U8 



two. The new nerve is now in all essential particulars similar to its 

 condition in the adult already described. 



The embryonic history shows that the new nerve is present in 

 embryos 10 mm in length, that it arises on the dorsal summit of the 

 primary fore-brain, on each side of the neuropore, in close connection 

 with elements of the disappearing neural crest. Its fibres are formed 

 before those of the olfactory nerve. I have not been able to locate 

 their neuroblasts with certainty, but the appearances in many sec- 

 tions, give ground for the belief that they are mainly derived from 

 the neural crest. 



Comment. 



Inasmuch as the nerves described in this paper possess a gan- 

 glion, and exist in the adults of so many selachians, it is rather to be 

 wondered at that they were not described earlier. In searching the 

 literature for mention of this nerve, I first turned with confident ex- 

 pectation to the work of Johannes Müller, for, very few anatomical 

 details escaped him in any material upon which he worked, but, 

 neither in his paper on Mustelus canis^) nor in the beautifully illu- 

 strated monograph of Busch ■^), produced under his direction is there 

 any indication in his figures or text of this nerve. Mention of it is 

 also lacking in the papers and monographs of Leuket and Gratio- 

 LET^), Miklucho-Maclay-*) and Rohon^), but, in Fritsch's*^ classi- 

 cal Memoir (1878), occurs the first, and only, reference I have been 

 able to find, to this nerve in any Selachian, prior to my paper of 1899. 

 And Fritsch's reference is so slight as to be merely casual. In his 

 figures of Galeus canis (Taf. I, Fig. 6), he represents two forwardly 

 directed strands in front of the prosencephalon. In the drawing they 

 stand out like stiff bristles and appear to spring directly from the 

 anterior surface of the fore-brain, on each side of the median depres- 

 sion. In the explanation of figures this structure is designated super- 

 numerary nerve ("überzähliger Nerv [Galeus]"), but there is no further 

 reference to it, and, therefore, the observation takes rank as a frag- 

 mentary one. Fritsch shows no ganglion, nor gives any indication 

 as to its brain attachment, centrally, or its terminus peripherally. The 



1) Ueber den glatten Hai des Aristoteles etc., 1842. 



2) De selachiorum et ganoideorum encephalo, Berolini 1848. 



3) Anat. comparee du Systeme nerveux, 1839 — 1857. 



4) Beiträge zur vergleichenden Neurologie der Wirbeltiere. I. Das 

 Gehirn der Selacbier, Leipzig 1870. 



5) Das Zentralorgan des Nervensystems der Selachier, Wien 1878. 



6) Untersuchungen über den feineren Bau des Fischgehirns, Berlin 

 1878. 



