38 
Northcutt 
Podocnemis this cell group is particularly 
well developed, as large as the pars ventralis 
of the lateral geniculate nucleus, and its de- 
velopment appears to reach a peak in these 
pleurodires. 
In all the reptiles I have examined, the 
dorsal optic nucleus lies lateral to nucleus 
dorsolateralis anterior, but does not appear 
to be a division of this nucleus. In lizards 
the dorsal optic nucleus begins rostrally, 
lateral to nucleus dorsolateralis anterior, but 
continues caudally to lie adjacent to nucleus 
rotundus, and ends just rostral to nucleus 
geniculatus pretectalis (Figs. 10, 12). 
In reexamining the retinal projections in 
a number of reptiles with the new autoradio- 
graphic methods, a second new retinal target 
has been discovered medial to the dorsal 
optic nucleus. This nucleus has been termed 
nucleus dorsocentralis (Fig. 10). A slip of 
the optic tract turns medially and passes the 
caudal pole of nucleus dorsolateralis anterior 
to enter a population of cells that lie imme- 
diately dorsal to nucleus rotundus. Papez 
(1935) described this cell group in Chelonia 
and termed it the nucleus (centralis) lateralis 
(his Figs. 5, 6). This new retinal target, 
nucleus dorsocentralis, has also been seen in 
Caretta (Bass and Northcutt, in prepara- 
tion) and in Caiman (Braford and North- 
cutt, in preparation), but not in Sphenodon. 
A third and a fourth retinal target, not 
previously described to my knowledge, are 
also revealed in autoradiographic prepara- 
tions of Gekko and Iguana (Butler and 
Northcutt, 1977). Both of these targets lie 
medial to the lateral geniculate nuclei. Optic 
fibers stream through the geniculate nuclei 
and terminate in the dorsal half of the pars 
dorsalis of nucleus ventrolateralis (Figs. 
lOB, 12B) and a previously undescribed cell 
group termed the intercalated optic nucleus 
(Fig. 11). Each of these new optic nuclei 
receives optic fibers via separate and distinct 
fascicles leaving the optic tract, and each 
nucleus is cytologically distinct from the 
adjacent geniculate nuclei. Thus, five sepa- 
rate and distinct cell populations (pars dor- 
salis of the lateral geniculate nucleus, inter- 
calated optic nucleus, dorsal optic nucleus. 
nucleus dorsocentralis and pars dorsalis of 
nucleus ventrolateralis) exist in the rostral 
thalamus of many reptiles which receive 
direct retinal projections. The small size 
and close proximity of these nuclei will com- 
plicate an experimental analysis of their 
efferents. In an attempt to determine their 
efferent projections, it is probably impossible 
to place electrolytic lesions in these nuclei 
because of the problem of interrupting axons 
of passage. However, the newer retrograde 
horseradish peroxidase transport method 
should allow resolution of the efferents of 
these retino-recipient thalamic nuclei. 
Ten years ago comparative neuroanato- 
mists were wondering if nonmammalian ver- 
tebrates would possess a homolog of the 
mammalian pars dorsalis of the lateral genic- 
ulate. Now we are faced not with a single 
nucleus but with five possible candidates. A 
similar situation exists in birds. Five dis- 
tinct cell populations located dorsal to the 
pars ventralis of the lateral geniculate nu- 
cleus receive direct retinal projections in 
birds (Reperant, 1973; Miceli, Peyrichoux 
and Reperant, 1975). Unlike mammals, in 
which only a single ipsilateral retino- 
thalamo-cortical projection has been re- 
ported, birds appear to possess at least two, 
and perhaps three, such pathways (Hunt and 
Webster, 1972; Karten, Hodos, Nauta and 
Revzin, 1973; Miceli, Peyrichoux and Rep- 
erant, 1975). Not only does the number of 
pathways in birds appear to be greater than 
in mammals, but one pathway is ipsilateral 
and at least one is bilateral. 
The topography in lizards as well as the 
number of rostral thalamic nuclei that re- 
ceive direct retinal projections are more 
similar to birds than to mammals, and it is 
likely that additional retino-thalamo-telen- 
cephalic projections will soon be discovered 
in reptiles. Until much more work has been 
done on the visual system of lizards and 
other reptiles, it is impossible to decide 
which, or how many, of the retino-recipient 
thalamic nuclei are homologous to the dorsal 
geniculate nucleus of mammals. Equally 
important, it now appears likely that lizards 
will be shown to possess several different 
