1886.] 



On the Cranial Nerves of the Newt. 



95 



the other, with the nerve-root. The nerve-elements of this mass are 

 probably derived from the nerves, and the sense-elements from the 

 epiblast. This description holds for the 5th, 7th, and 9th nerves. We 

 have not ascertained whether it is also true of the 10th. The com- 

 mon rudiment of the 7th and 8th nerves fuses with the sensory 

 epiblast at two points — one behind the other. The posterior of these 

 gives rise to the ear, the anterior to the sense-organ of the 7th nerve. 

 Soon after the fusion has taken place, the common rudiment of the 

 two nerves divides into two complete trunks, one innervating the ear, 

 and the other the sense-organ of the 7th nerve. 



At the point where the 5th nerve is fused with the skin, the Gas- 

 serian ganglion is formed. Later, it begins to separate from the 

 epiblast, retiring from the surface, but remaining still attached to it 

 by a cord of cells, which constitutes the ophthalmic branch of the 5th 

 nerve. At the same time, a main trunk grows out from the ganglion and 

 soon divides into two branches — the inferior and superior maxillary. 



The 7th and 9th nerves develop similarly to each other, but in a 

 somewhat different way from the 5th. In each the main trunk passes 

 onwards from its point of fusion with the epiblast towards the cor- 

 responding visceral cleft and fuses again with the epiblast of its dorsal 

 wall, the 7th nerve, as usual, innervating the 1st cleft, and the 9th 

 nerve the 2nd cleft. The clefts at this time are not perforated, but 

 represented merely by lateral outgrowths of the fore-gut, which have 

 fused with the epiblast at the points of contact. The ganglion which 

 has been formed on the upper part of the trunk in connexion with 

 the sensory epithelium next sinks inwards, remaining attached to the 

 epiblast by a nerve-branch, as in the case of the 5th nerve. The 

 lower part of the trunk becomes completely detached from its con- 

 nexion with the epiblast, and gives off two branches, one behind, and 

 one in front of the visceral cleft. Thus the first two clefts are each 

 provided with a post-branchial and a prse- branchial branch, derived 

 from the 7th and 9th nerves. 



We believe that all these branches of the cranial nerves are derived 

 from the original outgrowths from the brain, none of them being split 

 off from the external epiblast, as Mr. Beard has described in Elasmo- 

 branchs, and Mr. Spencer in Amphibia. The fusion of some of the 

 nerves with the skin seems to be comparable with what is known to 

 occur in vertebrate embryos generally, in the nose and ear, where the 

 nerve grows out from the brain to fuse with the sensory epithelium, 

 and we take all such cases to be merely instances of the innervation 

 of sense-organs. The innervation is generally completed very early 

 in the nose and ear, and, in the Newt, it appears to take place rather 

 eai'lier than usual in the mucous canals. Our observations seem to 

 point distinctly to this conclusion rather than to the view that the 

 nerves are, even partially, derived from the external epiblast. 



