Cell cycle during early cleavage. Cell cycle progression in Nothobranchius furzeri was visualized by fluorescent ubiquitination-based cell cycle indicator (FUCCI). From (A) to (H) the last synchronus division (fourth) is shown, all nuclei are yellow and cells perfectly synchronized in cycling. (I) Represents the first division where asynchrony starts. (J) last synchronous dark stage. (K-L) later stages when cells are clearly asynchronous and cells in different phases can be recognized at the same time point.