268 THE DEVELOPMENT OF THE CHICK 



nearly than any other cranial nerve to a ventral spinal nerve- 

 root. 



7 and 8. The Facial and Auditory Nerves. The ganglia of 

 these nerves at first form a common mass, the acustico-facialis. 

 But during the course of the fourth day the anterior and ventral 

 portion becomes distinctly separated from the remainder and 

 forms the geniculate ganglion; the remainder then forming the 

 auditory ganglia (cf. Fig. 102). The acustico-facialis ganglion 

 complex moves from its original attachment to the dorsal surface 

 of the brain and acquires a permanent root during the third day, 

 attached ventrally just in front of the auditory sac. 



(a) The seventh cranial or facialis nerve arises during the 

 fourth day from the geniculate ganglion which is situated just 

 above the second or hyomandibular branchial cleft. It grows 

 first into the hyoid arch (posttrematic branch), but towards 

 the end of the fourth day a small branch arises just above the 

 cleft and arches over in front of it and runs down the posterior 

 face of the mandibular arch (pretrematic branch). The origin of 

 the motor components is not known. 



(&) The further history of the auditory nerve is considered 

 with the development of the ear. 



9. The ganglion of the ninth cranial or glossopharyngeal nerve 

 (ganglion petrosum cf. Fig. 102) arises from the anterior part of 

 the postotic cranial neural crest as already described. Early on 

 the fourth day the ganglionic axones enter the base of the brain 

 just behind the auditory sac and establish the root, which con- 

 sists of four or five parts on each side. From the ganglion which 

 is situated at the summit of the third visceral arch a strong 

 peripheral branch develops on the fourth day, and extends into 

 the same arch; a smaller anterior branch develops a little later 

 which passes over the second visceral pouch and enters the 

 second visceral arch. About the same time an anastomosis is 

 formed with the ganglion of the vagus. 



10. The tenth cranial or vagus (pneumogastric) nerve is very 

 large and complex. Its ganglion very early shows two divisions, 

 one near the roots (ganglion jugulare) and the other above the 

 fourth and fifth visceral arches (ganglion nodosum cf. Fig. 102). 

 It arises by a large number of fine rootlets on each side of the 

 hind-brain behind the glossopharyngeus, and the roots converge in 

 a fan-like manner into the proximal ganglion; from here a stout 



