Quantitative analysis of cellular differentiation during early embryogenesis ofPlatynereis dumerilii.

As in many spiralian embryos with unequal cleavage, cleavage inPlatynereis follows an invariant pattern. Preceding each cleavage the cytoplasm is reorganized, allowing the spiral cleavage mode to produce cells with different cytoplasmic composition. The fertilized egg undergoes a dramatic ooplasmic segregation after the completion of the cortical reaction. As a consequence, a plug of clear cytoplasm becomes located at the animal pole. Once the four quadrants of the embryo have been established, the cleavage sequence of the D quadrant differs clearly from that of the other three quadrants. The results presented here suggest that differential distribution of the clear cytoplasm governs this sequence. The first quartet of micromeres, which will form the ectoderm and the cerebral ganglia of the head, is clearly bilaterally symmetrical from the onset of the third cleavage. Dorsoventral polarity and bilateral symmetry in the ectoderm of the trunk is expressed most markedly by the dorsal location of the large 2d cell, whose rapid proliferation is bilaterally symmetrical with respect to the median plane. As a result of this proliferation it comes to fill most of the posttrochal region (ectoderm, three pairs of anlagen for the setal sacs, and the ventral plate which forms the nerve cord). The other micromeres contribute only a minor portion of the ventral ectoderm and are involved in the formation of the stomodaeum. The mesentoblast, 4d, i.e. the stem cell of the primary mesoderm, forms at the sixth cleavage, also in a position on the dorsal mid-line. The daughter cells, which arise from 4d by strictly bilaterally symmetrical cleavage, form the mesodermal germ bands, which lie beneath the ectoderm. The trochoblasts are formed by asynchronously cleaving founder cells, but further cleavages in these cells are synchronous. This suggests that cell-cell interaction is involved in the development of this alleged mosaic embryo.