Chromosome behavior preceding secondary nuclei formation within a giant primary nucleus (50–100 μm in diameter) inAcetabularia calyculus was observed by the fluorescence emitted from 4′-6-diamidino-2-phenylindole (DAPI)-stained DNA.
Throughout the period when the large nucleolus was present in the primary nucleus, thin chromonemata were observed twining around the nucleolus. Nuclear division was initiated by degeneration of the sausage-shaped nucleolus into a number of spherical subunits soon after the initiation of cap formation. On the fourth day of cap development, the chromonemata became thicker and chromomeres appeared. They accumulated adjacent to the single spherical nucleolus. The lump of chromosomes became loosened and thick chromosomes were scattered in the nucleus. The peculiar shapes of chromosomes which suggest the existence of chiasmata were frequently observed until the chromosome segregation started. This sequence of chromosome behavior seems to be the prophase of meiotic division. Chromosome segregation, the “first meiotic division”, occurred on the seventh day of cap development, probably being accompanied by the “second meiotic division”. Immediately after nuclear division of the primary nucleus, secondary nuclei were formed and cyst formation started 24 hours after repeated mitoses of the secondary nuclei.