MOTOR NUCLEI IN PHYLOGENY 503 
Spino-occipital nuclei and roots (Nu. et rad. mot. Nn. spin. occ.). 
The dorsal portion of the somatic motor column of the cord, is 
continued upwards into the medulla in Polyodon in a manner in 
many respects recalling the relations prevailing in this region 
among selachians (figs. 10 and 18). The rostral end of this 
column is, however, not sharply defined as in sharks, and it 
is difficult to set a limit between the nucleus in question and the 
reticular elements of higher levels. A similar condition was 
noted in Bdellostoma. In figure 25 A, the level above which 
none of the emergent motor rootlets can be traced is taken to 
be the rostral limit of this motor column. 
Like the spino-occipital column in Selache, this nucleus in 
Polyodon les close to the ventricular gray, lateral to the fascicu- 
lus longitudinalis medialis, and the cells toward its rostral end are 
most dorsally placed. The emergent motor fibers pass very 
obliquely ventrad and caudad from this nucleus to reach the 
periphery of the bulb. In the latter respect these roots differ 
from the emergent spino-occipital fibers in Selache which pass 
almost directly ventrad (figs. 10 and 18). It should also be 
noted as a corollary to this observation, that in none of the 
selachians hitherto examined, does the spino-occipital nucleus 
extend so far rostrad of the first spino-occipital rootlet as does 
this column in Polyodon, above its first emergent fiber. Of the 
ganoids examined, Acipenser only seems to be an exception to 
this rule (figs. 17 and 25). The origin and relation of the spino- 
occipital nerves in Amia and Lepidosteus, and in Acipenser 
(except as noted above) closely correspond to those obtaining in 
Polyodon. 
Nucleus paramedianus (Nu. paramed.). On each side of the 
raphé, on the level of the first spino-occipital rootlet and extend- 
ing for some distance caudad, the nucleus paramedianus may be 
distinguished in Polyodon. Many internal arcuate fibers course 
across this ill defined nucleus, whose cells, as in Amia, are so 
diffusely distributed that it was not possible to indicate its out- 
line in the reconstruction chart. However, in both Acipenser 
and the young Lepidosteus specimens which Theunissen exam- 
ined, the nucleus paramedianus has been charted. 
