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		Results When observed before fixing, embryos from the control group had become early pluteus larvae with fully formed archenterons. Embryos from the sulfate-free group were still spherical, and had no observable archenterons after two days of incubation. Embryos stained both histochemically and immunofluorescently showed similar patterns. Alkaline phosphotase (AP) staining revealed activity throughout the midgut and hindgut of a fully formed archenteron in a control embryo (Figure 2A). However, AP activity was only apparent at the vegetal plate in a sulfate-free embryo (Figure 2B).   A. B. Figure 2. Fixed two-day old sea urchin embryos stained for alkaline phosphotase enzyme activity. (A) White arrow points to staining in a control embryo, in pluteus larva stage, which reveals enzyme activity throughout a fully formed archenteron or gut tube. (B) White arrow points to staining in an embryo raised in a sulfate-free environment, which reveals activity only at the posterior or vegetal plate and no discernable archenteron formation. Immunofluorescent staining for vegetal plate and posterior archenteron cells revealed staining throughout a fully formed archenteron in a pluteus larva control (Figure 3A). A ring of immunofluorescent staining in an embryo raised in a sulfate-free enviroment appeared only at the vegetal plate, with no discernable archenteron formation (Figure 3B).   A. B.   Figure 3. Fixed two-day old sea urchin embryos stained with 5C7, an antibody to vegetal plate and posterior archenteron cells. (A) White arrow indicates staining throughout a fully formed archenteron in a control embryo in pluteus larva stage. (B) The white arrow points to a ring of stained vegetal plate and archenteron cells that have not formed a discernable archenteron.