MOTOR NUCLEI IN PHYLOGENY 505 
VII nucleus. The fibers pass rostrad a short distance and form 
a root which courses laterad below the motor VII nucleus to 
gain the ventro-lateral periphery (fig. 19). 
The arrangement and relations of the constituents of the 
caudal visceral motor column and their motor roots in Acipenser, 
Amia, and Lepidosteus, are essentially similar to those described 
above in Polyodon. 
Abducens nucleus and roots (Nu. et rad. N. VI). In the teg- 
mentum, medial and somewhat ventral to the rostral end of the 
motor VII nucleus, a poorly defined and scattered collection of 
cells forms the abducens nucleus. In Polyodon, as in Lepi- 
dosteus, the presence of numerous reticular elements in this 
neighborhood makes it difficult to define the exact limits of this 
small nucleus. Thus, in Polyodon it is only possible to indicate 
the approximate outline of the abducens nucleus by means of 
dotted lines. Its rostro-caudal extent in figure 25 represents 
the limits beyond which no emergent radicles could be followed 
centrally. In studying Lepidosteus, Theunissen also experi- 
enced this difficulty so that, though he described the nucleus in 
question, he has not indicated it in his reconstruction chart 
(fig. 25D). In Amia and Acipenser, however, the abducens 
nucleus is somewhat less diffusely arranged and its position is 
indicated in the reconstruction charts (fig. 25 C and B). 
Three thread-like rootlets emerge in series from the abducens 
nucleus in both Polyodon and Acipenser, while four such root- 
lets can be identified in Lepidosteus and Amia. In each case 
they course almost directly ventrad to the periphery of the 
bulb. 
The position of the emerging abducens rootlets in ganoids 
midway between the exit levels of the motor VII and IX roots, 
corresponds closely to the arrangement of these roots in sharks. 
Also in regard to the position of the abducens nucleus with ref- 
erence to the exit level of the motor glossopharyngeus, the two 
groups closely resemble one another. In sharks, however, the 
abducens nucleus lies upon a more dorsal plane than it does in 
ganoids. 
