NERVOUS SYSTEM OF THE VERTEBRATA. 



377 



nerve, which is probably the remains of a part of the commissure which 

 connected the posterior branches of the vagus, at a stage in the evolu- 

 tion of the Vertebrata, when 

 the jjosterior visceral clefts 

 were still present. These 

 branches of the vagus are 

 probably partially preserved 

 in the ramifications of the 

 intestinal stem of the vagus 

 (Gegenbaur). The origin of 

 the ventral commissure, con- 

 tinued as the intestinal 

 bi-anch of the vagus, has not 

 been embiyologically worked 

 out. 



The lateral nerve may 

 very probably be a dorsal 

 sensory branch of the vagus, 

 whose extension into the pos- 

 terior part of the trunk has 

 been due to the gradual back- 

 ward elongation of the lateral 

 line', causing the nerve sup- 

 plying it to elongate at the 

 same time [vide Section on 

 lateral line). 



In the Chick the com- 

 mon rudiment for the vagus 

 and glossopharyngeal nerves 

 (Marshall), which has already 

 been spoken of, subsequently 

 divides into two parts, an 

 anterior forming the glosso- 

 pharyngeal nerve, and a 

 posterior forming the vagus 

 nerve. 



The seventh and au- 

 ditory nerves. As shewn 

 by Marshall's and my own 

 observations there is a com- 

 mon rudiment forthe seventh 



and auditory nerves. This rudiment divides almost at once into two 

 bi-anches. The anterior of these pursues a straight course to the hyoid 

 arch (fig. 271 A, VII.) and foi-ms the rudiment of the facial nerve; the second 

 of the two (fig. 271 A, au.n), which is the rudiment of the auditory nerve, 

 develops a ganglionic enlargement and, turning backwards, closely hugs 

 the ventral wall of the auditory involution (fig. 272). 



The seventh or facial nerve soon becomes more complicated. It early 

 develops, like the glossopharyngeal and vagus nerves, a branch, which 



' The peculiar distribution of branches of the fifth and seventh nerves to the lateral 

 line, which is not uncommon, is to be explained in the same manner. 



Fio. 271. Views of the head of Elasmo- 

 branch embryos at two stages as transparent 

 objects. 



A. Pristiurus embiyo of the same stage as fig. 

 28 F. 



B. Somewhat older Scjllium embryo. 



III. third nerve ; V. fifth nerve ; VII. seventh 

 nerve ; au.n. auditory nerve ; gl. glossopharyngeal 

 nerve ; Vff. vagus nerve ; fb. fore-brain ; pn. pineal 

 gland; mb. mid-brain; /t6. hind-brain ; f».r. fourth 

 ventricle; cb. cerebellum; ol. olfactory pit; op. 

 eye ; cm. V. auditory vesicle ; m. mesoblast at base 

 of brain ; ch. notochord ; ht. heart ; Vc. visceral 

 clefts ; eg. external gills ; pp. sections of body 

 cavity in the head. 



