52 
Northcutt 
this aspect of the ridge than we do about its 
possible motor function. However, isocortical 
evolution in mammals is characterized by 
marked changes in motor control. Most iso- 
cortical areas not only project upon the 
striatum, but also have considerable connec- 
tions with the brainstem reticular formation, 
as well as having indirect influence on cranial 
nerve motor nuclei and spinal motor neurons. 
Similar descending motor populations occur 
in the avian dorsal ventricular ridge (Zeier 
and Karten, 1971; Nottebohm, Stokes and 
Leonard, 1976). In Serinus, a neuronal popu- 
lation located in the caudal and lateral ridge 
projects directly to midbrain and hypoglossal 
nuclear structures after receiving input from 
neurons adjacent to the auditory division of 
the dorsal ventricular ridge (Nottebohm et 
al., 1976). Thus, the avian dorsal ventricular 
ridge, like mammalian isocortex, can be char- 
acterized as consisting of a series of separate 
sensory areas, each analyzing a single sen- 
sory modality, and forming connections with 
the striatum as well as bypassing the stria- 
tum to directly influence lower brainstem 
centers. While Hoogland (1975) has not 
reported such connections from the dorsal 
ventricular ridge of Tupinamhis, telen- 
cephalo-medullar and spinal pathways have 
been discovered in amphibians (Kokoros, 
1973; Kokoros and Northcutt, 1977) as well 
as in birds and mammals. Such broad distri- 
bution among tetrapods suggests that rep- 
tiles almost certainly will also possess com- 
parable cortico-bulbar pathways. One set of 
pathways may project from the different 
sensory areas of the ridge directly to the 
striatum, and a second pathway may project 
to the brainstem which arises from a dis- 
tinct ridge population (s) receiving projec- 
tions from adjacent ridge sensory areas. 
The evolution of the dorsal thalamus and, 
to a lesser extent, the pretectum is closely 
tied to the evolution of the dorsal ventricular 
ridge in lizards. Since the dorsal thalamus 
largely consists of centers that project to the 
ridge and other telencephalic centers, hyper- 
trophy of these telencephalic centers is cor- 
related with hypertrophy of the dorsal 
thalamus. 
As noted earlier, part of the avian pretec- 
tum projects to nucleus rotundus (Benowitz 
and Karten, 1975), and it is likely that at 
least part of the pretectal evolution in lizards 
is related to a retino-tecto-pretecto-rotundal 
pathway to the telencephalon. In other ver- 
tebrates, part of the pretectum also projects 
to the tectum (Trachtenberg and Ingle, 1974; 
Wilczynski, 1976), and it is likely that most 
of the variation seen in the pretectum of 
lizards will eventually be correlated with 
trends in the development of the optic tectum. 
Studies by Ewert (1970) and Ingle (1973) 
have implicated the amphibian pretectum in 
various aspects of visually mediated preda- 
tor-prey behavior. Lesions of the pretectum 
result in changes in the receptive field prop- 
erties of tectal neurons, with the result that 
anurans attack large, inappropriate objects, 
as if the objects were insects, rather than 
responding normally by fleeing from such 
objects. These studies suggest a complex role 
for the pretectum, mediated both through 
the tectum and lower brainstem centers. 
The hypertrophy of the pretectum in lizards 
possessing the iguanid pattern of organiza- 
tion makes these animals particularly attrac- 
tive for the initiation of similar functional 
studies in determining the role of the pre- 
tectum in lizard behavior. 
The most striking changes in the optic 
tecta of lizards are the relative size increases, 
or decreases, in the various laminae (Figs. 
15, 16). At present it is not possible to assign 
the various laminae to different functional 
categories with certainty, i.e. as terminal 
fields of specific incoming afferent systems 
or as the cells of origin of specific outgoing 
circuits. However, in reviewing our present 
information regarding the tectum, it is ob- 
vious that it is organized in this manner. In 
this context, the lamination of the super- 
ficial tectal zone is particularly interesting. 
It primarily consists of three laminae of 
incoming retinal fibers and their terminal 
laminae. Thus each retinal fiber lamina ap- 
pears to possess a separate terminal neuropil. 
A similar pattern of organization is seen in 
bony fish and amphibians. In these taxa, each 
lamina consists of a distinct retinal efferent 
