680 
The callosum has apparently been completely overlooked hitherto. 
Prof. Osporn in his valuable paper on the origin of the corpus callosum ') 
remarks: „In regard to the other groups of fishes [than Dipnoi] I still 
adhere to the hypothesis that the commissura interlobularis is a primitive 
form of the whole transverse commissural system of the hemispheres, 
thus representing both the anterior commissure and the corpus 
callosum.“ In this view we at one time shared, but have since 
described in Lepidosteus a distinct commissure far cephalad of 
the anterior commissure at a point near the separation of the pallium 
from the axial lobe. This we considered the morphological homo- 
logue of the callosum?). In the Drum, the callosum is very well 
developed and lies far cephalad of the anterior commissure and on 
the opposite side of the ventricle. Short reflection is sufficient to 
show that in a group like fishes where the mesal walls of the cortex are 
undeveloped it is morphologically impossible that the callosum should be 
merged with the anterior commissure. The former is a commissure of the 
roof, the latter of the base of the primitive prosencephalic vesicle. GORONO- 
WITSCH has shown that the fish brain suffers a considerable ventral flex- 
ture and I have pointed out that this has resulted in the formation of a 
sylvian fissure or fold. By reason of this fold and the failure of the 
tectum to develop and rotate caudad the callosum lies in contact with 
the lamina terminalis upon the cephalo-ventral aspect of the cere- 
brum, immediately dorsad of the olfactories. That there is no mistake 
in the identification is also shown by the connection of its fibres with 
the pyramidal cells of the central lobe which, as above suggested, 
represent the motor regions of the cortex. Moreover, we have been 
gratified by discovering that the fornix system and hippocampal com- 
missure are well developed and sustain the proper relations to the 
callosum as thus located. 
The hippocampal commissure arises in the hippocampal lobe entad 
of the proximal end of the radix lateralis and passes cephalad and 
crosses immediately behind the callosum. It is accompanied by fibres 
which diverge caudad near the meson to enter a double mass of cells 
homologous with the corpus fornicis, from which fibres arise to pass 
toward the thalamus. These relations are unmistakable in a conti- 
nuous series of horizontal sections. 
1) Morpholog. Jahrb., Bd. XII, p. 539. 
2) Journal of Comparative Neurology, Vol. I, p. 167. Hoır 
Zool. Jahrb., Bd. IV, mentions what he calls an olfactory commissure 
in the young herring which may prove identical with the above. 
