180 VISION 



striatal ridge jutting into the lateral ventricle to mark the position of the 

 striatum (Figure 6B, D). In Mustelus and Triakis, the same segment of the 

 telencephalic wall is characterized by cell groups with high acetylcho- 

 linesterase activity— a reliable marker of the striatum in other vertebrates. 



In neonate Platyrhinoidis, the ventrolateral telencephalic wall (Figure 7) is 

 intermediate between that of Squalus and Mustelus. There is no distinct 

 striatal ridge, but a similar cell group can be recognized lying ventromedial to 

 the lateral pallium and adjacent to the lateral ventricle (st, Figure 7B, C). 

 However, the telencephalon of adult Platyrhinoidis is more similar to that of 

 Raja, and the lateral ventricles are reduced to approximately half their neo- 

 nate volumes. 



In Raja (Figure 15) and the more advanced batoids, the lateral ventricles 

 are so reduced that the telencephalic hemispheres are essentially solid masses 

 of neural tissue. In these taxa a chevron-shaped nucleus of larger clustered 

 cells (st, Figure 15A, B, C) occupies the rostral half of the hemisphere and 

 lies ventromedial to the lateral pallium and nucleus a as does the striatum in 

 Notorynchus and Squalus, and is therefore a reasonable candidate for the 

 batoid striatum. However, histochemical and experimental anatomical 

 studies are needed to test hypotheses regarding striatal homologues in all 

 elasmobranchs. 



When summarized, the quest for a striatal homolog in chimaeras seems as 

 fruitless as the scholastic question: How many angels can dance on the head 

 of a pin. Kappers and Carpenter (1911) noted the strong, medially directed 

 flexure of the caudal hemispheric wall in chimaeras particularly when the 

 brain is viewed dorsally (Figure 8), and termed this caudal ridge the epi- 

 striatum, thus homologizing most of the telencephalic wall with the striatum 

 of other vertebrates. Holmgren (1922) recognized both rostral and caudal 

 striatal components in Chimaera, but noted he had no particular reason for 

 doing so. His rostral component occupied the slight ventricular bulge of the 

 lateral hemispheric wall, visible in Figure 28B, and the caudal component 

 was defined as the bulk of the caudal hemispheric wall, visible in Figure 28C. 

 Kuhlenbeck and Niimi (1969) reached conclusions similar to those of 

 Kappers and Carpenter, and included cell groups w, x, and y of the present 

 review in their striatal homolog. Faucette (1969a, 19696) recognized three 

 separate divisions of the striatum, essentially encompassing the lower half of 

 the ventrolateral hemispheric wall (excluding area superficialis basalis) 

 throughout the rostrocaudal extent of the telencephalon. 



Analyses of the chimaerid lateral hemispheric wall are difficult, due to 

 poor cellular differentiation in this area, and are further complicated by the 

 caudally thickened unevaginated part of the wall (Figure 28C). At present, it 

 is impossible to recognize a pallio-subpallial boundary for this part of the 

 telencephalic wall. Therefore, its cell masses cannot be interpreted. Analysis 

 of the chimaerid telencephalon will progress only by experimental study. 



Finally, the telencephalon medium of elasmobranchs (Figures 6E, F, 7E, 

 10D, 15E) is formed by the ascending and descending forebrain bundles (fb), 

 the preoptic area (pa), and the caudal part of nucleus a. Earlier in this 

 section, I suggested that nucleus a might be the elasmobranch homolog of 



