Sellaphora blackfordensis: two slightly different focuses of the centre of the same partly expanded auxospore. The two gametic nuclei (arrows) are very closely associated but unfused. The margin of each nucleus is made visible by the shell of dictyosomes (Golgi bodies) that surrounds it,  which appears as a series of short lines when the nucleus is in optical section. A tiny nucleolus can just be made out in each nucleus in the right-hand photograph.