During the Mid Blastula Transition Stage (MBT) a series of calcium transients arise at a narrow zone of the presumptive dorsal ectoderm. These transients establish a long-distance permissive signal that coordinates the spatio-temporal events that lead to the ventralization of the embryo. However, given the fact that these transients are generated at the dorsal side of the MBT blastula, it is of interest to understand how they can induce ventral features in the blastomeres that are located at the opposite side of the calcium source. In order to propose a plausible hypothesis to answer this question, this review is not only an exhaustive revision of the literature about calcium and the MBT in Xenopus, but tries to integrate this knowledge into a dynamical vision of the role of calcium in the ventralization of the blastula. In this dynamical scenario, the frequency of the calcium signal can have a central role in the control of the activation of the genes that impel the expression of the dorsal and ventral features of the blastula. In this form, calcium could exert a dual regulation of the inductive signals that patterns the midblastula. However, the scarce experimental and theoretical literature about the generation and role of calcium in the mid blastula transition makes this objective hard to be reached. However, this work tries to identify the main unsolved problems in this field and propose a series of hypothesis that can promote further experimental and theoretical research.