550 DAVIDSON BLACK 
plexes have been elaborated to produce movement of the pharyn- 
geal plates, a most striking difference in the arrangement of the 
caudal viscero-motor nuclei (especially the glossopharyngeus) 
is to be observed. It is significant, also, that the motor IX 
nucleus is subject to such wide variations in its relations (and 
consequently in its reflex connections) among teleosts, while in 
no other group of the vertebrate series does this nucleus display 
such a lack of conformity to type pattern. 
Abducens nucleus and roots (Nu. et rad. N. VI). Among 
teleosts the abducens nucleus is evidently composed of two more 
or less separated subnuclei which are situated ventrally in the 
tegmentum and from each of which emerges a single rootlet. 
The more rostral rootlet usually emerges on the exit level of the 
motor VII root or a very short distance caudad of this structure 
though among the Pleuronectidae it characteristically emerges 
rostrad of the motor VII root. The more caudal rootlet usually 
emerges at a level which is relatively more rostrad than that 
of the frontal abducens rootlet of selachians (figs. 17 and 41). 
In exceptional cases the abducens nerve may arise from the 
brain stem from three rootlets as in Lophius. 
The most rostrad of the abducens sub-nuclei lies either on 
the level of the emergent motor VII root or but a short distance 
caudal of this structure and the second sub-nucleus is situated 
slightly caudad of the exit level of the second abducens rootlet. 
In Tinca Kappers has described four abducens sub-nuclei, 
three of which are represented in his chart of the motor nuclei 
(fig. 41 B), while the fourth part, which lies mediad of the ros- 
tral ventral sub-nucleus and on the same level, could not be 
represented in such a reconstruction. 
The relations of the abducens nucleus and of its emergent root- 
lets noted here have already been fully described and discussed 
by Kappers (63, 64, 66). This author has shown that the fronto- 
ventral position of the abducens nucleus in teleosts as compared 
with selachians stands in direct relation with the great develop- 
ment of the ventral tecto-bulbar paths in the former animals. 
Among flat-fish the relatively enormous development of the 
tecto-bulbar tract is to be directly correlated with the fact 
