<prompt> Gastrulation occurs during which week after feilization?1st2nd3rdNone of the above  <response>: The most characteristic event occurring during the third week is gastrulation, which begins with the appearance of the primitive streak, which has at its cephalic end the primitive node. In the region of the node and streak, epiblast cells move inward invaginate to form new cell layers, endoderm and mesoderm. Ref: Langmans embryology 11th edition Chapter 5. 