158 JoURNAL OF CoMmPARATIVE NEuROLOGY. 
The pallium has its two halves fused in the median plane. 
Its most important region is the pair of pallial eminences. Each 
of these has a special extension of a lateral ventricle. The _ 
neurones of a pallial eminence are not arranged in layers, but 
it is practicable to recognize three distinct forms: (1) The neu- 
rones of the tractus pallii, the largest variety, their axones com- 
prising the tractus pallii. (2) Commissural and associative neu- 
rones, the axones of which are distributed in the pallium itself, 
a special decussation of commissural fibres occurring in the 
median plane. (3) CayAL neurones, lying tangential to the 
limitans externa. 
3 The pallium of Mustelus is regarded by the writer asa 
primitive representative of the stem giving origin to the pallia 
of amphibians, reptiles, birds, and mammals. It anticipates 
the olfactory connections of the reptilian brain. The tractus 
pallii is interpreted as giving the olfactory sense a hold on the 
nervous system in addition to that provided by the epistriatum 
and striatum, and the neuroporic and postolfactory nuclei. The 
phylogenetic development of the pallium of selachians is be- 
lieved to have been the outcome of the great dependence placed 
upon the olfactory sense by these animals. 
SrecTIoN IX. GENERAL SUMMARY AND CONCLUSION. 
Special problems relative to the several parts of the brain 
have already been discussed in the foregoing pages; while sum- 
maries have been placed at the close of each of the sections. 
There yet remain for consideration a few topics of more general 
scope growing out of the study as a whole. Before proceeding 
to these questions, however, it will be desirable briefly to review 
the most important results which have been obtained. 
I. General Summary. 
The Oblongata of Mustelus has become only slightly diver- 
gent from the structural plan of the primitive neural tube. 
The ventral cornu of the spinal cord is continued into the 
oblongata as the nucleus of the VI nerve, and as the scattered 
commissural and tract-neurones of the formatio reticularis. 
Se £y eee eee 
. 
q 
4 
