48 
Northcutt 
those of agamids, chamaeleonids, and 
iguanids. 
GENERAL DISCUSSION 
Three aspects of the central nervous sys- 
tem of lizards will be considered, with par- 
ticular emphasis given to the forebrain and 
midbrain. First, the neural features that 
differentiate reptiles from other tetrapods 
will be summarized. The evolution and pos- 
sible functional and behavioral significance 
of these neural features in reptiles, particu- 
larly in lizards, will then be explored. Finally, 
the implications of these neural characters 
and their distribution is used to speculate 
on the phylogeny of lizards. 
Comparison With Amphibians and 
Other Tetrapods 
Generally, the brains of reptiles are twice 
as large as those of amphibians of the 
same body weight (Ebbesson and Northcutt, 
1976). Most brain regions are involved in 
this difference rather than any single area, 
e.g., the optic tectum, accounting for the bulk 
of the increase. Substantial increase (2-4 
fold) in the volume of the cerebellum occurs 
in most lizards and is most likely related 
to increased importance of the limbs in loco- 
motion and behavior such as social signaling. 
Generally, most reptilian brain regions also 
possess more migrated neurons than do com- 
parable regions in amphibians, and the den- 
sity per unit area has probably doubled. 
One of the most striking differences be- 
tween amphibians and reptiles is the elabora- 
tion of a telencephalic dorsal ventricular 
ridge in reptiles (Fig. 18). This ridge ap- 
pears to form embryonically by a prolifera- 
tion of the lateral pallial wall (Johnston, 
1916; Kuhlenbeck, 1938; Kallen, 1962) which 
is also the most likely source of the ridge 
phylogenetically (Northcutt, 1974; Kicliter 
and Northcutt, 1975). The rostral half of 
the dorsal ventricular ridge is now under- 
stood to be the final target of multiple ascend- 
ing pathways that reach the telencephalon 
after synapsing, for the most part, in the 
dorsal thalamus (Fig. 19A). Thus, as the 
dorsal ventricular ridge differentiates in rep- 
tiles, so does the dorsal thalamus, since they 
are both parts of ascending sensory systems 
that stretch along the entire longitudinal axis 
of the brain. These pathways terminate not 
only in the dorsal ventricular ridge, but also 
in the striatum. At present it is not known 
whether the same neurons have projections 
to both regions or whether separate popula- 
tions located in the thalamus project to the 
two telencephalic targets. Amphibians clearly 
differ from reptiles in that the dorsal thala- 
mic nuclei of amphibians project massively 
upon the striatum and only sparsely upon the 
pallial formations (Kicliter and Northcutt, 
1975). Thus, it appears that the amphibian- 
reptilian transition was marked not only 
by an increase in the size of the entire brain, 
but also by the development of a lateral 
pallial region with a concomitant shift in the 
dorsal thalamic efferents from primarily sub- 
pallial to pallial targets in the telencephalon. 
Birds have clearly retained this pattern 
of telencephalic organization (Cohen and 
Karten, 1974) and have increased the brain- 
body ratio by some seven fold (Jerison, 
1973). Mammals, on the other hand, lack a 
dorsal ventricular ridge but have developed 
isocortex which is now believed to be homo- 
logous, in large part, to the dorsal ventricular 
ridge of reptiles and birds. It seems unlikely 
that the therapsid reptiles ever possessed a 
fully developed dorsal ventricular ridge since 
the telencephalon in all living birds and rep- 
tiles is approximately equal in width and 
length, while the endocasts of the extinct 
therapsids (Hopson, 1969) clearly reveal a 
telencephalic outline far more like that of 
living amphibians. It seems unlikely that 
theropsid reptiles would first have developed 
a dorsal ventricular ridge and then subse- 
quently have migrated neurons back out onto 
the lateral and dorsal surface of the telen- 
cephalon to form the laminated aggregates 
that we recognize as isocortex in mammals. 
The simplest explanation is that both modern 
reptiles and therapsids independently devel- 
