At the onset of the embryonic period, the appear(s). it will eventually become the neural tube.

Answer;

Primitive streak.

At the onset of the embryonic period, the primitive streak appears. It will eventually become the neural tube.

Explanation;

The primitive streak is a transient structure which forms on day 15 of human development.

It marks the start of gastrulation; the process in which the inner cell mass in converted into the trilaminar embryonic disc.

It originates from the anterior epiblast.

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