36 
Northcutt 
of the lateral forebrain bundle (Fig. 12). 
Throughout much of its rostro-caudal extent 
it is difficult to separate nucleus entopedun- 
cularis from the more dorsal nucleus ven- 
tromedialis. Again, nothing is known about 
the efferent projections of nucleus entope- 
duncularis. 
When the efferent projections of nucleus 
ventrolateralis and nucleus ventromedialis 
are known, it will be possible to decide 
whether these nuclei are homologous to parts 
of the dorsal or ventral thalamus of mam- 
mals. The striatum and spinal cord of mam- 
mals project to both the dorsal and ventral 
thalamus. Thus the afferents presently de- 
termined for the nuclei of the presumed 
ventral thalamus of lizards cannot resolve 
this problem. However, the striato-recipient 
dorsal thalamic nuclei (ventral anterior and 
ventral lateral thalamic nuclei) of mam- 
mals project to isocortex, while the striato- 
recipient ventral thalamus (subthalamic 
nuclei) projects back on the striatum. 
If nucleus ventromedialis projects to the 
dorsal ventricular ridge, this nucleus should 
be considered part of the dorsal thalamus of 
lizards and homologous to part of the dorsal 
thalamus of mammals. If nucleus ventro- 
medialis projects to the striatum, it should 
be considered homologous to part of the 
ventral thalamus of mammals. 
Nucleus geniculatus lateralis occupies the 
lateral edge of the ventral thalamus. It be- 
gins rostrally at the level of area triangu- 
laris and continues caudally under the ros- 
tral pole of the optic tectum (Figs. 10-14). 
Most workers have recognized a pars dor- 
salis and a pars ventralis. The pars dorsalis 
forms an oval-shaped cluster of cells dor- 
sally and can be easily separated from the 
pars ventralis which is divided into a medial 
cell plate and a lateral neuropil (Figs. 10- 
14). Both the pars dorsalis and pars ven- 
tralis receive a bilateral retinal projection 
in Lacerta (Armstrong, 1950), Iguana and 
Anolis (Butler and Northcutt, 19716), 
Xantusia (Butler, 1974), Gekko (Northcutt 
and Butler, 1974) and Tupinambis (Cruce 
and Cruce, 1975). Butler and Northcutt 
(1971a) and Foster and Hall (1975) have 
reported bilateral tectal projections to the 
pars ventralis of the lateral geniculate nu- 
cleus in Iguana. At present nothing is known 
regarding the efferent projections of the 
pars ventralis. 
The pars dorsalis of the lateral geniculate 
nucleus in lizards has been assumed to be a 
part of the dorsal thalamus and has further 
been assumed to project to the dorsal cortex 
of the telencephalon, as does a similarly 
named nucleus in turtles as reported by Hall 
and Ebner (1970). However, this assump- 
tion must now be questioned on the basis of 
the discovery of four new retino-recipient 
nuclei in the rostral dorsal thalamus of liz- 
ards and other reptiles (Figs. 10,12; Butler 
and Northcutt, 1977). Most recent studies on 
the visual system of lizards have recognized 
a bilateral retinal projection to a small oval- 
shaped cluster of cells that caps the cell plate 
and neuropil of the pars ventralis of the 
lateral geniculate nucleus. This cell cluster 
has been assumed to be the most dorsal tar- 
get of the optic tract in the rostral thalamus 
of lizards {Lacerta, Armstrong, 1950; Ig- 
uana and Anolis, Butler and Northcutt, 
19716; Tupinambis, Ebbesson, 1972, Cruce 
and Cruce, 1975; Gekko, Northcutt and But- 
ler, 1974). However, in describing the reti- 
nal projections in Gekko (Northcutt and 
Butler, 1974), we were puzzled by the pres- 
ence of a retinal terminal field located dorsal 
to the cell group that we labeled pars dor- 
salis of the lateral geniculate nucleus (Fig. 
3, Northcutt and Butler, 1974). This termi- 
nal field was charted but not discussed. In 
the same year an autoradiographic study of 
the retinal projections in Sphenodon was 
undertaken (Northcutt, Braford and Land- 
reth, 1974), and an equally extensive retinal 
target was discovered dorsal to the pars 
dorsalis of the lateral geniculate nucleus and 
was termed the dorsal optic nucleus. Subse- 
quently, this same target has been identified 
in Caretta (Bass and Northcutt, 1975) and 
in Caiman (Braford and Northcutt, in prep- 
aration). This new target probably corre- 
sponds to the cell group identified by Knapp 
and Kang (1968) in Podocnemis as the pars 
dorsalis of the lateral geniculate nucleus. In 
