MOTOR NUCLEI IN PHYLOGENY aile 
spino-occipital nucleus is situated dorsally upon the fasciculus 
longitudinalis medialis, and, as in sharks, is lacking in ventral 
elements. This position is probably due to the same influences 
that were noted in the discussion of this region in Selache, viz., 
to the action of reflex impulses by way of the well developed 
dorsal arcuate fibers and posterior longitudinal bundles and to 
the comparatively slight development of ventrally situated reflex 
pathways. 
Allis (1. ce.) has suggested that ‘‘the tongue of the adult Amia 
may, therefore represent a condition of that organ in which its 
partial muscularization has been lost, rather than not yet ac- 
quired, as Gegenbauer (28) states to be the case for fishes in 
general.’”’ However, within the central nervous system, no 
evidence has yet been forthcoming to indicate that the spino- 
occipital nucleus in Polyodon or other ganoid was at any time 
more specialized than is this complex in modern sharks. But 
it is interesting to note that: in both sharks and ganoids the 
nucleus which innervates the forerunner of the tongue muscu- 
lature is placed, as in higher forms, in close proximity to the 
visceral motor center innervating the musculature of the 
foregut. 
Nucleus paramedianus (Nu. paramed.). The varying develop- 
ment of the nucleus paramedianus among ganoids is worthy of 
note in connection with the question of the homology of this 
nucleus and the inferior olive of mammals. In Amia, which 
probably represents the nearest relative of the modern teleost 
group, there is no well circumscribed nucleus paramedianus and, 
since this nucleus does not appear as a definitely circumscribed 
gray mass in teleosts, amphibians or reptiles, it is possible and 
even probable that the nucleus in question was represented in 
the ganoid stock merely by an undifferentiated reticular area. 
Thus, the nucleus paramedianus of selachians would appear 
to be a structure independently specialized within the group 
after its divergence from an ancestral type common to sharks 
and ganoid stock. 
In Polyodon, the acusticum and lobus lineae lateralis are di- 
rectly continuous with the cerebellum, with which they corre- 
