DEVELOPMENT OF ELASMOBRANCH FISHES. 199 



outgrowtlis 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 ti-unk 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. XIV. 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 0, be traced passing on a level with the 

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

 nating in branches to the skin in front of the eye (PL xvi. 

 fig. 3, VII. ; 4 a, vii. a). It courses close beneath the skin (though 

 this does not appear in the sections represented on account of 



