554 THE SENSES AND SENSORY ORGANS. 



it morphologically, as it has never been shown that the two 

 ends of a columnar cell are necessarily physiologically dis- 

 similar. 



The development of the nerve end organs in the Vertebrate 

 has never been actually traced to the epithelium of the 

 primary optic vesicle; nor have those of the Arthropod been 

 traced directly from the epithelial layer. In both cases their 

 origin from the cells of an earlier stage has been assumed 

 rather than proved. 



7. THE THEORY OF ARTHROPOD VISION. 



Mliller's Hypothesis. - -Johannes M tiller [197], in the year 

 1826, enunciated his well-known theory of ' mosaic vision.' He 

 concluded from the radial arrangement of the Arthropod eye 

 that the retinal image is direct and not inverted, that it con- 

 sists of a number of points of light corresponding to the 

 number of ommatca, hence the term mosaic, as he compared 

 the picture so formed with a piece of mosaic-work. 



Miiller conceived that each great rod is a very narrow 

 straight tube isolated from its fellows by a coat of opaque pig- 

 ment, capable of transmitting a very narrow pencil of light 

 from a point or from a very small surface in the direction of 

 the axis of the tube, to the sentient retina. 



It appears to me that Miiller considered the great rods as 

 essentially dioptric structures, although he at one time de- 

 scribed them as nerves [198], but since his exposition of the 

 theory these have been regarded, as has been already stated, as 

 nerve terminals. Indeed, Huxley in 1880 said :* ' The only 

 modification needed in the original form of the theory of mosaic 

 vision, is the supposition that part, or the whole, of the visual 

 rod is not merely a passive transmitter of light to a nerve 

 fibre, but is itself in some way concerned in transmuting the 

 mode of motion, light, into that other mode of motion which 

 we term nervous energy. The visual rod is, in fact, to be re- 



' Huxley, T. H.,' The Crayfish : an Introduction to the Study of Zoology,' 

 Internal. Sc. Scr., vol. xxviii., London, Paris, and Berlin, iSSo. 



