182 JOHN BEARD. 



involution. Along the posterior side of this hypoblasts depression 

 the intestinal branch of the vagns runs. Gegenbaur has regarded this 

 branch of the vagus as containing rudiments of post-branchial branches 

 of aborted clefts ; and I think that in the relationship of this intestinal 

 branch in Torpedo to rudiments of a sixth cleft we have a new support 

 for this view. 



The ramus intestinalis is, as Van Wijhe states, mainly made up of 

 the post-branchial branch of the last true visceral arch ; but, as just 

 stated, it must also contain portions of the post-branchial branches of 

 one or more aborted clefts. Certainly this is the case in Torpedo. 



In the question of the homology of this nerve I can only agree with 

 Van Wijhe in rejecting Balfour's view that the ramus intestinalis is a 

 commissure. 



The statement just made concerning aborted clefts is also in accord- 

 ance with Van Bemmelen's researches on the thymus. His discovery 

 of thymus elements behind the vagus is mentioned by Dohrn 1 in his 

 last great work, as supporting his view that Vertebrates formerly 

 possessed many more gill-clefts than they do at present. The question 

 will be returned to later on. 



It is thus seen that in Torpedo at any rate the vagus contains the 

 elements of at least four segmental nerves and the rudimentary 

 portion of a fifth. 



The first one of the lot is, shortly after its first development, slightly 

 separated from the fused mass which contains the sense organs and 

 ganglionic portions of the rest. 



Hence vagus I. can be treated alone. As mentioned before, its 

 post-branchial branch passes along the posterior wall of the second 

 branchial cleft to the musculature of the cleft. The skin takes no 

 part in its formation. Above the cleft the main nerve fuses with the 

 skin, and there, as in other cases, the ganglion and primitive branchial 

 sense organ are formed. In this case too — and fig. 34 shows it fairly 

 we ll — the sensory thickening must be considered as taking part in the 

 formation of the ganglion. 



Later, the ganglion separates from the skin, and, along with this 

 separation, the sensory thickening grows forwards and takes also a 

 dorsal direction, a supra-branchial nerve splits off, and the sense organs 

 formed are part of the supra-temporal branchial sense organs (fig 51, 



1 Dohrn, "Studien zur Urgeschichte, &c, No. vii, 'Mittheil. a. cl. Zool. Stat, zu Neapel,' 

 Bd. vi, Heft 1. 



