Cellular origins of bilateral symmetry in glossiphoniid leech embryos

In the embryos of glossiphoniid leeches, as in other spirally cleaving embr yos, there is ambiguity as to how the early cleavages in the embryo relate to the bilateral symmetry of the adult. Traditionally, we have aligned the B-D axis of the 4-cell stage with the anterior-posterior (A-P) axis of the adult. This requires that the first cleavage be skewed with respect to the A-P axis. Here, we re-examine the fates and behaviours of early blastomeres and conclude that a more accurate representation of the embryo is to have the first cleavage plane transverse to the A-P axis.