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. 
Miiller’s Hypothesis.— Johannes Miiller [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 ommatea, 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,’ 
Internat. Sc. Ser., vol. xxviii., London, Paris, and Berlin, 1880. 
