Question

First cleavage plane is normally

• A. Right angle to entry site of sperm.
• B. Parallel to entry site of sperm.
• C. Right angle to pole axis.
• D. Random

Answer 1

The correct option is A Right angle to entry site of sperm.

Shortly after fertilization, the zygote undergoes cleavage, a series of rapid mitotic divisions with no period of growth during each cell cycle. For this reason, although the cell number increases, the embryo does not increase in size. The zygote initially divides to form a 2-celled embryo. The first division is vertical i.e., right angle to the point of entry of sperm and splits the egg into two equal cells. Then each of these cells undergoes mitosis and divides, bringing the number of cells to 4. Repeated divisions increase the number of cells, called as blastomeres, that make up the embryo. At about the 32-cell stage the embryo is a solid ball of blastomeres, called as morula.