Here's why:

* Rapid Division: Cleavage is characterized by rapid mitotic cell divisions without significant cell growth. This results in a larger number of smaller cells.

* Blastomeres: The resulting cells from cleavage are called blastomeres.

* Morula: As cleavage progresses, the blastomeres form a solid ball of cells called a morula.

* Blastocyst: Eventually, a fluid-filled cavity forms within the morula, creating a blastocyst.

So, cleavage is the key process that drives the early development of an embryo from a single fertilized egg into a multicellular structure.