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RESEARCH PRODUCT

On the Establishment of Polarity in Polychaete Eggs

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The study of Spiralian development began about a hundred years ago when Whitman (1878) published a description of the development of the leech, Clepsine marginata. Subsequently, Spiralian development became popular because the developmental fate of each individual blastomere can be determined precisely from the cell lineage. About the turn of the century several extensive papers describing the cell lineages of various molluscs (e.g. Blochmann, 1881, 1883; Kofoid, 1895; Conklin, 1897; Wierzejski 1905) and annelids (e.g. Wilson, 1892; Mead, 1897; Woltereck, 1904) appeared. From these studies it became clear that the general principles of mollusc and annelid development (and to a much lesser degree polyclad development; see Wilson, 1898) are identical. In annelid and mollusc embryos the first, second and third quartets of micromeres contribute to the ectoderm, the micromere of the fourth quartet within the dorsal quadrant is the stem cell for the mesoderm, and the macromeres together with the other micromeres of the fourth quartet form the entoderm. The invariant cleavage pattern as well as the apparant early determination of the blastomeres led to the assumption that the spiralian egg is a developmental mosaic. This means that the determination of blastomeres is governed by qualitative differences in cytoplasmic factors which are localized in specific regions of the ooplasm and correctly distributed by the cleavages. Due to these factors blastomeres would develop and differentiate autonomously. This theory of a strict mosaic development was supported by blastomere isolation experiments (Wilson, 1904a,b). The results of these experiments showed that isolated blastomeres develop as if they were still part of the whole embryo. This was, however, proven for larval cells only. Wilson demonstrated that a presumptive trochoblast cell when reared, became ciliated. Regarding this observation Wilson wrote: