In the 4 cell stage C. elegans embryo the P1 blastomere induces endoderm formation in the posterior daughter of the EMS blastomere. The P1 and EMS blastomeres can be extracted from the embryo and cultured alone or in combination. They will divide and you can use an antibody to determine if any endodermal cells are formed. You have identified a mutant called mom-2 (more of mesoderm, meaning less of endoderm) in which no endoderm forms. How would you determine if the mom-2 gene product acts in the P1 or EMS cell?