322 THE MORPHOGENESIS OF THE CENTRAL- NERVOUS SYSTEM 



THE SPINAL CORD 



The spinal portion of the neural tube is at first nearly straight, but is 

 bent with the flexure of the embryo into a curve which is convex dorsally. 

 Its wall gradually thickens during the first month and the diameter of its 

 cavity is diminished from side to side. By the end of the first month, 



Neural cavity 



Marginal layer 



Dorsal root 



Ependymal layer 



Spinal gangl 



Mantle layer 

 Dorsal ramus 



Ventral 

 root 



Nene trunk _. Sympathetic ganglion 



FIG. 325. Transverse section through a 10 rnm. human embryo at the level of the arm buds, 

 showing the spinal cord and a spinal nerve of the right side. X 44. 



three layers have been developed in its wall, as described on p. 301 (Fig. 

 325). These layers are the inner ependymal layer, which forms a narrow 

 zone about the neural cavity, the middle mantle layer, cellular, and the 

 outer marginal layer, fibrous. 



The Ependymal Layer is differentiated into a dorsal roof plate and a 

 ventral floor plate (Fig. 326). Laterally, its proliferating cells contribute 

 ncuroblasts and neuroglia cells to the mantle layer. The proliferation of 

 cells ceases first in the ventral portion of the layer, which is thus narrower 

 than the dorsal portion in 10 to 20 mm. embryos (Figs. 325 and 326). 

 Consequently, the ventral portion of the mantle layer is differentiated first. 

 The neural cavity is at first somewhat rhomboidal in transverse section, 



