Final answer:
During cleavage, the number of cells increases as the zygote divides repeatedly, eventually forming a blastula or a blastocyst in mammals, which includes an inner cell mass and a fluid-filled cavity.
Step-by-step explanation:
During cleavage, the number of cells in the developing embryo increases. The process of cleavage involves rapid, multiple rounds of cell division without an increase in the overall mass of the organism. This means that the large single-celled zygote divides into many smaller cells, increasing the cell number. Eventually, the cells rearrange themselves to form a hollow ball called a blastula, or more specifically a blastocyst in mammals, which includes a fluid-filled or yolk-filled cavity known as the blastocoel and the embryoblast or the inner cell mass.During cleavage, the number of cells increases without an increase in mass.
The zygote undergoes rapid cell division, resulting in the formation of a blastula, which is a hollow ball of cells. Each cell within the blastula is called a blastomere. In mammals, the blastula develops into a blastocyst, characterized by an inner cell mass that will become the embryo. Therefore, the correct answer is a) increases.