During which stage of development does the embryo progress from a single cell to multiple cells?

The correct answer is the stage of cleavage. This stage occurs immediately after fertilization, when the single-celled zygote begins to divide through a series of mitotic divisions. These initial divisions, known as cleavage divisions, produce smaller and smaller cells called blastomeres, resulting in the formation of a multicellular structure without an increase in overall size. This process typically leads to the creation of a solid ball of cells called a morula, and later a hollow structure called a blastula, preparing for further developmental processes. Cleavage is crucial as it sets the stage for subsequent key developmental events such as gastrulation, where the cells will begin to differentiate and form distinct layers. Understanding this stage is fundamental in cell biology and embryonic development as it represents the transition from a unicellular zygote to a multicellular embryo, setting the foundation for more complex developmental processes.