Houser, The Neurones of a Selachian. 103 
Fibres are also to be seen in numbers which do not termin- 
ate in the tuberculum acusticum at all. Some of these remain 
in the central core, continuing anteriorly ; while others are 
branches of fibres which have taken a course into the acusticum 
for partial termination there. All of these pass upward to the 
cerebellum ; they will be duly considered in Section V. 
a. Theoretical Conclusions.—It has already been pointed 
out that acustico-lateral components are not present as such in 
spinal nerves. Waiving the question of the origin of the sys- 
tem as a whole, it will not be out of place to consider here cer- 
tain problems relative to its primary centre, the tuberculum 
acusticum. 
The position which the tuberculum acusticum occupies in 
_the oblongata is indeed significant, superposed, as it is, on the 
cranial representative of the dorsal cornu, the general cutaneous 
nucleus. This fact can only be taken to mean that the tuber- 
culum acusticum is phylogenetically the younger of the two 
structures in question. In Subsection 6 it was shown that the 
general cutaneous nucleus has the archaic structural features of 
the dorsal cornu capped with a molecular layer continuous with 
the cerebellar crest of the acusticum; and that there are pres- 
ent here both the molecular cells and the neurones of PURKINJE. 
Now this-structural continuity may signify that the acusticum 
has been derived from the general cutaneous nucleus in the 
phylogeny of the vertebrate nervous system. Of course it will 
be necessary to have a thorough study of the embryology of 
this region in the lower vertebrates before such a conclusion 
can receive unqualified support. Then, too, the rise of the 
tuberculum acusticum has been but one feature in the evolution 
of the lateral line system as a whole, and so we may conf- 
dently expect some assistance from the further study of the 
affinities of its sense-organs. 
There is somewhat more solid ground for the belief that 
the tuberculum acusticum has itself given grigin to a very im- 
portant region of the brain, the cerebellum. The full evidence 
on this point will be presented in Section V, Subsection 6. 
