102 JOHN BEARD. 
the branchial sense organs of that segment, may remain very 
small or may increase to a very considerable length, but in any 
case the nerve connecting the whole length of the thickening 
with its ganglion is split off from the thickening, and split off 
simultaneously with the growth of the latter. 
11. The prz-branchial nerve is also formed as the ganglion 
separates from the skin, and is probably in all cases also split 
off from the epiblast in front of each cleft. 
12. Of the development of the R. pharyngeus nothing can 
be here recorded, but I think from the nature of the case that 
this nerve also probably arises from the cells on the upper wall 
of the cleft. 
Thus, as the general result of these observations, the existing 
views of the development of the dorsal root of a cranial 
nerve will have to undergo some modification. That in Elas- 
mobranchs the main root of the nerve is a direct outgrowth 
from the ueural ridge, as stated by Balfour and Marshall, is 
certainly true. The shifting and acquisition of a secondary point 
of attachment described by Marshall also seem to take place. 
The post-branchial branch also appears to arise from the direct 
outgrowth from the neural ridge, but in the formation of the rest 
the epiblast probably plays a part. In the case of the supra- 
branchial branches this is certain, and it is highly probable in 
the case of the ganglion. That the other branches, viz. the 
pre-branchial and R. pharyngeus of Van Wijhe, are derived 
from the skin is probable, and in one case it can be proved, 
viz. the pre-branchial nerve of the hyoid. 
Having now got a general view of the development of a typical 
cranial nerve, the various nerves may be considered. In 
the above schema we have the key to all the cranial nerves. 
Some, such as the ninth or glosso-pharyngeal, we shall find to 
fit in pretty exactly with the schema. But in others the story 
that ontogeny often omits or distorts ancestral history is also 
repeated. 
Some of the branches may be absent, even in the ontogeny, 
while others may be abnormally developed. Others, again, 
may be partially fused with neighbouring nerves, as has 
