EMBRYOLOGY OF TADPOLE AND CHICK 575 



It is important that the student note how the mesenchyme of the 

 mandibular arch is formed by the process of growth just described. The 

 mesodermal and ectodermal cells have so intermingled at the point of 

 fusion that the separate cells of ectoderm and mesoderm are now indis- 

 tinguishable. (Fig. 334.) 



The dorsal and superficial cells of the crest ganglion retain their 

 nervous character and come into close relation to the large placode close 

 to them, but the superficial portion of the placode, which is sensory, now 

 disappears. The deep or ganglionic portion not only enlarges, but 

 divides into two parts. The anterior portion becomes the ophthalmic 

 ganglion of the ophthalmic branch of the V nerve. The fibers of this 

 branch grow cephalad through the dorsal head region, and also grow 

 medially and connect with the medulla oblongata. 



The posterior portion of the placode ganglion fuses with the crest 

 ganglion to form the Gasserian ganglion or trigeminal ganglion. 



It is from the cells of the trigeminal ganglion that fibers arise which 

 run to the medulla on the dorsal side, and these fibers form the sensory 

 root of the V nerve. 



Then, too, there are fibers which grow out from the ganglion to pass 

 to the surface of the head to form the cutaneous branch of the V nerve, 

 while the fibers which pass in front of and behind the mouth are called 

 the mandibular and maxillary branches respectively. 



All of these branches as well as those from most of the branchi- 

 omeric nerves can be seen before the opening of the mouth. 



THE FACIAL AND AUDITORY, OR THE VII AND VIII NERVES 



Both of these nerves are derived from the acustico-facialis crest 

 ganglion and the placode associated with it. The VII nerve is connected 

 with the hyomandibular cleft, while the VIII nerve is a purely sensory 

 (auditory) nerve, and so not one of the branchiomeric series. 



The greater portion of the crest ganglion, as with the V nerve, con- 

 tributes to the mesenchyme of the hyoid arch, although the nervous por- 

 tion of the crest ganglion is more extensive than that of the V nerve, 

 which is due to the fact that a greater portion of the original ganglion 

 retains its nervous function. 



The superficial or nervous character of the placode does not dis- 

 appear in this case, but keeps on becoming larger, after which it sinks 

 below the surface of the head and invaginates to form the auditory sac. 

 (Fig. 334.) 



The deep placode ganglion cells which are in connection with this 

 sensory epithelium remain in contact with the sac to form the root of 

 the VIII nerve. 



The remaining portion of the placode ganglion joins with the nerv- 

 ous portion of the crest ganglion to form the ganglion of the VII nerve. 

 It is from this ganglion that fibers pass to the medulla and to the hyoid 



