162 



EMBRYOLOGY 



comes more and more restricted and finally is nothing more than a lining of 

 the central canal. The differentiating nerve cells become concentrated around 

 the central region of the cord, forming the gray matter, while the white 

 matter remains near the surface. Within the gray matter various regions begin 

 to differentiate. The most clearly marked is the motor region, which has sharp 

 boundaries. The number of cells that differentiate into motor neurons in this 

 motor region is dependent on the amount of muscle present in the surround- 

 ings. In grafting experiments in which muscle is added to or taken away from 

 a specific region, the number of cells in the motor region of the spinal cord 

 increases or decreases accordingly. 



CENTRAL CANAL 



O 



O 



EPENDYMAL 

 q\ LAYER 

 O 



m 



J0-J-NEUROBLAST 

 '0 



L WHITE MATTER 

 ANTERIOR COMMISSURE 



EPENDYMAL 

 LAYER 



CENTRAL CANAL 

 WHITE MATTER 

 MOTOR AREA 



DORSAL HORN 

 GRAY MATTER 



EPENDYMAL 

 LAYER 



CENTRAL 



CANAL 



WHITE MATTER 



ENTRAL HORN 



Fig. 95. Differentiation within 

 the spinal cord. Top: An early 

 stage, with a large central canal 

 lined by an ependymal layer. Neuro- 

 blasts are concentrated in an outer 

 layer, while the white matter is at 

 the periphery of the cord. The re- 

 gion of crossing of nerve fibers is 

 termed the anterior commissure. 

 Colter : The central canal and epen- 

 dymal layer are relatively smaller, 

 and a large area of nerve cells sur- 

 rounds the ependymal layer. The 

 motor region of the cord becomes 

 sharply delimited. Bottom: General 

 structure of the fully differentiated 

 nerve cord. The ventral horn cor- 

 responds to the motor area previ- 

 ously described. 



