Three gastrulation-stage embryos are presented. The left hand panel shows a wild-type embryo whose cells have properly migrated towards the vegetal pole via epiboly; towards the dorsal side via convergence; and to the interior via internalization. The arrowhead marks the division between external (epiblast) and internalized (hypoblast) layers. The middle panel shows a loss-of function double mutant embryo that is recessive for two nodal-related genes: squint (sqt) and cyclops (cyc). Internalization is essentially eliminated in these embryos. The right hand panel shows an embryo that has been co-depleted of Lefty 1 and Lefty 2 proteins, two antagonists of Nodal signaling, via injection of anti-sense morpholino oligonucleotides. Because Lefties antagonize Nodals, their depletion leads to hyperactive Nodal signaling. Such embryos exhibit excessive internalization and a breakdown of epiboly.