r>90 



THE NEEVOUS SYSTEM 



glia fibers) form a loose-meshed syncytium, the myelospongium, for the 

 support of the developing neuroblasts. Peripherally it is of denser texture 

 and relatively free of nuclei, and is designated the marginal velum. Coin- 

 cident with this process of neuroglia 

 histogenesis, the neuroblasts enlarge, 

 proliferate extensively and differentiate 

 into neurons, their processes axon and 

 dendrons arising as sprouts from the 

 originally spherical neuroblasts. 



The motor neurons remain located 

 in the ventral horn of the spinal cord 

 (Fig. 515), their axons (efferent fibers) 

 passing out through the ventral root to 

 unite with the fibers of the dorsal root 



^*L '." WM 



^^^^WL^. 



FIG. 510. SECTION THROUGH MEDULLARY 

 PLATE OF CLOSING NEURAL GROOVE OF 

 RABBIT EMBRYO. 



Two germinal cells are visible. (After 

 His.) 



FIG. 511. SECTION THROUGH 

 WALL OF LATER NEURAL 

 TUBE OF RABBIT EM- 

 BRYO, SHOWING A STAGE 

 IN THE DIFFERENTIATION 

 OF THE EPENDYMA CELLS 

 AND THE FORMATION OF A 

 MYELOSPONGIUM. 



(After His.) 



to form a spinal nerve. The sensory neurons arise external to the cord 

 from neuroblasts of the linear neural crests, and migrate some distance 

 ventrolaterally, where they aggregate into metameric oval masses which 

 differentiate into the spinal ganglia. The differentiation process includes 

 both neuroglia and neuron formation. The sensory neuron located in the 

 dorsal (spinal) ganglia is also originally spherical or oval; subsequently it 

 becomes bi-polar; and finally, by a process involving the retraction of the 

 cell body (cyton) from its two processes and the fusion of these processes 

 proximally, it becomes a unipolar cell. The definitive axon thus divides in 

 the manner of a T or Y. Its central process, or axon proper, passes into 

 the dorsolateral portion of the cord as an afferent fiber and here makes 



