How does the laying of the axial complex of organs in chordates occur?

The formation of axial organs — the neural tube, chorda, and digestive (intestinal) tube — begins in the embryos of chordates after gastrulation is complete.
Ectoderm cells located near the primary mouth begin to rapidly divide and form a neural plate, which stretches along the entire dorsal side of the embryo. Subsequently, the lateral edges of the plate rise, and its central part falls, forming a neural groove. The edges of the groove are closed, and it turns into a neural tube lying under the ectoderm – the germ of the central nervous system.
A chord is formed from a group of endoderm cells – an elastic rod extending in the embryos of all chordates under the neural tube. Subsequently, in vertebrates, the chord is completely replaced by the spine and only in some fish it persists throughout life. The rest of the endoderm gives rise to the digestive (intestinal) tube, located under the chord.
The axial complex of the organs of the embryo determines the bilateral symmetry of the body. The stage at which the axial organs are laid is called neurula.

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