NEURAL CRESTS AND THEIR DERIVATIVES 161 



owing to the migration of the pigment cells of the grafted neural crests of X. 

 The neural crests also furnish the cells which appear to lay down the myelin 

 sheaths over the surfaces of the nerve fibers. These sheath cells have been 

 traced to the neural-crest region by removing the neural crests and observing 

 that the nerve fibers lacked sheath cells. Finally, the bulk of the neural-crest 

 cells differentiate into the sensory nerve fibers. The cell bodies of the nerves 

 remain in the spinal ganglia, while the fibers grow into the skin, establishing 

 sensory endings there. The number of these cells which will differentiate 

 into sensory neurons is dependent upon the amount of skin in the periphery. 



Fibers from the spinal ganglia also grow into the spinal cord, forming 

 the dorsal roots of the spinal ganglia. The ventral roots of the spinal ganglia 

 are formed by fibers from cells in the ventral region of the "spinal cord 

 (Fig. 94, D). These cells form motor fibers which connect with the muscle 

 end plates in the limb muscles and the muscles of the back. Thus a stimulus, 

 such as a touch of the skin, will reach the spinal cord through the sensory 

 fibers via the spinal ganglia and the dorsal root. In the spinal cord the motor 

 region is stimulated, impulses go out through the ventral root to the muscles, 

 and contraction results. 



The development of pathways within the cord through differentiation of 

 neuroblasts leads to the development of a coordinated muscular response. 

 The gradual increase in complexity of the nerve-muscle pattern has been 

 worked out by experiments in the amphibian embryo. Suggested readings on 

 this subject will be found on page 312. 



Transformation of neural tube 

 into spinal cord 



The neural tube itself arises as a thick-walled tube, showing no internal 

 differentiation. Soon, however, we see two major divisions — an ependymal 

 layer of cells lining the central canal and an outer layer of neuroblasts on 

 either side (Fig. 95) . The cell bodies of the neuroblasts form the gray matter 

 of the spinal cord, while the nerve fibers and their myelin sheaths make up 

 the white matter. Some nerve fibers cross from one side of the cord to the 

 other. Such a collection of fibers is called a commissure. 



Further differentiation within the cord is characterized by a sharper 

 separation of these primary divisions (Fig. 95). The ependymal layer be- 



