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place. This idea has been supported by the highest authority. In 
Oscar HERTwIG’s Text-book of Embryology there is no vagueness as 
to the interpretation put upon the olfactory by that distinguished 
Embryologist, and, his interpretation is also that of a very large 
number of our best morphologists. He says: ,,Finally in treating of 
the development of the cerebrum there is still to be considered an 
appendage to it, the olfactory nerve. This part as well as the 
optic nerve, is distinguished from the peripheral nerves by its entire 
development, and must be considered as a specially modified portion 
of the cerebral vesicle. The older designation of nerve is therefore 
now more frequently replaced by the more appropriate name of olfac- 
tory lobe” (p. 448, Mark’s Translation). Also (p. 464): “The olfac- 
tory nerve is not equivalent to a peripheral nerve trunk, but, like the 
optic vesicle and optic nerve, a special part of the brain produced by 
an evagination of the cerebral hemispheres.” 
This way of looking at the matter must lead to confusion, for 
it assumes that the ,,tractus” of selachians for example, which is a 
derivative of the lobe, is the equivalent of the olfactory nerve, and 
disregards the true nerve which is also present, and which has an 
existence before the lobe begins to appear. The interpretation is 
based on an extensively modified condition not on a primary one. 
As will be shown later, the lobe, tractus and nerve exhibit varied 
conditions in different vertebrates and are much confused in the 
literature. 
About three years ago the writer undertook to determine the 
embryonic history nerve partly by dissections rather than by exclusive 
dependence on sections and reconstructions. It follows, of course, that 
a dissection will give truer pictures of relationship than the most care- 
fully made reconstructions. I have found in practice that embryos 
6 mm long offer no particular difficulties to the dissection of the 
central nervous system and the nerves. This method of work has 
justified itself by bringing to light in Acanthias a new cranial nerve 
(nv in all the Figs.), connected with the olfactory epithelium, whose 
presence has been overlooked by the many workers on the head of 
that selachian. It lies in such a position that it does not show to 
advantage in any of the conventional planes of making sections and, 
therefore, it is not difficult to understand why it has not been de- 
scribed before. 
Fig. 1 shows the front surface of the brain of an Acanthias 
embryo 25 mm in length. The brain-wall has been completely exposed 
by removing the overlying layers of epiblast and mesoblast, and the olfac- 
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