[Invariance of asymmetric cleavage during structural changes in Pulmonata zygotes]

The zygotes of Lymnaea stagnalis L. were centrifuged and, as a result, normal and small size zygotes were obtained. The small zygotes either contained the major part of heavy yolk granules and were characterized by complete cortical polarity, or were devoid of heavy yolk and significant part of vegetal cortex. Following the trypsinization of zygotes, the reciprocal rotation of sister blastomers around the spindle axis was observed during the first cleavage division. The direction of rotation did not depend on zygote size, cytoplasm and cortex composition, or the direction of cleavage furrow with respect to the axis of stratification of the zygote content and was always dexiotropic (as in the control zygotes). The dexiotropic rotations of blastomeres were also observed during the division of the trypsinized zygotes with the tripolar mitotic apparatus. The data obtained are considered in favour of the hypothesis of spiral contractile ring which accounts for hereditary dissymmetry of early cleavage in mollusces.