When calcium is released from internal stores in living cells the resulting wave of increased concentration can travel without deformation (continuous propagation) or with burst-like behavior, (saltatory propagation). We present the fire-diffuse-fire model in order to illuminate the differences between these two modes of propagation. We show that the calcium release wave in immature Xenopus oocytes is saltatory while the fertilization wave in mature oocytes is continuous. Whether the propagation is continuous or saltatory depends on the ratio of the time that a single site remains open to the time it takes for calcium to diffuse between neighboring release sites. If this ratio is large enough the propagation is continuous and if it is small enough the propagation is saltatory. In the continuous case the number of sites that release simultaneously is large while in the saltatory case only a single site releases at a time.