Experimental change of cytoplasmic composition can convert determination of blastomeres inPlatynereis dumerilii (Annelida, Polychaeta).

Early development ofPlatynereis dumerilii is characterized by an extremely constant cleavage pattern in which the volumes and cytoplasmic contents of the blastomeres show remarkably little variability (Dorresteijn 1990). In order to test the necessity of a precise partitioning of the cytoplasm, we have stratified the ooplasm by mild centrifugation (10 min at 300 g) after completion of meiosis but before first cleavage. The cytoplasm of the zygote stratifies randomly with respect to the pre-existing animal-vegetal axis, but first cleavage follows the animal-vegetal axis dividing the plasm before it has rearranged to its normal distribution. As usual, first cleavage is unequal in the majority of centrifuged eggs. Different sorts of cytoplasm are always distributed abnormally in comparison to normal two-cell stages. Under two circumstances this leads to the formation of double trunk structures in the young worm. Such 'double monsters' either originate from zygotes whose clear cytoplasm has been distributed equally to the two daughter blastomeres at first cleavage, or from unequal two-cell embryos whose larger blastomere cleaved equally at second cleavage forming blastomeres with equal lots of clear cytoplasm. Cell-lineage could be followed in an individual embryo of the latter category and showed the existence of two adjacent D-quadrants, giving rise to a double monster with a forked trunk. Embryos of the former category give rise to two opponent D-quadrants and double monsters with a four-sided trunk. As in the normal embryo, the amount of clear cytoplasm in a blastomere is positively correlated with the speed of its cell cycle, and endows the cell with D-quadrant developmental capacities as can be judged by the cleavage pattern.