ELASMOBRANCH BRAIN ORGANIZATION 179 



A similar pattern of amygdaloid development may exist in chon- 

 drichthians. A more scattered cell group can be recognized as a ventral 

 continuation of the lateral pallium in chimaeras (cell group z, Figure 28) and 

 in elasmobranchs (cell group a, Figures 10, 15). In both sharks and skates, 

 nucleus a receives secondary olfactory fibers and can be traced caudally to 

 where it eventually replaces a more medioventral cell group, area superficialis 

 basalis. 



In land vertebrates, the pallial division of the amygdala receives secondary 

 olfactory fibers from a specialized part of the olfactory system (the vomero- 

 nasal organ), whereas the basal amygdala receives substantial input from the 

 lateral pallium. A similar condition exists in chondrichthians. The area super- 

 ficialis basalis lies adjacent to nucleus z of chimaeras (nucleus a of elasmo- 

 branchs) and can be traced caudally into the lamina terminalis where it 

 clearly fuses across the midline in chimaeras (Figure 28B) and in Noto- 

 rynchus. Area superficialis basalis does not receive a direct olfactory input. 

 However, it does receive massive input from the olfactory-dominated lateral 

 pallium (Ebbesson 1972). These relationships suggest that nucleus a (nucleus 

 z in chimaeras) is homologous to the pallial amygdala of land vertebrates and 

 that area superficialis basalis is homologous to the basal amygdala of land 

 vertebrates. In land vertebrates these amygdaloid divisions have different 

 histochemical properties, as well as differential projections to widely scat- 

 tered forebrain centers, including the hypothalamus. Thus the proposed 

 homologies with elasmobranchs can be tested by histochemical studies. If 

 these studies confirm the proposed homologies, experimental tracing 

 methods should reveal well-developed hypothalamic connections with nu- 

 cleus a and area superficialis basalis. 



The telencephalic floor, or subpallium, consists of several nuclei in addi- 

 tion to area superficialis basalis. Ventromedially, two distinct cell masses lie 

 above the area superficialis basalis and the medial pallium (Figures 6, 7, 10, 

 15, 28). These nuclei probably represent the lateral and medial septal nuclei 

 (Is, ms, Figures 6, 7, 10, 15, 28). In their topography and structure, they 

 closely resemble similarly named nuclei in land vertebrates. However, noth- 

 ing is known about their connections in elasmobranchs. 



The ventrolateral telencephalic wall contains a number of cell groups I 

 have tentatively labeled the striatum (st), based on their topography and the 

 high acetylcholinesterase activity in sharks and batoids (unpublished observa- 

 tions). Among the chondrichthians I have examined, the topography of the 

 striatum is clearest in Notorynchus. In this species, the striatum can first be 

 recognized as a rapidly growing cellular ridge ventromedial to the lateral 

 pallium (st, Figure 10 A). As the ridge is traced caudally, it divides into 

 distinct dorsal and ventral components. 



The dorsal component is rapidly replaced by the expanding nucleus a 

 (Figure 10B). However, the ventral component can be traced further cau- 

 dally (Figure 10B, C), where it comes into close contact with, and may 

 merge with, the forebrain bundles (Figure 10D). A similar striatal ridge can 

 be recognized in Squalus (Figure 6A, C). In galeomorphs, however, the ven- 

 trolateral telencephalic wall is greatly thickened, and there is no trace of a 



