DEVELOPMENT OF ELASMOBRANCH FISHES. 191 



portant memoir on the Comparative Anatomy and development 

 of this orofan \ 



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 

 nerv^es (excluding the optic and olfactory nerves and the nerves 

 of the eye-muscles) from their first appearance to their attain- 

 ment 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 xiv. fig. 3 v), 

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

 the extreme dorsal summit of the brain, in identically the 

 same luay as the dor^sal 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 embry- 

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

 trace of anterior or ventral root has been met 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 

 independently of it, opposite the anterior end of the thickening 

 of the epiblast to form the auditory involution. It arises pre- 

 cisely like the fifth, from the extreme dorsal summit of the 

 neural axis (PI. XIV. fig. 4(X, vii). So far as I have been able 

 to determine, the auditory nerve and the seventh proper possess 

 only a single root common to the two. There is no anterior 

 root for the seventh any more than for the fifth. 



1 W. Miiller, Ueber Entwicklung und Bau d. Hj'pophysis u. d. Processus 

 infundibuli cerebri, Jenaische Zeitschrift, Bd. vi. 



