lead segments 
;omites 
;ranchiomeres 
ten. eutaneous 
omatie motor 
'isc. sensory 
'isec. motor 
The Cranial Nerve Components of Petromyzon. 195 
8 9 10 11 12 13 14 15 | 16 
+ b) 6 7 8 9 10 11 
br.1 br.2 br.3 br.4 br.5 br.6 br.7 | 
comp’'t | comp’t |sp.d.1 | sp.d.2 |sp.d.3 |sp.d.4 |sp.d.5 |sp.d.6 |sp.d.7 
inIX inX | | | 
— |sp.v.1 |sp.v.2 |sp.v.3 |sp.v.4 |sp.v.5 |sp.v.6 |sp.v.7” |sp.v. 
comp’t | comp’t | comp’t | comp’t | comp’t | comp’t | comp’t | comp’t | eomp’t 
inIX IHK IR IR In. lin Koh N Xu... AN X, in in 
br.1 br.2 br. 3 br. 4 br.5 br.6 | sp.d.6 ! sp.d.7 
| comp’t | comp’t | comp’t | comp’t comp’t |, comp’t | comp’t | comp’t | comp’t 
an Ol Ma Dein 2, - Mi EN HER in Rn N Ti in 
| br.1 br.2 br, 3w|w hr 4 br.5 br.6 | sp.d.6 | 8p.d.7 
From this it appears that one ventral spinal root is present anterior 
to the first independent dorsal root. This is not surprising in view 
of the persistence of all the somites as permanent myotomes. If it 
be thought that the eutaneous roots ($ and 9) have been unwarrantably 
assigned to the vagus, that they belong to a separate segment from 
the vagus proper, and that their presence gives evidence in favor 
of the view that the vagus represents two segments, reference to 
the above table or to plate V, will show that we should then be led 
to the absurd concelusion that these roots belong with ». 2, sp.d. 1 
with v. 3, sp.d. 2 with v. Zete; or, in other words, that each dorsal 
nerve is one and one-half segment anterior to its ventral nerve. 
In accordance with the above considerations and in agreement 
with the account of the embryology given by KoLTZorr, it is be- 
lieved that the branchial apparatus originally extended to the front 
edge of somite 10 (leaving out of account branchial sac 8 which 
appears in the ontogeny, and others which have been lost), and that 
the spinal nerves ineluding d. 5 and v. 6 belonged to this branchial 
region. The expansion of the branchial apparatus has, therefore, 
carried the last gill slit backward through the length of ten spinal 
segments, since it now lies in the plane of the 15'% dorsal spinal. 
Sp.v.6 was the nerve of the first post-branchial myotome, and it 
is this myotome 10 which supplies the first bud for the sub-branchial 
musculature. We should expect that the nerve sp.v. 6 would be tlıe 
first to contribute to the »hypoglossus«. In fact, sp.v. 4 is the first 
to eontribute to the epibranchial trunk, but it is impossible to know 
whether any of its fibers go around to the sub-branchial musecles. 
In P. Planeri eight myotomes, 10 to 17 inclusive, send buds into 
the sub-branchial muscles. In P. dorsatus possibly a larger number 
