Cortical contraction and ooplasmic movement in centrifuged or artificially constricted eggs ofCiona intestinalis

To discover the force causing bipolar ooplasmic segregation just after fertilization in ascidian eggs (Ciona intestinalis), cortical contraction and cytoplasmic movement were examined by centrifugation and by artificial constricting techniques. In the centrifuged eggs, the surface constriction appeared independently of cytoplasmic stratification. The yolk layer and the sub-centripetal layer moved toward the vegetal pole in the peripheral region. In the eggs which were artificially constricted by partially broken chorion and then fertilized, the inner cytoplasm always flowed from the vegetal sphere into the animal sphere during bipolar segregation. The direction of this cytoplasmic movement was independent of sphere size. This shows that the force causing cytoplasmic movement is supplied by the contraction of the vegetal-side cortex. It is suggested that the contracting cortex pushes the inner cytoplasm toward the animal pole and drags the peripheral cytoplasm toward the vegetal pole.