Forebrain and Midbrain of Lizards 
41 
representative taxa of other families (Teii- 
dae, Varanidae, Iguanidae, Agamidae, and 
Chamaeleonidae) these subdivisions are very 
striking (Fig. 14B). There is a marked 
enlargement of the dorsolateral division of 
the pars extensa, a region that has been 
termed the dorsal and ventral pretectal 
nuclei (Butler and Northcutt, 1973). The 
ventromedial division has been termed the 
medial pretectal nucleus. These divisions of 
the pars extensa do not receive direct retinal 
projections but do receive substantial pro- 
jections from the ipsilateral optic tectum 
(Foster and Hall, 1975). The targets of the 
dorsal and ventral pretectal nuclei are pres- 
ently unkno-wn, but it is likely that these 
nuclei are involved in an as yet undescribed 
visual pathway, as their hypertrophy is cor- 
related with enlargement of the visual sys- 
tem. The connections of the medial pretectal 
nucleus are unknown. 
The lateral pretectum of lizards is formed 
by three nuclei, all of which receive direct 
retinal input ; nucleus geniculatus pretectalis, 
nucleus lentiformis mesencephali, and nu- 
cleus posterodorsalis (Figs. 11, 14). In 
Gekko these nuclei receive both contralat- 
eral and ipsilateral retinofugal projections 
(Northcutt and Butler, 1974). At present 
nothing is known regarding the efferent 
pathways of these retino-recipient pretectal 
nuclei. These lateral pretectal nuclei, like 
the divisions of the pars extensa, are par- 
ticularly well developed in agamids, chamae- 
leonids, iguanids, teiids, and varanids. 
The pretectum of Sphenodon is very simi- 
lar to that of lacertid and gekkonid lizards 
in that pars extensa of nucleus lentiformis 
thalami is poorly developed. However, nu- 
cleus geniculatus pretectalis lies far more 
medial than in any of the lizards I have 
examined. Beyond this difference, the pre- 
tectum of Sphenodon is similar to that of 
lacertid or gekkonid lizards, and the same 
retino-recipient targets have been identified 
(Northcutt, Braford and Landreth, 1974). 
RESULTS AND DISCUSSION: 
OPTIC TECTUM 
The optic tectum comprises the bulk of 
the midbrain roof in lizards. It is a highly 
differentiated cortex divided into a number 
of alternating cellular and fibrous layers 
(laminae). P. Ramon (1896) divided the 
optic tectum of chamaeleons into 14 layers 
which he numbered from the ventricular to 
the pial surfaces. Leghissa (1962) has em- 
ployed a similar numbering sequence in his 
analysis of tectal evolution in vertebrates. 
Huber and Crosby (1926, 1933) believed that 
many of the layers described by Ramon were 
subdivisions of more extensive units, and 
they divided the optic tectum in a different 
manner recognizing only six layers. Senn 
(1966, 1968) has reported on the tectal 
organization and its development in a num- 
ber of lizards and believes that Ramon’s sub- 
divisions can be recognized in most lizards. 
Most recent studies (Butler and Northcutt, 
1971a, 1971b, 1973; Butler, 1974; Northcutt 
and Butler, 1974 ; Butler and Ebbesson, 1975 ; 
Cruce and Cruce, 1975) have utilized 
Ramon’s nomenclature as it is more useful 
hodologically than the nomenclature of 
Huber and Crosby, particularly when con- 
sidering the retinal input to the optic tectum. 
There is considerable morphological var- 
iation among the optic tecta of lizards, but 
before describing this variation it is neces- 
sary to describe the individual laminae and 
summarize their connections. In describing 
the laminae it should become obvious that 
these laminae can be grouped into three 
major zones that possess very different func- 
tions; and it is very likely that the optic 
tectum is not a single morphological char- 
acter, or unit, under a single selective pres- 
sure, but is, in fact, part of several circuits 
under very different selective pressures. 
The initial description of the tectal lam- 
inae will be based on the optic tectum of 
Iguana since the tectum is particularly well 
developed in this taxon, and the laminae can 
be easily recognized (Fig. 15). 
Lamina 1 consists of ependymal cells lin- 
ing the ventricle. These cells possess apical 
