The Cranial Nerve Components of Petromyzon. 193 
in placing the first dorsal (»Vagus-Anhang« of HATScHER) imme- 
diately in front of the first permanent ventral in P. Planeri. The 
nerve in P. dorsatus which corresponds to this is sp.d. 2, the second 
nerve which passes beneath the N. lineae lateralis and has a dorsal 
ramus passing up lateral to that nerve. In front of this, sp.d. 1 
has the same disposition and in front of it are two general eutaneous 
roots which in the foregoing description have been reckoned with 
the vagus. These roots follow close behind the roots of the vagus, 
have a common ganglion mesial to and separate from that of the 
lateral line nerve and closely related to the roots of the vagus, which 
have not yet fused into a common trunk. That these roots should 
be reekoned with the vagus is consistent with our expeetation that 
the vagus should have a general cutaneous component as in other 
vertebrates. The objeetion to assigning the two roots to a single 
segment can scarcely have weight in view of the fact that they have 
a common ganglion and that a little farther back in the same animal 
a dorsal nerve which is undoubtedly single, sp.d. 3, has two roots 
as far apart as these two. The objection to grouping all the six 
roots of the vagus into a single segment seems more serious. 
According to the view of the vagus which the writer has re- 
cently expressed (23), it is to be regarded not as a nerve formed 
by the fusion of the nerves of two segments but as the nerve of 
one segment, into the root of which there have been colleeted by a 
gradual process the fibers belonging to the roots of the next following 
branchial nerves. It is the visceral sensory and motor eomponents 
(i. e. those which are related to the gills) which are thus eolleeted 
while the general eutaneous components of the branchial region for 
the most part remain free. Upon the basis of the information which 
we previously had of cyclostomes it was thought that one such 
dorsal (general eutaneous) nerve was colleeted into the vagus in 
Gnathostomes and that the three following remained only as embryonie 
rudiments. The fourth following forms a permanent root in Acan- 
thias only. Now if sp.d. 2 of P. dorsatus is the »Vagus-Anhang«, 
which is rocognized as fusing with the vagus in Gmnathostomes, 
sp.d. 1 must have the same fate, and we are led to the probable 
conclusion that the general eutaneous components of two nerves are 
eolleeted into the vagus in Gnathostomes along with the visceral 
sensory and motor components of as many nerves as there are 
branchial segments following the vagus. The general cutaneous 
components in the vagus form only dorsal (oceipital) rami and 
