198 G. ELLIOT SMITH. 
Jacobson’s organ terminate. Hence I have distinguished it as 
the “ganglion of Jacobson’s organ.” The further 
significance of this arrangement I have discussed in the 
‘ Anatomischer Anzeiger.’ 
Separating both ganglionic masses from the second or nerve- 
cell layer there is a distinct layer of nerve-fibres (y), which 
form the root bundles of the “ olfactory [internal and external | 
radiations” of Edinger. It corresponds to the molecular layer 
or “ Randschleier” of the cortex cerebri. 
The surface regions of the hemisphere are already differen- 
tiated into two histologically distinct districts, (1) a basal 
region (comprising the tuberculum olfactorium and the pre- 
commissural area), which presents an amorphous and un- 
differentiated structure; and (ii) a dorso-lateral region—the 
pallium,—which already exhibits characteristically specialised 
features. Interposed between these regions on either side 
there are two districts—the hippocampus and pyriform lobe— 
which present intermediate or transitional features. These 
divisions have a further and much wider significance. The 
undifferentiated basal regions—including not only the pre- 
commissural area and tuberculum olfactorium, but also the 
prosencephalic part of the olfactory bulb, the corpus striatum 
and lamina terminalis (“septum lucidum ’”’)—are the ancient 
parts of the brain phylogenetically, and develop early in 
ontogeny. The transition regions appear next in phylogeny— 
the hippocampus and then the pyriform lobe. The highly 
differentiated pallium appears last, and continues to increase 
in size and in histological specialisation long after the other 
regions have reached the acme of their evolution, or are even 
in their decline. Thus in the Prototheria the hippocampus 
and pyriform have already reached the height of their develop- 
ment, whereas the pallium progressively increases in size and 
complexity in the Hutherian series up to Homo, in whom the 
lowest stratum—the olfactory—the tuberculum olfactorium 
and precommissural area are disappearing. 
