190 G. ELLIOT SMITH, 
commissural area” extends from the mesial aspect of -the 
olfactory bulb in front to the lamina terminalis behind (vide 
figs. 2 and 4, p. a.). The tuberculum olfactorium (locus per- 
foratus anticus) and the precommissural area (gyrus subcallosus 
of Zuckerkandl) correspond to the “ posterior olfactory lobule” 
of His. The whole of the mesial wall of the cerebrum 
dorsalto the precommissural area forms part of the 
rudimentary hippocampus (figs. 2, 6, and 7, hip.). It will 
be seen, therefore, that the hippocampus extends as far forwards 
as the olfactory bulb (the characteristic histological structure 
may be recognised at a distance of 150 u behind the olfactory 
ganglion). The part of the region in immediate relation to 
the precommissural area is the rudimentary fascia dentata, 
the dorsal part being the “ Anlage” of the cornu Ammonis. 
As it is traced backwards the hippocampus is found to pass to 
the dorsal side of the foramen of Monro (fig. 15) and choroid 
fissure, the ependymal choroid fold being attached to the 
ventral margin of the ‘‘ Anlage” of the fascia dentata. The 
hippocampal rudiment is co-extensive with the fissura choroidea, 
and forms its dorsal boundary (figs. 9 to 13). In the greater 
part of its extent this hippocampal “ Anlage ” is bulged into 
the lateral ventricle, giving rise to a shallow trough (the 
“ Bogenfurche” of Schmidt, fissura arcuata). To this 
trough the name “hippocampal furrow” (“ Ammons- 
furche ” of Mihalkovics) may very appropriately be applied. 
It has an exact morphological significance, being strictly con- 
fined to the region which is to become hippocampus. It is 
clear, therefore, from the relations of the anterior extremity 
of the hippocampus, that the true Bogenfurche cannot form 
the ‘ fissura prima” of His. 
The position occupied by the hippocampus in this foetus— 
entirely dorsal to the foramen of Monro and fissura choroidea 
—is typical of its true and essential relations in the whole 
Vertebrate series. The recognition of the homologous region 
in the sub-mammalian brain constitutes the key to the whole 
question of the nature of the commissures, because all the 
commissural fibres arising in this area must belong to the 
