SummaryThe author gives a short description of the optical principles of phase-microscopy. Differences of refractive index in the preparation are changed into differences of light-intensity. Principal cellular structures visible in living cells : nucleolus, spindle, centrosome, mitochondria, vacuoles, cilia, etc.Original observations on the formation and structure of cytoplasmic vacuoles : in erythrocytes of Amphibia, erythroblasts and reticulocytes of Mammals (vacuoles stained vitally and related to ribonucleoproteins) ; in erythrocytes : rod-shaped vacuoles related to lipoid-membrane changes ; in various types of leucocytes and nucleated red-blood cells : juxta-nuclear vacuoles originating from an accumulation of water between the cytoplasm and the nuclear membrane. Studies in phase-contrast of nuclear destructionin vitro(«pycnonecrosis»). Similarity with nuclear changes after irradiation or the action of radiomimetic mitotic poisons. Possibilities of the phase-contrast microscope in routine pathological work.