A comparative study of amitotic division activity of trophoblast cells by constriction and by extrusion in blastocysts of American mink during the obligatory period of delayed implantation has been carried out. The frequency of occurrence of amitotic figures was found to be nearly 10% at the onset of renewal of blastocyst growth (the blastocyst size was 0.4 mm in diameter), and nearly 20% at the stage of active growth (0.9 mm), as well as at the stage of expansion prior to blastocyst attachment to the uterine wall (1.7 mm). The ratios between the frequencies of division by extrusion and by constriction were 2:1, 5:1, and 4:1 at the three stages, respectively. We suggest that the cells generated via different amitotic ways play different roles in trophoblast differentiation.