Visual System in a Lizard 
81 
of either tritiated leucine (50 /xCi in 50 j«.l) 
or tritiated proline (250 fiCi in 25-50 ju.1). Sur- 
vival times ranged from 1 to 13 days. The 
brains were cut on a freezing microtome at 
25 ju,. The sections were mounted on subbed 
slides, dipped in Kodak liquid emulsion 
NTB2, and stored in light-tight boxes at 
5° C for times varying from 1 to 3 months. 
The slides were developed with D19 and 
stained with cresyl violet. 
RESULTS 
The results are illustrated in Figure 1 as 
a series of drawings of coronal sections 
through the diencephalon of the Tegu lizard 
taken from a description of the area by Cruce 
(1974). Certain parts of the brain (cortex, 
tectum, and part of the optic tract) have not 
been included in these drawings. The projec- 
tions seen in Figure 1 are based on the find- 
ings in a Tegu lizard which survived 3 days 
after an intra-ocular injection of 250 jitCi of 
proline. The results from the other animals 
were essentially the same. 
In this animal the density of silver grains 
or label (indicating the presence of labeled 
protein molecules) was sparser above the 
fibers of the optic tract than above terminal 
areas (Fig. 2). Therefore we are dealing 
with the rapid phase of axonal transport, 
illustrating that the density of silver grains 
allows us to distinguish between areas re- 
ceiving terminals of retinal axons and areas 
where fibers are merely passing through on 
their way to termination (i.e., labeled protein 
molecules in axons). 
Retinofugal fibers project bilaterally to 
nuclei of the ventral thalamus, dorsal 
thalamus, pretectum, and tectum. The hypo- 
thalamus and nucleus of the basal optic tract 
receive a contralateral projection only. In 
all cases of bilateral projections, the density 
of the contralateral projection is greater than 
that of the ipsilateral one. 
Dorsal Thalamus 
Throughout the rostral-caudal extent of 
the dorsal lateral geniculate nucleus (OLD) 
retinal fibers terminate bilaterally (Figs. 
1-5 to 1-7 and Fig. 2). On both sides the 
density of terminal labeling is not evenly 
distributed within the nucleus. At the level 
of Figure 1-6, two distinctive patches of 
labeling can be seen in the GLD; this con- 
figuration does not correspond to the bound- 
aries of the nucleus as defined in a cyto- 
architectonic study (Cruce 1974). 
Within the contralateral dorsal thalamus a 
sparse but definite patch of label is located 
in a part of the nucleus dorsolateralis (DL) 
and extends beyond the limits of that nucleus 
(Figs. 1-6, 1-7). 
Ventral Thalamus 
The only cell group of the ventral thalamus 
which receives a projection from the retina 
in the Tegu lizard is the ventral lateral 
geniculate nucleus (GLV). The boundaries of 
the GLV as drawn in Figures 1-5 to 1-8 are 
based on a cytoarchitectonic description in 
nonexperimental material. In the experi- 
mental material obtained in this study 
patches of label were seen outside these 
boundaries. We consider these areas to be 
further subdivisions of the ventral lateral 
geniculate, and one of them has been labeled 
as pars ventralis (GLVv) in Figures 1-5 
and 1-6. 
At its most rostral level the contralateral 
GLV is completely filled with label, but only 
a small patch of label is present ipsilaterally 
in a ventral location. More caudally the GLV 
consists of a lateral neuropil (with few cells) 
which is densely labeled on the contralateral 
side ; a medial cell plate does not contain any 
label. Presumably the terminals in the lateral 
neuropil make synaptic contact with the 
dendrites of neurons located in the medial 
cell plate. Ipsilaterally the more caudal part 
of the GLV contains a patch of label which 
is located in a dorsal position. The ipsilateral 
projection areas overlap those seen on the 
contralateral side. 
