200 G. ELLIOT SMITH. 
“The Precommissural Area of the Median Cortex.” 
The general relations of this region have already been 
described. In a scheme (fig. 5) which indicates the hypo- 
thetical arrangement of parts in the anterior wall of the 
primitive brain-tube it will be seen that the olfactory bulbs 
(olf. g), situated at each lateral angle, are separated by a 
broad plate of nervous tissue. With the development of 
the hemispheres this plate becomes bent forwards on either 
side of the middle line. The lateral part, which now forms a 
sagittally placed wall (p.a.), is the “ precommissural area,” 
and the median band connecting the two “ aree ” is the lamina 
terminalis (/.¢.), which later develops into the septum lucidum. 
His calls the “ precommissural area” part of the “ olfactory 
lobe.” In fact, as the homologue of Zuckerkandl’s “ gyrus 
subcallosus,” it must form part of His’s “ posterior olfactory 
lobule.” On morphological and histological grounds, however, 
the precommissural area cannot be separated from the rudi- 
ment of the “septum lucidum” (compare figs. 7 and 9), both 
having equal claims to be considered as parts of the “ olfactory 
lobe ” or rhinencephalon. 
The Hippocampus. 
As the pallium (fig. 15, p.) is traced round the supero-mesial 
angle of the hemisphere the cellular elements will be found to 
undergo a striking rearrangement. The molecular or super- 
ficial layer (fig. 15) will be found to become very much broader 
than it is in the pallium, and, by comparison with later stages 
of Perameles, is readily recognised as the homologue not 
only of the molecular layer, but also of the stratum lacunosum 
and stratum radiatum of the adult hippocampus. The cells 
of the superficial nerve-cell layer lose their densely packed 
arrangement, and the cells of the second, third, and fourth 
layers become directly continuous with a column of somewhat 
scattered cells, which produce the pyramidal and polymorphous 
cells of the hippocampus. As this cell column approaches the 
margin of the hemisphere the cells become more densely packed 
