394 J. B. JOHNSTON 
gations of neurones which there is reason to think are concerned 
with specific functions and to determine the position, extent and 
limits of these cell masses. Criteria depended upon are the 
position, size, form and grouping of the cells, their relation to 
fiber tracts and their relation to neighboring cell-masses. The 
more important aggregations of neurones are either separated 
from neighboring masses by cell-free zones, or they present a 
sudden transition to other masses whose cells differ markedly 
in size, position, staining qualities, etc. Sometimes sulci mark 
the lines of demarcation. Typical sections have been drawn to 
show the arrangement of cells in various regions and the lines 
of demarcation between the aggregations of neurones. To gain 
an understanding of the morphological relations, dissection and 
modelling have been practiced. After the various cell masses 
had been studied and their limits determined, a model was made 
of the right hemisphere together with a small adjacent part of 
the diencephalon. In making the plates for this model the limits 
of the several cell-masses were drawn under the projection micro- 
scope. The plates were then cut along these limiting lines 
but the parts not separated from one another. The model was 
then piled and fastened together as a whole and the several 
parts separated at last along the lines of the cuts. In this 
manner it was possible to pile the model accurately in the form 
of the control brain and since each part was strongly fastened 
together there was no distortion of the parts. 
I am indebted to Dr. Wm. F. Allen for the beautiful photo- 
graphs of this model. 
DESCRIPTION 
As is well known, the turtle has fairly large hemispheres whose 
caudal poles lie at the sides of the mesencephalon. The dis- 
position of the hippocampal formation and the olfactory tubercle 
in the medial wall have been described in a previous paper. 
Other features in this wall will be taken up in later sections. The 
lateral wall shows a broad depression where the hemisphere 
connects with the brain stem which at once suggests comparison 
with the insular region in the mammalian brain (fig. 1). 
