Eggs of ascidians are fertilized while arrested in metaphase of the first meiotic division. In the egg of the european species Phallusia mammillata the first polar body is extruded 5 minutes after fertilization.The second polar body is emitted after 25 minutes.Mitosis and first division take place 35 and 45 minutes respectively after sperm has entered the egg.
In Phallusia sperm enters preferentially in the animal hemisphere. The fertilizing sperm triggers a series of calcium signals that last until the meiotic cell cycle is completed. The first calcium wave (fertilization wave) initiates a spectacular actomyosin-driven cortical contraction in the general direction of the animal vegetal axis.This first major phase of cortical and cytoplasmic reorganisation defines the exact position of the contraction pole in the vegetal hemisphere (see drawing). The location of the contraction pole corresponds to the future site of gastrulation 3hrs after fertilization. The sperm entry site and the direction of the calcium wave at fertilization thus determine the future Dorsal side of the embryo. Four identifiable cytoplasmic and cortical domains are stratified around the contraction pole (2 domains, the red ER-rich cortical domain and the green mitochondria-rich subcortical Myoplasm domain are shown in the drawing).The introduced sperm centriole (centrosome) and chromosomes are carried vegetally by the cortical contraction. Their location prefigures the location of the Posterior side of the embryo where the first unequal cleavages and primary muscle cell differentiation will take place due to the positionning of maternal determinants.

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