126 



VISION 



J 





/ 



<? 



.0- 

 <0 



** 



. o 





o 

 .c- 



.<? 



OLFACTORY BULBS 



TELENCEPHALON 



DIENCEPHALON 

 MESENCEPHALON 



CEREBELLUM 

 MEDULLA 



o° 



/ 



52 



«F 





2? 



^ 



i? 



19 



12 



27 



11 



14 



33 



47 



18 



Weight of brain subdivisions as percent of total brain 



Figure 1 Relative development of major brain divisions in a number of cartilaginous 

 fishes: Hydrolagus colliei (ratfish), Squalus acanthias (spiny dogfish), Etmopterus hilianus 

 (blackbelly dogfish), Mustelus canis (smooth dogfish), Scyliorhinus retifer (chain dog- 

 fish), hums oxyrinchus (mako shark), Prionace glauca (blue shark), Carcharhinus milberti 

 (sandbar shark), Sphyrna lewini (scalloped hammerhead shark), Platy rhino idis triseriata 

 (thornback skate), Raja eglanteria (clearnose skate), Rhinobatos productus (guitarfish), 

 Myliobatis freminvillii (bullnose ray), Dasyatis centroura (roughtail stingray). 



In addition to examining the histology of a number of embryo and adult 

 brains, I also used experimental methods to determine the projections of the 

 olfactory bulb and retina in fetal and adult Squalus acanthias and Mustelus 

 canis and in adult Raja eglanteria. Two techniques were used: the Fink- 

 Heimer method for staining degenerating axons and their terminals after 

 experimental lesions and the autoradiographic method for tracing labeled 

 proteins. The technical details of these procedures can be found in Northcutt 

 and Butler (1976) and Northcutt (1976, 1977a). 



RESULTS 



General Considerations 



The cartilaginous fishes comprise at least two groups, the holocephalons 

 (chimaeras) and the elasmobranchs (sharks, skates, and rays). These groups 

 are believed to share a common ancestor (Schaeffer and Williams 1977). The 

 chimaeras, or ratfishes, possess brains (Figure 8) very similar to those of 

 elasmobranchs but clearly distinct from them in a number of neural char- 

 acters. All chimaeras possess olfactory bulbs arising from the rostral pole of 

 the telencephalon, while the olfactory bulbs of elasmobranchs arise laterally 

 via elongated olfactory peduncles or tracts (Figure 9). In Chimaera and 

 Hydrolagus, the olfactory bulbs arise directly from the telencephalic hemi- 

 sphere (Figure 8), while the bulbs of some rhinochimaerids and callorhinchids 

 arise via elongated peduncles (Garman 1904, Kuhlenbeck and Niimi 1969). 



