BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 109 
ment of this cleft is very obvious in horizontal longitudinal 
sections of certain stages, and is represented in fig. 47. Here 
there is a considerable hypoblastic depression (cl. v1) of the 
pharynx just behind the last or fifth branchial cleft. 
Corresponding to it is a shallower but still marked epiblastic 
involution. Along the posterior side of this hypoblastic de- 
pression the intestinal branch of the vagus 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 his 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 
accordance with Van Bemmelen’s researches on the thymus. 
His discovery of thymus elements behind the vagus is men- 
tioned by Dohrn’ 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 con- 
tains 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 1 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 
1 Dohrn, “Studien zur Urgeschichte, &c.,” No. vii, ‘Mittheil. a. d. Zool. 
Stat.zu Neapel.,’ Bd. vi, Heft 1. 
