134 PROFESSOR MARSHALL, 



brain rather than a nerve in the strict sense of the word. 1 Instead of 

 developing like the other nerves, the olfactory was stated to arise as 

 a hollow outgrowth from the anterior part of the cerebral hemisphere 

 — the so-called olfactory lobe or vesicle : it was also stated to arise 

 considerably later than the posterior cranial nerves. 



It is now known that these supposed distinctions between the 

 olfactory and the other nerves do not really obtain, 2 but, on the 

 contrary, that the olfactory nerves develope in precisely the same 

 way as the other cranial nerves ; that they arise at first from the 

 • upper part of the fore-brain and gradually shift downwards, acquiring 

 by so doing a secondary connection with the cerebral hemispheres, of 

 which they are at first completely independent ; and, finally, that the 

 olfactory lobe or vesicle so far from being the earliest part to be 

 developed is actually the last, no vestige of it appearing in the chick 

 until the seventh day of incubation, in the salmon till long after 

 hatching, or in dogfish until stage of Balfour's nomenclature. 



If, then, the olfactory nerve agrees in all important features of its 

 development with the other cranial, and the spinal nerves, the further 

 question at once suggests itself, — has it segmental value f An exami- 

 nation of the evidence at our disposal, which is unfortunately far from 

 complete, shows that there is much to be said in favour of such a 

 view ; thus, applying to the olfactory nerve the several tests of the 

 metameric value of cranial nerves in the order given above, we obtain 

 the following results : — 



1 . The olfactory nerve develops very early : the actual date of its 

 first appearance is very difficult to determine, and has not yet been 

 ascertained with certainty in any case, but in both the chick and 

 the dogfish it appears at a very early stage of development, and in 

 the chick, indeed, an attempt has been made to show that the 

 olfactory nerve is " one of the first nerves in the body to appear," 4 

 arising before any of the spinal nerves. There is also evidence, 

 though as yet inconclusive, in favour of the origin of the olfactory 

 nerve in the chick from the neural crest. 



1 Vide, e.g. , Huxley, Anatomy of Vertelrated Animals, p. 71 ; and Gegenbaur, Elements 

 of Comparative Anatomy, English Translation, p. 515. 



" Marshall. "Morphology of Vertebrate Olfactory Organ," Quart. Journ. of Micros. 

 Science," July 1879 ; and Balfour, Comparative Embryology , vol. ii. 1881, pp. 336 and 382. 



3 I have dealt with this question at some length in a former paper on ". The Morphology 

 of the Vertebrate Olfactory Organ," Quart. Journ. of Micros. Science, July 1879, to which 

 I would beg to refer the reader who may desire further details than I can give here. 



1 Marshall, "The Development of the Cranial Nerves in the Chick," Quart. Journ. of 

 Micros. Science, Jan. 1878, p. 23. 



