Final answer:
During embryonic development, migration in the development of the postumbilical gut is accomplished through embryonic folding, forming a tube called the primitive gut lined by the endoderm, which becomes part of the gastrointestinal system.
Step-by-step explanation:
The direction of migration during the development of the postumbilical gut involves a complex process where an embryo transitions from a flat sheet of cells into a three-dimensional structure through embryonic folding. Initially, the embryo undergoes lateral and head-to-tail folding, creating a C-shape and incorporating a portion of the yolk sac, which eventually forms part of the gut structure and protrudes with the umbilical cord from the abdomen. The endoderm, one of the primary germ layers formed during the gastrulation process, lines the tube created by this folding, known as the primitive gut, and is crucial in forming the gastrointestinal tract, liver, pancreas, and lungs.