ORGANS OF VISION. 509 
observations of His presuppose, and it appears to me more 
probable that the neuroblastic cells penetrate the epithelial 
layer of the olfactory area than that the cells of this layer 
become converted into olfactory cells. The investigation of 
the origin of the olfactory cells presents great difficulties, and 
cannot be said to have been settled, and the origin of the 
auditory cells is still more obscure. In the latter case at least 
it cannot be said that they do not originate from the neuroblast 
of the auditory nerve, although on theoretical grounds it may 
be held that they originate from the surface epithelium. 
The great difficulty introduced by such a theory, however, is 
the original discontinuity of the nervous tract, and the difficulty 
in understanding how the nerve-fibres originating from the epi- 
thelial elements find their way to the central nerve-terminals, 
a difficulty which vanishes when the direct continuity of the 
neuroblastic tissue is assumed to exist from its first origin. 
The origin of the retina from the brain-vesicle in Vertebrates 
is one of the best-established facts in their embryology, and 
can be demonstrated without difficulty; it is therefore, I 
conceive, unwise to rely on the investigations of His and 
on incomplete analogies between other sensory terminals as 
evidence ‘that the retina in Arthropods originates from the 
cutaneous epiblast, and it appears to me that these have 
been repeatedly used as an argument in favour of Grenacher’s 
views, and are the strongest argument which is capable of 
being adduced in their favour. It is certain that the great 
rods of the compound eye are derived from the cutaneous 
epiblast, and this certainly, to my mind, is a strong argument 
against their function as end organs. The view that they 
are end organs can only be justified by showing that end 
organs have such an origin in other cases, and this does not 
appear to have been proved. Whilst it is equally certain that 
the retina in Vertebrates at least is derived from the neural 
and not from the cutaneous epiblast; and as structures exist 
beneath the great rods derived, like the vertebrate retina, from 
the neural epiblast, this is good @ priori evidence, I think, that 
they are probably the true retina. 
