Sensory Discrimination: Vision 159 



and Hess's overthrown for all the subjects except the larvse 

 of the blowfly ; that is, the insects were more strongly af- 

 fected by blue than by yellow or green. But the blowfly 

 larva was more strongly affected by green than by any 

 other colored light. It responded, in other words, as Hess 

 and his pupil Weve (779) had found it to do. 



41. The Problem of Visual Qualities: Amphioxus 

 and Fish 



The vertebrate eye differs in origin and structure from 

 any form of invertebrate eye, the most striking difference 

 in structure being the location of the pigmented layer of 

 the retina behind the nerve fibre layer, a location which is 

 responsible for the existence of the blind spot in the verte- 

 brate eye, where the trunk of the optic nerve breaks 

 through the retinal layers. Another point of unlikeness 

 consists in the fact that the invertebrate optic nerves do 

 not cross on their way to the brain, while in the verte- 

 brates there is either total or partial crossing of the fibres. 



The reactions of Amphioxus to light offer as chief evi- 

 dence that they are accompanied by a specific sensation 

 quality the fact that they may be fatigued independently 

 of other reactions. The only structures suggesting a visual 

 function are pigment spots on the back near the head, and 

 other pigment spots distributed down the back. Amphi- 

 oxus makes negative responses to light, especially when the 

 light, from which heat rays have been extracted by passing 

 it through water, is directed at any point on the back, the 

 most sensitive region lying just behind the eye-spot (406, 

 543). Fatiguing the light reactions had no effect on re- 

 sponse to other forms of stimulation (543). Attempts to 

 test the color "preferences" of Amphioxus by illuminating 



