The phrase that best describes the process represented in the diagram is a zygote dividing by mitosis.
The fertilization (fusion) of an egg by a sperm cell produces a single cell which is call zygote. After the fertilization of an egg, the zygote starts a sequence of cell division called cleavage. During this process, the zygote divides into two identical cells through the process of mitosis. Mitosis is the term referring to the duplication of the genetic material and the division of a cell into two identical cells.
