Question

Select the ONE statement from the list below that is TRUE and enter the letter identifying the true statement at the beginning of your answer (I.e., “x” is true). Then, below your answer, explain in detail why each of the remaining four choices are not true (I.e., “y” is not true because..., “z” is not true because..., etc.)

A. During the process of blastocyst formation, the number and size of cells increase.

B. The blastocyst implants into the endometrial lining about 36 hours after fertilization.

C. In the early embryo, cells of identical genetic makeup become structurally and functionally different from one another through the process of gastrulation.

D. Umbilical arteries carry oxygen rich blood from the placenta to the fetus.

E. The movement of the fetus is not detected during the first trimester of pregnancy.

Answer 1

• OPtion E -The movement of the fetus is not detected during the first trimester of pregnancy Don't expect to feel any fetal movement just yet (except in the form of the queasiness, fatigue and headaches, that is). Your baby is far too tiny, and buried far too deeply within the protective cushioning of your womb, to make a blip on your belly radar. He or she could dance a jig, and you'd feel neither a stamp nor a hop.
• Option A is wrong because in humans, blastocyst formation begins about 5 days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells. The blastocyst has a diameter of about 0.1–0.2 mm and comprises 200–300 cells following rapid cleavage (cell division),meaning increase in cell number but not size.
• Option B is wrong because in humans, blastocyst formation begins about 5 days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells.
• Option C is wrong because during gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism. During gastrulation, many of the cells at or near the surface of the embryo move to a new, more interior location.
• Option D is wrong because emerging from the placenta is the umbilical vein, which carries oxygen-rich blood from the mother to the fetal inferior vena cava via the ductus venosus to the heart that pumps it into fetal circulation. Two umbilical arteries carry oxygen-depleted fetal blood, including wastes and carbon dioxide, to the placenta.