820 THE NERVOUS SYSTEM 



their sheath-like coverings are of two kinds, viz., myelinated fibers with 

 neurilemma and unmyelinated (Remak's) fibers with a thin neurilemma. The 

 latter are found especially among the sympathetic nerve fibers of the cerebro- 

 spinal series. (See Ranson, '39, p. 51.) 



It may be observed here, parenthetically, that the myelinated fibers of the 

 brain and spinal cord differ from the myelinated fibers of the peripheral 

 nervous system in that the sheaths are formed by an investment of neuroglia 

 fibers and nuclei and not by a neurilemma sheath. Many naked axons also 

 are present in the central nervous system. 



C. Morphogenesis of the Central Nervous System 



1. Development of the Spinal Cord 



a. Internal Changes in the Cord 



During the early development of the spinal cord described above the fol- 

 lowing areas are evident: 



( 1 ) the ependymal layer, 



(2) the mantle layer, and 



(3) the marginal layer. 



The further development of these areas results in the formation of a thin 

 dorsal roof plate and a ventral floor plate mainly from the ependymal layer 

 (fig. 353 J and K). Somewhat later the neural cavity of the cord is reduced 

 by the apposition and fusion of the dorso-lateral walls of the lumen immedi- 

 ately under the dorsal plate, leaving a rounded central canal below located 

 near the floor plate (fig. 355A-E). Synchronized with these events the lateral 

 walls of the neural tube expand greatly as the mass of cells and fibers increases. 

 During this expansion, the two dorsal parts of the lateral walls move dorsad 

 and mediad and in this way come to lie apposed together in the median plane 

 above the central canal. This apposition forms the dorsal median septum 

 (fig. 355D and E). The dorsal roof plate becomes obliterated during this 

 process. Ventrally, also, the lateral portions of the neural tube move toward 

 the mid-ventral line below the central canal. However, the two sides do not 

 become closely apposed, and as a result the ventral median fissure is formed 

 (fig. 355D and E). 



During the growth and expansion of the two lateral walls of the neural 

 tube, the neuroblasts of the nucleated mantle layer in the dorsal or alar 

 plate of the spinal cord increase greatly in number and form the dorsal (or 

 posterior) gray column (fig. 355A-E). The developing neuroblasts of the 

 dorsal gray column become associated with the dorsal root fibers of the spinal 

 nerves. Ventrally, the neuroblasts of the mantle layer increase in number in 

 the basal plate area of the spinal cord and form a ventral (anterior) gray 

 column. The ventral root fibers of the spinal nerves emerge from the ventral 



