Simplify the following text for a Grade 11 native English-speaking person.
Original: Prophase II : The chromosomes become visible , the nuclear envelope disappears and the centrioles form the spindle fibers .
Simplified: In prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids . Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division .