Virtual sections of Bouin's-fixed, PTA-stained fish specimens. Top two sections: Polyodon, 10 days post-hatching, horizontal sections through the head. Upper section shows the optic nerves, layers in the retina, and sections through the jaw adductor muscles. Middle section is more dorsal and shows neurocranial cartilage and otic chambers. Both sections show differential staining of brain tissues and the olfactory organs. Voxel size 4.3 μm. Bottom: European grayling (Thymallus thymallus) fixed in formalin and stained with PTA. This section shows the external naris (at top), the olfactory epithelium, and the olfactory nerve, as well as cranial cartilage. Voxel size 2.1 μm.