MCAT Biology Question 31: Answer and Explanation
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Question: 31
1. Which of the following developmental stages has the greatest nuclear-to-cytoplasmic ratio?
- A. Eight-cell embryo
- B. Morula
- C. Blastula
- D. Zygote
Correct Answer: C
Explanation:
The question is asking us to determine the developmental stage with the greatest nuclear-to-cytoplasmic material ratio. During the series of rapid mitotic divisions known as cleavage, the number of cells increases dramatically, but the overall volume of the embryo does not change. In fact, the volume of the embryo does not significantly increase until after the blastula has already implanted. As such, a high ratio of nuclear to cytoplasmic material will be found at the stage with the greatest amount of cells during early development. From the given choices, the stage with the greatest number of cells is the blastula. This is further supported by the fact that the cells are displaced from the center by the blastocoel.