322 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



larged and is constricted at two points to form the three primary brain vesicles. 

 The caudal two-thirds of the neural tube, which remains smaller in diameter, con- 

 stitutes the anlage of the spinal cord. 



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 ca\'ity is di- 



Neural cavity 



Maii,iinl h\ir 



Dors il } ( I 



Epdihm il 1 1\ I 



Spm il ^ HI I n 



Manllc I n 

 Dorsal ram II 



'C-' 



^ 



Mj 





Nerve trunk Sympathetic ganglion 



Fig. 325. — ^Transverse section through a 10 mm. human embryo at the level of the arm buds showing the 

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



minished from side to side. By the end of the first month three layers have been 

 developed in its wall as described in Chapter X, 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 neuroblasts and 

 neuroglia cells to the mantle layer. The proUferation of cells ceases first in the 



