HISTOGENESIS OF THE NERVOUS SYSTEM.
The elements of the nervous system are developed from the outer germ-layer or the ectoderm. As we have already seen, the brain and the spinal cord arise from a broad medial strip of ectoderm. Here the medullary plate is formed, which is bounded nally by the cuticle-plate. The medullary plate sinks and, at the same time, projects with its edges above the level of the embry onic area ; in this way is formed the medullary groove, bounded by the med ullary ridges. The medullary groove closes and becomes the medullary or neural tube.
The medullary tube, the direct successor of the medullary plate, consists at first of closely pressed epithelial cells, each of. which extends the entire ness of the layer. The wall of the entire tube, therefore, originally exhibits the character of a single-layered columnar epithelium, whose cells are bounded by the membrana limitans externa on the one side, and by the menzbrana limitans interna on the other (Fig. 97). Each epithelial cell encloses a large nucleus. In the inner zone, other large cells are irregu larly scattered between the epithelial elements, from which they are clearly distinguished by their round form and transparent homogeneous protoplasm. His designated these as the
The epithelial cells multiply rapidly and, consequently, become laterally compressed and elongated. Their nuclei lie at different heights and give rise to the appearance of a three- to six-celled layer. In reality, however, the cells completely retain the character of a single-layered columnar epithelium.
Some of the epithelial cells are early transformed. They grow into the spongio blasts of His, from which are developed the supporting elements, the ependyma and neuroglia cells.
Others of the epithelial cells change to pear-shaped elements and become the neuro blasts, which are transformed into the nerve-cells.
Both kinds of cells, the spongioblasts and the neuroblasts, therefore, are derivatives from the original ectodermic elements of the medullary plate. The above mentioned "germ-cells" of His are nothing more than cells of the primary medullary area under going mitosis and represent elements, whose division supplies the material for the increase of the indifferent ectodermal cells, on the one hand, and of their derivatives, the spongio blasts and the neuroblasts, on the other.