400 J. B. JOHNSTON 



the indusium is continued by a cortical band which reaches to 

 the olfactory bulb. Beneath this is a band of lightly staining 

 cells, loosely and irregularly arranged, which represents the pri- 

 mordium hippocampi. It is directly continuous with the less 

 compact part of the septum pellucidum above described. Fi- 

 nally, this part of the septum pellucidum in its caudal and lateral 

 part is connected with the hippocampus proper by columns of 

 cells between the bundles of the hippocampal conamissure as in 

 the opossum. See the description of figures 63 and 64. There 

 is therefore a continuous formation which begins at the olfactory 

 peduncle, divides at the genu into indusium and septum pelluci- 

 dum and unites again in the hippocampus behind. In other 

 words, the hippocampal and callosal commissures are imbedded 

 in one formation which in part is developed into hippocampus 

 and in part remains in the primitive condition. The septum 

 pellucidum includes the larger part of this undeveloped primor- 

 dium hippocampi. 



The median plane of the brain of the mole is reconstructed in 

 figure 55. There is an enormous naassa intermedia and the third 

 ventricle between it and the anterior commissure is reduced to 

 a very narrow slit. The hippocampal commissure is nearly hori- 

 zontal in position and the septum pellucidum very narrow dorso- 

 ventrally. It is largely occupied by fibers of the fornix superior. 

 The feature of really striking importance about this section is 

 the existence of a band of non-nervous tissue leading from the 

 neuroporic recess out to the surface below the genu of the corpus 

 callosum. In Weigert sections this has almost the appearance 

 of a canal and is occupied by blood vessels. In the sections at 

 either side of the median plane the fibers of the precommissural 

 bundle, which are interrupted in this figure, pass on into the 

 fimbria. When this is compared with the condition in the mam- 

 malian embryo it is evident that this is a vestige of the great 

 sagittal fissure which has been closed up by the encroachment 

 of the paraterminal body below and the pallial commissures 

 above. If this figure be compared with the sagittal sections of 

 the bat's brain, it will be seen that a quite similar line of demar- 

 cation between the paraterminal body and the septum occurs in 

 the bat and that the cell-free space is occupied by blood vessels. 



