512 DAVIDSON BLACK 
branchiales muscles (which, as is sharks, are innervated by 
spino-occipital nerves), are developed from. all the branchial 
myotomes in Acipenser (Edgeworth, Vetter, Furbringer). 
That a rostral migration of the spino-occipital nucleus should 
occur pari passu with the reduction and absorption into the 
head of occipital elements, is rather to be expected and further 
evidence of such a displacement of this nucleus is not lacking. 
Thus, if the distance between the first emergent spino-occipital 
root and the emergent motor IX nerve be measured in sharks 
and ganoids (figs. 17 and 25), a comparison will show this dis- 
tance to be the same in Lepidosteus and Selache and almost the 
same in Acipenser, Amia and Hexanchus, though a greater dis- 
crepancy is evident in the case of Polyodon. In other words, 
though certain occipital nerves are known to have been lost: in 
ganoids, those that remain have been displaced rostrad. 
Rostral displacement of the spino-occipital column must 
have been preceded first by the loss of its peripheral motor ele- 
ments (motor perikaryons), followed by the reduction and modi- 
fication of the remaining coordination elements of the nucleus. 
This is indicated in ganoids as in Bdellostoma by the difficulty 
with which the rostral end of this nucleus is defined. The 
column passes over gradually into the inferior reticular nucleus 
and thus appears to extend a considerable distance further ros- 
trad of the exit level of its first root than is really the case (vide 
supra). 
It should also be noted that the peripheral pre-hyal hypo- 
branchial musculature, at least in Amia, shows evidence of con- 
siderable individual variation, so that a condition may obtain 
here somewhat analogous to that already pointed out in Bdellos- 
toma, where the individual variation was so great that a ques- 
tion arose as to whether the extent of the somatic motor column 
in the reconstruction chart in figure 7 could be taken as repre- 
sentative of the species. 
Aside from this difficulty of delimitation, the rostral end of 
the somatic motor column in ganoids shows unmistakable evi- 
dence of specialization, so that it differs considerably from the 
anterior horn in the cervical region. The rostral end of the 
