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primitive streak) in the posterior epiblast. Once formed, however, the streak secretes its Nodal antagonist--the Lefty protein--which prevents further primitive streaks from forming. Eventually, the Cerberus-secreting hypoblast cells are pushed to the future anterior of the embryo, where they contribute to ensuring that neural cells in this region become forebrain rather than more posterior structures the nervous system. As the primitive streak reaches its maximum length, transcription of the Sonic hedgehog gene

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