138 JouRNAL OF ComMPARATIVE NEUROLOGY. 
Ependymal fibres extend through the entire thickness of 
the midbrain wall, branching but feebly. 
Section VII. Tue INTERBRAIN. 
The research of EpINGER, Das Zwischenhirn, ('92), presents 
an account of the fibre-tracts of the interbrain of Selachii and 
Amphibia as demonstrated by the WEIGERT method. It remains 
for me to add to the results of that work a description of the 
morphology of the neurones proper to the interbrain of Mustelus. 
I. The Thalamus. 
The thalami of a selachian are so small in proportion to 
the other parts of the brain (Fig. 1, 7%.) that certain of the 
older anatomists were thereby caused to overlook the inter- 
brain entirely ; see Section II. The small size of the thalamus 
is, I find, the expression of a low degree of organization. Sev- 
eral investigations have made it clear that the thalamus of a 
mammal has several well-defined thalamic nuclei. The thalamus 
of Mustelus, however, has remained ina condition of such 
primitive simplicity that it is not practicable to institute very 
strict comparisons between its neurones and those which are 
characteristic of higher forms. Before such comparison can be 
of much value, a study must be made of thalami having inter- 
mediate degrees of development. 
It has seemed to me advisable to distinguish but two col- 
lections of neurones in the thalamus of Mustelus. One group 
represents a differentiation of the ancient central gray matter ; 
this I have designated the nucleus strati grisei. The other col- 
lection is certainly the one from which the several geniculate 
nuclei of higher vertebrates have been derived; this I have 
called the nucleus geniculatus. 
a. The Nucleus Strati Grisetz.—As has just been mentioned, 
‘this collection of neurones represerits a differentiation of the 
primitive central gray matter. The nucleus strati grisei has 
retained its original situation next the third ventricle (Fig. 24, 
m. S. g.). It forms a broad zone just within the limitans interna, 
