Determinants of Spatial Organization

In theories on the influence of cytoplasmic determinants on development, the polar lobe formed by eggs of many annelids and molluscs has played an important role. This polar lobe is a transient protrusion at the vegetal pole of the egg, in which part of the cytoplasm is set apart during cleavage (cf. Figs. 1, 2). At the end of the first cleavage it fuses with one of the blastomeres (CD) of the 2-cell stage. At second cleavage a lobe is formed again, which flows into the D-blastomere at the 4-cell stage. In this way the vegetal region of the egg is shunted to the Dquadrant, which will become the dorsal quadrant of the future embryo.