180 
logically as well as, probably, functionally, and the two regions 
are very closely associated in development. They resemble one 
another, moreover, in the fact that they both transmit fornix 
fibres, and, like the tuberculum olfactorium, present a very 
atypical cortical structure, which consists of diffusely scattered 
small polyhedral cells, and which has no ventricular medullary 
coating. Meyer (26) is of opinion that these regions ought not 
to be called cortex, and calls them “ ganglion masses.” This pre- 
commissural area is the homologue of the “intraventricular 
lobe” of Herrick (17), the ‘‘ Fornix-leiste” of Edinger (9), the 
anterior part of the “septum pellucidum” in Reptiles according 
to Meyer (25), the “Ammonswulst” of Rabl-Rickhard (31). It 
forms the anterior part of the region called “septal area” by 
Huxley and Flower (11). It is pale in color, from the fact that 
a large number of fornix fibres, which Huxley distinguished as 
‘“‘precommissural fibres” lie in its substance, separated from the 
surface by a slight grey coating. 
The rest of the grey matter of the mesial hemisphere wall is 
divided into two almost equal parts by a well-marked regularly- 
arched sulcus—the fissura arcwata (Arnold) or hippocampal 
fissure (fig. 3, hf). The fissure begins in front, just behind the 
olfactory bulb, and slightly nearer the dorsal than the ventral 
margin of the hemisphere. From this point it curves backwards 
parallel to the margin of the hemisphere to terminate at the 
“temporal” pole. The external lip of this fissure is the subicu- 
lum cornu Ammonis. It is pale, from the presence of the super- 
ficial medullary lamina of the hippocampus. Corresponding to 
the concavity of the fissure, there is a broad band of grey matter 
concentric to the subiculum. This is the fascia dentata, which is 
also pale from the presence of medullated fibres in its molecular 
layer. Lying internal (concentric) to the fascia dentata (fd. ) is 
the broad white band of the fimbria (fi./, separated from the 
fascia dentata by a shallow fimbrio-dentate sulcus. The great 
width of the fimbria is striking, especially when seen in trans- 
verse section (fig. 5, f.). The posterior part of the vertical por- 
tion of the fimbria (just that part, as it happens, which in fig. 3 
is shaded, corresponding to the depression for the mesencephalon) 
is quite grey in color from the fact that the cornu Ammonis may 
be seen through the very thin layer of alveus fibres in this region. 
In the whole extent of the choroid fissure the fimbria is related 
to it, forming either its dorsal or posterior boundary. 
Immediately behind the precommissural area there is the thick 
mass of grey matter which is developed from the lamina termin- 
alis. It is bounded below by the anterior commissure, which 
really lies in it, and dorsally it is separated from the fascia 
dentata in Ornithorhynchus by the hippocampal commissure (37) 
