38 



E. F. MACNICHOL, JR. 



Fig. 3. Stained section through Limulus eye showing seven ommatidia. Sections 

 moderately bleached and stained with Lee Brown modification of Mallory's aniline 

 blue stain. (Courtesy of W. H. Miller). 



much as shown in Fig. 6 except that the nerve bundles do not show up as 

 well as in the slide which was silver stained. The black clumps at the left are 

 the ommatidia, the nerve fibers come out toward the right and disappear where 

 the section passes through a part of the nerve fibers. They then reappear at the 

 extreme right of the figure where they converge to form the optic nerve. Lateral 

 connections between the nerve fibers are also apparent. They appear to sub- 

 serve an inhibitory effect which will be discussed later. 



Due to the pigment in their capsules the interior of the ommatidia cannot be 

 seen, though some of the details of the structure can be seen in an occasional 

 unit which has been sliced open by the cut. 



A chlorided silver reference electrode was placed in the solution and a micro- 

 pipette held in a saline filled chlorided silver tube lowered over the ommatidia. 

 The electrodes were connected through our special input circuit and a D.C. 

 amplifier to an oscillograph and loud speaker. Usually the puncture of an om- 

 matidium caused a large steady deflection of the oscilloscope trace, about 50 

 millivolts, the pipette being negative, but there was no response to illumination. 

 As a pipette was thrust further inward in small increments the potential would 

 abruptly go back to zero, then negative again and then back to zero again, 

 several times. The negative deflections were taken to be the resting potentials 

 of the retinula cells which occupy most of the volume of the ommatidium. As 



