1. By means of pencil electrode, simultaneous intra-and extracellular recordings were made of the action potentials in the lateral eye of the horseshoe crab (Tachy_pleus tridentatus). A method of compensation was combined to minimize the capacitive coupling between the inner and outer pipette of the pencil electrode and also to reduce the effective input capacitance of each pipette.
2. The obtained result confirmed the conclusion in a separate paper (12) that the ommatidial action potential has its origin a little distance from the site of impulse generation. Discussion was made more about the mechanism.
3. The membrane resistance of photosensitive cells within ommatidia was found to re-increase during illumination in the wake of sudden decrease at the onset of illumination, providing an explanation of the fact that the off-deflection far smaller than the on-deflection in the extracellularly recorded ommatidial action potential.
4. The light adaptation affected much the height of initial peak of the ommatidial action potential, but its effect on the following steady level of depolarization was small.
5. Occasionally, the membrane was inflicted a local inactivation by the tip of microelectrode.