126 JOHN BEARD. 
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 long after hatching, or in the dogfish until 
stage O of Balfour’s nomenclature.’’! 
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 the writer’s 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 following 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 
development 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 glosso- 
pharyngeal and vagus, has 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 communication 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- 
1 Marshall, ‘Segmental Value of Cranial Nerves,’ p. 13. 
