Asatisfactorytechnique has been developed which allows sections through selected mitotic cells of human or newt tissue cultures to be examined by electron microscopy. The cells are grown on carbon‐coated cover‐slips, selected, fixed, and marked whilst being observed in the phase‐contrast light microscope, and embedded in Araldite. The cover‐slip is stripped off when the Araldite block has hardened. The required cell is identified again under phase contrast, and then sectioned using a diamond knife which allows serial sections beginning at the block face to be cut with assurance. Sections are picked up on special grids with a large central hole permitting the whole of several serial sections to be viewed without obstruction. The chromosomes are stained by immersing the grids in 2 p.c. aqueous uranyl acetate, often followed by further staining with lead hydroxide. Some features of their morphology were described, but they show little evidence of highly organized fine structure immediately below the limit of resolution of the light mic