Forebrain and Midbrain of Lizards 
25 
rant of the ridge area. In these taxa, there 
are a number of cytological criteria that 
allow subdivision of the DVR into anterior 
and posterior parts. The ridge can be di- 
vided into a superficial, or dorsal, cellular 
plate; and a central core throughout its en- 
tire rostro-caudal extent (Fig. 2). The cells 
of the plate are considerably larger than 
those of the core region and are very similar 
to the cells that form dorsal cortex. Ros- 
trally, the superficial plate can be divided 
into at least three areas in a medial to lateral 
plane on the basis of cell density and differ- 
ences in the cross-sectional thickness of the 
plate. In Gekko, these cytological subdivi- 
sions of the rostral superficial plate coincide 
with the high density SDH zones, and it is 
very likely that they correspond to visual, 
somatic, and auditory thalamic projection 
targets respectively. However, experimental 
studies are needed to confirm this hypothesis. 
As the superficial plate of the DVR is 
traced caudal to the anterior commissure, 
the thickness of the plate decreases and be- 
comes more uniform throughout its lateral 
to medial extent. Additionally, the cells of 
the plate form clusters of uniform size which 
are rather evenly spaced (Fig. 2). This is 
particularly striking in gekkonids such as 
Sphaeroductylus. 
Gekkonids also possess an additional strik- 
ing cytological feature in the ridge. In these 
taxa, giant neurons are scattered among 
smaller cells throughout the superficial cell 
plate of the dorsal ventricular ridge. In the 
pars posterior of the DVR there is an in- 
creased tendency for these giant cells to be 
located in the center of the individual cell 
clusters. Thus, in gekkonids at least two 
different neuronal populations constitute the 
superficial cellular plate of the DVR. 
In addition to gekkonids, the DVR can be 
divided into a superficial cellular plate and 
core in all representatives that I have ex- 
amined of the following saurian families; 
anguids, cordylids, dibamids, gerrhosaurids, 
helodermatids, lacertids, lanthanotids, pygo- 
podids, scincids, xantusiids, and xenosaurids. 
However, the iguanids, agamids, chamael- 
eonids, teiids, and varanids possess a DVR 
that is organized very differently (Figs. 
3,7,8). In these taxa, the ridge is greatly 
expanded and cannot be subdivided into a 
superficial plate and core based on differ- 
ences in either cell density or cell size. At 
best, a slightly higher density of cells along 
the extreme ventricular edge of the ridge is 
seen in some of these taxa and suggests a 
trace of the superficial cellular plate seen in 
other families. In lizards with this second 
type of ridge development, it is almost im- 
possible to recognize anterior and posterior 
subdivisions on cytological criteria. As in 
the gekkonids, the SDH activity is much 
higher in the rostral half of the ridge than 
in the caudal half and, as in lizards with the 
first type of ridge development, the anterior 
ridge can be divided into four regions based 
on cell density and histochemical differences 
(Figs. 3,4,7,8). There are no cellular clusters 
to demarcate the ridge into pars anterior 
and pars posterior, and the only nonexperi- 
mental cytological criterion may be a lower 
density of cells in the pars posterior. 
To date, very little is known regarding 
the descending projections of the DVR. 
Hoogland (1975) reports that lesions of the 
rostral DVR in Tupinambis reveal descend- 
ing pathways that do not extend beyond the 
telencephalon. Projections were described to 
the ipsilateral striatum as well as to other 
parts of the DVR. When parts of the caudal 
ridge were invaded, an additional projection 
was observed to the ventromedial hypothala- 
mus. Sligar and Voneida (1976) have re- 
ported similar results in Tupinambis. 
Dorsal ventricular ridge as a taxonomic 
feature. The dorsal ventricular ridge 
(DVR) of lizards appears to be a useful 
taxonomic feature for several reasons. It 
possesses low intraspecific variation, is re- 
gressive in only a few taxa (such as some 
burrowing forms), is most likely a poly- 
genic feature since the pars anterior is re- 
lated to a number of different sensory mo- 
dalities (at least three), and possesses a 
number of different characters or states that 
