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The development of the olfactory nerve given above shows that 
in Acanthias two elments are joined in the olfactory nerve: the dorsal 
median nerve, hitherto unrecognized, and the laterally placed main 
olfactory. Fibres from both are closely commingled in the olfactory 
membrane, but these nerves have independent and widely separated 
connections with the brain-wall. The former is the first to appear, 
it arises on the dorsal summit near the median plane in close con- 
nection with the disappearing neural crest. The lateral bundles, which 
are differentiated a little later, outstrip the mesial element in develop- 
ment and become the main olfactory. Both groups of fibres arise 
long before the lobus is developed, and we are brought to recognize 
that the olfactory nerve is an independent structure and is not derived 
from the lobe. The main olfactory is composed of fila olfactoria 
terminating in glomeruli. 
Following the development step by step in Acanthias, we see that 
the elongated slender connective between brain and the bulb at the 
base of the nasal cup, which in the adult condition simulates a nerve, 
is a secondary formation gradually produced by modification of the 
lobe. It is, in reality, a modified part of the brain and its designation 
as “nerve” is inappropriate. The true nerve is the group of relatively 
short fila olfactoria terminating in the glomeruli of the bulb. 
Fortunately, we have this means of recognizing the limits of the 
olfactory. 
A comparative study of the olfactory in other animals shows that 
in the Selachii we are dealing with a highly modified condition which 
is not typical. There is great variation as regards the anatomical 
condition in the different vertebrate classes. ‘The nerve is sometimes 
short and sometimes elongated, sometimes thick and sometimes slender, 
a more fundamental difference is, that the nerve-like connection between 
olfactory cup and brain is not identical in all cases. Sometimes, as 
in the Selachii, it is a tractus derived from the lobe, in other cases 
it is composed of the fila olfactoria and the lobe remains in close 
connection with the brain. In the latter case it is the true olfactory 
nerve, in the former it is more like a stalk upon the distal end of 
which the olfactory fibres are borne. 
In all our discussions we need first of all to distinguish between 
nerve proper (fila olfactoria with glomeruli), bulb, tractus and lobe. 
However close the superficial resemblance may be, there can be no 
homology between a hollow tractus derived from the lobe and a similar 
appearing structure composed of fila olfactoria. As far as known a 
tractus is always developed in the Selachii. In Protopterus and the 
