A74 DAVIDSON BLACK 
into two parts: a large celled nucleus (nucleus magnocellularis) 
extending approximately over the length of the superficial origin 
of the motor trigeminal roots, and a smaller celled nucleus 
(nucleus parvicellularis) which is incompletely separated from 
the former and becomes directly continuous caudally with the 
motor facial nucleus (figs. 4, 5 and 6). 
I have been unable to demonstrate in Bdellostoma such a 
complete correspondence between the two motor roots of the 
trigeminal nerve and the two cell groups of its-nucleus of ori- 
gin, as was done in the case of Myxine by Rothig and Kappers 
(88). It is presumable, however, that the first root takes its 
chief origin from the nucleus magnocellularis, though some of 
the fibers of the second root appear to originate there also. 
The motor facial nucleus may be traced down to within a short 
distance of the rostral end of the vagus column. It is difficult 
to set a definite limit to this nucleus caudally, as the cells in 
this part gradually become scattered. No root fibers from any 
source, however, can be traced into this diffuse caudal portion, 
which occupies a position somewhat more mediad than the 
caudal visceral motor nucleus. 
The abducens, trochlear and oculomotor nerves are absent in 
Bdellostoma, in which there is no functional eye and conse- 
quently no intrinsic nor extrinsic musculature to require 
innervation. 
Method of reconstruction. The relations described above are 
graphically represented in figure 7C. The nuclei and motor 
roots in question are plotted as if projected upon the mid- 
sagittal plane. In view of earlier discussions of this method of 
reconstruction, it is only necessary to mention here briefly the 
following points: 
1. Only the nuclei of one side of the brain stem (right or left) 
are plotted in these charts. 
2. The longitudinal extent and relations of the nuclei and 
emergent roots are accurately plotted. 
3. The dorso-ventral extent and relations of the nuclei are 
indicated only approximately. For this reason the charts have 
been supplemented by projectoscope drawings of transverse 
