The Lacertilian Forebrain 
67 
in other lizards, such as Iguana, it is com- 
posed mainly of nuclear groups, having only 
a faint suggestion of a layer of clumped cells 
on the medial edge (Fig. 1). In its cyto- 
architecture, the DVR in Gekko, a type I 
lizard (see Northcutt, this conference), is 
somewhat similar to that of snakes, turtles, 
and Sphenodon, whereas the DVR in Iguana, 
a type II lizard, is similar to that of the 
thecodonts — crocodiles and birds. The sig- 
nificance of these variations in morphology 
remains to be determined. 
EVOLUTION OF THE VERTEBRATE 
FOREBRAIN 
Before discussing the data on the con- 
nections and organization of the lacertilian 
forebrain in detail, I would like to provide a 
basis for a broader perspective by reviewing 
the history of thought on some aspects of 
vertebrate forebrain evolution. 
The classical work of Elliot Smith (1910), 
Herrick (1948), and Ariens Kappers, Huber, 
and Crosby (1936) developed the first co- 
herent picture of the evolution of the fore- 
brain. They analyzed the normal embryology 
and the topographical relationships of nuclei 
and fiber tracts in various species of non- 
mammalian vertebrates. In cross sections of 
the telencephalons of most vertebrates, sev- 
eral distinct regions can be recognized which 
appear to be fairly constant across classes. 
The dorsal portion, or pallium, can generally 
be divided into a medial and a lateral zone, 
between which lies a more or less developed 
dorsal zone. The ventral portion, or basilar 
region, generally contains nuclear groups 
and the main fiber bundles of ascending and 
descending systems. In reptiles and birds, a 
portion of the dorsal part of the telen- 
cephalon is expanded and bulges into the 
lateral ventricle ; it is thus referred to as the 
dorsal ventricular ridge (Johnston 1915). 
Whereas the cell somas of the medial, dorsal, 
and lateral pallial areas in reptiles are 
arranged in laminae, the cell somas of the 
dorsal ventricular ridge have a tendency to 
be grouped into nuclei. Based on the latter 
observation and on comparisons with mam- 
malian embryology, anatomists concluded 
that the dorsal ventricular ridge was homo- 
logous to the mammalian basal ganglia, 
terming it the “corpus striatum.” 
The reptilian pallial cortices were homolo- 
gized with mammalian cortices on the basis 
of fiber connections and position, and by a 
complex argument involving the forebrain 
commissures. On the basis of its connections 
with the olfactory tracts, the lateral lamina 
of cells was identified as piriform cortex. It 
was more difficult, however, to argue that 
the medial wall was hippocampal. 
In 1910, Elliot Smith reviewed the earlier 
views concerning the massive corpus cal- 
losum which connects the two hemispheres of 
the mammalian neocortex. He cited Owen as 
the first to recognize that there is no corpus 
callosum in marsupials and monotremes. 
Owen’s view had been vigorously opposed by 
Huxley and others, on the grounds that 
de novo origin of the corpus callosum in 
eutherian mammals was untenable in terms 
of evolutionary theory. However, Elliot 
Smith supported Owen’s view with his analy- 
sis of the brains of foetal monotremes. He 
showed that these brains have two rostral 
commissures which resemble the anterior 
and dorsal commissures of all vertebrates. 
While the dorsal commissure in eutherian 
mammals is the corpus callosum, in the 
foetal monotreme the dorsal commissure 
clearly connects the medial walls of the 
hemispheres, and the medial wall matures 
into the hippocampal cortex. By then com- 
paring the foetal monotreme brain with that 
of adult reptiles, Elliot Smith concluded that 
the cells in the medial wall of the reptilian 
cerebral hemisphere, which give rise to the 
dorsal commissure, were also hippocampal. 
Since the lateral cortex of reptiles was 
identified as piriform, the medial cortex as 
hippocampal, and the dorsal ventricular 
ridge as basal ganglia, the only remaining 
portion of the dorsal telencephalon to be 
dealt with was the dorsal cortex. The latter 
was therefore identified, as by default, as 
the homolog of mammalian neocortex. 
