Soru

To prepare a wet-mount slide of onion epidermal cells for observation under a compound light microscope, a technician must execute several precise steps. First, the technician uses forceps to peel a single, paper-thin layer of epidermal tissue from the inner curve of a fresh onion section. This thin layer is immediately placed flat on the center of a clean glass slide, ensuring the tissue does not fold over itself. Next, a single drop of iodine solution is applied directly onto the tissue; this stain binds to starch and proteins, making the cell walls and nuclei clearly visible. Finally, the technician holds a plastic coverslip at a forty-five-degree angle against the slide, touching the edge of the iodine drop, and slowly lowers it over the onion specimen to avoid trapping air bubbles.

According to the described procedure, which of the following steps must be performed immediately before the technician adds the drop of iodine stain to the onion specimen?