Figure 2. Uptake of ³H-histamine into the ocellus of nauplius larvae as revealed by autoradiography. (A) A whole mount of a nauplius larva viewed from the side. (B) A section of a nauplius larva through the ocellus, viewed from the top. The pigmented ocellus (arrows in A and B) is obvious within this transparent animal and in sections. (C) Brightfield and (D) epipolarization illumination of an ocellus in a 2-µm section of a nauplius that had been incubated in ³H-histamine. Accumulation of silver grains over the ocellus cannot be distinguished from pigment in brightfield illumination but is obvious in epipolarized light, which reflects from the silver grains but not from the pigment. The insert shows a section through another eye at a different orientation in which the pigment arms form two back-to-back cups around the photoreceptors; the right arm of the upper cup is incomplete. (E) Brightfield and (F) epipolarization illumination of a 2-µm section of a nauplius that had been incubated in ³H-serotonin. There is no accumulation of grains over the ocellus. Scale bar in D applies to C–F.