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 corpus luteum secretes estrogens and progesterone. the latter hormone causes changes in the uterus that make it more suitable for implantation of the fertilized ovum and the nourishment of the embryo. if the egg is not fertilized, the corpus luteum becomes inactive after 10–14 days, and menstruation occurs.

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