Cones of living amphibian eye: selective staining.

The outer segments of cones are selectively stained by the chlortriazinyl dye Procion Yellow injected into the vitreous humor. Since the dye does not cross nerve cell membranes, the selective staining of cones is further evidence for structural differences between rod and cone outer segments. Specifically, it is believed that cone saccules are open to extracellular space whereas rod saccules are not.