542 DAVIDSON BLACK 
tion taken by the emergent fibers. On the other hand, rostral 
or caudal nuclear migration and emergent root displacement 
may keep pace so that the direct emergence of a motor root 
from its nucleus does not altogether exclude the possibility of 
nuclear displacement. 
The presence of both dorsal and ventral cell groups in the 
occipito-spina] motor column in teleosts in contrast to selachians 
and ganoids has already been pointed out by Kappers (66 and 
74). This author has shown that the more ventral situation of 
certain of the motor e’ements of this nucleus in teleosts is pos- 
sibly one of the direct expressions of the strong reflex influence 
of the tractus octavo-motorius cruciatus which courses together 
with the tecto-bulbar fibers along the ventro lateral periphery 
of the bulb at this level (see also Wallenberg, 96). 
In the occipito-spinal nucleus of teleosts the ventral cell group 
differs in many histological details from the dorsal cells of this 
nucleus (Kappers, 66). Some light is shed on the different 
functions of these two cell groups when the peripheral distribu- 
tion of the first two precervical motor roots in ganoids is 
compared with that in teleosts. 
In Polyodon, where the ventral cell group is lacking’ in the 
most rostral portion of the precervical motor column, the first 
somatic motor nerve of this series does not furnish branches to 
any lateral (pectoral fin) muscles but is distributed solely to 
ventral (hypobranchial) spinal musculature (Danforth). The 
second nerve may also be entirely restricted in its motor dis- 
tribution to ventral musculature in some individuals (Fur- 
bringer). Indeed it is a general rule for ganoids that the first 
spino-occipital nerve never participates in the formation of the 
brachial plexus, though the second nerve usually does so. Simi- 
larly in selachians the upper members of the motor spino- 
occipital series take no part in the innervation of pectoral fin 
musculature (brachial plexus)—and no ventral cell group is 
present in the rostral end of the spino-occipital column in 
these forms. 
In Ameiurus (McMurrich and Wright, l. ¢.), however, the 
first precervical somatic motor root is distributed not only to 
