In embryology, during which week is the primary gut formed?

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Formation of the primitive gut tube occurs when the embryo undergoes folding during the fourth week. After gastrulation establishes the three germ layers in week three, the embryo must fold cephalocaudally and laterally to convert the flat trilaminar disc into a cylindrical tube. This folding brings the endoderm into continuity as a complete, hollow tube that lines the future gut and communicates with the yolk sac. That conversion to a gut tube happens in the fourth week, laying the groundwork for the foregut, midgut, and hindgut regions that will develop later. Earlier weeks set up fertilization, implantation, and gastrulation, but the actual gut tube forms with the fourth-week folding.