THE SEGMENTATION OF THE BODY. 



159 



evidence there can be little doubt that such a segmentation does 

 take place, and that it possesses segments corresponding to those 

 of the body. A group of cells in each segment, afterwards those 

 of the anterior horn, sends out processes to all the muscles of the 

 primitive body segment in which it is situated. The anterior 

 root of a spinal nerve is thus formed. Other motor cells send out 

 processes which reach viscera through the white rami communi- 

 cantes and sympathetic system (Fig. 128). 



post. div. 

 post. root. gang, 

 ant. div. 

 vert. gang, 

 renal gang, 

 semilunar, gang. 

 Auerbach's plex. 



Fig. 128. 



epibl. 

 post. root. gang. 



neur. canal 

 vert. gang. 



a ^ — renal gang. 

 *^~~~~~—semilun. gang. 



Fig. 129. 



Fig. 128.— Diagram of the Nerve System of the 11th Dorsal Segment. 

 Fig. 129. — A diagram showing the derivation of the Parts of the Nerve System of 

 the 11th Segment in the Embryo. 



(2) Besides the anterior horn cells, two other nerve groups 

 become connected with each segment. At the point where the 

 medullary plates are cut off from the epiblast to form the neural 

 canal, a crest, the neural crest, grows out on each side (Fig. 129) 

 composed of the cells which formed the junctional line between 

 medullary plates and epiblast. A group of these neuroblasts grows 

 into each segment and forms the posterior root ganglion. Each 

 neuroblast within the ganglion sends out a process which bifur- 

 cates, one branch or fibre growing into the cord and ending in the 

 posterior column and cells of the posterior horn, the other passing 

 to the skin, muscles, etc., of the segment. The posterior nerve 

 root is thus formed by the ingrowing processes of the cells of the 

 posterior root ganglion, and thus the body segment in which the 

 outgrowing processes are distributed is brought into sensory com- 

 munication with the central nervous system. The anterior and 

 posterior roots unite to form a spinal or segmental nerve. Like 

 the artery it divides into a posterior division for the epaxial part 



