Tardigrades are poorly studied aquatic arthropods. Now a 3D time-lapse recording of the development of the tardigrade Thulinia stephaniaefrom embryogenesis to hatching provides new information on the ancestral mode of early development in Arthropoda (see p. 1349). By tracking the fate of individual cells in living embryos, Hejnol and Schnabel reveal that T. stephaniae embryos undergo an irregular indeterminate cleavage pattern. Furthermore, early embryogenesis is highly regulative; even after laser ablation of half the embryo at the two- or four-cell stage, normal juveniles develop, a degree of recovery from blastomere ablation not previously seen in protostome embryos. Because tardigrades are a basal lineage within the Arthropoda, a comparison of these new details of tardigrade development with those of other protostomes, such as crustaceans, suggests that indeterminate cleavage and regulatory development are part of the ground pattern of Arthropoda and probably all Ecdysozoa.