Normal development and perturbations. Normal sea urchin embryos (top) develop two primary axis: the animal-vegetal axis and the oral-aboral axis. Nuclearization of β-catenin in cells on the vegetal side initiates endomesoderm specification. Later on the ectoderm is divided into an oral and aboral side, which is comparable to the dorso-ventral axis in vertebrates. Treating embryos with lithium chloride leads to enhanced nuclearization of β-catenin and, as a result, a shift in cell fate toward vegetal and formation of excess endomesoderm (left). Conversely zinc sulfate treatment prevents endomesoderm formation (right). The molecular basis for zinc sulfate action is unknown, as is the effect of these drugs on the ectoderm.