. The development of the chick; an introduction to embryology. Birds -- Embryology. THE NERVOUS SYSTEM 235 A narrow non-nucleated margin, known as the marginal velum, appears in the lateral Avails of the neural tube external to the nuclei (Fig. 138). This is occupied by the outer ends of the epithelial cells. At this time, therefore, three zones may be distinctly recognized in the walls of the neural tube, viz., (1) the zone of the germinal cells, including also the inner ends of the epithelial cells, (2) the zone of the nuclei of the epithelial cells, (3) the marginal velum. No distinctly ner
. The development of the chick; an introduction to embryology. Birds -- Embryology. THE NERVOUS SYSTEM 235 A narrow non-nucleated margin, known as the marginal velum, appears in the lateral Avails of the neural tube external to the nuclei (Fig. 138). This is occupied by the outer ends of the epithelial cells. At this time, therefore, three zones may be distinctly recognized in the walls of the neural tube, viz., (1) the zone of the germinal cells, including also the inner ends of the epithelial cells, (2) the zone of the nuclei of the epithelial cells, (3) the marginal velum. No distinctly nervous elements are yet differentiated. Such elements, however, soon begin to appear: Fig. 139 repre- sents a section through the cord of a chick embryo of about the end of the third day; it is from a Golgi preparation in which the distinctly nervous elements are stained black, and the epithelial and germinal cells are seen only very indis- tinctly. The stained elements are the neuroblasts, and it will be observed that they form a layer roughly intermediate in position between the marginal velum and the nuclei of the epithelial cells. They are usually regarded as derived from germinal cells that have migrated from their central position outwards; but it is mi3.^ M/4f. Fig. 139.—Transverse section through the spinal cord and ganglion 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 epithelial cells. However this may be in such an early stage, it is certain that the neuroblasts formed later are derived from germinal cells. It will be observed that each neuroblast consists of a cell- body and a process ending in an enlargement. The process arises as an outgrowth of the cell-body, and forms the axis cylin- der or axone of a
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