A NEW CRANIAL NERVE IN SELACHIANS. 51 



fissure between the two divisions of the olfactory nerve, branching unequally as 

 described for other specimens. The main trunk of the nerve unites, as in other cases, 

 with the outer bundle of the lateral division of the olfactory nerve. There its fibres 

 mingle with those of the fila olfactoria and pass between the folds of the nasal mem- 

 brane. Some of the fibres of the new nerve reach the extreme antero-lateral portion 

 of the olfactory cup, and others terminate more centrally. 



The brain of a half-grown Squalus acanthias, about 90 centimetres in length, is 

 represented by Figure 16 (PI. VI). It shows very well a gradation between the 

 "pup" stage and the adult. In the "pup" stage (Fig. 15) the olfactory lobe is well 

 formed, but there is no tractus; in the stage represented by Figure 16 the tractus 

 is forming and the olfactory nerve proper is being removed from its former position 

 near the brain. The tractus with its neurons of the second order form the link 

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

 condition in the adult already described (p. 43). 



The embryonic history shows that the new nerve is present in embryos 10 milli- 

 metres in length, that it arises on the dorsal summit of the forebrain on each side of 

 the neuropore in close connection with elements of the disappearing neural crest. 

 Its fibres are formed slightly before those of the olfactory nerve. I have not been 

 able to locate their neuroblasts with certainty, but the appearances in many sections 

 give some ground for the belief that they are derived from the neural crest. 



There are numerous rounded cells distributed among the fibres of the new nerve 

 that give it a different appearance, under the microscope, from the bundles of fila 

 olfactoria. These rounded cells are not seen in the smaller branches; hence the 

 difficulty of distinguishing the two sets of fibres is here greatly increased. 



IV. PREVIOUS OBSERVATIONS ON THE NEW NERVE. 



The anatomy and embryology of the new nerve having been considered, there 

 remain two general questions: (1) Are there any references in zoological literature as 

 to the presence of this nerve in the selachians? and (2) Has it been observed in any 

 other vertebrates? 



The nerve was not described in any selachian prior to the appearance of my 

 paper in 1899, and the only reference I can find as to its existence is in the memoir by 

 Fritsch (78). In his figure (PL I, Fig. 6) of the brain of Galeus canis he represents part of 

 the middle portion of the new nerve in the form of two short slender strands extend- 

 ing from the forebrain straight outward and nearly parallel to each other. These 



