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THE GERM LAYER DERIVATIVES 



the hind legs. The tenth, with a branch from the ninth, forms the 

 ischio-coccygeal plexus which sends branches to the urinary bladder, 

 cloaca, genital ducts, and posterior lymph hearts. 



The Neural Crest Derivatives 

 The Sympathetic Nervous System. 



The development of the sympathetic nervous system is not under- 

 stood clearly but it is believed also to arise from the neural crests, by 

 a downgrowth along the afferent trunk to the spinal nerve and thence 

 to the dorsal aorta and the viscera beginning at about the 6 mm. stage. 

 The sheath cells probably come from the neural tube itself. Just before 

 hatching there may be seen clusters of cells around the junction of the 

 dorsal and ventral nerve roots, and these clusters will give rise to the 

 sympathetic ganglia which become associated with the dorsal aorta 

 and the viscera. These ganglia are also interconnected by paired longi- 

 tudinal strands on either side of the spinal cord. The fibers which join 

 the sympathetic ganglia with the spinal nerve are known collectively as 

 the ramus commuiiicans. 



The sympathetic system arises with the vagus (cranial nerve X), 

 and its connecting fibers extend through the jugular foramen of the 



Efferent nerve 



Sclerotome 

 (skeleton) 



Spinal cord 



Dorsal root ganglion 



Afferent nerve 



Myotome (striated muscle) 



Peripheral nerve 



1 



Spinal nerve 



Notochord 

 Dorsal aorta 



Sympathetic ramus (communicans) 

 Sympathetic ganglion 

 Sympathetic nerve to gut 



Relation of the spinal and the sympathetic nervous systems. 



