ELASMOBRANCH BRAIN ORGANIZATION 177 



myelinated tract that courses among the cells of this ventral plate is second- 

 ary olfactory fibers. Earlier elasmobranch studies also assumed that this 

 ventral plate received olfactory projections, but recent experimental studies 

 have not confirmed these assumptions. Similar experimental studies do not 

 exist for chimaeras, but I have not been able to trace the myelinated fibers 

 of the ventral plate back into the olfactory bulb, and I suspect that these 

 fibers, like those of elasmobranchs, are not secondary olfactory fibers. 



Finally, analyses of the number of distinct telencephalic areas in chon- 

 drichthians and their homologies with other vertebrates have been handi- 

 capped by the extensive cell migrations, the loss of distinct cell-free zones, 

 and the reduction in ventricular volume that characterize galeomorph sharks 

 and batoids (Figures 6, 7, 15). Holmgren (1922) stressed embryological 

 studies as a fruitful approach to these problems, and fortunately he chose to 

 study one of the simplest shark brains (Squalus acanthias). I have examined 

 embryos of Squalus as well as the telencephalic histology of a number of 

 adult sharks and batoids. Based on these studies is my belief that chon- 

 drichthians, like many other vertebrates, possess three main roof (pallial) 

 formations (Figures 6, 7, 10, 15, 28). The lateral pallium (lp) receives the 

 main olfactory input and is probably homologous to the lateral pallium of 

 land vertebrates. The dorsal pallium (dp and en) is divided into inner and 

 outer laminae (Figures 6, 7, 10, 15). In elasmobranchs the outer lamina, 

 unlike that of other vertebrates, continues across the midline, forming an 

 interhemispheric bridge (Figures 6, 7, 10, 15). The inner lamina undergoes 

 extensive evolution in elasmobranchs. In Notorynchus, the inner lamina of 

 the dorsal pallium is poorly developed, forming only a slight bulge in the 

 roof of the lateral ventricle (Figure IOC, D). In squalids, the inner lamina is 

 better developed and caudally forms a thickened mass termed the central 

 nucleus by Ebbesson (1972) (Figure 6E). In squalids the central nucleus does 

 not fuse at the midline but remains a distinct and separate cell group. In 

 galeomorphs, the central nucleus is extensively hypertrophied, resulting in a 

 massively thickened interhemispheric bridge (Figure 6D, F). The central nu- 

 cleus receives substantial ascending sensory projections from the thalamus of 

 the diencephalon (Ebbesson and Schroeder 1971, Ebbesson 1972, Schroeder 

 and Ebbesson 1974). This pallial center is now known to receive visual, 

 lateralis, and trigeminal sensory inputs (Cohen et al. 1973, Piatt et al. 1974). 



The details of these projections are still preliminary, but they suggest that 

 the central nucleus is divided into a number of functional areas, rather than 

 consisting of a single cellular population with multimodal properties. A 

 number of distinct cytological subdivisions can be recognized in that pallial 

 region termed the central nucleus (Figures 6, 7, 10, 12, 15, 17), and further 

 studies will likely demonstrate that differences in the development of this 

 pallial complex are correlated with differential sensory specialization among 

 elasmobranchs. 



The dorsal pallium of batoids is more like that of galeomorph than 

 squalomorph sharks. All batoids examined to date have reduced lateral ven- 

 tricles and well-developed central nuclear complexes. Platyrhinoidis (Figure 7) 

 reveals the simplest pallial development among batoids and is most 



