THE LATER DEVELOPMENT OF THE FROG 141 



The remainder of the placode ganglion joins with the nervous 

 portion of the crest ganglion, forming the ganglion of the VII 

 nerve, from which centripetal processes grow out connecting 

 the ganglion with the medulla, while centrifugal processes 

 extend into the hyoid arch and neighboring regions (hyoman- 

 dibular and palatine nerves). 



The Glossopharyngeal and Vagus (Pneumogastric) Nerves 

 (IX and X}. These are the nerves of the remaining visceral 

 clefts, the first to fourth branchial clefts (third to sixth visceral 

 arches). The IX nerve is limited to the region of the first 

 branchial cleft, while the X nerve is distributed around the 

 remaining gill clefts, and is to be regarded as a compound nerve, 

 equivalent to several branchiomeric nerves. The crest gang- 

 lion of these nerves forms the large posterior section of the 

 crest in the head region (Fig. 44, E, H}. Essentially it resem- 

 bles that of V; it rapidly grows very large, extends farther 

 ventrally and posteriorly, and contributes to the formation of 

 mesenchyme to a considerably less extent than the ganglion of 

 the V nerve. The placode of the IX nerve develops typically; 

 its superficial sensory portion disappears, while its ganglionic 

 portion comes only slightly into relation with the crest ganglion. 

 Posterior to this, the large placode of the X nerve appears simul- 

 taneously and has a similar early history, save that it fuses 

 more extensively with the nervous portion of the crest ganglion. 

 When the fibers grow out from the IX and X ganglia they pass 

 together into the medulla as a single root. The IX ganglion 

 is only incompletely separated from that of X by the passage 

 of the anterior cardinal vein (see below). From the IX por- 

 tion of the ganglion, which is thus almost wholly placodal in 

 origin, fibers grow peripherally into the usual relation with the 

 first branchial cleft, while from the mixed ganglion of the X 

 nerve, branches grow out with typical relations to all the remain- 

 ing clefts. 



From the vagus ganglion certain other important processes 

 grow out posteriorly. From the placode a considerable tongue 

 of cells grows posteriorly, forming the rudiment of the sense 

 organs of the lateral line (see below), and accompanying this, 



