The phase in which a cell begins to split the cytoplasm and where daughter cells first become visible in mitosis is called telophase.
During mitosis, telophase is the final stage, following anaphase. In this phase, the chromosomes that have been separated during anaphase begin to de-condense back into chromatin. At the same time, the nuclear membranes start to reform around each set of chromosomes, effectively creating two distinct nuclei within the cell.
As telophase progresses, the process of cytokinesis also begins. Cytokinesis is the physical division of the cytoplasm, which results in the formation of two daughter cells. In animal cells, this occurs through a cleavage furrow that constricts the cell membrane, while in plant cells, a cell plate forms. Therefore, it is in telophase that we not only see the reformation of the nuclei but also the physical changes that indicate the cell is nearing its division into two separate, identical daughter cells.