42 THE NERVOUS SYSTEM 



The spinal cord of a 20 mm. human embryo presents well-defined ependymal, 

 marginal, and mantle layers. Figure 22 should be compared with the appear- 

 ance presented by a cross-section of the spinal cord in the adult (Fig. 55). The 

 mantle layer with its many nuclei differentiates into the gray matter of the spinal 

 cord, which contains the nerve-cells and their dendritic processes. The mar- 

 ginal layer develops into the white substance as a result of the growth into it 

 of the axons from neuroblasts located within the mantle layer. These form 

 association fibers which ascend or descend through the marginal layer and serve 

 to connect one level of the neural tube with another. It is not until these 

 longitudinally coursing axons develop myelin sheaths that the white substance 

 acquires its characteristic coloration. 



The cavity of the neural tube is relatively large, and at the point marked 

 "neural cavity" in Fig. 22 a groove is visible. This is the sulcus limitans. It 

 separates the dorsal or alar plate from the ventral or basal plate. The mantle 

 layer of the alar plate develops into the dorsal gray column which, like the other 

 parts developed from this plate, is afferent in function. The afferent fibers, 

 growing into the spinal cord from the spinal ganglia, either terminate in this 

 dorsal column or ascend in the posterior part of the marginal zone to nuclei 

 derived from the alar plate in the myelencephalon. Most of the association 

 fibers which run in the marginal layer have grown out from neuroblasts located 

 in the dorsal column. The mantle layer of the basal plate gives rise to the 

 ventral gray column. From the neuroblasts in this region grow out the motor 

 fibers of the ventral roots and spinal nerves. 



From what has been said it will be clear that the entire nervous system is 

 ectodermal in origin. The nervous element proper or neurons are derived from 

 the neuroblasts; the supporting tissue of the brain and spinal cord, the neuroglia, 

 is derived from spongioblasts ; while the neurilemma of the peripheral nerves is 

 the product of sheath cells which have migrated out from the spinal ganglia 

 and possibly also from the neural tube. 



