494 DAVIDSON BLACK 
Discussion 
Spino-occipital complex. Among sharks the spino-occipital 
nerves are distributed to both epibranchial and hypobranchial 
spinal musculature and the pre-hyal elements of the hypo- 
branchial musculature are well developed (25 and 97). Unlike 
cyclostomes, there is evidence of a definite specialization of the 
rostral end of the spino-occipital motor column in sharks, whereby 
this nucleus becomes dorsally placed, loses its ventral elements, 
and terminates abruptly a very short distance rostral of the 
exit level of the first spino-occipital root. In consequence of 
this, the spino-occipital rootlets in sharks pass from their nucleus 
of origin almost directly ventrad. These differences between 
the spino-occipital complex of selachians and that of cyclo- 
stomes, are significant in view of Furbringer’s findings: that the 
lower sharks mark a stage in vertebrate evolution where all tend- 
ency toward caudal extension of the branchial apparatus comes 
to an end, so that from sharks onward, there is a definite tend- 
ency toward reduction of gill area together with assimilation 
into the head (and consequent reduction) of cervical segments. 
The very evident variation in the number of gills among 
individuals of Bdellostoma (100) mustexercise no small influence 
upon the development and stability of the motor center inner- 
vating the somatic structures affected by this variation. This 
would partly explain the lack of any appearance of fixation or 
specialization of the rostral end of the motor spino-occipital 
column in these species. 
On the other hand, the relative constancy of the development 
of the pre-hyal coraco-mandibularis elements as well as the 
epibranchial spinal musculature in sharks (subspinalis and inter- 
basales, innervated by Furbringer’s occipital nerves (v), w, x, 
y, and z), would tend to produce a relatively constant arrange- 
ment of the rostral end of the motor spino-occipital column 
from which these nerves arise; for progressive reduction of the 
subspinalis and interbasales among sharks becomes evident only 
when the higher and lower selachians are compared or when this 
region is studied ontogenetically and it would thus appear that 
