MOTOR NUCLEI IN PHYLOGENY 541 
ptecervical motor root and that of the motor IX is usually 
greater in teleosts than in ganoids and always greater in teleosts 
than in certain ganoids (e.g. Lepidosteus). 3) In teleosts a 
sharp line of demarcation between the motor occipito-spinal 
nucleus and the coordinating elements of the nucleus motorius 
tegmenti cannot be drawn. In this respect ganoids stand in an 
intermediate position between selachians and teleosts.. 4) The 
elements composing the nucleus motorius tegmenti are arranged 
within the caudal reticular area on-.a plan almost exactly like 
that of the rostral occipito-spinal motor nucleus—the only dif- 
ference between the two lies in the absence of neurones of the 
peripheral efferent type in the former nucleus, the coordination 
neurones alone being represented there. 5) The occipito-spinal 
motor nucleus of teleosts resembles the caudal portion of the 
spino-occipital nucleus of selachians in the arrangement of its 
constituent neurones. 
Evidence of rostral migration. 1) Though it is known that 
at least four of the most rostral spino-occipital nerves are not 
represented in the Pleuronectidae, yet in Hippoglossus the 
occipito-spinal’ nerve b is placed as near the exit level of the 
motor IX nerve as the most rostral occipital nerve is in selachians, 
(and even nearer than is the case in most of the latter animals). 
2) In most Pleuronectidae the distance between the exit level 
of the motor IX nerve and that of the first precervical motor 
root is less than in Amia, Acipenser, or Polyodon. 3) The 
somatic motor nucleus projects further rostrad of its first emer- 
gent root in teleosts than is the case in either sharks or ganoids. 
4) The most positive evidence of rostral migrations, however, 
is to be seen among teleosts in the very oblique caudal] course 
taken by the emergent occipito-spinal roots (especially those of 
the nerve b)—as if the rostral displacement of their peripheral 
attachments had not kept pace with that of their motor nucleus. 
Thus in view of Kapper’s earlier work (1. c.) it may be said that, 
in the absence of manifest mechanical influence, positive evi- 
dence of nuclear displacement may safely be adduced when a 
motor root takes a more or less indirect emergent course, and 
that the direction of this displacement is indicated by the direc- 
