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MCAT Question A Day - 5/21/13 - Answer!

If fertilization usually occurs in the fallopian tubes, and implantation takes place 4–7 days later, what developmental stage actually implants in the endometrium?

A.  Zygote
B.  Solid ball of cells
C.  Hollow ball of cells
D.  Ball of cells with three germ layers

The correct answer is (C). If the zygote forms at fertilization, there will be two cells within 24–30 hours, and additional cleavage divisions occurring every 10–12 hours thereafter. Within 4–7 days, there will be at least 64–128 cells. This stage is the hollow ball of cells called the blastula (blastocyst in humans). The 32-cell solid ball of cells (morula: choice B) would form earlier, while the gastrula (choice D) would form later.