518 DAVIDSON BLACK 
Abducens nucleus,and roots (Nu. et rad. N. VI). The position 
of the poorly defined abducens nucleus in all ganoids midway 
between the exit levels of the motor VII and IX roots, is deter- 
mined in all probability by the termination of the acoustic nerve 
at this level. Kappers (64) has already pointed out the im- 
portance of this factor as well as that of the dorsal arcuate 
fibers of both the crossed and direct octavo-motor tracts in 
determining the position of the abducens nucleus in sharks. The 
nucleus in Polyodon is small in correspondence with the slight 
functional development of the eye and lies somewhat ventrally 
in the tegmentum between the tractus tecto-bulbaris and the 
fasciculus longitudinalis medialis and is traversed by the arcuate 
fibers of the tractus octavo-motorius. 
In all ganoids the abducens is placed more ventrally in the 
tegmentum than in sharks and appears to be characteristically 
more closely associated with the tractus tecto-bulbaris in the 
former animals. This apparently would indicate that the tractus 
tecto-bulbaris was more highly developed in ganoid ancestors 
than in the modern representatives of the group, for otherwise 
it is difficult to account for this ventral position of the abducens 
nucleus in the presence of such evidently well developed dorsal 
acustico-motor tracts. 
Such a conclusion is strengthened by the relations obtaining 
in the oculomotor and trochlear nuclei in Polyodon to be noted 
subsequently and also by the findings of Danforth (1. c.) in his 
dissections of the eye-muscles in this form. According to this 
author, considerable variation was met with in connection with 
these muscles and in one case he records the complete suppres- 
sion of the external rectus on the left side as evidence of a retro- 
grade tendency in this area. Further, Allis (Il. ¢.) regards the 
type of arrangement of the eye-muscles in ganoids as more 
specialized than in selachians, and, though this generalization 
may be subject to some question, yet the fact remains that, 
like Polyodon, in Amia the external rectus muscle also varies in 
its development. 
Motor trigeminal nucleus and root (Nu. et rad. mot. N. V). In 
all ganoids the motor V nucleus occupies a dorsal position in the 
