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.

the  frequency of a wave represents  the number of wavelengths that pass a specific point per second. the unit for  frequency  is hertz (hz).

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emerging idea only occurs as a raw idea that could appear out of nowhere. meanwhile, to reach scientific consensus , that idea must be structured and tested over and over again through various experiments. the scientific consensus transforms the raw idea into idea that is more reliable through the process of scientific elimination.