MOTOR NUCLEI IN PHYLOGENY 489 
through the ventral portion of the VII motor nucleus to reach 
the lateral side of the posterior longitudinal bundle. Here it 
turns and courses caudad to its cells of origin in the posterior 
visceral column lateral to the fasciculis longitudinalis medialis. 
This mode of origin is essentially similar to that described by 
Kappers in Scyllium (72, p. 384, fig. 1.). 
In Selache and in Hexanchus the ascending motor VII root 
may be traced caudad as far as the motor glossopharyngeal 
fibers extend. It results from this that the motor VII nucleus 
overlaps that of the glossopharyngeus and the two nuclei are 
quite as intimately associated as are those of the sensory VII 
and IX roots. This condition is indicated in the reconstruction 
charts in figure 17 where the caudal limit of the descending VII 
and IX motor roots is marked by a heavy line. 
The motor vagus nucleus in Selache extends for a long dis- 
tance into the upper part of the cord and in consequence it over- 
laps the somatic motor column to a much greater degree than 
was the case in Bdellostoma. Its exact caudal extent was only 
determined by cutting an additional series of sections of the 
cervical cord. For comparison the posterior visceral column in 
Hexanchus was re-examined and it was found the vagus nucleus 
in this form could indeed be traced further caudad than had 
been indicated in previous reconstructions, though its exact 
caudal limit was not determined. 
Abducens nucleus and roots (Nu. et. rad. N. VI). The abducens 
nucleus in Selache occupies a position about midway between the 
exit levels of the motor VII and IX roots. Its cells of origin 
are scattered among the fibers of the posterior longitudinal 
bundle and its fibers converge to form three rootlets which pass 
directly ventrad and emerge in series from the ventral periphery 
of the bulb (fig. 12). The.nucleus in Selache is not quite so 
dorsally situated as in Hexanchus or Heptanchus, but its rela- 
tion to the vestibulo-motor fibers is essentially similar in all 
three forms. 
The position of the emergent roots of the abducens at a 
more caudal level than those of the motor VII nerve is character- 
istic of sharks and also of ganoids and amphibia. In the case 
