Fig. 7. AO staining of nematocysts after fixation in the presence and absence of chelators of divalent ions. Animals were fixed in 4% formaldehyde in the presence of PBS (A), Tris buffer (B), or Tris buffer followed by incubation in PBS (C). Capsules in A stain bright red with AO. Capsules in B stain weakly with AO; the insert shows an exploded capsule that is empty of pG for comparison. Capsules in C also stain weakly with AO indicating that post-treatment cannot improve AO staining. Identical results were obtained when 10 mM EDTA (in Tris buffer) was used in place of PBS. Bar, 10 µm.