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.