CLEAVAGE PATTERNS AND MESENTOBLAST FORMATION IN THE GASTROPODA: AN EVOLUTIONARY PERSPECTIVE

The larger gastropod taxa are characterized by distinctive cleavage patterns. The cell stage at which the mesentoblast is formed appears to be crucial. In none of the taxa is it formed earlier than the 24- and not later than the 63-cell stage. A heterochronic shift from late to early mesentoblast formation appears to coincide with successive steps in gastropod evolution. Comparison of the early cleavage patterns appears to be a powerful method for investigating the evolutionary relations between major gastropod taxa.