The super paramagnetic beads were injected with glass needles into the yolk of the zebrafish embryo to ensure the integrity of the zebrafish embryo (Protocol for pulling needles in Supp.). We performed the injections at 4 hpf and the injection needle was inserted from either the vegetal pole or the lateral side and beads were deposited close to the yolk membrane (Figure 1b). We found that performing the injections at extremely low pressure (5-10 psi) and long injection duration (100-150 ms) avoided the dispersion of beads. By applying a strong constant magnetic field after the injection of the beads with a permanent magnet the beads were attracted and clump. This aggregation of beads preserved the single beads from translating through the yolk and eased the rotation.