196 JOHN BEAED. 



as the other cranial (segmental) nerves : 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 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 loug after hatching, or 

 in the dogfish until stage 0. of Balfour's nomenclature." 1 



For the rest it is hardly necessary to repeat here the evidence 

 advanced by Marshall of the segmental nature of the olfactory nerve. 

 Though in my opinion not quite conclusive, it is of value so far as it 

 goes, and it will be summarised later on after additional evidence 

 has been adduced in favour of the segmental nature of the olfactory 

 nerve. 



But Marshall recognises in the olfactory organ the rudiment of a 

 gill-cleft, and, as I am led to a somewhat different view, it may be of 

 advantage to give a summary of Marshall's reasons for this opinion. 



For the detailed account the reader is referred to the paper on 

 " The Morphology of the Vertebrate Olfactory Organ." The folio wing- 

 abstract is taken from Wiedersheim's ' Lehrbuch der Vergleichenden 

 Anatomie,' p. 375. The epitome there given is so concise and clear 

 that I do not feel it necessary to offer any excuse for reproducing it 

 here. 



Starting from the fact that the olfactory nerve agrees in its develop- 

 ment with the other cranial nerves — that is, that it represents a spinal- 

 like nerve which springs from the neural ridge — Marshall regards the 

 olfactory groove as a primitive gill-cleft, which in exactly an analogous 

 position to that in which the true gill-clefts are supplied by branches 

 of the glossopharyngeal and vagus, lias an anterior (upper) and a 

 posterior (lower) branch of the olfactory nerve, these branches being 

 respectively in front of and behind the supposed olfactory cleft. The 

 Schneiderian folds of the nasal mucous membrane are comparable to 

 the gill-filaments of fishes. As a consequence of the above view a com- 

 munication between the nasal and oral cavities must once have existed 

 in all Vertebrates, including fishes. Leaving aside the fact that such 

 a condition is still present in Myxinoids, traces of it are to be seen in 

 the naso-oral groove of Selachians, and also in the development of other 

 fishes. Thus Marshall found in salmon embryos obvious diverticula of 



1 Marshall, ' Segmental Value of Cranial Nerves,' p. 13. 



