the division of cells in the developing animal egg in a spiral manner (see Fig. 109 ). Most invertebrate groups exhibit this type of cleavage with the exception of echinoderms, protochordates and a few smaller groups.
The conservation of the early cleavage patterns and the specification of the developmental fate of blastomeres with identical geometrical positions strongly argue for the assumption that spiral cleavage has been adopted only once, and is very refractory to evolutionary modifications (Buss 1987).
We believe that nemerteans exhibit a derived developmental condition, and agree with previous reports that the ancestral spiralian developmental condition was one in which equal, quartet spiral cleavage occurred, and quadrant fates and axial properties were established epigenetically via inductive interactions (26).