CELL MASSES IN THE FOREBRAIN ADT 
thickening on the other hand is relatively more extensive. In 
this form the direct course of the superficial fibers of the crus 
up to the pallial thickening is much more clear than in Cistudo. 
It is hoped to test the disposition of these fibers experimen- 
tally. If the above conclusions drawn from the study of sec- 
tions prove to be correct, it is evident that the pallial thickening 
must be regarded as the chief source of the fibers descending 
from the pallium to the brain stem. It should be noted that the 
pallial thickening occupies the same position in the hemisphere 
as that occupied by the motor area in the lower mammals, except 
that there is no appreciable frontal association area seen in front 
of it. Also, the descending fibers lie in front of the sensory radi- 
ations as they do in the mammalian internal capsule. It seems 
probable that the pallial thickening is chiefly motor (efferent) 
in function and that the ventricular ridge and a large part of the 
general pallium are sensory and associational. However, it is 
probable from my present results that the sensory and motor 
areas are not wholly independent. Also the whole subject of 
localization within the sensory field is untouched in the present 
paper. 
SUMMARY AND COMMENTS 
The advantage offered by the turtle brain for the study of the 
evolution of the mammalian cerebral hemisphere is that the turtle 
occupies such an intermediate position between fishes and mam- 
mals that it is possible from the standpoint of the turtle brain to 
recognize the homologous structures in all three groups and to 
explain the changes which have taken place from the brain of 
the fish to that of the turtle and from the brain of the turtle to 
that of the marsupials and mammals. This is due to the fact 
that among living reptiles the turtles are most nearly related 
to the ancestors of mammals and still possess a relatively simple 
and slightly specialized brain. In some points there would be 
great advantage from the study of the dipnoan brain, to bridge 
the gap between the fishes and reptiles. This gap is much 
wider than that between the reptiles and mammals, since neither 
the specialized fishes nor the amphibians stand in the line of 
