52 A NEW CRANIAL NERVE IN SELACHIANS. 



appear to arise from the anterior surface of the forebrain on each side of the median 

 depression. This paired structure is designated by Fritsch as a supernumerary nerve 

 (" uberzahliger Nerv [Galeus] '). It is not described, however, and there is no sug- 

 gestion as to its connection with the olfactory cup or with the ventral surface of the 

 brain. It would appear that part of this nerve had been removed by dissection, 

 and, Fritsch recognizing the remainder as an undescribed nerve, designated it as 

 "supernumerary." Although his carefully made figures embrace the brains of some 

 other forms described in this paper, the presence of the new nerve is not indicated 

 in any of them. 



As to the second question, the occurrence of the new nerve in other vertebrates 

 than selachians, the publications of Pinkus are of importance. Pinkus ('94) pub- 

 lishes in the Anatomischer Anzeiger under the title "Ueber einen noch nicht beschrie- 

 benen Hirnnerven des Protopterus annectens," a preliminary account of a new nerve 

 in this fish. Soon after there appeared in his complete paper ('95) a further 

 description and a reconstruction showing the position of this nerve on the ventral 

 surface of the brain. At the time of writing my paper in the Anatomischer Anzeiger 

 I called attention to this work of Pinkus, but I was not ready to say that the nerve 

 described in Squalus acanthias corresponded to that in Protopterus, for the nerve 

 in Squalus and in the skate is dorsal where it connects with the brain-wall, and in 

 Protopterus it is ventral. At that time the nerve was not known to exist in any 

 selachian except the two forms mentioned. Since observing it in Mustelus, Carcharias, 

 and other sharks in which it has a ventral position, I am strongly inclined to the 

 view that the nerve described by Pinkus is homologous with the new nerve in the 

 selachians. 



Pinkus describes the nerve in the adult Protopterus, but gives no facts as to its 

 embryonic history. His summary relating to this nerve in his complete paper ('95, 

 p. 332) is as follows: "Ein bisher nur bei Protopterus nachgewiesener, markloser 

 Nerv, welcher am Vorderende des Recessus prseopticus das Zwischenhirn verlasst, 

 lagert sich dem Olfactorius an und verlauft neben ihm bis an das Vorderende der 

 Nase, wo er in einen Zellhaufen am Dach der vorderen Nasenoffnung sich verliert. 

 Eine kolbige Anschwellung dieses Nerven, welche durch die Einlagerung gross- 

 kerniger, von alien anderen nervosen Zellen des Protopterus anscheinend ver- 

 schiedenen Zellen bedingt ist, macht es wahrscheinlich, dass wir es hier mit einem 

 neuen Organ zu thun haben." 



In the selachians I do not find the mass of cells described by Pinkus in Protop- 

 terus, but I have traced the fibres of the new nerve between the folds of the nasal 

 epithelium. 



