective point, the cells change their form. On the surface appear small humps and jagged protuberances. These projections later elongate and become compact branches beset with small knobs. By the further development of the knobs and the manifold division of the outgrowths, the later protoplasmic processes or dendrites of the cells arise. In this manner the nerve-cell, or rather the neurone, is formed as an independent individual. It includes the cell-body and the outgrowing protoplasmic processes or dendrites and sends out the delicate net or neurite, which in its later development becomes the nerve-fibre.