4H DEVELOPMENT OF ELASMOBRANCH FISHES. 



rower and narrower, and is finally reduced to a solid cord, 

 which in its turn disappears. The remaining vesicle then be- 

 comes divided into lobes, and connects itself closely with the 

 infundibulum (PL 16, figs. 5 and 6 pt}. The later stages for 

 Elasmobranchs are fully described by W. Miiller in his im- 

 portant memoir on the Comparative Anatomy and development 

 of this organ 1 . 



Development of the Cranial Nerves. 



The present section deals with the whole development (so 

 far as I have succeeded in elucidating it) of the cranial nerves 

 (excluding the optic and olfactory nerves and the nerves of the 

 eye-muscles) from their first appearance to their attainment of 

 the adult condition. My description commences with the first 

 development of the nerves, to this succeeds a short description 

 of the nerves in the adult Scyllium, and the section is completed 

 by an account of the gradual steps by which the adult condition 

 is attained. 



Early Development of the Cranial Nerves. Before the close 

 of stage H the more important of the cranial nerves make their 

 appearance. The fifth and the seventh are the first to be 

 formed. The fifth arises by stage G (PL 15, fig. 3 v), near the 

 anterior end of the hind-brain, as an outgrowth from the extreme 

 dorsal summit of tJie brain, in identically the same way as the 

 dorsal root of a spinal nerve. 



The roots of the two sides sprout out from the summit of 

 the brain, in contact with each other, and grow ventralwards, 

 one on each side of the brain, in close contact with its walls. I 

 have failed to detect more than one root for the two embryonic 

 branches of the fifth (ophthalmic and mandibular), and no trace of 

 an anterior or ventral root has been tnet with in any of my sections. 



The seventh nerve is formed nearly simultaneously with or 

 shortly after the fifth, and some little distance behind and inde- 

 pendently of it, opposite the anterior end of the thickening of 

 the epiblast to form the auditory involution. It arises precisely 



1 W. Miiller, "Ueber Entwicklung und Bau d. Hypophysis u. d. Processus in- 

 fundibuli cerebri," Jenaische Zeitschrift, Bd. vi. 



