82 
BULLETIN OF THE LABORATORIES 
brates. In the case of Didelphys the hippocampus and related struc- 
tures are strongly not to say predominatingly developed. The motor 
cortex as such is thrown well cephalad and the fornicate gyrus is car- 
ried forward along the mesal surface, as may be seen from an inspec- 
tion of the transverse sections of Plate A. Thus it happens that the 
caudal portion of the dorsal commissure system is much more highly 
developed than the cephalic or callosal portion. The later consists of 
few fibres which spring from the region about the anterior prolonga- 
tion of the splenial fissure, if this term may be applied to the fissure 
which bounds the cephalad continuation of the fornicate gyrus. The 
separation of the callosum and hippocampus commissure must be as- 
cribed in great part to the folding of the hippocampus and its compres- 
sion by the contact of the thalamus which leaves but one available 
path — that persued by the fimbriae. It may be supposed that the ce- 
phalad point of fixation of the hippocampus is determined by the for- 
nix bundles, which necessarily enter the corpus fornicis at a nearly 
constant point. That the fibres belonging to the fornix system are dis- 
tinct from those of a commissural character was suggested by Stieda 
and seems quite probable from our observations. 
The anterior commissure evidently is the chief coordinating com- 
missure of the frontal portion of the cerebrum. 
The easiest solution of the problem of the relations of the cal- 
loso-hippocampal commissure with the praecommissura would be to 
homologize the former with the dorsal, the latterwith the ventral com- 
missures of the cord. Yet the anterior commissure receives fibres from 
almost the entire surface of the cerebrum. 
The Praecommissura. Of the three divisions of the anterior com- 
missure which may be recognized, the so called olfactory portion has 
been sufficiently discussed in connection with the olfactory. The 
frontal portion is closely associated with it and these two are together 
less than the temporal branch. The fibres of the praecommissura hug 
the ventricles and are perforated by bundles from the peduncles. 
Dorsad the commissure is bounded by the very large, nearly 
quadrangular, body of the fornix. Longitudinal sections of the brain 
at the median fissure show that the two hemispheres are connected by 
(i) a delicate membranous tela which springs from the cephalo-dorsal 
tuberosity of the thalamus and passes cephalad to unite with a conspic- 
uous projection of tlie lamina terminalis cephalad of the hippocampal 
commissure and dorsad of the anterior commissure. This connection 
