Forebrain and Midbrain of Lizards 
45 
identify these laminae, and the Golgi prepa- 
rations of Butler and Ebbesson (1975) sug- 
gest that this region in their material was 
poorly impregnated. However, both of these 
studies report bipolar and horizontal ele- 
ments in these laminae. 
Lamina 12, in addition to possessing 
retinal fibers, appears to contain fibers of 
nonretinal origin; only a fraction of the 
fibers of this layer degenerate following 
removal of the retina (Northcutt and Butler, 
1974) . Recently Foster and Hall (1975) have 
reported that in Iguana, tectal efferents exit 
in laminae 8, 12 and 14. 
Senn (1968, 1970) has suggested that the 
tectal laminae can be grouped into three 
main zones based on their embryonic develop- 
ment. The recognition of these zones has 
been strengthened by recent experimental 
studies (Foster, Lymberis and Hall, 1973; 
Northcutt and Butler, 1974; Foster and Hall, 
1975) . Laminae 8 through 14, which pri- 
marily consist of retinofugal fibers and their 
terminal neuropils, have been termed the 
superficial zone. Laminae 6 and 7 constitute 
the central zone, consisting primarily of bi- 
polar neurons whose outer dendrites ramify 
in the superficial retino-recipient zone and 
whose inner dendrites probably receive tel- 
encephalic and ascending medullar inputs. 
Finally, laminae 1 through 5, containing 
neurons that do not migrate away from the 
embryonic layer, constitute a periventricular 
zone. 
Many lizards such as Anelytropsis, An- 
niella, Dihamus, Feylinia, and Typhlosaurus 
are characterized by extensive reductions in 
the visual system which are correlated with 
reductions in the superficial and central 
tectal zones. The finding that none of these 
species shows reductions in the periventric- 
ular tectal zone suggests that the periventric- 
ular zone is not under strong selective pres- 
sures related to vision, but may be more 
influenced by nonvisual parameters. 
Recently Foster and Hall (1975) have 
demonstrated that laminar lesions of the tec- 
tum in Iguana reveal distinctly different out- 
puts depending on which tectal laminae are 
involved in the lesion. If the superficial tec- 
tal zone is lesioned (laminae 8-14), projec- 
tions are traced rostrally to the pars ventralis 
of the lateral geniculate nucleus and caudally 
to the magnocellular division of nucleus 
isthmi. If lesions are extended ventrally to 
include lamina 7, a massive projection to 
nucleus rotundus is observed. Only when 
lesions include the periventricular zone (cel- 
lular laminae 3 and 5) are the descending 
tecto-bulbar and tecto-spinal pathways in- 
volved. Unfortunately, Foster and Hall were 
unable to identify the laminae of origin of 
the projections to the pretectum. Their 
results, however, strongly support Senn’s 
concept that the tectum is arranged in a 
series of zones subserving different functions 
mediated by distinctly different pathways. 
In this context, it is particularly interesting 
to note that the descending tectal pathways 
do not terminate directly on the motor nuclei 
of the cranial nerves, but end within the 
lateral and medial reticular formations of 
the brain stem. Whatever visual functions 
are mediated by the tectum are still at least 
one to two neurons removed from the final 
common motor pathway. 
Two major patterns of tectal variation can 
be recognized in lizards (Senn, 1966). In 
agamids, chamaeleonids, iguanids, teiids, and 
varanids, lamina 14 is always the main optic 
fiber layer, and lamina 12 is at least half the 
thickness of lamina 14 as seen in the trans- 
verse plane (Fig. 15). Laminae 8 and 10 
contain many cells closely packed in tight 
rows. Lamina 7 is never present as a single 
layer but is divided into inner and outer sub- 
divisions with the neurons of the inner sub- 
division scattered within lamina 6. Com- 
pared to gekkonids, lacertids, and scincids, 
the periventricular laminae appear reduced, 
particularly lamina 5. This pattern is clearly 
seen in Iguana, and for purposes of brevity 
will be subsequently referred to as the 
iguanid tectal pattern. 
Anguids, cordylids, dibamids, gekkonids, 
gerrhosaurids, helodermatids, lanthanotids, 
lacertids, pygopodids, scincids, xantusiids, 
and xenosaurids possess a distinctly differ- 
ent tectal pattern (Fig. 16) that I refer to 
as the lacertid tectal pattern. In these taxa 
