How many cleavages are completed in 16 celled stage of egg

The correct answer is 4.

To understand why 4 cleavages are required to reach the 16-cell stage of an egg, we can look at the pattern of cell division during early embryonic development. Cleavage is a series of rapid mitotic cell divisions where the single-celled zygote divides into multiple cells (blastomeres) without increasing in overall size.

During typical holoblastic (complete) cleavage, the number of cells doubles with each successive round of division. This geometric progression can be expressed mathematically as:
Number of cells = 2ⁿ
where _n represents the number of completed cleavages.

Let us trace this step-by-step:
1. First Cleavage (1st division): The single-celled zygote (1 cell) divides to produce 2 cells (2¹ = 2).
2. Second Cleavage (2nd division): The 2 cells divide simultaneously to produce 4 cells (2² = 4).
3. Third Cleavage (3rd division): The 4 cells divide to produce 8 cells (2³ = 8).
4. Fourth Cleavage (4th division): The 8 cells divide to produce 16 cells (2⁴ = 16).

Therefore, by the time the embryo reaches the 16-cell stage (often representing the morula stage in many organisms), exactly 4 rounds of cleavage divisions have been completed.