The movie shows several sequences showing changes in birefringence in oocytes of the marine annelid Chaetopteris that had formed meiotic spindles before exposure to centrifugal force in the Centrifuge Polarizing Microscope developed by S. Inoue. Spindles appear dark or bright according to their directionality with respect to that of the polarized light.
See S. Inoue et al. 2001. Centrifuge polarizing microscope II. Sample biological applications. J Microsc 201:357-367.