270 THE CHICK. 



its development has already been described as that of a typical 

 cranial nerve. 



VIII. The auditory, or eighth cranial nerve (Fig. 115, vm) 

 is, in the chick, continuous with the facial nerve from its first 

 appearance. It is a short stout nerve, which at a very early 

 period, about the fiftieth hour, comes in contact with the auditory 

 epithelium, and fuses with this. The subsequent development 

 of the nerve consists mainly in its division, distally, into 

 branches supplying the several special patches of the auditory 

 epithelium, and will be described more fully in the section 

 dealing with the development of the ear. 



So far as the chick is concerned, there appears to be no 

 reason for separating the facial and auditory nerves from each 

 other. The two together make up a typical cranial nerve, of 

 which the auditory nerve represents the cutaneous branch, 

 greatly hypertrophied in consequence of the large size and 

 importance of the sensory patch, i.e. the internal ear, which it 

 supplies. 



IX. The glosso-pharyngeal, or ninth cranial nerve (Figs. 115, 

 ix, and 119, NB), is at first continuous with the pneumogastric 

 or tenth nerve, a single elongated strip of the neural ridge on 

 the roof of the hind-brain, immediately behind the ear, giving 

 origin to both these nerves. The strip divides, before the end of 

 the second day, into an anterior or glosso-pharyngeal portion, 

 and a posterior or pneumogastric portion. 



The glosso-pharyngeal develops as a typical cranial nerve ; 

 it early acquires connection with a sensory patch of the surface 

 epiblast, and its main stem, beyond this point, runs downwards 

 along the first branchial arch (Fig. 115). The root of attach- 

 ment of the nerve to the brain early becomes multiple, consisting 

 of four or five small rootlets, which spread out in a fan-like 

 manner on entering the brain. The multiple character of the 

 roots of the glosso-pharyngeal nerve is of interest, as showing 

 that the similarly multiple nature of the roots of the third nerve 

 is not incompatible with a possible origin of this latter from the 

 neural ridge. 



X. The pneumogastric, or tenth cranial nerve (Fig. 115, x) 

 arises from the posterior part of the outgrowth from the neural 



