516 DAVIDSON BLACK 
From the above it becomes evident that, though the caudal 
portion of the motor vagus nucleus is well formed in ganoids, 
yet in these animals the elements homologous with those form- 
ing the bulk of the accessory nucleus of higher forms must neces- 
sarily be few. It is also evident that, apart from the question 
of the homology of the muscles whose innervation has been noted 
with similarly named muscles in other forms (Herrick, 34) the 
variations in peripheral motor distribution in this region within 
the ganoid group appear to be faithfully reflected in the arrange- 
ment of the central motor nuclei. 
Motor facial and glossopharyngeal nuclei and roots (Nu. et Bae 
mot. Nn. VII-IX). The arrangement of the motor nuclei form- 
ing the rostral part of the caudal viscero-motor column in gan- 
oids is, inter se, typically selachian, consisting as it does of a nu- 
clear mass from which arise the motor facial and glossopharyngeal 
roots and which is continuous caudally with the motor vagus 
nucleus. There is, however, a significant difference to be ob- 
served in the relations to neighbouring structures of this part 
of the caudal viscero-motor column as a whole in the two 
groups. 
In every case among ganoids the chief bulk of the elements 
forming the motor facial nucleus, lies rostrad of the exit level 
of the motor IX root. In selachians, on the other hand (at 
least among the more primitive members of the group) the reverse 
is true and the motor VII nucleus lies chiefly caudad of the exit 
level of the motor LX root. 
The different position of the motor VII nucleus with refer- 
ence to the exit level of the motor glossopharyngeus in these 
two groups might of course be due to a shifting of the motor 
IX root. However, the relation of the nucleus in question to 
that of the abducens nerve and to its root in both ganoids and 
sharks would tend to negative such a supposition. 
It has been pointed out already that the position of the 
abducens nucleus with reference to the exit level of the motor 
glossopharyngeus, is practically the same in both ganoids and 
sharks, so that for purposes of comparison here it is reasonable 
to consider this nucleus as a more or less fixed point. 
