Fertilized mouse eggs were examined between 27 and 34 h after the superovulating injection of human chorionic gonadotrophin (HCG). Out of 1334 eggs examined, 432 were already at the 2-cell stage; the remaining 902 at the 1-cell stage were examined in detail. All chromosome preparations of the first cleavage mitosis were classified into groups corresponding with the stages of prometaphase (early and late), metaphase (early or ‘prechromatid’, ‘chromatid’ and ‘late chromatid’) and anaphase.

An indirect estimate was made of the duration of the first cleavage mitosis and of its component stages from the incidence of stages observed at different time intervals after the HCG injection. Fertilized eggs were also observed at 37°C by time-lapse cine-photography and the interval between the disappearance of the pronuclei and the beginning of telophase of the first cleavage division was determined. The progress of eggs fertilized in vitro was also observed under normal culture conditions. A close correlation was observed between the indirect method of assessing the mitotic time and the direct values obtained from the studies on time-lapse and in vitro culture. The effect of temperature on the mitotic time was also examined by the time-lapse method.

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