176 



VISION 



2 mm 



Figure 28 Transverse sections through (A) rostral, (B) mid, and (C) caudal levels of the 

 right telencephalic hemisphere of the ratfish, Hydrolagus colliei. The levels of these 

 sections are indicated in Figure 8. as, area superficialis basalis; f, fornix; It, lamina ter- 

 minalis; mp, medial pallium; ot, olfactory tract; sm, stria medullaris; sn, septal nuclei; tc, 

 tela chorioidea; w, cell group w (possible lateral pallium); x, cell group x (possible dorsal 

 pallium); y, cell group y (possible striatum); z, cell group z (possible pallial amygdala); 1, 

 pallial-subpallial boundary of Holmgren (1922) and Kuhlenbeck and Niimi (1969); 2, 

 pallial-subpallial boundary of Faucette (1969a, 1969b). 



Faucette, that the pallio-subpallial boundary is low on the lateral tel- 

 encephalic wall. However, it is quite possible that the cell group Faucette 

 (1969a, 19696) recognized as lateral pallium (cell group w, Figure 28A) is 

 actually part of the dorsal pallium or a nonolfactory segment of the lateral 

 pallium. Cell group w in Hydrolagus is characterized by high cell density, and 

 a well-developed pathway of myelinated fibers courses to and/or from the 

 cell group along the ventrolateral telencephalic wall, in a pattern reminiscent 

 of the lateral forebrain bundle in other vertebrates. Thus, cell group w may 

 have connections with the dorsal thalamus and may represent part of a 

 pallial development similar to the dorsal ventricular ridge of land vertebrates. 

 I believe that the more sparsely celled area, which lies lateral to cell group w 

 and receives secondary olfactory fibers, is a more likely homolog of the 

 lateral pallium. 



Holmgren (1922) believed that a more dorsal cell group represented the 

 lateral pallium in chimaeras, but he suggested that the main olfactory target 

 was a more ventral area (cell groups w and x, Figure 28), which he termed 

 the lateral olfactory nucleus. Kuhlenbeck and Niimi (1969), however, argued 

 that this telencephalic region in chimaeras is homologous to the striatum of 

 other vertebrates. 



Finally, all studies to date have interpreted a ventrally located cell plate 

 (as, Figure 28) in the telencephalon of chimaeras as homologous to the 

 olfactory tubercle of other vertebrates and have assumed that the heavily 



