40 



E. F. MacNICHOL, JR. 



Fig. 6. Section through Limulus eye showing cross connections between the nerve 

 fibers (silver stained). (Courtesy of W. H. Miller.) 



Since the retinula cells occupy most of the space within an ommatidium the 

 electrode should have passed through at least one every time the pipette was 

 thrust into an ommatidium so we do not think they give rise to any electrical 

 activity in response to light. The eccentric cell, on the other hand, occupies a 

 very small portion of the total volume and is the only other histologically iden- 

 tifiable structure within the ommatidium. 



We think that it is necessary to obtain some contact with an eccentric cell 

 to obtain activity. This view has been confirmed by direct visual observation. 

 On several occasions we have been able to see the cell bodies of intact eccentric 

 cells in ommatidia that have been torn open when the eyes were sliced. It w r as 

 possible to penetrate these exposed cells under direct visual control and we ob- 

 tained large spikes in response to light when the electrode was thrust into them. 

 We have never obtained any sort of electrical activity, other than a resting 

 potential, from the exposed retinula cells. Therefore, we are tempted to indict 

 the eccentric cell as the structure in which the spikes originate. However, the 

 situation is more complicated. 



Three types of activity in response to light have regularly been found in 

 ommatidia as shown in Fig. 7. 



