8o 
BULLETIN OF THE LABORATORIES 
Univ., VoL VI,) that there is no direct connection between the tractus 
praecommissuralis cruris, or internal olfactory tract, and the olfactory 
substance. On the other hand, the fibres of the tract seem to be 
largely derived from the cortex cruris and its forward extension into 
the pes. Of this relation we have been long convinced in the opos- 
sum, as well as lower vertebrates, though it would be premature to 
decide that there is no secondary connection with the pero. A few 
general considerations might be advan ced to indicate the bearing of 
the conclusion suggested. 
1. The olfactory, as a sensory nerve, should be derived from 
the dorsal part of the primitive medullary tube or embryonic vesicle. 
According to the above suggestions, such an origin is the only one. 
The three superficial radices ultimately reach the gyrus fornicatus. 
Now, if the callosum be considered the partial homologue of the 
dorsal commissure of the cord, of which there can be little doubt, the 
cephalic part of the hippocampus, i. e. gyrus fornicatus, is the homo- 
logue of the most dorsi-mesal part of the vesicle. The commissura 
fornicis is also a portion of the dorsal commissural system and the 
connected parts of the hippocampus are morphologically part of dor- 
si-meson. The enormous development of the parietal part of the first 
embryonic vesicle which serves to bring much of the ventral surface 
dorsad, especially cephalad, causes a revolution correlated with the 
flextures which obscure the primitive simplicity, but in the rodents 
and marsupials, it is nevertheless sufficiently obvious. 
2 . The olfactory, as a sensory nerve, should have a trophic 
ganglion. This might lie upon the nerve itself, occupying a distinct 
fossa in the skull, like the Gasserian ; it might be carried peripherad 
and become associated with the end organ, as in the auditory ; or it 
might fuse with the brain itself to form an apparently organic unity. 
To us tlie latter seems the actual state of the case. If the tracts are 
really superficial, lying, for the most part, ectad to the neuroglia layer 
and, in spite of subdividing into several bundles, reach the same part 
of the brain; and if the so-called deep olfactory tract or olfactory 
bundle of the praecommissura springs, like other fascicles of that 
commissure, from cortex cells, there* seems to be no reason for doubt- 
ing (what is a priori so probable) that the adhesion of the pero to the 
pes is a comparatively subordinate character. In this case, while 
there may be more or less fusion and interblending of the two, there 
