The time lapse movie spans fertilization, ooplasmic segregation, and early divisions in the egg of the tunicate Phalusia mammilata. Contrast is provided by both polarizing microscopy and differential interference microscopy. The entrance of the sperm (upper right) stimulates waves of contraction in the cytoplasm. Polar bodies are formed and extruded, after which male and female pronuclei migrate toward each other, and nuclear fusion occurs. Finally, the zygote proceeds through mitosis and cleavage stages.