Which of the following events is involved in cleavage of the zygote during week 1 of development?
**Core Concept**
Cleavage of the zygote during the first week of embryonic development involves rapid mitotic divisions that increase the number of cells without significant growth, resulting in smaller blastomeres due to constant cell division with minimal cytoplasmic expansion.
**Why the Correct Answer is Right**
During cleavage, the zygote undergoes successive mitotic divisions that produce more cells (blastomeres) while the total cell mass remains constant. As each division occurs, the cytoplasm is partitioned equally, leading to a decrease in the size of individual blastomeres. This is a key feature of early embryonic development, especially in the first few days post-fertilization, before implantation. The process is not meiotic and does not produce differentiated cells.
**Why Each Wrong Option is Incorrect**
Option A: Meiotic divisions occur only in gamete formation, not in cleavage. The zygote undergoes mitotic, not meiotic, divisions.
Option B: Blastomeres remain undifferentiated during early cleavage; differentiation begins later in development.
Option C: Cytoplasmic content actually decreases per cell due to equal division, not increases.
**Clinical Pearl / High-Yield Fact**
In early cleavage, blastomeres are small, undifferentiated, and mitotically active—this is why the embryo is sensitive to environmental insults and nutrient availability before implantation.
✓ Correct Answer: D. A decrease in size of blastomeres