144 JOURNAL OF COMPARATIVE NEUROLOGY. 
5. Summary of the Interbrain, 
The thalamus is small in size and has remained on a low 
plane of differentiation. It is practicable to distinguish but 
two thalamic nuclei. One, the nucleus strati grisei, has become 
defined from the central gray matter for the reception of fibre- 
terminations, chiefly those of the tractus strio-thalamicus from 
the striatum ; the axones of the nucleus give origin to the trac- 
tus thalamo-tectalis. The other thalamic nucleus, the nucleus 
geniculatus, receives collateral branches trom optic fibres. It 
is wholly inferior to the tectum mesencephali as an optic ter- 
mination, but it represents the specialized geniculate nuclei of 
higher forms. : 
The two nuclei habenulae do not exhibit great disparity 
in size. An olfactory tract, the tractus olfacto-habenularis, 
terminates here; the neurones of the nuclei give origin to the 
tractus habenulo-peduncularis. 
The lobi inferiores are the seat of a crowded group of neu- 
rones, and the lobes are thereby given a large size. A neurone 
has a widely spreading dendritic top. Its axone branches in a 
T-shaped manner, 
Supporting elements are strongly developed. The neuro- 
glia is remarkable for the mat-like interlacing of its branches. 
Ependymal fibres often ramify to a striking degree; they ex- 
tend through the whole thickness of the neural tube. 
Section VIII. Tue ForesRAIN. 
The key to the understanding of the vertebrate forebrain 
was given by Rasi-Rickuarp (83) when he formulated the 
theory of the membranous pallium for the teleost. The exten- 
sion of the generalization to other groups has been productive 
of results which fall into place in an almost schematic way. A 
quite remarkable series is given by the fishes. This series be- 
gins with a condition in the teleost where the entire roof of the 
forebrain remains non-nervous, and culminates in the dipnoid 
with a pallium having the essential morphological characters 
pertaining to all brains of a higher order, 
