SUMMARY: Despite tremendous differences in size and yolk content, the eggs of two sibling species, Platynereis dumerilii and P.massiliensis, show the same asymmetrical mode of cytoplasmic segregation. The largest part of the yolk-free cytoplasm is shunted into the D-quadrant after second cleavage. The specific cleavage behavior of the D-cell-line suggests a causal relation between cytoplasmic specification, speed of cell cycles and determination. We have expanded our morphometric analysis onto the development of Pomatoceros triqueter, a polychaete with equal cleavage. However, we could show that small though significant size differences between the quadrants may foreshadow the future dorsoventral polarity of this embryo. The largest quadrant cleaves asynchronously with respect to the other three quadrants. Like in Platynereis, the speeds of cell cycles of blastomeres are positively correlated with the volume of cytoplasm.
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Abt. 1