178 VISION 



comparable in this respect to intermediate galeomorphs. The pallium of Raja 

 (Figure 15) reflects only an intermediate level of the complexity reached by 

 batoids. Comparison of a series of sharks and a series of batoids, Noto- 

 rynchus—Squalus—Mustelus—Sphyrna and Platyrhinoidis — Raja—Dasyatis, 

 suggests that parallel development of the dorsal pallium characterizes the 

 advanced sharks and batoids. More detailed studies are needed to determine 

 if comparable dorsal pallial regions receive the same sensory inputs and have 

 hypertrophied in both sharks and batoids. 



It is now impossible to determine whether chimaeras have similar pallial 

 specializations. Analyses by Holmgren (1922) and Kuhlenbeck and Niimi 

 (1969) suggest that the chimaerid pallium is restricted in volume and pos- 

 sesses neither interhemispheric neural bridges nor pallial specilizations com- 

 parable to those of elasmobranchs. On the other hand, Faucette (19696) in- 

 terpreted a dorsolateral cell group (cell group x, Figure 28) as homologous to 

 part of the dorsal ventricular ridge— an avian and reptilian pallial complex 

 that receives thalamic sensory projections from a wide array of different sen- 

 sory systems. The dorsolateral cell groups (groups x and/or w, Figure 28) of 

 the chimaerid telencephalon may represent specializations of the dorsal and/ 

 or lateral pallia, an interpretation consistent with their topography. How- 

 ever, all interpretations are merely speculative until experimental informa- 

 tion establishes the connections and functions of the various telencephalic 

 cell groups in chimaeras. 



A more medial pallial group (mp) borders the dorsal pallium in chon- 

 drichthians (Figures 6, 7, 10, 15, 28), and in elasmobranchs it fuses across 

 the interhemispheric bridge (Figures 6, 7, 10, 15). Nothing is known about 

 its connections, but its topography suggests strongly that it is homologous to 

 the medial pallium (hippocampal complex) of land vertebrates. 



Discussions of other vertebrate pallia frequently include an additional 

 telencephalic area— the pallial or cortical division of the amygdala. The ver- 

 tebrate amygdaloid complex is not a single nucleus, but consists of two to 

 eight major nuclei divided into basal (subpallial) and pallial divisions. The 

 topographical positions of these nuclei are easily recognized in early em- 

 bryonic stages of mammals, or in vertebrates with simple pallial develop- 

 ment, such as amphibians (Northcutt 1974). The pallial division of the 

 amygdala arises laterally as a caudoventral continuation of the lateral pal- 

 lium. It expands medially, occupying the caudal floor of the telencephalon, 

 and fuses with the rostral preoptic area of the hypothalamus. As the pallial 

 amygdala courses caudally and medially, it lies adjacent to a second cell 

 group, the basal amygdala which arises medially beneath the septal nuclei 

 and also courses caudally. Upon reaching the lamina terminalis, however, the 

 basal amygdala fuses with the contralateral basal amygdala. Thus, the sim- 

 plest pattern of amygdaloid development occurs in amphibians (Northcutt 

 1974) and consists of a mediobasal amygdaloid cell group in the shape of a C 

 (whose arms are directed rostrally and whose base runs in the lamina ter- 

 minalis) and a more lateral pallio-amygdaloid cell group running caudally 

 from the lateral pallium through the lamina terminalis to the preoptic area of 

 the hypothalamus. 



