*A*, total steady-state outward current measured at 0 mV. The number of cells measured at the various ages (P0-P26) was (from left to right): 2, 3, 29, 10, 8, 8, 6, 4, 11, 7, 2, 4, 12, 5, 11, 3, 5, 1, 5, 4. *B*, development of *I*_{K,n}. Number of cells: 6, 2, 8, 8, 2, 2, 9, 7, 2, 4, 12, 5, 11, 3, 5, 1, 5, 4. *C*, increase in membrane capacitance measured at −84 mV with age. Number of cells: 2, 3, 29, 13, 12, 8, 6, 4, 11, 7, 2, 4, 15, 5, 11, 3, 5, 1, 5, 4. *D*, zero-current potential. Number of cells: 2, 3, 29, 12, 12, 5, 4, 3, 10, 7, 2, 4, 12, 4, 11, 2, 5, 1, 5, 3. Fits to the data in *B*-*D* are according to a sigmoidal logistic growth curve:where *A* is the size of the current, membrane capacitance or the membrane potential, *k* is a slope factor and *t*_{½} is the age at which *A* is half-way between *A*_{max} and *A*_{min}. Values for *t*_{½} and *k,* respectively, were: *B*: 11.9 days, 0.36 day^{−1}, *C*, 11.7 days, 0.36 day^{−1}; *D*: 8.9 days, 0.52 day^{−1}. *E* and *F*, half-maximal activation (*V*_{½}) and slope factor (*S*), respectively, for the activation curves, from fits to single Boltzmann curves. Number of cells: 2, 3, 28, 10, 8, 6, 6, 4, 8, 5, 1, 3, 12, 5, 11, 3, 5, 1, 5, 3. *G*, examples of electromotile responses to voltage steps from −64 to about +50 mV; 23 °C. *H*, development of electromotility. Left ordinate and •: cell shortening to voltage steps from −64 to +50 mV (in nm). Right ordinate and □: percentage of cells with measurable motile responses. *n*, number of cells. Error bars indicate ±s.e.m. in all panels.