Derivation of High-Purity Definitive Endoderm from Human Parthenogenetic Stem Cells Using an in Vitro Analog of the Primitive Streak oleh Nikolay Turovets, Jeffrey Fair, Richard West, Alina Ostrowska, Ruslan Semechkin, Jeffrey Janus, Li Cui, Vladimir Agapov, Irina Turovets, Andrey Semechkin, Marie Csete, Larissa Agapova