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. 
(b) 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 
eanglion petrosum ef. Fig. 102) arises from the anterior part of 
the postotie 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 
