124 VISION 



Table 1. Chondrichthian Central Nervous Systems described in literature 

 or examined by author.* — Continued 



(Aronson 1963, Okada et al. 1969) 

 *Sphyrna lewini 

 *Sphyma tiburo 



Sphyrna zygaena 



(Okada et al. 1969) 



*Those examined by author. 



Brain: Body Data 



Brains of a number of elasmobranch species (Table 2) were perfused or fixed 

 by emersion in AFA.* All specimens were adults based on gonadal tissues 

 and reported adult body lengths. AFA fixation results in an 8-9% reduction 

 in brain weight, and all brain weights reported are uncorrected for this 

 reduction. Reported body weights are from fresh, unfixed material. Addi- 

 tional data from values cited by Crile and Quiring (1940), Quiring (1950), 

 Ridet et al. (1973), and Bauchot et al. (1976) were used, and they are noted 

 in Table 2. 



Data for relative development of major brain divisions (Figure 1) were 

 obtained by immersing AFA-fixed brains in fixative and dissecting the fol- 

 lowing brain divisions for weighing: olfactory bulbs, telencephalon (including 

 olfactory peduncles), diencephalon, mesencephalon, cerebellum, and 

 medulla. The caudal boundary of the telencephalon was considered to be a 

 plane extending from the rostral border of the optic chiasm. The caudal 

 boundary of the diencephalon was considered to be a plane extending from 

 the rostral pole of the optic tectum to the caudal pole of the infundibulum. 

 The optic nerves were not included in the weight of the diencephalon, but 

 were transected within 2 mm of the chiasm. The cerebellum was considered 

 to include all tissue lying dorsal to a rostrocaudal transection just below the 

 ventral lip of the cerebellar auricle. The caudal boundary of the medulla was 

 set at the level of the first complete cervical spinal nerve. All cranial nerves 

 were transected at the base of the brain, and neither they nor the meninges, 

 blood vessels, and chorioid plexus of the fourth ventricle were included in 

 the brain division weights. 



Each brain division was blotted immediately before being weighed. A 

 Mettler analytical balance (Model H10) was used for all measurements. The 

 accuracy of 10 repeated measurements on small brain divisions (0.003 g) was 

 ±1.6%. 



Histology 



The brains of embryos as well as adults were fixed in AFA, dissected from 

 the heads, embedded in paraffin, and sectioned at 15 jitm in the transverse 



*90 ml 80% ethanol, 5 ml formalin, 5 ml glacial acetic acid. 



