Question

What happens in cells of embryo during cleavage?

• A. Decrease in cytoplasm.
• B. Increase in cytoplasm and decrease in nuclear material.
• C. Decrease in both.
• D. Both remains constant.

Answer 1

The correct option is A Decrease in cytoplasm.

Shortly after fertilization, the zygote undergoes cleavage, a series of rapid mitotic divisions with no period of growth during each cell cycle, resulting in a decrease in cytoplasm content. 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. 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. Eventually, anywhere from 64 to several hundred blastomeres form the blastula, which is usually a hollow ball with a fluid-filled cavity, the blastocoel. Cleavage partitions the zygote into many small cells, that serve as the basic building blocks for subsequent development. At the end of cleavage, the small cells, that make up the blastula begin to move about with relative ease, arranging themselves into the patterns necessary for further development.