MOTOR NUCLEI IN PHYLOGENY 521 
sentially selachian character of the communis area in these 
forms. Further, the simplicity of the arrangement of the 
elements composing the caudal viscero-motor column is in itself 
an indication of the absence of any great specialization of the 
derivatives of the hyo-branchial myomeres in ganoids. 
Oculomotor and trochlearis nuclei and roots (Nu. et rad. N. III 
and IV). The small size of the oculomotor and trochlear nuclei 
and their roots in all ganoids is indicative of the relatively slight 
functional importance of the eye in these forms as compared with 
sharks. In discussing the abducens nerve, mention was made 
of the evidence furnished by the structure of the oculomotor 
and trochlear nuclei in support of the view that ganoid ancestors 
were possessed of a more highly developed optic apparatus than 
the modern representatives of this group. This evidence rests 
upon the following facts:—Though these nuclei and their roots 
are both relatively and absolutely smaller than in sharks, yet 
the oculomotor nucleus presents distinct signs of specialization, 
so that within it may be distinguished two secondary nuclei 
such as elsewhere are found among fish only in teleosts. Fur- 
ther, the definite gap which exists between the oculomotor and 
trochlear nuclei in all ganoids, is a condition which also obtains 
elsewhere among fishes only in certain teleosts (e.g., Rhombus, 
Hippoglossus, ete.) in which the visual apparatus is highly 
developed. 
In conclusion, it may again be emphasized that the arrange- 
ment of the motor nuclei in Polyodon furnishes additional evi- 
dence of the importance of neurobiotactic influences in the de- 
termination of nuclear pattern. The predominantly dorsal 
position occupied by the motor nuclei in this form and in other 
ganoids is in correspondence with the relative importance of the 
dorsal reflex pathways (posterior longitudinal bundle, octavo- 
motor tracts, ete.). In this respect ganoids resemble sharks. 
The more ventral position of the abducens nucleus in ganoids 
than in sharks is explained on the hypothesis that the tecto- 
bulbar paths were functionally more highly developed in the 
ganoid stock than in the modern representatives of the group. 
This hypothesis is borne out by the evidence furnished by the 
