NERVOUS SYSTEM OF THE VERTEBRATA. 465 



same manner as other nerves ; and Marshall (No. 355), to whom we are 

 indebted for the greater part of our knowledge on the development of this 

 nerve, has proved that it arises prior to the differentiation of the olfactory 

 lobes. 



The earliest stages in the development of the nerve have not been 

 made out. Marshall, as already stated, finds that in the Chick the neural 

 crest is continued in front of the optic vesicles, and holds that this fact is 

 strong a priori evidence in favour of the nerve growing out from it. As 

 mentioned above, note on p. 456, I cannot without further evidence accept 

 Marshall's statements on this point. In any case Marshall has not yet been 



FIG. 273. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN 

 EMBRYO OF ScYLLiUM. (Modified from figures by Marshall and myself.) 



c.A. cerebral hemispheres; ol.v. olfactory vesicle; olf. olfactory pit; Sch. Schnei- 

 derian folds; 7. olfactory nerve. The reference line has been accidentally taken 

 through the nerve to the brain ; pn. pineal gland. 



able again to find an olfactory nerve till long after the disappearance of the 

 neural crest. The olfactory nerve at the next stage observed forms an out- 

 growth of fusiform cells springing on either side from near the summit of 

 the fore-brain ; and at fifty hours it ends close to a slight thickening of the 

 epiblast forming the first rudiment of the olfactory pit, with the walls of 

 which it soon becomes united. 



The growth of the cerebral hemispheres causes its point of insertion in 

 the brain to be relatively shifted ; and on the development of the olfactory 

 lobes (vide pp. 444, 445) it arises from them (fig. 273). In Elasmobranchs 

 there is a large development of ganglion cells near its root. From Marshall's 

 figures these appear also to be present in the Chick, but they do not seem to 

 have been found in other forms. In both Teleostei and Amphibia the 

 olfactory nerves are at first extremely short. 



Marshall holds that the olfactory nerve is a segmental nerve equivalent 

 to the third, fifth, seventh etc. nerves. It has been already stated that in my 

 opinion the origin of the olfactory nerves from the fore-brain, which I hold 

 to be the ganglion of the praeoral lobe, negatives this view. The mere fact 



a in. 30 



