The temporal and spatial relationships between cortical contraction, sperm trail formation, and pronuclear migration in fertilizedXenopus eggs.

The cortical contraction begins 4 min after insemination and one minute after prick activation. During the next 4 min, the pigment margin moves 15 degrees toward the animal pole. The cortex then relaxes to the prefertilization level over the next 10 min. Contrary to earlier estimations, the cortical contraction occurs during the same time span as the wave of cortical granule exocytosis. We suggest that the two events may result from a common stimulus. The sperm trail (ST) forms during the relaxation of the cortex. The ST first appears as a conically-shaped trail of pigment in the cytoplasm; it then elongates into a funnel-shaped trail as the male pronucleus migrates into the egg. The base of the cytoplasmic ST can be seen on the surface of the egg as a circular condensation of pigment. The male and female pronuclei migrate at a constant rate of 12 μm per minute. The male pronucleus migrates by the enlargement of its aster, whereas, it appears that the female pronucleus is dependent on the male aster for its motion.