In order to clarify whether or not the electronegative olfactory mueosal potentials (EOG) are generator potentials, the effects of changed ionic enviroment were studied. The EOG decreased in amplitude and in some cases nearly or completely disappeared, when Na + in the bathing Ringer solution was replaced .by sucrose, Li +, choline +, tetraethylammoninm + (TEA), or hydrazine. In the K+-free Ringer solution, the negative EOG's initially increased and then decreased in amplitude, In Ringer's solution with increased K +, the negative EOG's increased in amplitude. When K + was increased in exchange for Na + in Ringer's solution, the negative EOG's decreased, disappeared, and then reversed their polarity (Fig. 6). Next, when the K + was replaced by equimolar sucrose, Li +, choline +, TEA+, hydrazine, or Na +, the reversed potentials recovered completely only in Na+-Ringer's solution, but never in the other solutions, Thus, the essential role of Na + and K + in the negative EOG's was demonstrated. Ba ++ was found t ° depress selectively the electropositive EOG, but it hardly :decreased and never increased the negative EOG. Hence, it is concluded that Ba ++ interferes only with CI-influx, and that the negative EOG's are elicited by an increase in permeability of the olfactory receptive membrane to Na + and K +, but not to CA-. From the ionic mechanism it is inferred that the negative EOG's are in most eases composites of generator and positive potentials.Since the pioneer work by Hosoya and Yoshida (1937) in the dog, and by Ottoson (1954Ottoson ( , 1956 in the rabbit and frog, the electrical phenomena elicited in the olfactory epithelium by application of odors have been extensively studied and much new information has been obtained. Besides the electronegative slow potentials of the " o n " type found by the above workers, and named "electro-olfactogram ( E O G ) " by Ottoson (1956), electronegative po-

We have determined the distribution and localization of the monoamine octopamine in the prosomal central nervous system of the horseshoe crab, Limulus polyphemus, by light and electron microscopic immunocytochemistry. Sixteen discrete clusters of octopamine-like immunoreactive neurons are situated bilaterally in the tritocerebrum and circumesophageal ring of fused thoracic ganglia. Two pairs of anterior clusters are located laterally in the cheliceral and first pedal ganglia; the remaining six pairs of clusters are located ventromedially in the second through fifth pedal ganglia, chilarial ganglia, and opercular ganglia. The immunoreactive somata range from about 40 to 100 microns in diameter and occur in clusters of 12-24 cells. There is extensive distribution of octopamine-immunoreactive nerve fibers in Limulus; dense fiber tracts course anteroposteriorly through the central nervous system, and most neuropil regions are innervated by immunoreactive processes and terminals. This wide distribution of octopamine-like immunoreactivity provides an anatomical basis for the several effects of octopamine in Limulus. We determined the subcellular localization of octopamine by postembedding immunoelectron microscopy. The immunogold-labelled terminals are morphologically unique; they contain large, distinctively shaped dense-core granules, typically cylindrical with a prominent indentation in one end. These large granules are 100-150 nm in diameter and range from 150-400 nm in length. The dense labelling of these unusual granules with immunogold particles indicates that octopamine is sequestered in or associated with the granules.

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