Name the stage where differentiation of the cells occur

Cell differentiation, in a multi-cellular organism, is the process through which a simple immature cell evolves to a matured specialized cell with a specified function. An initial cell that is capable of differentiating into any type of specialized cell is called totipotent. In mammals, only the zygotic cell and the cells derived from first few mitotic cell divisions from zygote are totipotent. This process of cell differentiation is started immediately after fertilization of a female egg. The fertilized egg/ zygote undergoes multiple rounds of cell division forming blastocyst where the initial cell differentiation occurs.
In later stages of growth, organism maintains a fixed population of a class of cells throughout their life cycle. These cells, called stem cells or pluripotent cells, are capable of differentiating into many, but not all, specialized cells. For example, Hematopoietic stem cells and Mesenchymal stem cells.