80 
Cruce & Cruce 
tiles, bilateral retinofugal projections have 
been consistently observed in lizards and 
snakes, e.g., Lacerta vivipara (Armstrong, 
1950), Tupinamhis nigropunctatus (Ebbes- 
son, 1970; Cruce and Cruce, 1975), Iguana 
iguana (Butler and Northcutt, 1971), Anolis 
carolinesis (Butler and Northcutt, 1971), 
Vipera aspis (Reperant, 1973), Gekko gecko 
(Northcutt and Butler, 1974a), Xantusia 
vigilis (Butler, 1974), Thamnophis sirtalis 
(Halpern and Frumin, 1973), Natrix sipedon 
sipedon (Northcutt and Butler, 19746) but 
are not usually seen in turtles, e.g. Chelydra 
serpentina (Knapp and Kang, 1968a), 
Podocnemis unifilis (Knapp and Kang, 
19686), Pseudemys scripta (Hall and Ebner, 
1970), Caretta caretta (Bass and Northcutt, 
1975) or crocodilians, e.g.. Caiman crocodilus 
(Braford, 1974), Caiman sclerops (Reperant, 
1975) or tuatara, e.g., Sphenodon punctatus 
(Northcutt et al., 1975). In reptiles possess- 
ing bilateral retinal projections, these fibers 
terminate in parts of the thalamus and in 
some of the pretectal nuclei. Until recently, 
only contralateral terminals had been seen 
in the tectum. In lizards the first reports of 
bilateral retinotectal projections were those 
of Butler (1974) and Northcutt and Butler 
(1974a) ; subsequently we also saw bilateral 
retinotectal projections (Cruce and Cruce, 
1975). Reperant (1973) reported ipsilateral 
retinotectal connections in the snake, Vipera 
aspis. 
Many of the previous experimental studies 
have used the method of tracing the antero- 
grade axonal degeneration which results 
from destruction of the retina; this destruc- 
tion may be produced by removing the eye 
(i.e. enucleation) or swabbing the retina. The 
axons and terminals of the destroyed retinal 
cells then undergo a process of degeneration. 
After a certain time, the animal is sacrificed 
and its brain removed for histological pro- 
cedures. The brain is cut in thin sections and 
stained by a reduced silver stain, usually 
either the Nauta-Gygax (1954) or Fink- 
Heimer (1967) stain which demonstrates 
degenerating axons. 
Recently a new technique to study antero- 
grade axonal connections (Cowan et al.. 
1972) has become available. This method is 
based on principles of active function of 
neurons rather than on degeneration result- 
ing from cell destruction and death. This 
technique of autoradiographic tracing of 
anterograde axonal connections utilizes the 
process of transport of protein molecules 
along the axon from nerve cell body to its 
terminals. Radioactive amino acids are in- 
jected into the vicinity of nerve cell bodies. 
The amino acids are incorporated into pro- 
tein in the somata of neurons. These radio- 
active protein molecules are then transported 
along the axon to the terminals, and their 
entire course may be visualized by standard 
autoradiographic methods. The presence of 
labeled molecules is indicated by the presence 
of developed silver grains in the emulsion 
overlying the brain section. Depending upon 
how long the animal survives after the injec- 
tion, labeled protein molecules will be concen- 
trated either in higher proportion in the 
nerve terminals (rapid transport with short 
survival times) or equally in both terminals 
and axons (slow transport with longer sur- 
vival times). 
The results presented in this paper were 
obtained by using the autoradiographic trac- 
ing technique to study the projections of the 
retina in the Tegu lizard. This technique has 
also recently been employed by Northcutt 
et al. (1974), Bass and Northcutt (1975), 
and Reperant (1975) to study retinal con- 
nections in other species of reptiles. In an 
earlier report (Cruce and Cruce, 1975), we 
compared the results obtained using auto- 
radiography with those obtained using the 
anterograde axonal degeneration method 
(Nauta-Gygax and Fink-Heimer stains). We 
found that the autoradiographic results con- 
firmed those derived from the degeneration 
method ; in addition the autoradiographic 
technique was able to elucidate some aspects 
of the projection which were ambiguous with 
the degeneration method. 
METHODS 
Seven Tegu lizards {Tupinamhis nigro- 
punctatus) were given intra-ocular injections 
