Explanation:
During implantation, the trophoblast differentiates into two distinct layers: the inner cytotrophoblast, and the outer syncytiotrophoblast.
The second statement best explains the type of blood vessel and the function of these vessels' valves - <span>These are veins, which prevent the backflow of blood because the walls have little muscle to pump blood back to the heart.
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Veins are large blood vessels which carry blood back to the heart. <span>Veins have a system of valves to prevent back-flow.</span>
Answer:
a change from one day to the next day
