Transmission electron micrograph showing the extensive accumulation of darkly stained heterochromatin at the nuclear periphery typical of many differentiated cells. Heterochromatin is excluded from regions of nuclear envelope with nuclear pores. This organization is illustrated here in a bone marrow plasma cell from guinea pig. Figure 114 from Chapter 4 (Nucleus) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the corresponding chapter is available on the ASCB's BioEDUCATE website.