A. Fully developed embryo
B. Type of stem cell
C. Type of brain cell
D. Part of the blood system
E. Very early stage of the embryo
Is the answer A or E? Explain the correct answer. If it's A, explain why it can't be E. If it's E, explain why it can't be A.
In the first stages of embryonic development, a single-celled zygote undergoes many rapid cell divisions, called cleavage, to form a blastula, which looks similar to a ball of cells.
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). About seven days after fertilization,the blastocyst undergoes implantation, embedding into the endometrium of the uterine wall.
So, option A ( Fully developed embryo ) is the correct answer.
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