396 J. B. JOHNSTON 



among the fimbria fibers and as a mass of pure gray matter on 

 the ventricular surface of the fimbria until it overlaps the rostral 

 end of the hippocampus. Here the primordium and the cortex 

 meet as a common bed for the fimbria and the caudal portion 

 of the hippocampal commissure which lies beneath the splenium. 

 The condition in the rat also indicates the essential continuity 

 of the primordium hippocampi and the hippocampus proper. 

 This subject is more fully discussed in the following section. 



In the turtle (fig. 18) the primordium hippocampi behind the 

 foramen is small and is occupied by the fimbria. The fimbrio- 

 dentate sulcus continues back in the medial wall of the hemi- 

 sphere as far as the fimbria can be recognized, some distance 

 beyond the end of the choroid fissure (fig. 20). 



EXAMINATION OF MEDIAL SURFACE OF HEMISPHERES AND OF 

 SAGITTAL SECTIONS 



The medial surface of the opossum's hemisphere is drawn in 

 figure 22. The reader will notice the deep hippocampal fissure 

 above the fascia dentata and that it is suddenly obliterated ros- 

 trad by the deep in-folding of the dorsal wall at the sulcus olfac- 

 torius. The fascia dentata is narrow over the anterior pallial 

 commissure and grows wider rostrad. Beneath it is the broad 

 bundle of precommissural fibers of the fornix system. The an- 

 terior pallial commissure stands in the path of these fibers. The 

 fimbrio-dentate sulcus begins between the dorsal end of these 

 fibers and the fascia dentata and extends caudad over the com- 

 missure. The locus of the neuroporic recess is clearly seen in 

 the so-called recessus inferior. Upon this surface no sulcus is 

 visible between the primordium hippocampi and the area parol- 

 factoria. It has probably been obliterated by the great bundles 

 of precommissural fibers running dorso-ventrally across it. The 

 course of the boundary line, as indicated by the internal struc- 

 ture above described, is shown in figure 86. 



. In figures 35 and 34 are drawn a nearly median sagittal sec- 

 tion through the region surrounding the commissures and a sec- 

 tion of the anterior pallial commissure and hippocampal forma- 

 tion lateral to the median plane. In the most medial section it 



