Fig. 1. (A) Immunoreactive opsin in the pineal organ (arrow) of the halibut embryo. Note the undifferentiated lateral eyes (e). (B) Herring embryo, with opsin-immunoreactive pineal cells (arrow), at an earlier developmental stage than the halibut in A. (C) Parasagittal section through the brain of a halibut embryo showing the generally low level of differentiation. The earliest differentiated neuronal cell bodies and axonal pathways are immunoreactive for acetylated 
-tubulin (arrows). Note the absence of immunoreactivity in the optic tectum (ot). (D) Axonal connections (arrowheads) between the pineal organ (pin) and deep brain regions in the halibut embryo; pc, posterior commissure. (E) Expression of ultraviolet-opsin mRNA in the pineal organ of the halibut embryo. (F) Expression of ultraviolet-opsin mRNA in the pineal organ of the halibut larva. (G) The adult pineal organ of halibut (transverse section at the level of the end-vesicle) displays a large number of photoreceptors expressing green-opsin mRNA; bv, blood vessel; asterisk, lumen of the pineal organ. (H) Photoreceptors in the ventral retina of the halibut larva express ultraviolet-opsin mRNA (arrows). Scale bar: A, 50µm; B, 140µm; C, 80µm; D 30µm; E,F, 12µm; G, 40µm; H, 15µm.
