THE NERVOUS SYSTEM 



551 



ever, widens out slightly at the level of the longitudinal sulcus. Finally, 

 the lateral walls fuse together, except in the ventral region, where a 

 portion of the lumen is left as the central canal of the cord. The plane 

 of fusion of the ependymal cells persists in the adult as the dorsal septum. 

 On the ventral side of the cord, however, a median fissure is formed as a 

 result of the increase in thickness of the lateral walls and the failure of the 

 floor-plate to grow. 



The mantle layer becomes the gray matter of the cord. As a result 

 of unequal growth of this layer, dorsal, lateral and ventral gray columns 



ependymal 

 layer 



D. 100mm. E. Adult 



Fig. 454. — Transverse sections through spinal cord of the pig at various ages. Note 

 especially the parts of the adult cord derived from the ependymal, mantle, and marginal 

 layers of the embryonic neural tube. (From Patten's "Embryology of the Pig.") 



develop. By the time the embryo is three months old, the gray matter 

 has assumed in cross section its characteristic H-shape. 



In the early stages of the spinal cord, two kinds of cells are differ- 

 entiated, germinal cells which become neuroblasts, and non-nervous 

 spongioblast cells. Following the outgrowth of the neurite from a neuro- 

 blast, a number of dendrites grow in the opposite direction. According 

 to the theory of neurobiotaxis of Kappers, the neurite is stimulofugal, 

 that is, grows away from the source of stim.ulus, and the dendrites are 

 stimulopetal, that is, they grow towards the stimulus. Nerve cells of two 

 kinds arise, motor, which are Hmited to the anterior and lateral gray 

 columns, and association cells, which may lie in the dorsal column. 



Two types of supporting cells develop from the spongioblasts, ependy- 

 mal cells, with elongated processes which extend radially from the central 

 canal to the periphery of the cord, and neuroglia cells, which have shorter 



