460 



J. B. JOHNSTON 



Figs. 60 and 61 Sections farther caudad. Here the hippocampal cortex below 

 the corpus callosum becomes smaller (60) and then insensibly merges with the 

 undifferentiated cells of the septum. In other words, we have here a hippo- 

 campus without a cortical layer and this is the equivalent of the polymorphic 

 layer and of the primordium hippocampi of the selachian. A well developed 

 nucleus parolfactorius medialis appears in figure 61, quite comparable to that 

 in the bat and mole but placed somewhat more caudally. 



Fig. 62 Section at the level of the neuroporic recess showing how the primor- 

 dium hippocampi becomes the bed of the hippocampal commissure. On the left 

 side where the cells are drawn, all the white space is filled by commissural fibers. 



Fig. 63 Rat, parasagittal section near the median plane. The hippocampal 

 flexure and the upper end of the facia dentiata are cut. The section cuts length- 

 wise of the fornix superior where that pushes between the hippocampal com- 

 missure and the corpus callosum toward the splenium. From the septum (p.h.) 

 to the right many cells extend back among the fornix fibers and blend with the 

 polymorphic layer of the hippocampus. 



