Answer :
The correct answer is embryonic folding. The ectodermal and mesodermal germ layers' explosive expansion is principally responsible for the lateral folding of the embryo. These layers start to fold and merge toward the midline as they grow.
The longitudinal and transverse folding that results from embryonic expansion, particularly of the neural tube, converts the flat trilaminar embryonic disk into a more cylindric embryo. The folding don't happen one at a time sequentially; they happen concurrently. Flexion, or curving, separates the embryo from the embryonic membranes, to which it is finally only connected by a slender stalk, the umbilical cord. Flexion turns the embryo into a sort of "tube." The dorsal part of the embryo grows more quickly than its ventral portion, which causes the embryo to curve around the umbilical region as its long axis increases quickly due to core growth being greater than peripheral growth.
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