THE STRUCTURE OF VISUAL CELLS 47 



vertebrates. When light plays upon the retina of fishes, 

 amphibians, some reptiles and birds, the rod myoid elongates 

 and the cone myoid contracts. In darkness the opposite 

 response occurs. Since these phototropic reactions are 

 directly opposite in character to the same stimulus, it is 

 obvious that the rods and cones represent two different and 

 distinct elements. The existence of intermediate forms seems 

 very doubtful (Osawa, 1898). 



