194 J. B. Johnston 
these components of the following two or more segments are quite 
lost owing to the reduction of post-otice myotomes and corresponding 
ectodermal area. 
Returning now to the vagus root, it seems to the writer that 
we have here a condition of incomplete concentration of the elements 
going to make up the vagus of Gnathostomes, whose relative posi- 
tions may be explained at least in part by reference to the conditions 
in Myxine. In Myzine it is stated (8) that the vagus and glosso- 
pharyngeus are represented by a single root. 'The writer has offered 
in explanation of this the suggestion that the process of collecting 
the roots of more caudal branchial nerves into those more cephalie 
has gone one step farther in Myzine than in other forms so that 
the roots of all the branchial nerves have been colleeted into the 
glossopharyngeus. Whether due to a tendencey of the same sort or 
not, we see in P. dorsatus that the visceral sensory root of the vagus 
(root 4) lies far forward near to the visceral sensory and motor root 
of the glossopharyngeus (root 3). Following it are the three motor 
roots and lastly the two general eutaneous roots. We may imagine 
that the visceral sensory root has shifted forward and that the motor 
have followed it, while the general eutaneous have not been affected. 
This would imply that the original position of the proper vagus root 
was in the vieinity of root 8 or 9, and if so the interval between 
it and the glossophäryngeus would be more nearly equal to that 
between the glossopharyngeus and facialis. 
While these considerations are indireet and conjectural, they 
are consistent with the direct evidence upon which these eutaneous 
roots were assigned to the vagus; namely: the presence of a single 
&anglion in elose relation with the vagus roots, the contribution of 
fibers to the vagus roots proximal to the first ganglion of the vagus 
trunk, and the absence of a dorsal ramus having typical relations 
to the lateral line nerve and the myotomes. 
The segmental relations in the post-otie region of P. dorsatus 
are expressed in the following table which should be compared with 
the similar table in (23): 
