430 J. B. JOHNSTON 
The hippocampal formation 
With the expansion of the lateral brain vesicle and the forma- 
tion of a caudal pole the massive hippocampal primordium in 
the roof of the selachian forebrain becomes spread out into a 
thin medial wall of the expanding vesicle. At the same time a 
part of the choroid roof of the median ventricle is carried out in 
the medial wall of the lateral ventricle and forms the choroid 
plexus extending from the interventricular foramen to the tem- 
poral pole. The relations of the hippocampus to the secondary 
olfactory centers in the medial wall have been fully discussed in 
previous papers and the history of the hippocampus above the 
reptiles is well known from the work of Elliot Smith and others. 
The general pallium 
The force which has brought about the changes in the ol- 
factory centers mentioned above is the expansion of the general 
pallium. This is derived from the anterior end of the somatic 
sensory column (Johnston ’10 ¢ and ’11 a) which early in verte- 
-brate evolution extended up into the roof of the forebrain between 
the lateral olfactory area and the hippocampal primordium. 
This mass of cells received ascending fibers from the sensory 
centers in the thalamus and rapidly developed into an impor- 
tant collection of sensory and correlating centers for somatic 
relations. These centers proved of incomparable value to those 
vertebrates in which they were the more highly developed, 
through enabling them to extend the range of environment to 
which they could adapt themselves. 
This expanding general pallium was located between the 
olfactory area and the hippocampus and was connected with 
its fellow of the opposite hemisphere by a commissure (corpus 
callosum) running through the substance of the hippocampus 
and in close relation to the hippocampal commissure. Under 
these conditions the expansion of the general pallium brought 
about great changes in the adjacent structures, as already noted. 
The first effect was an expansion distinctly in the dorsal or 
pallial portion of such a lateral forebrain vesicle as exists in the 
