422 DEVELOPMENT OF ELASMOBRANCH FISHES. 



becomes split into two parts, an anterior to form the seventh 

 nerve, and a posterior to form the auditory nerve. The gan- 

 glionic part of the auditory nerve is derived from the primitive 

 outgrowths from the brain, and not from the auditory involu- 

 tion. I do not feel perfectly confident that an independent 

 origin of the auditory nerve might not have escaped my notice ; 

 but, admitting the correctness of the view which attributes to 

 the seventh and auditory a common origin, it follows that the 

 auditory nerve primitively arose in connection with the seventh, 

 of which it' may either, as Gegenbaur believes, be a distinct 

 part the ramus dorsalis or else may possibly have formed 

 part of a commissure, homologous with that uniting the dorsal 

 roots of the spinal nerves, connecting the seventh with the 

 glossopharyngeal nerve. In either case it must be supposed 

 secondarily to have become separate and independent in con- 

 sequence of the development of the organ of hearing. 



My sections of embryos of stage K and the subsequent 

 stages do not bring to light many new facts with reference to 

 the auditory nerve : they demonstrate however that its gan- 

 glionic part increases greatly in size, and in stage O there is a 

 distinct root for the auditory nerve in contact with that for the 

 seventh. 



The history of the seventh nerve in its later stages presents 

 points of great interest. Near the close of stage K there may 

 be observed, in the living embryos and in sections, two branches 

 of the seventh in addition to the original trunk to the hyoid 

 arch, both arising from its anterior side ; one passes straight 

 forwards close to the external skin, but is at first only traceable 

 a short way in front of the fifth, and a second passes downwards 

 into the mandibular arch in such a fashion, that the seventh 

 nerve forks over the hyomandibular cleft (vide PI. 15, fig. 2, VII. ; 

 15 a, VII.). My sections shew both these branches with great 

 clearness. A third branch has also come under my notice, 

 whose course leads me to suppose that it supplies the roof of 

 the palate. 



In the later stages my attention has been specially directed 

 to the very remarkable anterior branch of the seventh. This 

 may, in stages L to O, be traced passing on a level with the 

 root of the fifth nerve above the eye, and apparently termi- 



