During prophase, the first stage of nuclear division, the chromatin gradually condenses and the chromosomes become visible as discrete bodies. In early prophase, the chromosomes appear as slender, elongate threads that slowly shorten and thicken. By late prophase, it may be possible to see that each chromosome now consists of two longitudinal halves, the chromatids, joined at a constriction called the centromere. As prophase continues, the nucleoli become smaller and eventually disappear, and the nuclear membrane disintegrates. Prophase ends when the nuclear membrane has completely disappeared.