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The embryonic shield as organizer in the fish embryo. A donor embryonic shield (about 100 cells from a stained embryo) is transplanted into a host embryo at the same early-gastrula stage. The result is two embryonic axes joined to the host's yolk cell. In the photograph, both axes have been stained for sonic hedgehog mRNA, which is expressed in the ventral midline. (The embryo to the right is the secondary axis.) (After Shinya et al. 1999; photograph courtesy of the authors.)