DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 87 



Fig. 26, which represents a longitudinal section through the 

 hind brain and mouth of a chick of 123 hours, shows the glosso- 

 pharyngeal (IX) with its ganglion and its two branches, with 

 their relations to the second visceral cleft, very clearly. 



A point of considerable interest, shown in figs. 22, 23, and 26, 

 is that the glossO'pharyngeal is not attached to the hind brain by a 

 single root like the fifth and seventh, but by a variable number — 

 four or five — of small roots which spread out in a fan-like manner 

 and enter the brain separately. In this respect they resemble 

 the vagus roots and the posterior roots of the spinal nerves, and 

 difi'er from the anterior cranial nerves, which would appear to be, 

 in this respect at least, more primitive. In figs. 23 and 26 the 

 anterior of these roots is seen to stretch forwards over the audi- 

 tory capsule, so as to bridge over to a great extent the gap 

 between the glosso-pharyngeal behind and the facial and auditory 

 in front. I regard this as an indication of a commissural con- 

 nection between these nerves. 



It is worth noticing that in the frog there is a dorsal commis- 

 sural cord connecting the roots of the facial and glosso-pharyngeal 

 nerves together. This, I would suggest, is part of the longitu- 

 dmal commissure derived from the neural ridge.^ 



The vagus. — This is derived from the posterior of the two 

 branches into which the outgrowth conmon to it and the glosso- 

 pharyngeal divides. It is at first rather smaller than the 

 glosso-pharyngeal, and even as late as the 122nd hour {vide 

 fig. 26, IX, X) there is hardly any appreciable difference in size 

 between the trunks of the two nerves. Its root soon acquires 

 the multiple character noticed in the glosso-pharyngeal (figs. 22, 

 23, and 26). 



The condition of the vagus at 122 hours is shown in fig. 16, X, 

 where the nerve is seen to run downwards and backwards parallel 

 to the glosso-pharyngeal for a certain distance. It then expands 

 into a very large fusiform ganglion, which overlies the third and 

 fourth visceral clefts. Beyond the ganglion the nerve is con- 

 tinued, as the intestinal branch, in close connection with the 

 walls of the alimentary canal. 



The anterior root of the vagus is continuous, without entering 

 the brain, with the posterior root of the glosso-pharyngeal (figs. 23 

 and 26). 



The auditory capsule in its early stages of development lies 

 very close to the hinder end of the brain, and its hinder border 

 is separated from the first protovertebra by an interval not ex- 

 ceeding the width of a single protobertebra. If we hold that the 



* Vide Huxley, 'Encyclopaedia Britannica,' vol. i, Art. "Amphibia," fig. 

 25, p. 766; also ' Proc. Zoological Society,' IS?^, p. xxxi, fig. 5, Sy. 



