These observations are consistent with, and extend, older electron microscopic work on polar body formation in eggs of Spisula and Tubifex (2, 7) and are of value for comparison with current findings on unequal cell division
in Saccharomyces cerevisiae and Caenorhabditis elegans (8, 9).
At around the 13-cell stage, the prospective axial cell, 4a, undergoes an unequal cell division
. The [5A.sup.11] pair divide equally and produce two pairs of daughter cells, [6A.sup.111] and [6A.sup.112] plus [6A.sup.111] and [6A.sup.112].