Forebrain and Midbrain of Lizards 
27 
Sphenodon reveal a single high density zone 
that is restricted to the outer half of the 
neuropil located beneath the cellular plate. 
This histochemical distribution suggests that 
most of the ascending thalamic efferents 
probably form synapses on the distal shafts 
of the apical dendrites of the plate cells, 
rather than on the proximal dendritic shafts 
or cell bodies as almost certainly is the case 
in most lizards. 
The caudal ridge of Sphenodon possesses a 
number of similarities with the caudal ridge 
in gekkonids. The caudal ridge in both taxa 
possess low SDH concentrations and discon- 
tinuous clustering of the superficial plate 
neurons (Figs. 2,9). Unlike most lizards, a 
nucleus sphericus cannot be recognized in 
Sphenodon, even though a vomeronasal or- 
gan has been described (Parsons, 1970). 
Finally, the cortices in Sphenodon are more 
similar to turtles and crocodiles than to liz- 
ards, since they form a continuous plate with 
no cell-free breaks. 
The DVR of Lialis and Xantusia are most 
similar to those of the gekkonids. Both Lialis 
and Xantusia possess a well differentiated 
peripheral plate with cell clusters and giant 
cells. The giant cells tend to be separated 
from the clusters, rather than located in 
their center as in gekkonids. The peripheral 
SPHENODON 
cd 
C 
Figure 9. Camera lucida drawings of transverse sections through the right telencephalic hemisphere of the 
tuatara, Sphenodon. Bar scale represents 3 mm. cd, dorsal cortex; cl, lateral cortex; cm, medial cortex; cn, 
core nucleus of the dorsal ventricular ridge; d, diencephalon; dvr,a,p, anterior and posterior divisions of the 
dorsal ventricular ridge; Is, lateral septal nucleus; ms, medial septal nucleus; na, nucleus accumbens; not, 
nucleus of the lateral olfactory tract; ot, olfactory tubercle; st, striatum; vm, nucleus ventromedialis. 
