Forebrain and Midbrain of Lizards 
51 
were responsible for the elaboration of the 
dorsal ventricular ridge? An examination of 
the organization and variation in ridge struc- 
ture among lizards suggests possible func- 
tions from which we can deduce these 
selective pressures. 
Possible Functional and Behavioral 
Implications of CNS Evolution in Lizards 
Considerable morphological variation in 
four brain regions of lizards (dorsal ventri- 
cular ridge, dorsal thalamus, pretectum, and 
optic tectum) has been described earlier. 
The observed variation is characterized by 
two patterns of organization that have been 
termed lacertid or igimnid patterns. The 
lacertid pattern is considered the more gen- 
eralized or ancestral pattern, as it is most 
similar to the organization of Sphenodon and 
turtles. Sphenodon and most lizards possess 
a dorsal ventricular ridge with a pronounced 
peripheral cellular plate and a poorly devel- 
oped central core region. Those lizards pos- 
sessing the iguanid pattern (agamids, cha- 
maeleonids, iguanids, teiids, and varanids) 
have greatly expanded ridges with little trace 
of the peripheral cellular plate. However, the 
same four subdivisions of the rostral dorsal 
ventricular ridge can be identified in lizards 
with either pattern (Figs. 2, 3). Clearly the 
expansion of the ridge is not due to the 
hypertrophy of any single sensory modality, 
but to an increase in neurons subserving all 
sensory modalities that reach the dorsal 
ventricular ridge. In this context it is inter- 
esting to note that lizards with the iguanid 
pattern of neural organization possess the 
highest brain-body ratios, with the teiids and 
varanids being the most encephalized (Platel, 
1976). 
While it is now clear that the evolution 
of the dorsal ventricular ridge in lizards is 
characterized by a quantitative increase in 
neurons, little is known about possible neu- 
ronal evolution within the ridge itself. In 
Sphenodon the peripheral plate neurons 
possess long multiple apical dendrites that 
extend into the central core. This pattern of 
organization is very similar to the condition 
seen in turtles (Northcutt, 1970) and in the 
lateral pallium of amphibians (Hoffman, 
1963). At present, it is impossible to charac- 
terize the neuronal types of the rostral dorsal 
ventricular ridge of lizards. Such a charac- 
terization would contribute greatly to our 
understanding of ridge evolution in lizards. 
Hypertrophy of the ridge could have taken 
place in one of two ways. There could simply 
have been an increase in the number of ridge 
cells with no change in the different types of 
cells present. If this were the case, then an 
increase in the number of neurons would 
probably increase the quantity of informa- 
tion processed per unit time, but no change 
in the quality of information processing 
would occur. However, if evolution of the 
ridge is characterized by the rise of new cell 
types with different integrative properties, 
as well as an increase in total cell number, 
those lizards with expanded ridges may proc- 
ess sensory information differently than 
other lizards. 
In this context it is particularly interesting 
to note that much of the evolution of iso- 
cortex among mammals is related to an in- 
crease in stellate and granular (intrinsic 
neurons) populations (Mitra, 1953). These 
populations are the primary targets of the 
ascending thalamic sensory pathways and 
they in turn form connections with the so- 
called “motor” neurons of isocortex. The 
evolution of these intrinsic neurons is be- 
lieved to underlie the integrative and plastic 
capabilities of mammalian isocortex (Jacob- 
son, 1975). Comparable populations of granu- 
lar neurons are now believed to have evolved 
in birds (Nauta and Karten, 1970), and their 
presence in reptiles would strongly suggest 
that the expanded ridges of lizards may be 
performing functions similar to mammalian 
isocortex or avian dorsal ventricular ridge. 
The complex social behavior of agamids and 
iguanids, and the active foraging predation 
of teiids and varanids, may be reflections of 
ridge hypertrophy. 
For the most part, I have concentrated on 
the sensory aspects of the dorsal ventricular 
ridge in lizards since we know far more about 
