54 



NERVOUS SYSTEM OF VERTEBRATES. 



constriction into a longer caudal segment for the vagus nerve 

 and a shorter cephalic segment for the glossopharyngeus. The 

 development of the ganglia and fibers of these nerves proceeds 

 in essentially the same manner as in the case of the spinal nerves, 

 except that the ganglia migrate somewhat farther from the brain 



crest. 



D.I' 



FIG. 30. Reconstruction of the peripheral nerves in a four weeks human 

 embryo, 6. g mm. long. Enlarged 16.7 diameters. From Streeter. Ot. v., audi 

 tory vesicle; i, 2, 3, visceral arches; C. i, first cervical nerve; D. i, first thoracic 

 nerve; L. i, first lumbar nerve; S. i, first sacral nerve. 



wall and come to lie outside the muscle somites and against the 

 ectoderm above the gill slits. The ganglion of the glossopharyn- 

 geus lies over or nearly over the second gill slit. The vagus is 

 connected with a series of ganglia equal in number to the number 

 of gill slits from the third onward. In cyclostomes (Figs. 28, A 

 and B) the ganglion of the vagus is formed first over the third 



