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 difficult}* 

 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 Grenadier'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 a priori evidence, I think, that 

 they are probably the true retina. 



