8 Journal of Comparative Neurology. 



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 the lamina terminalis cephalad of the hippocampal 

 commissure and dorsad of the anterior commissure. This connection 



