Light micrograph showing mitosis - telophase if whitefish blastula. No magnification given. Mitosis, the usual method of cell division, characterized typically by the resolving of the chromatin of the nucleus into a threadlike form, which condenses into chromosomes, each of which separates longitudinally into two parts, one part of each chromosome being retained in each of two new cells resulting from the original cell. The four main phases of mitosis are prophase, metaphase, anaphase, and telophase. Blastula, an animal embryo at the stage immediately following the division of the fertilized egg cell, consisting of a ball-shaped layer of cells around a fluid-filled cavity known as a blastocoel.