Discussion 
133 
DISCUSSION 
PARTICIPANT to BUTLER: Since the 
medial portion of dorsal cortex has descend- 
ing connections to the hypothalamus via the 
medial forebrain bundle (Lohman and Men- 
tink, 1972), would that field not be more like 
cingulate cortex in mammals? 
BUTLER: It does proect to the hypo- 
thalamus and certainly resembles mammalian 
limbic cortex in this regard. At the same 
time, in turtles this same topographical area 
has been shown to be the target of the dorsal 
lateral geniculate nucleus (Hall and Ebner, 
1970). 
MacLEAN: We have shown with micro- 
electrode and anatomical studies that the 
posterior part of the parahippocampal gyrus, 
not the entorhinal area, receives some visual 
input, which appears to be direct. 
PETERSON: We have animals with 
lesions identical to your cortical ones, which 
show severe behavioral deficits on successive 
reversals, which is a classic limbic deficit. 
BUTLER: One of the problems here is 
whether or not the entire dorsal cortex is 
limbic, and it may well be. On the other hand, 
if the visual retino-thalamo-telencephalic 
pathway in reptiles is homologous to the 
geniculostriate system in mammals, then 
part of the reptilian dorsal pallium is also 
neocortical in nature. We may well be dealing 
with a field homology, such that the dorsal 
pallium in reptiles is homologous as a field 
to both cingulum and striate cortex in mam- 
mals. Thus, the question of the origin of 
the corpus callosum enters here. In the turtle 
Podocnemis (Ware, 1974), there are massive 
DVR-DVR connections via the anterior com- 
missure; in contrast, these have not been 
found in the turtle Pseudemys (J. Hall, per- 
sonal communication), nor in Gekko. 
NORTHCUTT: In Podocnemis, however, 
we don’t yet know which areas in the rostral 
telencephalon correspond to the dorsal cortex 
and dorsal ventricular ridge in other reptiles, 
due to the hypertrophy of cell groups there. 
We are dealing with the problem that if the 
two cerebral hemispheres are going to talk 
to each other, they must do so by means of 
commissural systems which can develop in 
the lamina terminalis embryologically, or 
descend to sites such as hypothalamus or 
tectum, cross there, and turn rostrally again. 
We need to know how many fiber systems 
there are in the lamina terminalis. 
RAND: When you speak of teids, do you 
mean both macro and microteids? 
NORTHCUTT: The micro- and macroteids 
are very different. The telencephalon of mi- 
croteids look like that of lacertids, whereas 
the pretectum and tectum look like those 
structures in varanids. Parts of the microteid 
visual system are obviously well developed, 
but the DVR is not. It looks like the DVR in 
Sphenodon or in the gekkonids. If the CNS 
of microteids were primitive, then the de- 
rived condition of the CNS in macroteids 
would have to be considered a case of parallel 
evolution with varanids. However, microteid 
chromosomal studies suggest that microteids 
are advanced members of the teid radiation. 
Thus I suspect that the microteid DVR is 
secondarily simple due to their fossorial 
niche. 
W. CRUCE: The Type I DVR of Spheno- 
don seems to be more complex in terms of 
having a corticoid plate. Would you comment 
on how you define the type II as being more 
complex? 
NORTHCUTT: The DVR of Sphenodon 
consists of a single plate of cells with multiple 
apical dendrites, and what little Golgi infor- 
mation we possess suggests that there is a 
single cell type. The SDH distribution within 
DVR suggests that the thalamic input is 
restricted to the outer dendritic segments of 
the plate neurons. This pattern is similar to 
the pallial pattern in amphibians and primi- 
tive fishes such as Polypterus, and, therefore, 
I suspect it is close to the primitive pattern 
for reptiles. The Type II pattern can be 
derived from such a simple plate by migra- 
tion and differentiation of the corticoid plate. 
MacLEAN : Why downgrade neocortex by 
calling it DVR? Why not just be content with 
something like corticoid? That is the oddest 
neocortex I have ever seen! 
NORTHCUTT: Homology is an issue here. 
If you define isocortex (neocortex) as a lami- 
