I’ve annotated this movie to demarcate the phases of mitosis.  In a healthy sand dollar embryo at this stage, most cells in animal hemisphere still divide synchronously (in the vegetal hemisphere, the micromere lineage divides more slowly, and the macromere descendants slightly faster).

The chromosomes have begun to condense within the intact nucleus at the beginning of the movie.  Once the nucleus breaks down the fully-condensed chromosomes are herded toward the mid-plane of the forming mitotic spindle, presumably by assembling kinetochore microtubules.  In normal cells at this stage, there is little if any of the chromosome oscillations that many somatic cell lines exhibit.  Metaphase is likewise perfunctory, and anaphase is followed immediately by cytokinesis.

— text by George von Dassow