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Why do cells become smaller during cleavage even though their number increases?
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Animal development
The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.
During the process of cleavage, cells become smaller because there is almost no growth occurring between consecutive mitotic divisions.12
While the number of blastomeres increases through rapid cell division, the total volume of living matter within the embryo does not change substantially during this period. Because the original zygote is subdivided into an increasing number of smaller units without a corresponding increase in total mass, the size of each individual cell is reduced by approximately half at each division. Essentially, the large zygote is partitioned into smaller, more conventional-sized cells that serve as the building blocks for the future organism.123