548 DAVIDSON BLACK 
be situated on the level of its root exit intermingled with the 
elements of the caudal motor VII nucleus but completely sepa- 
rated from the motor X nucleus (e.g., Gadus), or again it may be 
intimately associated with the motor VII nucleus and form 
with the latter one large nuclear complex entirely apart from the 
motor vagus column (e.g., Lophius). 
The possible factors operating to produce the characteristic 
ventral displacement of the motor VII elements in teleosts have 
been discussed in detail by Kappers (62, 64, 72 and 73). This 
author has definitely excluded the possibility of the production 
of this displacement through purely mechanical influences. 
In addition he has shown that the ventral sequestration of the 
motor VII nucleus has undoubtedly been strongly influenced 
by the anterior gustatory tract among forms in which this 
pathway is well developed; while among those animals in which 
the anterior taste tract is relatively small, the ventral optic and 
tactile systems are most probably chiefly responsible for this 
displacement. 
Kappers has also shown that the characteristic geniculate 
course of the emergent motor IX root in teleosts has been pro- 
duced largely through the more or less mechanical traction exer- 
cised upon it during the ventral migration of the motor VII 
elements (72). However, with regard to the migration of the 
perikaryons of the motor [IX nucleus, no such direct mechanical 
influence can be demonstrated and there can be no doubt that 
the peculiarly variable relations of this nucleus among teleosts 
are due to the action of neurobiotactic forces of a nature similar 
to those which have brought about the ventral displacement 
of the motor VII nucleus in these forms. 
In teleosts the arrangement of the musculature innervated by 
the nerves V, VII, and IX has been strongly influenced in its 
development by the perfection among these forms of two char- 
acteristic organ complexes, viz., (1) the respiratory opercular 
mechanism, and (2) the pharyngeal tooth-bearing apparatus. 
The important effect of the development of the opercular type 
of respiration in ganoids upon the motor V and VII nuclear 
pattern has already been alluded to and will be discussed again 
