The time-lapse movie shows early development in the Lytechinus (sea urchin) embryo. Polarizing microscopy highlights the mitotic spindles which may show bright or dark contrast according to their orientation with respect to the polarized light. For clearer visualization, the embryo was flattened - this leads to abnormal spatial location of cells at the later divisions.
See: Inoue S (1964) Organization and function of the mitotic spindle. In, Primitive Motile systems in Cell Biology. pp549-598. Academic Press