136 PROFESSOR MARSHALL. 



5. The olfactory nerve is distinctly ganglionic near its root of origin 

 from the brain in Elasmobranchs and in the chick. 



It would thus appear that although the evidence is at present far 

 from conclusive, and although further information is needed on many 

 points, notably concerning the earliest stages of development of the 

 olfactory nerve, yet that the nerve answers fairly well to the tests of 

 segmental value as defined above ; and it is important to note that 

 the points in which it responds incompletely are precisely those on 

 which our knowledge of the nerve is avowedly imperfect, and that in 

 no case is a test directly contradicted. I am therefore disposed, while 

 fully admitting the need for further investigation, to rank the olfactory 

 nerve as the most anterior of the cranial segmental nerves, the nerve 

 belonging to the first head-segment. 



The segmental value of the olfactory nerve has recently been 

 advocated by Wiedersheim, who draws attention to the fact that in 

 Epicrium, and probably in other GymnopMona as well, there are on 

 either side two olfactory nerves, one dorsal and one ventral, the roots 

 of the two being perfectly independent, and some little distance apart. 1 

 Wiedersheim considers that these two roots are homologues of the 

 dorsal and ventral roots of a spinal nerve, and that by their discovery 

 the segmental rank of the olfactory nerve may be considered to be 

 established. 



A similar condition of the olfactory nerve in Pipa dor&igera has 

 been figured, though not described, by Fischer. 2 



These two cases, in both of which the additional root is the dorsal 

 one, tend strongly to confirm the view taken above of the primitive 

 connection of the olfactory nerve with the dorsal surface of the brain, 

 and therefore presumably with the neural crest ; but in the absence 

 of any observations on either the development or the physiological 

 properties of the two roots in question, I do not think that much 

 weight can be attached to Wiedersheim's suggestion of their homology 

 with the roots of a spinal nerve. 



Balfour 3 argues against the segmental value of the. olfactory nerve, 

 on the ground that it is incompatible with the views which he holds 

 concerning the primitive vertebrate mouth, and concerning the relations 

 between the nervous systems of vertebrates and invertebrates. His 



1 Wiedersheim, Die Anatomie der Gymnophionen, Jena, 1879, pp. 59, 60, and pi. iv. fig. 

 35, pi. vi. fig. 62. 

 " Fischer, Amphibiorum nudorum neurologice specimen primum, 1843, Tab. ii. fig. 1. 

 5 Balfour; Comparative Embryology, vol. ii. 1881, pp. 260-265 and 383. 



