In sea urchin embryogenesis it has been suggested that the initial territories are specified by a combination of the asymmetric distribution of cytoplasmic determinants and cell-cell interactions. At the 60-cell stage blastomeres clonally originated from founder cells divide the embryo into five distinct territories: small micromeres, large micromeres, vegetal plate, oral ectoderm and aboral ectoderm. The territories are identified by the expression of specific marker genes and their cell lineages (Davidson, 1989, 1991). The large micromeres are thought to play a role as an organiser and initiate a cascade of signal transduction toward overlying cells (Davidson, 1989). In this model the large micromeres induce the overlying veg2 tier, specifying the vegetal plate (Ransick & Davidson, 1993, 1995). The veg2 tier then induces the overlying cells, which include gut cells and cells of the prospective ectodermal territories (Wikramanayake et al., 1995; Wikramanayake & Klein, 1997). Thus, the large micromeres, which are the prospective primary mesenchyme cells (PMCs), play a key role in cell fate specification and axis determination during sea urchin embryogenesis. Previous data suggested that the large micromeres are autonomously specified to become PMCs by maternally inherited determinants (Okazaki, 1975; Kitajima & Okazaki, 1980). An important question in sea urchins embryogenesis is the identity and function of the proposed maternal determinants.