Fig. 3. Chemotactic response of cAR2/RI9 cells. (A) Positive response of aggregation-competent cells, 38 minutes after the cAMP-filled electrode (10^-1 M cAMP) had been introduced. The cells have moved chemotactically towards the tip of the electrode and piled up. The outline of the tip in the cell mass is indicated in black. (B) The cAMP concentration in the electrode was 10^-4 M, too low to be detected by the cAR2/RI9 cells. The cells remain evenly spread. The photograph was taken after about 38 minutes as well. Bar, 50 µm.