68 
Butler 
The telencephalons of anamniote verte- 
brates were believed to be almost completely 
dominated by olfactory input, and these 
animals were thus relegated to a much less 
important status in terms of evolutionary 
theory and development. Their forebrains 
appeared to have less cytological differentia- 
tion than those of land vertebrates and were 
not thought to contain any regions homolo- 
gous to neocortex. It was believed that only 
after vertebrates emerged onto the land were 
the auditory, visual, and somesthetic systems 
elaborated, and that these systems then in- 
vaded the telencephalon, claiming synaptic 
sites that had originally been olfactory. 
The findings of a number of recent experi- 
mental studies have shed new light on these 
concepts of vertebrate forebrain evolution 
and have necessitated a revision of the 
earlier conceptions of homologies. Studies 
using anterograde degeneration methods 
have shown that the olfactory system does 
not dominate the telencephalon in anamni- 
otes. In sharks (Ebbesson and Heimer, ’70), 
actinopterygian fishes (Scalia and Ebbesson, 
1971; Braford, 1973a; Braford and North- 
cutt, 1974), and amphibians (Scalia et al., 
1968a; Scalia, 1972; Northcutt and Boyce, 
1975) the olfactory projections to the tel- 
encephalon are confined to relatively small 
and discrete areas, as they are in reptiles 
(Scalia, 1968; Scalia et al., 1969; Heimer, 
1969) and mammals (Scalia, 1966; Heimer, 
1969). Thus, it would appear that in all ex- 
tant vertebrates the greater part of the 
telencephalon is potentially available for the 
reception of nonolfactory sensory system 
inputs, and the idea of the invasion of an 
olfactory-dominated telencephalon by these 
inputs in ancestral land vertebrates has been 
discarded. The question immediately raised 
by these findings was the nature of these 
olfactory -free regions in nonmammalian 
vertebrates. An examination of the organi- 
zation of ascending sensory systems was 
thus initiated; and, while few species have 
been studied to date, it is now possible to 
state a number of postulates regarding the 
evolution of both the dorsal thalamus and 
the telencephalon. 
Before turning to reptiles in detail, some 
of the relevant data on ascending sensory 
systems in nonmammalian vertebrates in 
relation to our revised concepts of forebrain 
evolution will be reviewed. In the visual sys- 
tem, retinal projections have been studied in 
one or more species from every extant verte- 
brate class. This sample, however, is not 
evenly distributed among classes, nor is it 
extensive enough to allow more than a few 
basic conclusions. While there is considerable 
variation in the presence of bilateral versus 
totally contralateral retinal projections 
(Braford, 19726; Northcutt and Butler, 
1976), in all nonmammalian vertebrates 
studied to date the retina projects to nuclei 
in the dorsal thalamus, ventral thalamus, 
and pretectum, as well as to the tectum, and, 
in most cases, to the basal optic nucleus in 
the mesencephalic tegmentum (e.g., Agna- 
tha: Northcutt and Przybylski, 1973; 
Osteichthyes : Ebbesson, 1968 (teleosts) ; 
Northcutt and Butler, 1976 (holosteans) ; 
Chondrichthyes : Graeber and Ebbesson, 
1972; Amphibia: Scalia et al., 19686; Rep- 
tilia: Hall and Ebner, 1970a (turtles) ; Butler 
and Northcutt, 1971a; Cruce and Cruce, 
1975, this conference (lizards) ; Northcutt 
and Butler, 1974a (snakes) ; Northcutt et al., 
1974 (rhynchocephalian) ; Braford, 19736 
(crocodiles) ; Karten and Nauta, 1968) 
(Aves). The tectum in turn projects to the 
dorsal thalamus, to a locus that in some 
cases overlaps topographically with the 
retinal target, as in amphibians (Scalia and 
Gregory, 1970), and in other cases does not, 
as in reptiles (Hall and Ebner, 1970a; 
Butler and Northcutt, 19716; Braford, 
1972a). 
Auditory and somesthetic projections have 
been studied in fewer species, but it has been 
established that in reptiles and birds these 
systems project to the dorsal thalamus. 
Auditory projections are relayed from the 
torus semicircularis (inferior colliculus) in 
the midbrain to a dorsal thalamic nucleus 
(Karten, 1967; Fritz, 1974a; Foster, 1974). 
Somesthetic inputs have also been identified, 
but generally appear to be less well repre- 
sented in the dorsal thalamus than the other 
