The outer cells of a blastocyst become the , an organ that offers protection and nourishment. once the blastocyst has implanted in the uterus, it is called an . after eight more weeks of development, it is called a .

The outer cells of a blastocyst become the placenta, an organ that offers protection and nourishment. Once the blastocyst has implanted in the uterus, it is called an embryo. After eight more weeks of development, it is called a fetus.

Explanation:

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mass

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

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