| 11. | The tissue undergoes convergent extension as the primitive streak regresses, or as the embryo gastrulates.
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| 12. | The formation of the primitive streak, through which gastrulation occurs, is induced by Koller's sickle.
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| 13. | Similarly, the posterior cells of Koller s sickle form the posterior region of the primitive streak.
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| 14. | They are derived from the posterior primitive streak region of the mesoderm in the gastrulating embryo.
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| 15. | Within birds and mammals, epiblast cells converge at the midline and ingress at the primitive streak.
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| 16. | Callebaut and Van Nueten observed the formation of a normal secondary primitive streak, mesoderm, and definitive endoderm.
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| 17. | In 1926, Ludwig Graper first studied the involvement of Koller's sickle in the formation of the primitive streak.
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| 18. | As the cells of Koller s sickle migrate during gastrulation, they form different portions of the primitive streak.
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| 19. | Even after implantation begins, a preembryo ( and proembryo ) may exist up until formation of the primitive streak.
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| 20. | The anterior cells of Koller s sickle become the anterior region of the primitive streak, known as Hensen's node.
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