CELL MASSES IN THE FOREBRAIN 429 
chiefly brought about by the great expansion of the dorsal pal- 
lium. The expansion of the pallium was accompanied by a great 
thickening of the lateral bundle and of the cell mass in which it 
was imbedded. The stretching of the lateral olfactory area 
accompanying the expansion of the pallium was followed by a 
collecting together of the olfactory centers into a ring around 
the striatal area. The striatum then, thickening and bulging 
outward, came to have its surface freed from the olfactory layer. 
That a complete ring of olfactory centers was formed around the 
striatal area was due to the connection which the medial ol- 
factory tract retained with the nucleus of the lateral olfactory 
tract (fig. 56). This connection of the lateral olfactory nucleus 
with the medial tract and diagonal band has produced the 
characteristic bending of a part of the lateral olfactory tract to 
reach its nucleus in the amygdaloid region (fig. 60). 
We have now to note how this condition in the turtle differs 
from that in the mammal. Although the gross appearance is 
much the same there is one essential difference, namely, that the 
striatal area has received a covering of general pallium, convert- 
ing it into the insula. This has come about by the further 
expansion of the pallium which has pushed the pyriform lobe 
down over the striatal area so that in mammals it runs along the 
ventral border of the insula. Now a part of the lateral olfactory 
tract Sweeps round in a curve to reach its nucleus in the tip of 
the temporal pole just as in the turtle. There is this difference, 
however, that the concavity of that sweeping curve is filled not 
by the ciate area but only by the tuberculum and the anterior 
perforate space. 
The striatal area has thus been the scene of two important 
migrations of brain substance in the history of vertebrates. 
Between the fishes and reptiles the olfactory area has spread over 
the striatum and left its surface free again in the turtle. Be- 
tween the turtle and mammals the general pallium has spread 
down over the striatum and left it covered by the insular cortex. 
