Transmission electron micrographs. The upper panel shows a section through a frog nucleated erythrocyte. The nucleus is largely filled with heterochromatin which appears to have a fibro-granular structure when fixed with formaldehyde and osmium tetroxide. The lower panel shows the filamentous conformation of chromatin in an insect spermatid nucleus. Figure 119 (upper, courtesy of Hans Ris) and 120 (lower, courtesy of David Phillips) from Chapter 4 (Nucleus) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.