174 VISION 



control of the melanophore-stimulating hormone (Wilson and Dodd 1973). 

 Electrical stimulation of the lobes in unanesthetized, free-swimming nurse 

 sharks (Ginglymostoma) evoke biting and mouthing of food (Demski 1977), 

 and Wilson and Dodd (1973) have demonstrated that the inferior lobes of 

 Scyliorhinus are characterized by extensive aminergic innervation and exert 

 inhibitory control of the release of the melanophore-stimulating hormone. 



Nothing is known about the efferent hypothalamic pathways in chondrich- 

 thians, but telencephalic, medullar, and perhaps spinal pathways exist, as in 

 other vertebrates. 



Telencephalon— The chondrichthian telencephalon, like that of other 

 vertebrates except actinopterygian fishes, consists of paired evaginated cere- 

 bral hemispheres and a caudal telencephalon medium (Figures 8, 9, 12, 13, 

 16, 17). The telencephalon medium in all vertebrates is a part of the em- 

 bryonic forebrain that is not carried laterally into the evaginating or everting 

 hemispheres. In all vertebrates, the olfactory bulbs arise by a secondary 

 evagination from the cerebral hemispheres. Such is the case in elasmo- 

 branchs; however, the olfactory bulbs and their peduncles (olfactory tracts) 

 arise far laterally (Figures 7C, D, 9, 12, 13, 16, 17). 



The olfactory bulbs in chimaeras (Figure 8) arise rostrally from the frontal 

 pole of the hemispheres and have distinct dorsal and medial divisions, which 

 appear to receive their input from dorsal and ventral halves, respectively, of 

 the olfactory organ. The olfactory bulbs of most sharks and skates (Figures 

 9, 12, 13, 16, 17) possess distinct medial and lateral divisions. Norris and 

 Hughes (1920) reported that the medial division of the olfactory bulb in 

 Squalus receives input from the medial and lateral lamellae of the olfactory 

 organ, while the lateral division receives input from only the lateral lamellae. 

 Daniel (1934), however, argued that the medial and lateral divisions of the 

 bulb receive input from the medial and lateral lamellae, respectively. Experi- 

 mental studies are needed to determine the olfactory epithelium's exact 

 pattern of projection onto the bulb. However, differences in bulbar divisions 

 and their peduncular development among elasmobranchs suggest important 

 topographical organization. 



Nieuwenhuys (1967) reviewed earlier literature on the histological organ- 

 ization of olfactory bulbs in chondrichthians and recognized four distinct 

 bulbar layers: an outer layer of primary olfactory fibers, a glomerular layer 

 formed by the dendrites of deeper mitral and large triangular cells, a mitral 

 layer formed by the mitral cell bodies and the secondary olfactory fibers, 

 and a deep granular cell layer (Figure 7C, D). The granular cells of chon- 

 drichthians, like those of lampreys, possess axons, and the secondary ol- 

 factory fibers arise not only from mitral cells, as in most vertebrates, but also 

 from large triangular cells within the granular layer. 



Until recently, all parts of the elasmobranch telencephalon were believed 

 to receive secondary olfactory fibers from the olfactory bulb (Backstrom 

 1924, Kappers et al. 1936, Aronson 1963). However, new experimental 

 studies on sharks and skates reveal that olfactory projections to the tel- 

 encephalon of elasmobranchs are as restricted as those in land vertebrates 



