Figure 1 Contractile vacuoles filling and emptying in a Chlamydomonas cell. Four pictures were taken about two minutes apart to illustrate the filling and emptying cycle of two contractile vacuoles in the cell. A, The two contractile vacuoles are empty and invisible at this magnification. B, One vacuole has filled (arrow). C, This vacuole is emptying and the second vacuole is filling. D, Both vacuoles have emptied again. Part of the boundary of the gelatinous envelope that surrounds the cell is visible in each picture (Micrographs courtesy B.E.S. Gunning)