Figure 8.

Molecular switch for induction of dorsal axis. Biliverdin is proposed to interact with a cortical factor to trigger or switch on the downstream activation of genes. When the chemical switch is turned on, the Nieuwkoop center and the Spemann–Mangold organizer are sequentially formed. The UV irradiation of biliverdin renders an ineffective photo-product, the chemical switch remains off, and the dorsalizing gene products that participate in the configuration of the dorsal axis are not formed. The result is adorsal teratology. MBT, midblastula transition.