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