The development of the spinal cord examined by autoradiography.

The wall of a recently closed neural tube is generally considered to consist of three concentric layers. Surrounding the lumen is found the ependymal zone characterized by large pale cells known as the germinal cells of His (1889). These cells are believed to give rise to the neuroblasts and some of the spongioblasts. Soon after formation the neuroblasts move away from the lumen to enter a densely packed nucleated zone referred to as the mantle layer. This layer consisting of spongioblasts and neuroblasts ultimately gives rise to the gray matter of the spinal cord. The outermost layer, known as the marginal layer, contains the fibres of the developing neuroblasts and later forms the white matter. In 1897 Schaper suggested that the primitive neural wall should not be divided into different layers. In his opinion the dividing cells at the lumen and the nondividing ones in the nucleated area (mantle layer) belonged to the same epithelial cell type.