576 THE EMBRYOLOGY OF THE FROG 



arch and associated regions, to form the hyomandibular and palatine 

 nerves. 



THE GLOSSOPHARYNGEAL AND VAGUS (PNEUMOGASTRIC) 

 OR IX AND X NERVES 



The remaining visceral clefts, that is, the first to fourth clefts, or 

 third to sixth visceral arches, are associated with the IX and X nerves. 



The IX nerve is limited to the first gill cleft alone, but the X nerve 

 is associated and distributed to the others. It is to be considered a com- 

 pound nerve, as it is made up of several branchiomeric nerves. 



The large posterior part of the neural crest in the head region is the 

 portion associated with the IX and X nerves. 



Its growth is much like that of the V nerve, though it does not 

 assist in forming so much mesenchyme. 



The superficial sensory portion of the placode of the IX nerve dis- 

 appears, and its ganglionic portion is only slightly related to the crest 

 ganglion. 



Posterior to this, the larger placode of the X nerve appears simulta- 

 neously, and passes through similar stages, but in this case there is a 

 more extensive fusion between it and the nervous portion of the crest 

 ganglion. 



The fibers from the IX and X ganglion pass out together to the 

 medulla as a single root. The anterior cardinal vein partially separates 

 the IX from the X ganglion. 



The fibers which pass out from the IX nerve portion of the ganglion 

 are practically all placodal in origin, and pass to the first branchial cleft, 

 while the fibers coming from the mixed ganglion of the X nerve are 

 connected with all the remaining clefts. 



It is well to pay considerable attention to the X nerve, as it is one 

 of the most important nerves in the body, being connected: with many 

 important, structures. 



.,..,, It is from the X ganglion that other processes than those just men- 

 tioned, also grow. A considerable tongue of cells grows -out posteriorly 

 to form the sense organs of the lateral line, shortly to be discussed, while 

 the fibers which are to become the lateral line nerves accompan}^ this 

 tongue (Fig. 340). These latter fibers are present only during the tad- 

 pole stage. 



Then there are branches which pass to the thoracic and abdominal 

 organs to form the visceral branch of the X nerve. 



As sensory nerves pass to a general center, and motor nerves pass 

 from a center to some outlying region, it is well to appreciate how some 

 of the nerves mentioned above come to be mixed, that is, having both 

 sensory and motor fibers running along side by side. 



The motor fibers of the branchiomeric series do not arise by sepa- 

 rate roots (Fig. 336) as do the sensory, but from neuroblasts in the walls 



