We present a simple and parameter-free nuclei tracking method for reconstructing cell dynamics in fluorescence 3D+t images of embryogenesis. The strategy is based on the use of the mathematical morphology operators directly in the 4D image. The morphological reconstruction of a marker -manually or automatically selected- in an initial spatio-temporal position generates a connected path over the time representing the cell migration. Thus, the processing provides a coherent spatiotemporal estimation of cell movement. The algorithm has been validated on in vivo images of early zebrafish and sea urchin embryogenesis acquired with two-photon laser scanning microscopy providing mean tracking rates above 98% per time step.