The relationship between retinula and eccentric cells in the lateral eye of Limulus polyphemus was studied using a double electrode technique which permitted simultaneous recording of light-initiated responses in two sense cells and the labeling of the cells for subsequent histological examination and identification. The following results were obtained: (a) light-initiated slow responses with and without superimposed spike potentials were recorded from retinula cells and from eccentric cells (only one eccentric cell yielded responses without superimposed spike potentials); (b) spike potentials recorded in different cells within the same ommatidium were always synchronous; (c) a complete absence of spike potentials was observed in two experiments in which no eccentric cells could be found in the ommatidia containing the labeled retinula cells; (d) the greatest differences in the characteristics of responses recorded simultaneously occurred in those recorded from retinula-eccentric combinations. The results indicate that there is only one source of spike potential activity within an ommatidium (presumably the eccentric cell) and that the light-initiated response of retinula cells may be independent of the eccentric cell response. The suggestion is advanced that the response of the retinula cell may "trigger" the eccentric cell response.
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