THE DEVELOPMENT OF THE NERVOUS SYSTEM 



589 



These gross developmental changes more appropriately considered in 

 detail in works of Embryology involve histogenetic processes in the walls 

 of the canal. The earlier of these processes are essentially alike in both 

 cerebral and spinal portions of the canal. Originally the ectodermal layer, 

 which subsequently forms the wall of the tube, 

 consists of a single layer of cuboidal cells similarly 

 undifferentiated. Shortly spherical and oval ger- 

 minal cells appear among the primitive ependymal 

 cells. These indifferent cells migrate peripherally 

 so that two layers can now be distinguished in the 

 wall; an inner nuclear and an outer non-nuclear or 

 marginal layer. Cell boundaries have meanwhile 

 become obliterated and the entire wall is now essen- 

 tially a dense syncytium. Still later the syncytium 

 assumes a looser texture, forming the myelospon- 

 gium, and the indifferent germinal cells give rise 

 to two types of cells : the neuroblasts, from which 

 differentiate the neurons; and the spongioblasts, 

 from which the neuroglia cells and fibers develop. 

 Shortly after the time when neuroblasts and 

 spongioblasts first become distinguishable the wall 

 of the neural tube can be conveniently divided into 

 three zones: (1) the ependymal layer, forming in- 

 ternally also a limiting membrane; (2) the inter- 

 mediate, middle, nuclear or mantle layer, containing neuroblasts, spongio- 

 blasts and indifferent cells; and (3) the outer non-nuclear or marginal 

 layer, bounded externally by an outer limiting membrane. A glia frame 1 - 



work pervades the entire width of 

 the wall. 



Neuroglia cells differentiate 

 from spongioblasts by a process 

 involving chiefly the formation of 

 glia fibers in their exoplasm. 

 These fibers apparently arise from 

 spongioplasmic fibrils of the pro- 

 toplasm. Many glia fibers subse- 

 quently become disposed extracel- 

 lularly. The primitive ependymal 

 cells become greatly modified into 

 stout, slightly branching fibers ex- 

 tending through the width of the walls of the neural tube. The main body, 

 of columnar shape* with its nucleus and a distal tuft of cilia, retains its cen- 

 tral position and forms an ependymal lining for the spinal canal and the 

 ventricles of the brain. The several neuroglia constituents (astrocytes and 



FIG. 503. CELL LIN- 

 ING THE NEURAL 

 CANAL OF THE NEW- 

 LY-HATCHED RAIN- 

 BOW TROUT, SHOW- 

 ING MITOCHONDRIA 

 IN AN EMBRYONIC 

 NERVE CELL. 



Moves' technic. X 

 2000. 



FIG. 509. SECTION THROUGH MEDULLARY 

 PLATE OF A RABBIT EMBRYO. 



The cells include a large round ger- 

 minal cell. (After His.) 



