106 W. H. TALIAFERRO 
would have to pass through the retinula in a distal direction. 
Otherwise the light would not be affected by the middle region. 
If the above suggestion is correct, the pigment would play 
no part in the localization of photic stimulation in the individual 
retinulae. We might, however, conceive that the middle region 
has no effect on photic stimulation, but that the rhabdome is 
itself so constructed that only light passing along its longitudinal 
Fig. 17. Diagram representing the relation between structure and photic 
stimulation in the individual retinula. A, one of the accessory cells which form 
the pigment-cup; 1, middle region of the retinula; N, nucleus of retinula; 
k, sensory rhabdome. Light striking the rhabdome parallel to the axis a results 
in stimulation. Light from the direction b or c does not result in stimulation. 
axis sets up stimulation. If this is true, light from either direction 
as long as it is parallel to the longitudinal axis might cause stimu- 
lation. The pigment, then, might serve to localize photic stimu- 
lation in that it would prevent light passing through the rhabdome 
in a proximal direction and allow light from the opposite direction 
to strike the rhabdome. Thus, in figure 17, the pigment is so 
placed that light along the arrow c cannot strike the rhabdome. 
While this suggestion ascribes a certain limited localizing function 
