Understanding how the embryonic germ layers become competent to form their characteristic tissue types is a problem of fundamental importance to developmental biology. Knowledge of how the endodermal layer is first determined and then differentiates has only recently begun to accumulate. In sea urchins, several different signals have been implicated in endoderm formation, beginning as early as the fourth cleavage division and continuing until just prior to invagination of the endoderm. Recent experiments in sea urchin embryos have shown that the activity of glycogen synthase kinase 3-β and entry of β-catenin into the nucleus during cleavage stages is required for mesoderm and endoderm formation (Emily-Fenouil et al., 1998; Logan et al., 1999), implicating the Wnt signalling pathway in this process. Overexpression of β-catenin leads to an exaggeration of endoderm and mesoderm in the embryo at the expense of ectoderm (Wikramanayake et al., 1998). Since this signal is required for both mesoderm and endoderm, some other signal must be present to differentiate between these two germ layers. Micromeres formed by the fourth cleavage division have the ability to induce endoderm (Ransick & Davidson, 1995). This induction can occur independently of the entry of β-catenin into the nucleus of the cells induced to form endoderm (Logan et al., 1999), indicating micromere induction acts through a different signalling pathway. Final determination of endoderm also requires cell interactions through the late mesenchyme blastula stage, since cells from embryos dissociated prior to that stage fail to develop into endoderm autonomously (Chen & Wessel, 1996). A sea urchin member of the hedgehog family of signalling molecules has been reported to be expressed in the vegetal plate, indicating it also may play a role in endoderm formation.