Summary The paper presents data on the dynamics of different mitotic phases at the second cleavage division in the eggs of sturgeon, Acipenser güldenstädti colchicus V. Marti at 11, 16.2 and 23°C, and in those of trout, Salmo trutta morpha lacustris Linné at 1.8±0.3°C. The duration of mitotic phases in all the experiments is expressed in the fractions of one mitotic cycle of the first cleavage divisions τ0. The movement of sister chromosome sets in sturgeon and trout eggs under cleavage proceeds in the following way: the slow process of disjunction of sister chromosomes during metaphase is followed by their fast movement at anaphase. At the transition of ana- to telophase the movement of chromosomes slows down. The separation of telophase nuclei then follows, which exceeds the movement of chromosomes at anaphase in both the velocity of movement and the distance covered. Elongation of the spindle at anaphase proceeds in parallel to the movement of chromosomes and continues up to the beginning of telophase. During telophase the distance between the centers increases. Cytotomy consists of two interrelated phenomena separated under pathological conditions, namely, of the analage of the furrow within the egg and of the ingrowth of the cortical layer. Each mitotic phase in sturgeon and trout eggs is characterised by definite dimensions of the mitotic figure, while ana- and telophase are also characterized by a definite distance between sister chromosomes. Relative duration of the same mitotic phases in three series of sturgeon eggs (at 11, 16.2 and 23°C) and in a series of trout eggs (at 1.8±0.3°C) are characterized by a close similarity. Relative duration of the period of furrow analge within the egg, unlike the phases of nuclear division, undergoes changes with temperature changes. One of the specific disturbances of mitosis at a temperature approaching the upper limit is the untimely separation of centrioles and the appearance of multipolar mitoses. In some eggs repetitive duplication of centrioles takes place under these conditions. Dimensionless characteristics of duration of the primary disjunction of chromosomes and of their rapid movement are very close: at the completion of the second maturation division and at the first cleavage divisions, in different animals (sturgeon, trout) and at different temperature (within the optimal ones). It can be suggested that the ratios found are of a general importance and reflect one of the mechanisms of a strict coordination of the complex of processes leading to the nuclear division.