MOTOR NUCLEI IN PHYLOGENY 473 
It is quite possible that further study of a large number of 
specimens may confirm Worthington’s observations and show 
that the spino-occipital nucleus overlaps the vagus area in some 
specimens to a greater extent than in the one charted in figure 7. 
Individual variation, especially in this part of the brain in 
Bdellostoma, is rather to be expected, so that Johnson’s obser- 
vation that “the segment of the glossopharyngeus also has a 
somatic motor nerve in Bdellostoma”’ (52, p. 194) should be 
modified to a certain extent. 
Motor vagal nuclei and roots (Nu. et rad. mot. N. X). The re- 
maining motor nuclei in this form are visceral in character and 
may conveniently be described as forming two columns of cells, 
one on each side of the bulb in the ventro-lateral area. Each 
column is further definitely subdivided into a rostral and a caudal 
portion (vide fig. 7). 
The caudal viscero-motor column corresponds to the caudal 
atero-ventral column described by Holm in Myxine (lLec.). It 
is the motor nucleus of the vagus and consists of a continuous 
column of medium sized cells from which five (?) vagus rootlets 
arise. It is probable that the first two very small rootlets 
should be considered together as the first vagus root, though the 
first small rootlet of this series in all probability represents the 
motor glossopharyngeus of other authors (q. v.). The pres- 
ence of a true glossopharyngeal nerve in Bdellostoma, however, 
is highly improbable (vide infra.). 
Rostrally the vagus nucleus begins a few sections above the 
level of its first motor rootlet, while caudally it extends for 
some distance below the level of the upper prolongation of the 
anterior horn. 
Motor trigemino-facial nuclei and roots (Nu. et rad. mot. Nn. 
V-VII). The rostral viscero-motor column (frontal latero- 
ventral column of Holm) contains the motor nuclei of the 
trigeminus and facialis (fig. 7 C). | 
The motor root of the trigeminus consists of two parts, of 
which the rostral is the larger. These two portions are only 
evident on the periphery of the bulb and at once unite to form a 
single trunk. The motor nucleus of this nerve is also divisible 
