392 Jounial of Comparative Neurology and Psychology. 



Rcsujiic of Literature. 



One of the earliest investigators to consider tlie development of 

 the olfactory nerve was Milnes Marshall' (187 8-187 9). He describes 

 the first appearance of the olfactory nerve in the chick, as "a small 

 outgrowth of spherical or slightly fusiform cells, arising on either side 

 from near the top of the forebrain." "This small process," Marshall 

 claims, "may be traced for a short distance in successive sections run- 

 ning downward and slightly outward, lying close to, but perfectly in- 

 dependent of the external epiblast. At this period there is hardly a 

 perceptible thickening of the epiblast at the spot where the olfactory 

 pit will shortly afterwards appear. It is toward this point the growth 

 is directed." 



It is evident from reading Marshall's papers that he began his 

 work with the preconceived idea that the olfactory nerve arises from a 

 continuation forward of the neural ridge. His observations are all 

 made from that standpoint. It will be seen later how, by failing to 

 observe the very first stages in the formation of the olfactory nerve, 

 and by making a slight error in the observation of other stages, he 

 might be led to believe that he had evidence of the origin af the olfac- 

 tory nerve from the brain. In order to further support his attempt to 

 homologize the olfactory with other nerves, he attempts to homologize 

 the nasal pits with a pair of gill clefts. Mixot, in his textbook of em- 

 bryology, points out that "Marshall failed to attribute weight to the 

 fact that the gill clefts are primarily evaginations of the endoderm 

 while the nasal pits are invaginations of the ectoderm." 



BalfouR'' who preceded Marshall in the examination of the ol- 

 factory nerve came to the conclusion that in the elasmobranchs 

 it originates from the peripheral end of each olfactory lobe. Thus, 

 Balfour, as Marshall, believes in a cranial origin for the olfactory 

 nerve. The onlv point in which he materiallv differs from Marshall 

 is in the fact that he believes that the nerve arises from the olfactory 

 lobe, while Marshall believes that, in both the elasmobranchs and 

 the chick, it arises before an olfactory lobe has been formed. 



1 Milnes Marshall. The Development of the Cranial Nerves in the 

 Chick, Quart, /ourn. Micros. Sc, Vol. XVIII, 187S. 



Morphology of the Vertebrate Olfactory Organ, Quart, Joiirn. Micros. Sc.^ 

 Vol. XIX, 1879. 



2 Francis Balfour. Journal of Anatomy and Physiology, Vol. XI, 1878. 



