MOTOR NUCLEI IN PHYLOGENY 507 
rostral end of the nucleus, the bundle curves ventro-laterad to 
reach the periphery. In this part of its course the fibers become 
arranged in the form of two distinct emergent bundles, one more 
rostrally placed than the other. These rootlets are separated 
only by a few sections, and it was not possible to demonstrate 
any specific localized relationship between the two rootlets on 
the one hand, and the two nuclear subdivisions on the other; 
each rootlet apparently derives its fibers from the nucleus as a 
whole, and not from any special portion of it. 
Aside from the partial division of the motor V nucleus and the 
double nature of its emergent root, the relations of this com- 
plex in Polyodon correspond closely to those in Lepidosteus 
(fig. 25 A and C). In all other ganoids examined, the nucleus 
in question is dorsally situated and its chief bulk is placed 
caudad of the level of its root entrance. The last mentioned 
relation is especially evident in Amia, where the nucleus ex- 
tends caudally well beyond the level of the emergent motor VII 
root. In selachians, on the other hand, the motor V nucleus is 
placed more nearly on the level of its root entrance and, while it 
always extends a considerable distance rostrad of this point, 
the nucleus never extends caudad of the exit level of the motor 
VII root (fig. 17). Among cyclostomes, however, the motor V 
nucleus in Petromyzon closely corresponds to that of Polyodon 
both in its position and its extent. Similarly in Myxine and 
Bdellostoma, the motor V nucleus agrees with that of Polyodon 
both in its rostro-caudal extent and in its relation to the exit 
levels of the motor V and VII roots, though their resemblance is 
somewhat obscured owing to the very ventral situation of the 
nucleus in question and its continuity with motor VII column 
in the former animals (figs. 7, 17 and 25). 
Oculomotor and trochlear nuclei and roots (Nu. et rad. Nn. IIT 
and [V. The oculomotor and trochlear nuclei are both small in 
Polyodon, as in other ganoids. The oculomotor nucleus lies 
upon the dorso-median aspect of the fasciculus longitudinalis 
medialis in the Sylvian gray and is placed entirely rostrad of the 
caudal border of its emergent root (fig. 25 A). 
