Fig. 2. Grey-scale representation of the density of the endoplasmic reticulum (parameter ) used in the simulations of Ca²⁺ oscillations and waves in ascidian eggs. The ER is assumed to be more concentrated in the cortex of the egg, with a maximal value in the animal cortex. Parameter measures the local ratio between the ER and the cytosolic volumes. Scale: black, =0.12; white, =0.07. This ER density is given by equations (6) and (7) with the following parameter values to characterize the gradient of ER-density from the periphery to the center and the shape of the pacemaker (see text): basal density of ER _B=0.07, _C=0.10, w=0.04, r_C=75 µm, h=0.04 and d=15 µm. a, animal pole; v, vegetal pole.