Electron- and light-microscope observations on the spleen of the newt Triturus cristatus: the surface topography of the mitotic chromosomes.

In polychromatophil erythroblasts, and in cells containing little or no haemoglobin, the surfaces of the mitotic chromosomes frequently bear finger-like projections averaging about o-i /* in diameter. Both the mitotic and interphase chromosomes in polychromatophil erythroblasts from some newts bear, in addition, extensive sheets of chromatin, about 315-465 A thick, depending on fixation, bounded on each side by nuclear envelope-like fragments. The remarkably constant width of the sheets in all cells indicates that chromosomes contain a constructional unit of fixed dimensions. This unit may have general significance, since other workers have found envelope-limited sheets of similar dimensions in one plant and several animal species. An hypothesis concerning the construction of chromosomes is discussed. It seeks to relate the thickness of the sheets of chromatin to the average diameter of the majority of the finger-like processes. The large intranuclear granules, 400 A in diameter, in polychromatophil erythroblasts survive in polysome-like configurations in dividing cells.