402 NILS HOLMGREN 
general pallium is under formation is much broader than in the 
preceding stage and occupies nearly the whole dorsal aspect of 
the ventricular layer (fig. 7, g.p.c.). From this cell-masses are 
streaming out below the primordial cortex (fig. 8, prim.c.), 
pressing the intermediate part of this cortex (the exterior layer of 
the general pallium) against the brain surface and partly con- 
fluent with it (fig. 8, g.p.c.). Since in the foremost part of the 
brain the lateral and medial parts of the primordial cortex are 
not exposed to this pressure, the primordial pallium is thus divided 
into three parts, a medial, a dorsal, and a lateral, respectively 
corresponding to the hippocampal, the general pallial, and the 
pyriform cortex, of which the pyriform is formed first. The 
general pallium (the inner layer) develops after the pyriform lobe 
and causes a frontal separation of the hippocampal pallium from 
the pyriform lobe. 
The hippocampal part of the primordial cortex rostrally is 
tolerably well differentiated. At the recessus neuroporicus the 
hippocampal pallium dorsal to this recess joins the corresponding 
part of the other hemisphere. Caudal to the neuroporic recess the 
hippocampal part is continuous with that of the other side, form- 
ing a thin sheet of cells passing the mid-line close to the ventricular 
layer. On each side of the mid-line the hippocampal cortex is 
confluent with the underlying general pallial rudiment. In 
summary, the hippocampal primordial pallium consists of: 1) 
a frontal part covering the medial part of the frontal pole of the 
brain hemisphere; 2) a commissural part dorsal to the recessus 
neuroporicus. 
In the front part of the brain the zona limitans lateralis is well 
marked. At the extreme lateral part of the forebrain the zona 
limitans is obsolete and the pyriform lobe is continuous with the 
lateral olfactory nucleus. Caudal to the bulbus olfactorius rudi- 
ment the zona grows more conspicuous. 
The evagination of the forebrain in this stage not being more 
than commenced, the septal structure is very short. The zona 
limitans medialis is not pronounced and the subpallial parts 
directly adjoin the hippocampal structure at the upper level of 
the neuroporic recess. The septum is filled with a dense mass of 
