THE HISTOGENESIS OF THE NERVOUS TISSUES 



307 



Those fibers which are first functional receive their myelin sheaths first. 

 The mycHnation of nerve fibers is only completed between the second and third 

 year (Westphal). Many of the peripheral fibers, especially those of the sympa- 

 thetic system, remain unmyelinated and supplied only with a neurilemma sheath. 

 The myehnated fibers, those with a myehn sheath, have a ghstening white ap- 

 pearance and give the characteristic color to the white substance of the central 

 nervous system and to the peripheral nerves. Ranson (Amer. Jour. Anat., vol. 

 12, 1911) has shown that large numbers of unmyehnated fibers occur in the peri- 

 pheral nerves and spinal cord of adult mammals and man. Those found in the 

 spinal nerves arise from the small cells of the spinal gangha. 



B 



Fig. 311 ^Ependymal cells from the neural tube of chick embryos. A, of first day; B, of third day. 



Golgi method (Cajal). 



The Development of the Supporting Cells. — The spongioblasts of the 

 neural tube (p. 302) differentiate into the supporting tissue of the central nervous 

 system. This includes the ependymal cells, which line the neural cavity, forming 

 one of the primary layers of the neural tube, and neuroglia cells and their fibers. 



We have described how the strands of the syncytium formed by the spongio- 

 blasts become arranged radially in the neural tube of early embryos (Fig. 305 D). 

 As the wall of the neural tube thickens, the strands elongate pari passu and form a 

 radiating branched framework (Fig. 302). The group of spongioblasts which 

 line the neural cavity constitutes the ependymal layer. Processes from these 



