Abstract: The present paper gives an account of our present knowledge about the physiology of early embryogenesis in the spiralians. A major trait of spiralian embryos is the rapid diversification among the blastomeres. Both cleavage asymmetry and ooplasmic segregation determine the dorsal fate of the largest (D-)quadrant in unequally cleaving embryos as well as the fate of larval structures in certain tiers of micromeres among both equally and unequally cleaving embryos. The cleavage pattern in the D-quadrant differs from that of the other quadrants and biases the rapid proliferation of the 2d- and 4d-cell line. Blastomeres within the D-quadrant play a role as organizers, and cellular interactions with these organizers makes the other cells adapt their cleavage strategies to accomplish a dorsoventral harmonious pattern. An analysis of cell fates of larval and post-larval cell line precursors in cleavage-arrested Platynereis embryos shows that the cells get specified asynchronously and by different mechanisms.
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