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.
Owenia collaris embryos undergo holoblastic spiral cleavage to form a coeloblastula, which, although exceptionally hollow (again like some nemerteans), conforms to the ancestral pattern for annelids (Okada, 1970).
The greatest number of phyla, collectively indicated as the Spiralia, has adopted spiral cleavage. It is very unlikely that the spiralian phyla have convergently developed the spiral style of cleavage.
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).